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NIST TN 2228
Wind-Driven Fire Spread to a Structure from Fences and Mulch
August 10, 2022
December 12, 2022
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Wind-Driven Fire Spread to a Structure from Fences and Mulch
Kathryn M. Butler; Erik L. Johnsson; Alexander Maranghides; Shonali Nazare;
Marco Fernandez; Mariusz Zarzecki; Wei Tang; Eric Auth; Rachel McIntyre;Michael Pryor; William Saar; Colin McLaughlin
December 12, 2022
https://doi.org/10.6028/NIST.TN.2228-upd1

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Date updated: December 12, 2022

NIST Technical Note
NIST TN 2228

Wind-Driven Fire Spread to a
Structure from Fences and Mulch
Primary Authors: Kathryn M. Butler and Erik L. Johnsson

Alexander Maranghides
Shonali Nazare
Marco Fernandez
Mariusz Zarzecki
Wei Tang
Eric Auth
Rachel McIntyre
Michael Pryor
William Saar
Colin McLaughlin
This publication is available free of charge from:https://doi.org/10.6028/NIST.TN.2228

NIST Technical Note
NIST TN 2228

Wind-Driven Fire Spread to a
Structure from Fences and Mulch
Primary authors: Kathryn M. Butler and Erik L. Johnsson

Alex Maranghides
Shonali Nazare
Marco Fernandez
Fire Research Division
Engineering Laboratory

Mariusz Zarzecki*
Wei Tang*
Eric Auth*
Rachel McIntyre*Michael Pryor*
William Saar*
Colin McLaughlin**Former NIST employee; all work for this publication was done while at NIST.

This publication is available free of charge from:https://doi.org/10.6028/NIST.TN.2228
August 2022

U.S. Department of Commerce
Gina M. Raimondo, Secretary
National Institute of Standards and Technology
Laurie E. Locascio, NIST Director and Under Secretary of Commerce for Standards and Technology

NIST TN 2228
August 2022
Certain commercial entities, equipment, or materials may be identified in this document in order to describe an
experimental procedure or concept adequately. Such identification is not intended to imply recommendation or
endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the entities,
materials, or equipment are necessarily the best available for the purpose.

NIST Technical Series Policies

Copyright, Fair Use, and Licensing Statements
NIST Technical Series Publication Identifier Syntax
Publication History
Approved by the NIST Editorial Review Board on 2022-07-15

How to Cite this NIST Technical Series Publication
Butler KM and Johnsson EL, Maranghides A, Nazare S, Fernandez M, Zarzecki M, Tang W, Auth E, McIntyre R,
Pryor M, Saar W, and McLaughlin C (2022) Wind-Driven Fire Spread to a Structure from Fences and Mulch.
(National Institute of Standards and Technology, Gaithersburg, MD), NIST Technical Note (TN) NIST TN 2228.
https://doi.org/10.6028/NIST.TN.2228

NIST Author ORCID iDs
Kathryn M. Butler: 0000-0001-7163-4623
Erik L. Johnsson: 0000-0003-1170-7370
Alex Maranghides: 0000-0002-3545-2475
Shonali Nazare: 0000-0002-0407-5849
Marco Fernandez: 0000-0002-4227-8866

Contact Information

kathryn.butler@nist.gov
erik.johnsson@nist.gov

NIST TN 2228
August 2022

Abstract
A series of field experiments was conducted to examine the effects on fire spread toward a
structure for combustible fences and mulch under conditions that may be encountered in a
wildland-urban interface (WUI) fire. The fire behavior of a variety of materials, designs, andconfigurations were studied under various wind conditions. The 187 experiments were split
into five categories: mulch only, fence only, fence plus mulch, parallel fences, and long range
firebrand experiments. Fence materials included western redcedar, California redwood, pine,
vinyl, and wood-plastic composites, and fence styles included privacy, lattice, and goodneighbor (board on board). The tested mulch types were shredded hardwood, mini pine barknuggets, pine straw, rubber, and artificial turf. A wind machine provided a mean wind speed
between 6 m/s and 14 m/s (13 mi/h to 31 mi/h). The fence and/or mulch bed was ignited by apropane burner on the ground at the end farthest from the structure. A small structure was
located between 0 m and 1.83 m (0 ft to 6 ft) downwind of the fence or mulch bed as a target
for flames and firebrands. A target mulch bed at the base of the structure tested the ability of
firebrands produced by the burning fence and mulch bed to ignite spot fires that threatened
the structure.
The experiments in this study demonstrated that combustible fences can be rapid conduits forfire and can potentially spread fire to attached or adjacent structures. Combinations ofcombustible items were found to increase the fire hazard disproportionately. Fire behavior
was classified as very high, high, medium, and low hazard. Rapid fire growth and large
flames were found for parallel fences and one type of wood-plastic composite fence. Goodneighbor fences carried flames from the ground to the top of the fence. Combustible fenceswith mulch at their base were found to be high hazard, transporting fire through thecommunity and providing a steady source of firebrands. Fire spread continuously over mulchbeds, with progress sometimes enhanced by the ignition of spot fires downwind. Medium fire
hazard was found for wood fences in the absence of mulch, with slow fire spread dominatedby glowing combustion with occasional small flames. Low fire hazard was expected fornoncombustible fences, with maintenance required to minimize the accumulation of finecombustible materials along the fence.
In all cases the fire progress was affected by the wind field; the structure created both upward
flow (enhanced by buoyancy) and a vortex that both deposited firebrands next to the structureand slowed flame spread on the ground. During most experiments, the burning mulch andfences produced firebrands that ignited spot fires in the target mulch bed. In long range
experiments, firebrands from fences and mulch beds caused ignitions over 47 m (155 ft)
downwind.
This study of fence fire spread is part of a series designed to better inform standards andcodes regarding placement of landscape features around homes that are at risk of exposure to
wildland-urban interface fires.
Keywords
Embers; fence fires; fences; firebrands; fire spread; mulch; parallel fences; structural
ignition; structure vulnerability; wildland urban interface fires; wind-driven fires; WUI fires.

iNIST TN 2228
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Table of Contents
Executive Summary ........................................................................................................... 1
Introduction ................................................................................................................14
Motivation...............................................................................................................14
Background ............................................................................................................18
1.2.1. Structure Vulnerabilities ....................................................................................18
1.2.2. Fence Studies ..................................................................................................18
1.2.3. Mulch Studies ...................................................................................................20
Approach................................................................................................................21
Objectives ..............................................................................................................22
Experimental Description ..........................................................................................23
Research Location and Site Description .................................................................23
Wind Field Generation............................................................................................24
2.2.1. Wind Machine...................................................................................................24
2.2.2. Flow Straightener .............................................................................................25
2.2.3. Velocity Profiles ................................................................................................25
Target Shed ...........................................................................................................26
Mulch Types and Preparation.................................................................................27
2.4.1. Mulch Pans ......................................................................................................27
2.4.2. Mulch Types .....................................................................................................28
2.4.3. Mulch Conditioning ...........................................................................................30
2.4.4. Target Mulch Bed .............................................................................................31
Fence Types, Materials, and Preparation ...............................................................32
2.5.1. Privacy Fences .................................................................................................32
2.5.2. Lattice Fences ..................................................................................................34
2.5.3. Good Neighbor Fences ....................................................................................34
2.5.4. Wood-Plastic Composite Fences ......................................................................35
2.5.5. Fence Support ..................................................................................................35
2.5.6. Parallel Fences.................................................................................................36
2.5.7. Fence Materials ................................................................................................37
2.5.8. Wood Conditioning ...........................................................................................38
Ignition Source .......................................................................................................38
Measurements .......................................................................................................392.7.1. Wind Speed Profiles .........................................................................................392.7.2. Ambient Wind Speed and Direction ..................................................................42
Data Acquisition .....................................................................................................42ii

NIST TN 2228
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2.8.1. Wind and Temperature Data ............................................................................422.8.2. Digital Video and Photographic Records ..........................................................42
Experimental Procedures .......................................................................................44
2.9.1. Weather Conditions ..........................................................................................44
2.9.2. Preparation .......................................................................................................44
2.9.3. Operations ........................................................................................................45
Parameter Summary ..............................................................................................46
2.10.1. Subject of Experiment ......................................................................................46
2.10.2. Firebrand Target ...............................................................................................47
2.10.3. Fence Material, Style, and Configuration ..........................................................48
2.10.4. Mulch Type .......................................................................................................48
2.10.5. Separation Distance from Structure ..................................................................49
2.10.6. Wind Speed and Direction ................................................................................50
Analytical Tools..........................................................................................................51
Video Analysis........................................................................................................51
3.1.1. Event Timing ....................................................................................................51
3.1.2. Conversion of Videos to Image Sequences ......................................................54
3.1.3. Mulch Experiment Flame Front Tracking ..........................................................54
3.1.4. Fence Experiment Flame Front Tracking ..........................................................56
Wind Analysis and Visualization .............................................................................57
Flow Simulations ....................................................................................................60
3.3.1. FDS Model of Mulch Experiments ....................................................................60
3.3.2. Flow Field, including Vortex at Base of Shed ....................................................61
3.3.3. Wind Effects on Firebrands and Fire Spread ....................................................63
Experimental Results .................................................................................................64
Mulch Only Experiments ........................................................................................64
4.1.1. Example ...........................................................................................................64
4.1.2. Fire Spread Behavior........................................................................................67
4.1.3. Type of Mulch ...................................................................................................71
4.1.4. Firebrand Spotting ............................................................................................83
4.1.5. Summary ..........................................................................................................87
Fence Only Experiments ........................................................................................89
4.2.1. Example ...........................................................................................................89
4.2.2. Fire Spread Behavior........................................................................................91
4.2.3. Type of Fence ..................................................................................................93
4.2.4. Firebrand Spotting ............................................................................................98

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NIST TN 2228
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4.2.5. Summary ........................................................................................................101
Fence and Mulch Experiments .............................................................................103
4.3.1. Example .........................................................................................................103
4.3.2. Fire Spread Behavior......................................................................................106
4.3.3. Type of Fence, combined with HW Mulch .......................................................117
4.3.4. Type of Mulch, combined with WRC Privacy Fence........................................132
4.3.5. Firebrand Spotting ..........................................................................................138
4.3.6. Summary ........................................................................................................143
Parallel Fence Experiments..................................................................................146
4.4.1. Example .........................................................................................................147
4.4.2. Type of Fence ................................................................................................150
4.4.3. Fire Spread Behavior......................................................................................152
4.4.4. Parallel Fences Without a Mulch Bed .............................................................159
4.4.5. Mixed Parallel Fences ....................................................................................161
4.4.6. Firebrand Spotting ..........................................................................................171
4.4.7. Discussion ......................................................................................................176
4.4.8. Summary ........................................................................................................176
Long-Range Firebrand Experiments.....................................................................179
4.5.1. Experimental Design ......................................................................................179
4.5.2. Firebrand Spotting ..........................................................................................179
4.5.3. Summary ........................................................................................................184
Discussion ................................................................................................................185
Hazardous Scenarios ...........................................................................................185
Limitations ............................................................................................................188
Conclusions .............................................................................................................190
Key Findings ........................................................................................................190
6.1.1. General Findings ............................................................................................190
6.1.2. Very High Hazard Configurations ...................................................................191
6.1.3. High Hazard Configurations............................................................................192
6.1.4. Medium Hazard Configurations ......................................................................194
6.1.5. Low Hazard Configurations ............................................................................194
Primary Recommendations ..................................................................................194
Recommendations for Future Work ......................................................................195
References .......................................................................................................................197
Appendix A.

Uncertainties...........................................................................................205

A.1. Experimental Setup ..............................................................................................207

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A.2. Wind Data ............................................................................................................211
A.3. Timing Data ..........................................................................................................213
A.4. Flame Spread Analysis ........................................................................................216
A.4.1. Mulch .............................................................................................................216
A.4.2. Fences ...........................................................................................................218
Appendix B.

Flammability Comparison of Fence Materials ......................................220

B.1. Test Sample .........................................................................................................220
B.2. Experimental Procedure .......................................................................................221
B.3. Test Results .........................................................................................................221
B.4. Ignitability and Extinction ......................................................................................223
B.5. Heat Release Rate ...............................................................................................223
B.6. Firebrand Generation and Char Residue ..............................................................224
B.7. Concluding Remarks ............................................................................................225
Appendix C.

Wind Characterization ............................................................................227

C.1. Measurement Distance from Wind Machine .........................................................227
C.2. Wind Profiles ........................................................................................................229
C.3. Measure of Wind Speed for Each Experiment ......................................................235
Appendix D.

Experimental Matrices............................................................................236

Appendix E.

Reading the Case Descriptions .............................................................244

E.1. Data Box A: Experimental Configuration ..............................................................244
E.2. Data Box B: Photographs Taken Before and During the Experiment ....................245
E.3. Data Box C: Flame Spread as a Function of Time ................................................245
E.3.1. Fences ...........................................................................................................245
E.3.2. Mulch Only .....................................................................................................246
E.4. Data Box D: Table of Timing Values and Environmental Factors..........................247
E.5. Data Box E: Applied Wind ....................................................................................248
E.6. Data Box F: Ambient Wind ...................................................................................249
Appendix F.

Case Details: Mulch Only .......................................................................251

Appendix G.

Case Details: Fence Only .......................................................................275

Appendix H.

Case Details: Fence and Mulch .............................................................286

Appendix I.

Case Details: Parallel Fences ................................................................334v

NIST TN 2228
August 2022

List of Figures
Fig. 1. Fence burning in Magalia, California during Camp Fire, 8 November 2018.
Photographs taken two minutes apart. CAL FIRE, used by permission. ..............................16
Fig. 2. Fence burning during Camp Fire as viewed from an ambulance. American Medical
Response (AMR) – Shasta County, used by permission. ....................................................16
Fig. 3. Ignited wood fencing in Waldo Canyon Fire in Colorado, 2012. Photo from Colorado
Springs Fire Department, used by permission. ....................................................................17
Fig. 4. Major components of the experiment (not to scale). .................................................23
Fig. 5. Aerial view of site used for experiments. Google Earth image with NIST overlay. ....24
Fig. 6. Photo of test site showing fan, flow straightener and target shed in an experiment ona mulch bed without a fence. ...............................................................................................25
Fig. 7. Target shed configurations for a) Series 1 and b) Series 2 experiments...................27
Fig. 8. Mulch types: a) shredded hardwood mulch, b) mini pine bark nuggets, c) pine straw,
and d) shredded rubber mulch. ...........................................................................................29
Fig. 9. Moisture analyzer used for measuring moisture content of mulch. ...........................31
Fig. 10. Target mulch bed and digital timer. ........................................................................32
Fig. 11. Fence types: a) western redcedar privacy, b) aged privacy, c) vinyl privacy,d) redwood lattice, e) pressure treated pine lattice, f) western redcedar good neighbor,g) wood-plastic composite #1, and h) wood-plastic composite #2........................................33
Fig. 12. Parallel privacy fences. Colorado Springs Fire Department, used by permission. ..36
Fig. 13. Double lattice fence................................................................................................37
Fig. 14. Propane burner for igniting fence and/or mulch, with torches exposed. ..................38
Fig. 15. Propane burner protected by Kaowool blanket and aluminum foil. .........................39
Fig. 16. Bidirectional probe array. .......................................................................................40
Fig. 17. Diagram of the bidirectional probe array used to measure the velocity field. ..........41
Fig. 18. Top view schematic of experimental setup showing placements of video cameras,
timer, and bi-directional probe array. Google Earth image with NIST overlay. .....................43
Fig. 19. Distribution of 187 experiments by subject of experiment, firebrand target, fence
type, mulch type, separation distance from the wall, and wind speed. .................................46
Fig. 20. Experimental configurations with combustible wall as target. .................................49
Fig. 21. Experimental configurations with shredded hardwood mulch bed at base ofstructure as target. ..............................................................................................................49
Fig. 22. Illustration of timing markers for Test A-29, showing a) end of ignition process,b) start to removal of propane burner, c) audio track for fan on, d) audio track for fan off,e) beginning of water application, and f) 5 s after the water is applied. ................................52
Fig. 23. Example of GUI with perspective lines for analyzing flame spread over a mulch bed
with time. .............................................................................................................................55
Fig. 24. GUI for selection of location points for fence video. ................................................57
Fig. 25. Visualization of zeroing data for 29 July 2016, used for tests A-57 to A-60.............59
Fig. 26. Visualization of wind speed and ambient data for Test A-57. .................................60
Fig. 27. FDS model results showing instantaneous velocity vectors, colored by wind speed.
Side view (a) shows velocities along the center plane, and top view (b) shows velocities in aplane 0.1 m (4 in) above the ground. ...................................................................................61
Fig. 28. FDS model results showing time-averaged contours of wind speeds in thex-direction. Side view (a) shows contours along the center plane, and top view (b) shows
contours in a plane 0.1 m (4 in) above the ground. ..............................................................62
Fig. 29. Time sequence of shredded hardwood mulch bed at medium wind speed separated
from shed wall by 0.91 m (3 ft) [Test A-83]. .........................................................................65
Fig. 30. Flame front location vs. time for Test A-83, with HW mulch at medium wind speed
and 0.91 m (3 ft) separation distance from structure, showing expanded uncertainty. .........66

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Fig. 31. Two modes of fire spread behavior in HW mulch beds: uniform flame fronts for a)
medium [Test A-42] and c) high [Test A-27] wind speeds, and flame spread through spotting
for b) medium [Test A-8] and d) high [Test A-10] wind speeds. ...........................................67
Fig. 32. Effects of dominant spread mode and wind speed on flame front location vs. timefor HW mulch beds at zero separation distance from structure. ...........................................68
Fig. 33. Effects of wind speed on flame spread for HW mulch at separation distances ofa) 0.91 m (3 ft) and b) 1.83 m (6 ft). .....................................................................................69
Fig. 34. Time evolution of firebrand spot fire ignited at wall of structure, beginning with firstsign of spot fire at tspot [Test A-2]. ........................................................................................70
Fig. 35. Effects of separation distance on flame spread for HW mulch at medium windspeed. .................................................................................................................................71
Fig. 36. Effects of HW mulch thickness on flame spread for high wind speed and separation
distance of 1.83 m (6 ft). Black lines mark distance of each end of the mulch bed from theshed. ...................................................................................................................................72
Fig. 37. Effects of HW mulch thickness on flame spread for medium wind speed and zeroseparation distance from shed. ...........................................................................................73
Fig. 38. Time sequence of pine bark mulch bed in medium wind speed with zero separationfrom shed wall [Test A-99]. ..................................................................................................74
Fig. 39. Fire behavior examples for pine bark mulch beds with a) medium [Test A-100],b) low [Test C-4], c) medium [Test A-72], and d) high [Test A-86] wind speeds. ..................74
Fig. 40. Effects of wind speed on flame spread for PB mulch at separation distances ofa) 0 m and 0.30 m (1 ft), b) 0.91 m (3 ft), and c) 1.83 m (6 ft). .............................................75
Fig. 41. Time sequence for pine straw mulch bed in medium wind speed [Test A-98]. ........76
Fig. 42. Effects of wind speed on flame spread for PS mulch at 0.91 m (3 ft) separation
distance...............................................................................................................................77
Fig. 43. Time sequence for rubber mulch bed in medium wind speed [Test C-27]. .............78
Fig. 44. Shredded rubber mulch extinguished after 6 min, showing upper layer of crumbly
residue and lower layer of virgin material. ...........................................................................79
Fig. 45. Effects of wind speed on flame spread for rubber mulch at 1.83 m (6 ft) separation
distance...............................................................................................................................79
Fig. 46. Time sequence for artificial turf in low wind speed [D-22]. ......................................81
Fig. 47. Time sequence for artificial turf in medium wind speed [D-23]. ...............................81
Fig. 48. Curled segments and char residue after burning of artificial turf [Test D-23]...........82
Fig. 49. Effects of wind speed on flame spread for artificial turf at 1.83 m (6 ft) separation
distance...............................................................................................................................82
Fig. 50. Spot fires in target mulch bed resulting from mulch experiments a) HW mulch at half
thickness in medium wind [Test A-43], b) HW mulch in high wind [Test A-80], c) HW mulch
in medium wind [Test A-42], d) HW mulch in low wind [Test A-104]. ...................................83
Fig. 51. Time to ignition of first spot fire vs. wind speed for mulch experiments. ..................85
Fig. 52. Time to ignition of first spot fire vs. separation distance for mulch experiments. .....85
Fig. 53. Time to ignition of first spot fire to put flames against the wall vs. wind speed formulch experiments. .............................................................................................................86
Fig. 54. Time to flames reaching the wall vs. wind speed for mulch experiments. ...............86
Fig. 55. Progression of glowing combustion over western redcedar privacy fence in highwind speed [Test A-26], showing a) ignition, b) erosion of board at t = 2.5 min, c) large
firebrand at t = 4.5 min, d) secondary ignition by firebrand at t = 7.5 min, and e) burn pattern
at t = 11.5 min. ....................................................................................................................90
Fig. 56. Firebrand spotting at 9 min after fan on for Test A-26. ...........................................91
Fig. 57. Time sequence for Test A-101, western redcedar privacy fence only (no mulch),with medium wind speed at 1.83 m (6 ft) separation from shed. ..........................................92

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Fig. 58. Location of the flame front on the fence as a function of time. Blue lines indicate
experiments on western redcedar privacy fences, and red lines are from redwood latticefence experiments. Line style indicates separation distance between the fence and theshed: 0 m (dotted), 0.30 m (1 ft) (dashed), 0.91 m (3 ft) (dot-dash), and 1.83 m (6 ft) (solid).
............................................................................................................................................93
Fig. 59. WRC privacy fence without mulch, in medium wind speed at separation distances ofa) 0 m [Test A-18], b) 0.30 m (1 ft) [Test A-21], c) 0.91 m (3 ft) [Test A-65],
and d) 1.83 m (6 ft) [Test A-30]. ..........................................................................................94
Fig. 60. WRC privacy fence without mulch, in high wind speed at separation distances ofa) 0 m [Test A-28], b) 0.30 m (1 ft) [Test A-32], c) 0.91 m (3 ft) [Test A-25],
and d) 1.83 m (6 ft) [Test A-91]. ..........................................................................................94
Fig. 61. Redwood lattice fence only, in medium wind speed at separation distances ofa) 0 m [Test A-82], b) 0.30 m (1ft) [Test A-113], c) 0.91 m (3 ft) [Test A-77],
and d) 1.83 m (6 ft) [Test-A-75]. ..........................................................................................95
Fig. 62. Vinyl privacy fence in high wind conditions at 0 m separation distance [Test A-34].
............................................................................................................................................96
Fig. 63. Time sequence for Test E-2, wood-plastic composite fence WPC1 alone (without
mulch), with low wind speed at 1.83 m (6 ft) separation from shed. .....................................97
Fig. 64. Final configuration of wood-plastic composite boards after Test E-2. .....................97
Fig. 65. Water being applied to the eaves of the shed after Test E-2. Smoke rising from theroof is visible. ......................................................................................................................98
Fig. 66. Time to ignition of first spot fire vs. wind speed for experiments with fence only. ....99
Fig. 67. Time to ignition of first spot fire to put flames against the wall vs. wind speed forexperiments with fence only. .............................................................................................100
Fig. 68. Time to flames reaching the wall vs. wind speed for experiments with fence only.
..........................................................................................................................................100
Fig. 69. Image sequence from video of western redcedar fence combined with shredded
hardwood mulch, at medium wind speed and 1.83 m (6 ft) separation distance fromstructure [Test A-29]. .........................................................................................................104
Fig. 70. Flame front location vs. time for Test A-29, showing expanded uncertainty..........105
Fig. 71. Char patterns on western redcedar privacy fence during Test A-29, from the left
camera view (ignition at the lower right). ...........................................................................105
Fig. 72. Burn patterns for low, medium and high wind levels for WRC privacy fences:a) Test A-102, b) Test A-29, and c) Test A-31 and RW lattice fences: d) Test A-107,e) Test B-79, and f) Test B-85. ..........................................................................................107
Fig. 73. Effects of wind speed on flame front location vs. time for WRC privacy fences andRW lattice fences shown in Fig. 72....................................................................................108
Fig. 74. Burn patterns at low wind speed for separation distances of 0 m, 0.30 m (1 ft),
0.91 m (3 ft), and 1.83 m (6 ft) for WRC privacy fences: a) Test A-106, b) Test A-105,c) Test A-95, and d) Test A-102 and RW lattice fences: e) Test B-66, f) Test B-74,g) Test B-59, and h) Test A-107. .......................................................................................109
Fig. 75. Effects of separation distance on flame front location vs. time for WRC privacy
fences and RW lattice fences at low wind speed. ..............................................................110
Fig. 76. Set-up for two-panel length western redcedar fence combined with shredded
hardwood mulch a) with and b) without a structure downwind [Tests C-19 and D-3,
respectively]. .....................................................................................................................111
Fig. 77. Two-panel length western redcedar fence combined with shredded hardwood
mulch, at low wind speed and 1.83 m (6 ft) separation distance [Test C-19], at t = 45 min.
..........................................................................................................................................112
Fig. 78. Single panel western redcedar fence combined with shredded hardwood mulch, at
low wind speed and 1.83 m (6 ft) separation distance [Test A-102], at t = 20 min..............112

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Fig. 79. Effects of fence length on flame front location vs. time at low wind speed. ...........113
Fig. 80. Two-panel length western redcedar fence combined with shredded hardwood mulchwithout a structure downwind, at about t = 12 min [Test D-3].............................................114
Fig. 81. Comparison of two-panel length fences with and without a structure downwind. ..114
Fig. 82. Setups for western redcedar privacy fence panels at a height above the shredded
hardwood mulch pan of a) 0 cm [Test A-102], b) 5.1 cm [Test C-10], c) 7.6 cm [Test C-11],d) 10.2 cm [Test C-14], and e) 15.2 cm [Test C-28]. ..........................................................115
Fig. 83. Comparison of results from privacy fences raised above the hardwood mulch panby a) 0 cm [Test A-102], b) 5.1 cm [Test C-10], c) 7.6 cm [Test C-11], d) 10.2 cm[Test C-14], and e) 15.2 cm [Test C-28]. ...........................................................................116
Fig. 84. Effects of height above hardwood mulch bed on flame front location vs. time for
WRC privacy fences shown in Fig. 83. ..............................................................................117
Fig. 85. WRC privacy fence with HW mulch at medium wind speed, at about t = 4 ½ min[Test A-29]. .......................................................................................................................118
Fig. 86. Effects of wind speed on flame spread for WRC privacy fence combined with HWmulch at separation distances of a) 0 m, b) 0.30 m (1 ft), c) 0.91 m (3 ft), d) 1.83 m (6 ft). 119
Fig. 87. Aged privacy fence, at medium wind speed, at t = 5.2 min [Test C-25].................120
Fig. 88. Effects of wind speed on flame spread for aged privacy fence/HW mulch atseparation distance of 1.83 m (6 ft). ..................................................................................120
Fig. 89. Image sequence for vinyl privacy fence attached to wall with medium wind speed[Test A-35]. .......................................................................................................................121
Fig. 90. Redwood lattice fence combined with shredded hardwood mulch, 3½ min after fan
on, at medium wind speed [Test B-79]. .............................................................................122
Fig. 91. Effects of wind speed on flame spread for RW lattice fence/HW mulch at separation
distances of a) 0 m, b) 0.30 m (1 ft), c) 0.91 m (3 ft), d) 1.83 m (6 ft). ................................123
Fig. 92. Pine lattice fence combined with shredded hardwood mulch, 3 ½ min after fan on,
at medium wind speed [Test B-81]. ...................................................................................124
Fig. 93. Effects of wind speed on flame spread comparing pine lattice and redwood latticefences at 1.83 m (6 ft) separation distance. Shredded hardware mulch was arranged at thebase of both fences. ..........................................................................................................124
Fig. 94. Time sequence for good neighbor fence at low wind speed [Test C-18]. ..............125
Fig. 95. Good neighbor fence experiments at a) t = 4.5 min for medium wind speed[Test C-22] and b) t = 2.6 min for high wind speed [Test C-23]. .........................................126
Fig. 96. Effects of wind speed on flame spread for good neighbor fence/HW mulch atseparation distance of 1.83 m (6 ft). ..................................................................................127
Fig. 97. Time sequence for wood-plastic composite fence #1 at low wind speed and 1.83 m(6 ft) separation distance [Test E-1]...................................................................................128
Fig. 98. Final configuration of wood-plastic composite boards after Test E-1. Yellow arrows
highlight the final positions of fallen fence boards. .............................................................129
Fig. 99. Time sequence for wood-plastic composite fence #2 at low wind speed and 1.83 m(6 ft) separation distance [Test F-1]. ..................................................................................130
Fig. 100. Collapse of horizontal planks shortly after t = 12 min [Test F-1]..........................130
Fig. 101. Final configuration of wood-plastic composite boards after Test F-1. .................131
Fig. 102. Flame spread as a function of time for wood-plastic composite fence experiments.
..........................................................................................................................................132
Fig. 103. WRC fence with HW mulch in medium wind speed with mulch thickness equal toa) 5 cm [Test A-29] and b) 2.5 cm [Test A-64], at t = 5 min. ..............................................133
Fig. 104. Effects of wind speed on flame spread for WRC privacy fence combined with half
thickness (2.5 cm thick) HW mulch at separation distances of a) 0 m, b) 0.30 m (1 ft),c) 0.91 m (3 ft), d) 1.83 m (6 ft). .........................................................................................134

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Fig. 105. Time sequence for WRC privacy fence and PB mulch in medium wind speed[Test A-74]. .......................................................................................................................135
Fig. 106. Time sequence for WRC privacy fence and PB mulch in medium wind speed[Test A-90]. .......................................................................................................................135
Fig. 107. Effects of wind speed on flame spread for WRC privacy fence combined with PB
mulch at separation distances of a) 0 m, b) 0.30 m (1 ft), c) 0.91 m (3 ft), d) 1.83 m (6 ft). 136
Fig. 108. Time sequence for WRC privacy fence in PS mulch at medium wind speed[Test A-109]. .....................................................................................................................137
Fig. 109. Effects of wind speed on flame spread for WRC privacy fence combined with PS
mulch at separation distances of (a) 0 m, (b) 0.91 m (3 ft), and (c) 1.83 m (6 ft). ...............138
Fig. 110. Time to ignition of first spot fire vs. wind speed for fence plus mulch experiments.
..........................................................................................................................................140
Fig. 111. Time to ignition of first spot fire vs. separation distance for fence plus mulch
experiments. .....................................................................................................................140
Fig. 112. Time to ignition of first spot fire to put flames against the wall vs. wind speed forfence plus mulch experiments. ..........................................................................................141
Fig. 113. Time to flames reaching the wall vs. wind speed for fence plus mulch experiments.
..........................................................................................................................................142
Fig. 114. Time to first spot fire as a function of nominal wind speed for experiments forfences with mulch (red), mulch beds only (blue), and fences only (gold). ..........................142
Fig. 115. Comparison of a) single (Test B-79 at t = 3 min) and b) double (Test B-75 at
t = 1.5 min) RW lattice fences in medium wind speeds. .....................................................146
Fig. 116. Time sequence for parallel WRC privacy fences with shredded hardwood mulch at
46 cm (18 in) spacing and low wind speed [Test C-6], showing first three minutes. ...........148
Fig. 117. Time sequence for parallel WRC privacy fences with shredded hardwood mulch at
46 cm (18 in) spacing and low wind speed [Test C-6], for times from 3 ½ min to 6 min. ....149
Fig. 118. Parallel western redcedar privacy fences with shredded hardwood mulch at 20 cm(8 in) spacing and low wind speed, at t = 5 min [Test B-82]. ..............................................150
Fig. 119. Double redwood lattice fence with pine bark mulch at low wind speed, at t = 3 min[Test A-103]. .....................................................................................................................151
Fig. 120. Double pine lattice fence with shredded hardwood mulch at low wind speed, atabout t = 3:15 [Test C-29]. Note spot fires in the target mulch bed at the base of thestructure caused by firebrands. .........................................................................................152
Fig. 121. Flames extending above and downwind of parallel WRC privacy fences at fence
panel spacings of a) 20 cm at t = 5:00 [Test B-82], b) 30 cm at t = 5:00 [Test C-5], c) 46 cm
at t = 4:00 [Test C-6], and d) 61 cm at t = 14:30 [Test C-7]. ...............................................153
Fig. 122. Fire behavior of parallel WRC privacy fences at fence panel spacings of a) 20 cm[Test B-82], b) 30 cm [Test C-5], c) 46 cm [Test C-6], and d) 61 cm [Test C-7]. ................153
Fig. 123. Fire behavior for parallel privacy fences spaced apart by 91 cm [Test C-8] showinga) end view and b) left side view at t = 18:00. ....................................................................154
Fig. 124. Fire behavior at t = 5:00 for two-panel length parallel privacy fences at 91 cm
spacing [Test C-13]. ..........................................................................................................155
Fig. 125. Fire behavior at t = 6:15 for two-panel length parallel privacy fences at 91 cm
spacing [Test C-13]. ..........................................................................................................155
Fig. 126. Extent of charring as a function of time for parallel WRC privacy fences withspacings between fence panels from 20 cm to 91 cm. ......................................................156
Fig. 127. Location of char front closest to the wall as a function of time for one- and twopanel length parallel WRC privacy fences with 91 cm spacing. Black horizontal lines indicate
positions of end and center posts. .....................................................................................157x

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Fig. 128. Double redwood lattice fences with hardwood mulch as a function of wind speed:a) low wind (Test A-103 at t = 3:00), b) medium wind (Test B-75 at t = 1:30), and c) highwind (Test B-62 at t = 1:30). ..............................................................................................158
Fig. 129. Location of char front closest to the wall as a function of time for parallel redwood
and pine lattice fences with HW mulch at various wind speeds. ........................................159
Fig. 130. Parallel fences without mulch: a) western redcedar privacy fence with 20 cm (8 in)spacing [Test B-84], b) redwood lattice fence at medium wind speed [Test B-80], and c) pine
lattice fence at low wind speed [Test C-20]. ......................................................................160
Fig. 131. Double redwood lattice fences without mulch at t = 4 min with low wind speed[Test A-110]. .....................................................................................................................160
Fig. 132. Cement board (noncombustible) parallel to WRC privacy fence with 31 cm (12 in)spacing, near end of experiment (t = 6 min), in camera views from a) near the fan, b) left
side, and c) right side [Test D-7]. Arrows indicate wind direction. ......................................162
Fig. 133. Cement board (noncombustible) parallel to WRC privacy fence with 46 cm (18 in)spacing, near end of experiment, in camera views from a) near the fan, b) right side, and c)left side [Test C-15]. Arrows indicate wind direction...........................................................163
Fig. 134. Comparison of char patterns near the end of experiments with parallel WRC fence/ cement board at a) 31 cm (12 in) spacing [Test D-7] and b) 46 cm (18 in) spacing [Test C15] and c) single WRC panel [Test A-102]. Arrows indicate wind direction. .......................164
Fig. 135. Extent of charring as a function of time comparing parallel WRC fence/cement
board combination at 31 cm (12 in) and 46 cm (18 in) spacings to single panel WRC fence.
..........................................................................................................................................164
Fig. 136. Vinyl privacy fence parallel to western redcedar privacy fence near end of test in
camera views from a) near the fan, b) right side, and c) left side [Test C-16]. ...................165
Fig. 137. Flame spread comparison of parallel WRC fence/vinyl fence combination at 46 cm(18 in) spacing to single panel WRC fence. .......................................................................166
Fig. 138. Pine lattice fence parallel to western redcedar privacy fence with 31 cm (12 in)spacing, at maximum flame length (t = 2:15), in camera views from a) near the fan, b) left
side, and c) right side [Test D-8]. .......................................................................................167
Fig. 139. Pine lattice fence parallel to western redcedar privacy fence with 46 cm (18 in)spacing, near end of test, in camera views from a) near the fan, b) right side, and c) left side
[Test C-17]. .......................................................................................................................168
Fig. 140. Comparison of flame spread patterns for WRC privacy fence parallel to pine latticefence at spacings of 31 cm (12 in) [D-8] and 46 (18 in) [C-17] with single panel WRC privacy
fence [A-102]. Arrows indicate wind direction. ...................................................................169
Fig. 141. Comparison of flame spread patterns for WRC privacy fence parallel to pine latticefence at spacings of 31 cm (12 in) [D-8] and 46 (18 in) [C-17] with single panel PL fence [C21]. Arrows indicate wind direction. ...................................................................................170
Fig. 142. Flame spread comparison of parallel WRC privacy fence/pine lattice fence
combination at 31 cm (12 in) and 46 cm (18 in) spacing to single panel WRC privacy fence
and pine lattice fence. .......................................................................................................171
Fig. 143. Time to ignition of first spot fire vs. wind speed for parallel fence plus mulch
experiments. .....................................................................................................................173
Fig. 144. Time to ignition of first spot fire to put flames against the wall vs. wind speed forparallel fence plus mulch experiments. ..............................................................................173
Fig. 145. Time to flames reaching wall vs. wind speed for parallel fence plus mulch
experiments. .....................................................................................................................174
Fig. 146. Comparison of times to ignition of first spot fire between single fences and parallel
fences with mulch as a function of wind speed. .................................................................174
Fig. 147. Time to first spot fire as a function of nominal wind speed for experiments on
single (blue) and parallel (green) fences only (without mulch) in which spotting occurs. ....175xi

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Fig. 148. Double lattice fence experiment without a structure and with a target mulch bed
situated 47.6 m (156 ft) from the downwind end of the fence [Test C-3]. ...........................180
Fig. 149. Shredded hardwood mulch experiment without a structure and with a target mulch
bed situated 47.0 m (154.5 ft) from the downwind end of the mulch bed [Test C-2]. ..........180
Fig. 150. Spot fire ignition resulting from double redwood lattice fence, showing a) the target
mulch bed at t = 10 min, after 2 min at high wind speed, and the double lattice fence atb) t = 7 min and c) t = 10 min after the fan was turned on [Test C-3]. ................................181
Fig. 151. Spot fire ignition in a) target mulch bed resulting from b) burning hardwood mulch
at t = 6 min, with high wind speed [Test C-2]. ....................................................................182
Fig. 152. Spot fire ignitions in a) target mulch bed resulting from b) burning hardwood mulch
at t = 16 min, after 2 min at high wind speed [Test C-2]. ....................................................182
Fig. 153. Setup for shredded hardwood mulch bed without a structure and with an elevatedtarget mulch bed situated 38.1 m (125 ft) from the downwind end of the mulch bed[Test D-1]. .........................................................................................................................183
Fig. 154. Spot fire ignition in a) target mulch bed resulting from b) burning hardwood mulch
at t = 6 min [Test D-1]. .......................................................................................................183
Fig. 155. Woodpile experiment without a structure and with a) a target mulch bed situated25.6 m (84 ft) from the downwind end of b) the woodpile [Test C-1]. .................................184
Fig. 156. Illustration of numerous fire spread pathways among neighboring parcels via linear
features and other combustibles. Inset photographs are from field observations andexperiments. Reproduced from [16], Fig. 10. .....................................................................186

Fig. B.1. Specimens of a) PVC, b) WRC, and c) WPC1 fence boards showing crosssectional and top views. ....................................................................................................220
Fig. B.2. Cone calorimeter samples of a) PVC, b) WRC, and c) WPC1 fence materials. ..221
Fig. B.3. Heat release data for PVC, WRC, and WPC1 fence materials exposed to
50 kW/m2 heat flux.............................................................................................................223
Fig. B.4. Digital images of (a) firebrand formation in WRC wood sample after flame
extinction, (b) flaky, ash residue of wood, (c) hard char residue of PVC sample, and d) flaky,woody residue of WPC1 sample after 800 s of exposure to 50 kW/m2 heat flux. ...............225

Fig. C.1. Distances between experimental elements. Distances to scale. .........................227
Fig. C.2. Locations of bidirectional probe array for separation distances of(A) 1.83 m, (B) 0.91 m, (C) 0.30 m, and (D) 0 m. Distances to scale. ................................228
Fig. C.3. Diagram of the bidirectional probe array used to measure the velocity field.
(Repeated from Fig. 17). ...................................................................................................230
Fig. C.4. Mean wind speed pseudocolor plots by wind speed and probe array location, over
all experiments. Dimensions to scale. ...............................................................................233
Fig. C.5. Horizontal weighted mean wind speed profiles 1.22 m (4 ft) above the ground. ..234
Fig. C.6. Vertical weighted mean wind speed profiles along the centerline. ......................234

Fig. E.1. Experimental case description. ...........................................................................244
Fig. E.2. Data Box A – Experimental configuration. ...........................................................245
Fig. E.3. Data Box B – Photographs taken before and during the experiment. ..................245
Fig. E.4. Data Box C – Flame spread as a function of time for fence experiments. ...........246
Fig. E.5. Data Box C – Flame spread as a function of time for mulch experiments. ..........247
Fig. E.6. Data Box D – Table of timing values and environmental factors. ........................248

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Fig. E.7. Data Box E – Applied wind. ................................................................................249
Fig. E.8. Data Box F – Ambient wind. ...............................................................................250

List of Tables
Table 1. Number of experiments producing spot fires for each mulch type. ........................87
Table 2. Number of experiments producing spot fires for each fence type. .........................99
Table 3. Number of experiments producing spot fires for each fence plus mulch type. .....139
Table 4. Number of experiments producing spot fires for each combination of parallel fenceswith mulch. ........................................................................................................................172
Table 5. Number of experiments producing spot fires for parallel fences without mulch. ...175

Table A.1. Uncertainty in wood fence dimensions .............................................................209
Table A.2. Uncertainty in dimensions for experimental setup ............................................211
Table A.3. Uncertainty budget for point velocity measurements ........................................212
Table A.4. Uncertainty in timing data ................................................................................215
Table A.5. Mulch experiments: Uncertainty in flame spread analysis ................................217
Table A.6. Fence experiments: Uncertainty in flame spread analysis ...............................219
Table B.1. Cone calorimetry data for PVC fence material at 50 kW/m2. ............................221
Table B.2. Cone calorimetry data for WRC fence material at 50 kW/m2. ...........................222
Table B.3. Cone calorimetry data for WPC1 fence material at 50 kW/m2. .........................222
Table C.1. Distances from fan for bidirectional probe array at various separation distances.
..........................................................................................................................................229
Table C.2. Number of experiments in each category of wind speed and array location. ....229
Table C.3. Weighted mean velocity for lower four probes along centerline in each categoryof wind speed and array location, in m/s............................................................................235
Table D.1. Mulch only. ......................................................................................................237
Table D.2. Fence only. ......................................................................................................238
Table D.3. Fence plus mulch – Western redcedar (WRC) privacy fences. ........................239
Table D.4. Fence raised above mulch bed ........................................................................240
Table D.5. Fence plus mulch – Other fences. ...................................................................241
Table D.6. Parallel fences involving western redcedar (WRC) privacy fence.....................242
Table D.7. Parallel lattice fences. ......................................................................................243

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Acknowledgments
The Frederick County Fire and Rescue Training Facility in Frederick, Maryland made thiswork possible by allowing us to perform the experiments on their property. NIST greatly
appreciates the cooperation and support from the leadership of Frederick County Fire,
including County Fire Chief Tom Coe, Assistant County Fire Chief David Barnes; Battalion
Chiefs Lenne Stolberg, Frank Malta, Dan Healy, Jon Newman, and Chris Mullendore;
Captains Jeff Shippey and Michael Webb; Lieutenants Chris Dimopoulos, Mike Moser, andJohn Arnold; scheduler Cheryl Riley; and the always helpful Lew Raeder, training logistics.
Special thanks to Jeremy Taylor of the Colorado Springs Fire Department (currently ofColorado Springs Utilities) for overcoming much shipping-related aggravation to send us
several aged fence panels to test in addition to some great fence photos.
The NIST technicians who contributed their technical expertise and hard labor to ordering
supplies for, instrumenting, setting up, carrying out, and cleaning up after the nearly two
hundred experiments in this study have earned our great appreciation. Many thanks to Phil
Deardorff, Laurean Delauter, Tony Chakalis, Steven Fink, and Mike Selepak for their expert
assistance to Rik Johnsson and Marco Fernandez in making these experiments happen.
Finally, this report has been greatly strengthened through careful review by NIST colleagues
Glenn Forney, Rodney Bryant, Tom Cleary, Jiann Yang, and Paul Reneke and external
reviewer Rusty Day of Portola Valley, California. Thank you all for your thoughtfulcomments.

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List of Abbreviations and Acronyms
AHJ

Authority Having Jurisdiction

BRI

Building Research Institute

FDS

Fire Dynamics Simulator

GUI

Graphic User Interface

HOA

Homeowners Association

HMM

Hazard Mitigation Methodology

HRR

Heat release rate

HWM

Shredded hardwood mulch

IBHS

Insurance Institute for Business & Home Safety

NFPA

National Fire Protection Association

NIST

National Institute of Standards and Technology

PHRR

Peak heat release rate

PBM

Pine bark mulch

PSM

Pine straw mulch

PVC

Polyvinyl chloride, commonly referred to as vinyl

RH

Relative humidity

RSS

Root sum square

RW

Redwood

WPC

Wood-plastic composite

WRC

Western redcedar

WUI

Wildland-urban interface

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Executive Summary
Wildland-urban interface (WUI) fires threaten communities in many locations around theworld. Fences and mulch have been identified as contributors to the spread of WUI fires
within communities. Once ignited, these fuels become sources that may ignite nearby objects
through direct flame contact, radiation, convection, and firebrands. The hazard of winddriven fire propagation and spread associated with fences varies greatly depending on theirdesign, material composition, configuration with respect to nearby materials and objects, andmaintenance.
It is important to understand the mechanisms by which these combustible landscaping
elements can transport fire to a home in order to find ways to address the hazard. Suchknowledge helps with proactive design, implementation, and maintenance within thecommunity. It informs homeowners on what they can do to protect themselves and theirproperties. It also helps fire departments to plan defensive strategies, placing resources andassigning tasks where they will be the most effective. The goal is to enhance the safety ofmembers of the public and first responders and to reduce structural fire losses.
CONTRIBUTION OF FENCES AND MULCH TO THE FIRE PROBLEM
WUI fire case studies performed by the National Institute of Standards and Technology(NIST) have identified fences and mulch as common contributors to the spread of WUI fires
within communities. In the Tanglewood Complex Fire, over 2.4 km of fences were found to
be damaged or destroyed within a community of 47 residential structures in which 17 homes
were destroyed and 4 were damaged. Instances of fires spreading along fences to structures
were observed in the Witch Creek Fire and the Waldo Canyon Fire. Post-fire fieldobservations have found that wood fences are often totally consumed, leaving behind only
the metal hardware (nails and screws) used during their assembly. In high fire and ember
exposure locations, fences that are partially burned have often been linked to specificdefensive actions, as related by first responders. In many WUI fires, firefighters haveremoved fences as part of their defensive strategy aimed at containing the fire or preventing
it from reaching a nearby structure. Such activities reduce resources allocated to life safety
operations and direct structure protection.
Fences, mulch, and other combustible landscaping elements can act as both potential ignition
sites from existing fires and sources of fire spread themselves. These materials can be ignited
by a wildfire through direct flame contact, radiation and convection from the flames, or
firebrands. Firebrands, also referred to as embers, are carried by the wind and may ignite
combustible materials in a community far downwind of the fire front. Once ignited, fences
and mulch may themselves ignite nearby objects, including a home, through the samemechanisms of direct flame contact, radiation, convection, and firebrands.
Fences are typically placed along the perimeter of parcels, and they can be connected directly
to neighboring fences or sit in close proximity to other fuels such as combustible sheds orwoodpiles. The way the fence is connected to a residential structure can also impact how thefire spreads from a burning fence and the probability of ignition.
Some fences release little or no energy while others burn vigorously. Fence and mulch
combinations can be divided into four fire spread hazard categories: very high, high,1

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medium, and low. Very high fire hazard fence configurations are those that threaten astructure through rapid fire spread and large flames extending above the fence. This study
found that parallel combustible surfaces fell into this category, as did certain wood-plastic
composite fences.
High fire hazard combinations of fence and mulch release a considerable amount of energy
while burning, igniting nearby combustibles through direct flame contact or radiation andconvection from the flames. This category also includes fences and mulch that generate
firebrands capable of igniting spot fires downwind. Most combustible fences, mulch, andfence-mulch combinations fell into this category, with the capability of acting as a wick tospread fire toward a structure in the time of minutes to an hour or so.
Fences and mulch are classified as medium fire hazard if they are combustible but do not
typically ignite nearby combustibles and do not cause significant downwind fire spread. In
this category were vinyl fences and single wood fences in the complete absence of adjacentfine fuels.
Although fences made of noncombustible materials such as stone, brick, or steel were notincluded in this study, they can be classified as low fire hazard. They do not burn on theirown but can trap and accumulate windblown debris along their length. If not removed,
combustible materials at the base of even noncombustible fences can transport fire.
Because of the linear character of fences, they can contribute to multiple fire pathways,
resulting in fire propagation both within and beyond the parcel of origin. This linearity andthe resulting extensive spatial fire and ember exposure can potentially increase the hazard to
multiple parcels and multiple structures in the vicinity of very high and high fire hazard
fences.
FENCE USAGE AND MAINTENANCE
Multiple considerations affect fence design and implementation. In addition to obvious
factors such as function, esthetics, and cost, permitting and installation requirements may be
imposed by Authorities Having Jurisdiction (AHJs) and homeowners associations (HOAs).
Some of these considerations impact the ignition propensity and fire behavior of fences.
Fences are often installed in contact with the ground to keep pets in a yard or unwanted
animals out. Unfortunately, this configuration may put those fences in proximity to
flammable mulch and to windblown debris accumulating at the base.
Fences are exposed to outdoor conditions, including extremes in temperature, precipitation,and UV radiation, over long time periods, typically with little or no fire prevention
maintenance. Leaf litter and other combustible debris may accumulate at the base, creating afine, dry, continuous fuel source analogous to the mulch beds in this study.
Vegetation planted near the fence may dry out, ignite in a WUI fire, and ignite the fence inturn. Interactions between the burning vegetation and fence may increase the intensity of thefence and enhance the fire spread.
Homeowners using fences to enclose their yards may erect fences near or at the property line.
When this practice is followed by two adjoining neighbors, it can result in a parallel fence
configuration that this study finds to be particularly hazardous.

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EXPERIMENTAL DESIGNThis report presents a study of fire spread from combustible fences and mulch beds to a
nearby structure in a wind field. The goal of the study was to assess the severity of the fire
hazard that fences and mulch posed to structures. The fire behavior of these fuels, including
flame spread rate, spotting due to firebrands, and downwind ignition potential, was observed
under various conditions of applied wind and proximity to a structure.
In these field experiments, a fence section, mulch bed or fence/mulch bed combination was
arranged perpendicular to the wall of a small structure and separated from it by a distance up
to the length of a fence panel. Beyond the fence and/or mulch bed was a large fan thatgenerated a wind field directed toward the structure. A fire was ignited using a propane
burner at the end of the object closest to the fan, and the fire was observed as it spread toward
the structure through flame contact and firebrand spotting.
In early experiments, the wall of the structure was covered by a sacrificial combustible layerand structure ignition was characterized. Only firebrands caught in the narrow space between
the structure and the pavement were found to cause incipient ignitions in these experiments.
To better characterize the ability of firebrands generated by the burning fence or mulch bed
to ignite combustible materials near the house, a second mulch bed was placed along the baseof the structure as a target. The wall was noncombustible in these experiments, and thestructure was considered threatened when the spot fire reached the wall. In every case, theexperiment was terminated using water suppression when fire reached the base of thestructure and before the structure itself ignited. The eaves and roof of the structure weremanually cooled when pyrolysis (smoking) was observed. The contributions of structure
geometry, wall cladding, and construction to structure ignition were not the focus of thisstudy.
Video analysis provided data to measure the progress of the fire along the mulch and fence.
The time for a firebrand to ignite a spot fire in the target mulch bed was recorded, along withthe time for the spot fire to reach the wall. Ambient wind and temperature were measured,and bidirectional probes recorded the speed of the wind reaching the test object.
A variety of fence and mulch materials, designs, and combinations were tested at three windspeed levels and four separation distances between the fuel source and the structure. The fourbasic configurations that were studied are illustrated in Fig. ES.1. Shown from left to right
are test setups for mulch only, fence only, fence plus mulch, and parallel fences. Fencematerials included western redcedar, redwood, pine, weathered wood, vinyl, and woodplastic composite. Privacy, lattice, and good neighbor (board on board) fence styles werestudied. Mulch types included shredded hardwood mulch, pine bark mulch, pine straw
mulch, rubber mulch, and artificial turf. Most fence experiments were performed with alength of one fence panel; a few experiments added a second fence panel length to test
whether the fire spread mechanisms had been fully captured. The three wind speed levels
were categorized as low (6 m/s or 13 mi/h), medium (10 m/s or 22 mi/h), and high (14 m/s or
31 mi/h). The four separation distances were 0 m, 0.30 m (1 ft), 0.91 m (3 ft), and 1.83 m(6 ft). Long-range firebrand spotting was studied in a small number of experiments without
the shed in place and with the target mulch bed at least 40 m (130 ft) downwind from thefirebrand source.

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Fig. ES.1. Configurations tested in this study at various separation distances (SD).

LIMITATIONSThis study was a survey of the fire behavior in wind of a variety of combustible fences andmulches near a structure. It illuminated the differences in behavior for selected materials andfence designs and demonstrated certain trends.
Limitations of this research include the following:
‒
‒
‒
‒
‒
‒
‒
‒
‒
‒

Combinations of fuels were limited.
Few experiments were repeated.
Distance downwind was limited for long-range spotting study.
Fuels were ignited at a single location on or near the ground.
Ignition was by gas burner rather than a natural source.
The orientation of wind to the structure wall was limited.
The mulch was preheated by heat conduction through the steel pan.
Accumulation of windblown debris was not considered in Fence Only experiments.
Effects of terrain were not studied.
Smoke toxicity was not included.

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KEY FINDINGSThe experiments in this study demonstrated a range of fire spread hazards from various types
and configurations of fences and mulch ignited close to a structure in a wind field. Generalfindings are listed first and followed by findings for configurations classified as very highhazard, high hazard, medium, and low hazard. The findings are labelled according to thefollowing categories:
FH

Fire Hazard

LSLife Safety

HR

Hazard Reduction – materials, assemblies, implementation/housekeeping

IC

Improved Characterization – recommended future work to characterize thesefuels more fully

General findings
The results from these experiments on fire spread demonstrated that:
F1. As combustible materials are combined, the hazard increases disproportionately.
(FH)
Fuel agglomeration provides significant increase in energy release and increases fire andember exposures. For a single combustible fence panel by itself, the fire behavior was limited
to glowing combustion near the area of ignition, with firebrands generating spot fires only onrare occasions. When a combination of a wood fence and shredded hardwood mulch was
ignited at the base, the flames remained over the lower half of the fence and progressed
steadily in the direction of the wind. Adding a second wood fence parallel to the first resulted
in flames engulfing the fences within a few minutes of ignition.
F2. Fences may impact egress. (LS)In a WUI fire, high and very high hazard fence configurations may result in a line of flamesclose to egress paths from a house or auxiliary dwelling. In one set of experiments on awood-plastic composite fence, the top and bottom frames distorted and allowed burning
boards to fall to either side. This created a 4 m (12 ft) wide zone of flames along the fenceline.
F3. Fire spread rates vary with fence material and design, wind speed, and fuelconfiguration, including the presence or absence of mulch. (FH)This report provides data on a variety of fence and mulch materials, designs, andconfigurations.
F4. Spot fires due to firebrands may ignite within a few minutes, even over a
distance of 47.6 m (156 ft) or more from the burning item, and may continue toignite long after the initial flaming combustion has subsided. (FH)
Firebrands capable of igniting spot fires downwind were generated by nearly all
combinations of fence and mulch tested in this study. All wood fences with mulch at the basecaused spot fires in the target mulch bed. Spot fires were often ignited within a few minutesof mulch and fence ignition. Shredded hardwood mulch and pine bark mulch burned and5

NIST TN 2228
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emitted firebrands for longer than an hour. Ignition of spot fires was also demonstrated fromfirebrands transported by the wind over distances as far as 47.6 m (156 ft) from the burning
item under high wind conditions and over a paved surface. The wind field may deposit
firebrands close to the wall of the structure. If a home is undefended during a WUI fire andnot properly hardened, these firebrands may pose a serious threat to the home.
F5. A standard test method is needed to evaluate the fire characteristics of fences.
(IC)
A standard test method is needed to assess the fire performance of fences. The method should
consider not only materials but assemblies and be carried out in a vertical orientation. Itshould be able to distinguish the fire behavior of various materials, including wood-plastic
composites, wood, and vinyl, and designs, including privacy, lattice, and good neighbor.
Very High Fire Hazard Configurations
Certain fence and mulch combinations were found to result in rapid fire spread and large
flames. In a region subject to WUI fires it’s advisable to remove these fuel sources if
possible. Standard tests used to evaluate the fire characteristics of fences should be
developed. Illustrations of the fire behavior of Very High Hazard fence and mulch
configurations are shown in Fig. ES.2.
The fire behavior of configurations of fences and mulch identified in this study as Very HighHazard supported the following findings:
F6. Rapid fire growth and large flames were found for parallel fences and one typeof wood-plastic composite fence. (FH)‒

Parallel wood privacy fences and double wood lattice fences were engulfed in flames
within a few minutes of ignition. Radiative exchange between the parallel burning
surfaces and convection of the hot gases trapped in the bounded space caused rapid
intensification of combustion and eruptive fire behavior. A large fire occurred evenwhen wood privacy fences were separated by 91 cm (3 ft). A parallel fence
configuration can arise when neighbors erect fences along both sides of a property
line.

‒

For a western redcedar privacy fence next to a pine lattice fence, the fire behavior
depended on spacing. Rapid fire growth and intense flames were found for a spacingof 31 cm (12 in). The char patterns on each fence were similar to those for the fencesindividually when the spacing between them was 46 cm (18 in).

‒

Limited testing indicates that ignition of certain wood-plastic composite fences canresult in high intensity fire behavior. For one of the two types tested, the fence burned
intensely, with large flames extending above the fence. The warped frame allowed
vertical boards 1.8 m (6 ft) tall to fall to both sides, creating a 3.7 m (12 ft) wide zoneof flames that could block egress and threaten property.

F7. Good neighbor fences serve as a ladder fuel to carry flames from the ground to
the top of the fence. (FH)‒

For good neighbor fences at low wind speed, the flames reached the top of the fence
downwind from the ignition point. This is due to radiative and convective heat

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transfer between the boards connected to alternating sides of the stringer, the samemechanisms that caused rapid flame growth between parallel fences. At higher windspeeds the maximum height of the fire stayed below the center stringer of the fence.
F8. Rubber mulch generates large flames initially and when disturbed. (FH)‒

Rubber mulch burned with black smoke and large initial flames, followed by a long
period of sporadic flaming as a top layer of crumbly solid residue slowed the flow ofoxygen to the unburned fuel beneath. Disturbing the mulch bed renewed the flamingas the unburned fuel was exposed to air.

Fig. ES.2. Examples of Very High Hazard fences and mulch: a) parallel privacy fences, b) double
lattice fences, c) wood-plastic composite #1 fence, d) good neighbor fence, e) rubber mulch.

High Fire Hazard Configurations. Many fence and mulch combinations exhibited fire
behavior in the medium hazard range, supporting fire spread and generating firebrands but
not progressing to full involvement with large flames. This section describes the behavior ofsome configurations that fall into this category. Illustrations of the fire behavior of HighHazard fences and mulch are shown in Fig. ES.3.
The fire behavior of configurations of fences and mulch identified in this study as HighHazard supported the following findings:

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F9. A fence with mulch at its base transports fire through the community andprovides a steady source of firebrands to ignite combustible material downwind.
(FH)‒

Wood privacy or lattice fences combined with mulch were more hazardous than either
the fence or the mulch bed separately. Adding fine combustible materials to the baseof a fence promoted fire spread along the base of the fence, allowing the combinationof fence and mulch to act as a wick transporting fire along the entire length of thefence. With ignition at the base of the fence, flames remained below half of the fence
height. In every case, firebrands ignited spot fires in the target mulch bed.

‒

Stringers slowed the upward fire spread in these experiments by limiting the flame
height on one side of the fence. In a WUI fire, however, they could provide locations
for firebrands to lodge and ignite new fires on the fence.

‒

The flame spread rate in the horizontal direction was similar for all wood fences,including privacy, lattice, and good neighbor fences. Away from the wind field near
the structure, the fire spread from the ignition point to the end of the single fence
panel was 2 min to 5 min for high and medium wind speeds and 7 min to 12 min for
low wind speeds.

‒

For one type of wood-composite fence, the fire remained below the halfway point of
the fence height. Horizontal boards fell out of the frame and burned in line with thefence.

‒

Lifting a fence 15 cm (6 in) above shredded hardwood mulch decoupled the burning
behavior of the fence from the mulch between posts. This conclusion may not holdfor mulches that burn with higher flames, such as pine straw or rubber mulch. The
benefits of raising the fence above the mulch may not be realized if a barrier is placed
between them to keep wildlife out or pets in. Combustible debris such as leaves orneedles that collect along the barrier will reduce the advantages of this design in a
fire.

F10. Fire spreads easily across the fine overlapping particulates of a mulch bed. The
fire intensity, rate of fire spread, production and size of firebrands depend on
the material properties and physical characteristics of the mulch. (FH)‒

Rubber mulch burned with black smoke and large flame initially and when disturbed.
See description under Very High Hazard.

‒

Pine straw mulch burned rapidly with high intensity. By itself, the pine straw mulch
was consumed without igniting spot fires. However, embedded combustible objects
were easily ignited by the intense flames. When combined with a western redcedar
privacy fence, the fire in the pine straw mulch ignited the fence quickly and spread tothe end of the panel within one minute. The burning wood fence then generated
firebrands capable of igniting spot fires.

‒

The fire spread behavior in a shredded hardwood mulch bed was affected by the flow
field, ignition of spot fires downwind, and the geometry of the mulch bed andstructure. Fire progressed in hardwood mulch beds through both continuous flame
spread and firebrand spotting. Spot fires allowed the fire to jump in the direction of
the wind.
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Fire spread more slowly over the mini pine bark mulch than over the shredded
hardwood mulch, often taking at least twice as long to reach the end of the mulch bed.
This likely results from the difference in texture – the chunks of mini pine bark mulch
do not ignite as easily as the long, thin particles that characterize the shredded
hardwood mulch.

F11. More information is needed on the fire behavior of a combustible fence next toan auxiliary structure. (IC)‒A WRC privacy fence separated from a noncombustible cement board by 31 cm(12 in) resulted in a char pattern similar to that of a single panel WRC privacy fence,with flame spread about three times faster. For a spacing of 46 cm (18 in), the fire
behavior was less intense for a WRC privacy fence in combination with either acement board or a vinyl privacy fence, as compared to a single panel WRC fence. The
char pattern remained below the bottom stringer, and the flame spread was at thesame rate or slower.

Fig. ES.3. Examples of High Hazard fence and mulch configurations: a) western redcedar (WRC)privacy fence with pine straw mulch, b) redwood lattice fence with shredded hardwood (HW) mulch,c) wood-plastic composite #2 fence with HW mulch, d) WRC privacy fence parallel to vinyl privacy
fence, e) HW mulch f) pine straw mulch.

Medium Fire Hazard Configurations: Some fences and mulch experiments demonstrated
very slow fire spread without flaming and little or no generation of firebrands. Illustrations of
the fire behavior of Medium Hazard fences and mulch are shown in Fig. ES.4.
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The fire behavior of configurations of fences and mulch identified in this study as Medium
Hazard supported the following findings:
Without nearby fine combustible materials, the fire spread over a single combustible fence isslow and dominated by glowing combustion. (HR)‒The fire spread over wood fences in the absence of fine combustibles was generally
slow and dominated by glowing combustion with occasional small flames. Woodfences produced large firebrands from pieces of the fence breaking off and small
firebrands from glowing combustion. However, spotting in the target mulch bed wasrare in these experiments. It should be noted that it may be difficult to keep a woodfence sufficiently clear of fine combustible materials to achieve the slow-growth fire
behavior. Windblown debris such as leaves and pine needles may accumulate before
and during a WUI event.

F12. Fence and groundcovers with added or inherent fire resistance reduce the flame
spread rate and the hazard due to flames and firebrands. (HR)‒Vinyl privacy fences did not support significant burning under the tested windconditions. With mulch at its base, vinyl privacy fences, including panel, bottom
frame, and fence post, blackened and distorted along the entire length of the fence.
Distortion allowed the boards to fall out of the bottom frame. No firebrands weregenerated.

‒

The single type of artificial turf tested in this study, with synthetic fibers made frompolypropylene with a urethane-coated backing, was difficult to ignite and exhibited
slow flame spread.

Fig. ES.4. Examples of Medium Hazard fences and mulch: a) western redcedar privacy fence without
mulch, b) redwood lattice fence without mulch, c) vinyl privacy fence with shredded hardwood mulch,d) artificial turf.

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Low Fire Hazard Configurations. Although fences made of noncombustible materials suchas stone, brick, or steel were not included in this study, they can be classified as low fire
hazard. They do not burn on their own and have been shown to provide protection against
radiant heat. However, any fence can trap and accumulate windblown debris along theirlength. If not removed, combustible materials at the base of even noncombustible fences
could potentially allow fire to spread.
F13. Noncombustible fences free of leaf litter and other combustible debris will not
spread fire. (HR)
Maintenance is required to reduce the accumulation of fine combustible materials along a
fence. This minimizes the amount of windblown debris such as leaves and pine needles that
can ignite during a WUI event.
PRIMARY RECOMMENDATIONS
The following recommendations for members of a residential community are intended toaddress both ember and fire (radiation, convection, and direct flame contact) exposure
hazards generated by combustible fences. Although the recommendations are intended
primarily for moderate to very high hazard WUI locations, they are expected to reduce local
fire hazards in any community.
For more detailed recommendations on spacings of combustible elements and hardening ofstructures and parcels, refer to the WUI Structure/Parcel/Community Fire Hazard Mitigation
Methodology report.1
R1.

Avoid parallel fences, to reduce exposure to large flames. Parallel fences can resultin highly hazardous fuel accumulation corridors that are difficult to access andmaintain. Spacing of 0.9 m (3 ft) between fences is not sufficient.

R2.

Avoid combustible fences where they can impact egress, to protect life safety.

R3.

Avoid proximity to other combustible fuels, to reduce fire intensity and limit fire
spread. This includes fuels above the fence and fuels across parcel boundaries. Avoid
mulch at base of fence.

R4.

Avoid proximity of combustible fences to residence, including neighboring
residence, to prevent direct ignition. The relationship between spacing and structure
to prevent structure ignition is a function of structure construction materials/assemblyand fence materials/design.

R5.

Replace combustible landscape features with noncombustible or low fire hazard
features when possible. Fire spread is more likely with wood and wood-plastic
composite fences than with fences made of vinyl or noncombustible materials such asstone, brick, or steel.

R6.

Keep fence and yard clear of debris, to reduce the amount of fuel and potential
pathways for fire.

A. Maranghides, E. Link, S. Hawks, J. McDougald, S. Quarles, D. Gorham and S. Nazare, “WUI Structure/Parcel/Community Fire Hazard
Mitigation Methodology,” NIST Technical Note 2205, National Institute of Standards and Technology, Gaithersburg, MD, 2022.
1

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R7.

Harden structures against firebrands, to prevent structure ignition from embers
produced by fences or other combustible sources.
RECOMMENDATIONS FOR FUTURE WORK

More research is needed to determine the vulnerabilities of structures to fence fires relative tofence types and materials, the proximity and connection of the fence to the structure, and thedesign and exterior materials of the structure itself. Once the vulnerabilities are better
understood, mitigation techniques such as material treatments and coatings can be exploredbeyond the simple solutions of increased separation and replacement of materials with
noncombustible options.
S1. Study the effects on fire behavior of closely spaced parallel wall surfaces incommunities.
The same radiation exchange and convective transport of hot gases between burning parallel
surfaces that led to eruptive behavior for parallel wood fences can potentially result in highly
hazardous situations for other closely spaced parallel surfaces, including residences. As one
approach, the Structure Separation Experiments project at NIST2,3 is addressing the questionof how far apart residences should be from other nearby structures.
S2. Continue to study the fire behavior of landscape features and potential
mitigation methods.
There are many combustible landscape features that may contribute to fire hazards in a parcel
or community. Work is ongoing at NIST to understand the interactions of fires on woodpiles,
landscape timbers, creosote-treated timber, and sheds. The work will include strategies for
mitigation. Together, these studies will inform existing and new codes and standards withquantitative fire spread mitigation and structure protection strategies based on experimental
data.
S3. Improve data collection methods.
The range of fire behaviors found for a variety of materials, designs, and configurations
suggests improvements in data collection methods that would be useful for future targetedstudies. For fence and mulch configurations resulting in large flames and very high hazard
conditions, the radiative and convective heat flux received at vulnerable locations could be
evaluated by heat flux sensors and/or infrared (IR) imaging. Firebrand fluxes, sizes, andenergy content could be assessed in future studies by new measurement technology,
including a three-dimensional firebrand tracking system under development at NIST.
S4. Use fire modeling to better understand the physics behind the fire behavior.
Modeling can be used to extend the understanding from this study to other configurations offences, mulch beds, structures, and other fuels, in order to identify other high hazard

2

A. Maranghides, S. Nazare, E. Link, K. Prasad, M. Hoehler, M. Bundy, S. Hawks, F. Bigelow, W. Mell, A. Bova, D. McNamara, T.
Milac, D. Gorham, F. Hedayati, B. Raymer, F. Frievalt and W. Walton, “Structure Separation Experiments: Phase 1 Preliminary Test Plan,”
NIST TN 2161, National Institute of Standards and Technology, Gaithersburg, MD, 2021.
3
A. Maranghides, S. Nazare, E. Link, M. Bundy, A. Chernovsky, E. Johnsson, K. Butler, S. Hawks, F. Bigelow, W. Mell, A. Bova, D.
McNamara, T. Milac, D. Gorham, F. Hedayati, B. Raymer, F. Frievalt and W. Walton, “NIST Outdoor Structure Separation Experiments(NOSSE): Preliminary Test Plan,” NIST TN 2199, National Institute of Standards and Technology, Gaithersburg, MD, 2022.

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configurations. The results in this report may be helpful in validating a physics-based firemodel, including:‒Fire behavior of fences as a function of material properties and design.

‒

Fire behavior of parallel fences compared to single fences.

‒

Dependence on parallel fence spacing for time at which flames engulf the fences.

‒

Dependence of fire behavior on parallel fence length, including the spacing for which
a second panel length results in explosive fire growth, and the time for fire to spreaddown a long fen‒

Char patterns for wood privacy fences, both individually and in parallel withcombustible and noncombustible fences and walls.

S5. Develop fire test(s) for evaluating fences and fence materials that represent theactual fire hazard.
A standard test method is needed to assess the fire performance of fences. The method should
consider not only materials but assemblies and be carried out in a vertical orientation. Itshould be able to distinguish the fire behavior of various materials, including wood-plastic
composites, wood, and vinyl, and designs, including privacy, lattice, and good neighbor.

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Introduction
The trees, grass, brush, and organic debris that make up wildland vegetation are not the only
source of fuel for wildland-urban interface (WUI) fires. Once such a fire reaches acommunity, its structures and landscape features add to and may come to dominate the firesources, magnifying the risk. Combustible elements in a neighborhood may transform frombeing the targets of flames and embers to fire sources themselves that threaten surrounding
properties and the people who live there. How and where we build, then, affects the
progression of a WUI fire.
The National Institute of Standards and Technology (NIST) is working to assess fire hazards
in our built environment and to develop mitigation methodology to harden it against ember
and flame exposures. This report on fences and mulch builds on a growing body of NIST
research studying fire behavior and how the materials, designs, and configurations present in
a community influence a WUI fire.
Motivation
The wildland-urban interface refers to areas where houses are adjacent to or intermixed withwildland vegetation. The regions where the WUI overlaps with high risk of wildland fires
due to fuel, weather, terrain, and sources of ignition are where these wildland fires pose thegreatest risk to lives and property. Effective methods are needed in these areas for protecting
people, homes, and communities from wildfires.
A large and growing number of people live in WUI areas in the United States. In 2010, theWUI covered 9.5 % of the total area of the conterminous U.S. and contained 43 million
homes (about 1 in every 3) sheltering 98 million people, or 32 % of the U.S. population [1].
This represented an increase of 33 % in land area and 41 % in the number of homes since
1990, due almost exclusively to construction of new houses in or close to wildlandvegetation [1]. Residents are attracted to the WUI due to the closeness to natural settings andamenities and to the relative affordability of housing farther from urban centers.
Demographic forces, including retirement, population growth, and population shifts, areexpected to continue expanding the WUI in the U.S. [2].
Residence in the WUI does not in itself put one at risk from WUI fires. Wildfires threatencommunities where the WUI intersects with areas where there is a significant risk of wildfire
ignition and spread. A fire can also ignite in wildland areas surrounded by a community, thenrapidly grow to put one or more communities at risk. Drought, wind, terrain, vegetation typeand health, and the presence of ignition sources contribute to the probability of development
of WUI fires. Conditions for wildfires are expected to worsen as climate change continues toenhance fuel abundance and drought conditions [3]. Humans have greatly expanded both thespatial range of wildfires and the length of the fire season, now accounting for 84 % of thetotal number of wildfires [4]. Balch et al. developed a map of fire risk for populated placesby combining population densities with wildfire risk [5]. The map shows that high andmedium WUI fire risk areas are scattered across the U.S., including New Jersey, Kentucky,
Tennessee, Florida, and the south Atlantic Seaboard in addition to midwestern and westernstates such as Texas and California.

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WUI fires can occur when wildland fires cannot be controlled, often due to extreme wind andfuel conditions, and spread into communities. Such fires have caused significant losses to lifeand property in the U.S., Canada, and other parts of the world including Australia andMediterranean Europe. The costs have increased in time; of the 20 most destructive fires in
California history, more than half occurred since 2017 [6]. At the top of this list is the CampFire of November 2018, which resulted in 85 fatalities and destroyed over 18 000 structures,
including 90 % of the homes in Paradise, CA. The Camp Fire was one of the costliest natural
disasters of 2018, with an overall loss of $16.5 billion as estimated by multinational
insurance company Munich Re [7]. The second most destructive fire on the same list was the
Tubbs Fire in Sonoma County in October 2017, which resulted in 22 deaths and over 5 000
structures destroyed. Research is urgently needed to better understand WUI fire-structureinteractions and to support changes to building and community designs and codes in order tomitigate the increasing losses from the growing number of WUI fire incidents.
NIST has carried out studies on several WUI fires, including the 2009 Witch Creek Fire in
California [8, 9], the 2011 Tanglewood Complex Fire near Amarillo, Texas [10, 11], and the2012 Waldo Canyon Fire in Colorado [12]. NIST is currently studying emergency
communications during the Chimney Tops 2 Fire near Gatlinburg, Tennessee, which killed
14 people and destroyed or damaged 2 500 structures in November 2016 [13]. The NISTCamp Fire WUI case study is currently ongoing, with the Camp Fire fire progression
timeline already published [14]. The current focus of the multi-year case study is on
notification, evacuation, and temporary refuge areas (NETRA), and the final major reportwill focus on responder actions and structure survivability.
Fences and mulch have been identified as common contributors to the spread of WUI fires
within communities. In the Tanglewood Complex Fire, over 2.4 km of fences were found to
be damaged or destroyed within a community of 47 residential structures (in which 17 homes
were destroyed and 4 were damaged) [10]. Instances of fires spreading to structures alongfences were observed in the Witch Creek Fire [8] and the Waldo Canyon Fire [12]. Figure 1
shows the progress of a fire from a wood privacy fence to a structure during the 2018 CampFire in California. Figure 2 shows a length of fence burning in the same fire. Because of theirlinear nature, it is possible for fences to spread fire over long distances. In the Waldo Canyon
Fire and many others, firefighters removed fences as part of their defensive strategy aimed atcontaining the fire [12], as shown in Fig. 3, reducing resources allocated to direct structure
protection. Mulch and other groundcovers also provide a continuous pathway for fire. A
report on 21 structures that were damaged by burning wildland vegetation in Virginia over a
two-year period found that the fire spread to most structures by means of leaf and tree litterthat covered the ground with a thickness between 8 cm and 13 cm [15].

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Fig. 1. Fence burning in Magalia, California during Camp Fire, 8 November 2018. Photographs taken
two minutes apart. CAL FIRE, used by permission.

wo
Fig. 2. Fence burning during Camp Fire as viewed from an ambulance. American Medical Response(AMR) – Shasta County, used by permission.

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Fig. 3. Ignited wood fencing in Waldo Canyon Fire in Colorado, 2012. Photo from Colorado Springs
Fire Department, used by permission.

Fences, mulch, and other combustible landscaping elements can act as both potential ignition
sites from existing fires (targets) and sources of fire spread themselves. These materials canbe ignited by a fire through direct flame contact, radiation, convection, or firebrands.
Firebrands, also referred to as embers, are carried by the wind and may ignite combustible
materials in a community far downwind of the fire front. Once ignited, fences and mulch
may ignite nearby objects, including a home, through direct flame contact or firebrands.
The protection of people and property in the WUI depends in part on improvements tobuilding and landscape materials, design, and maintenance practices. Efforts to improve
community resistance to fire include: WUI building code organizations, such as theInternational Code Council (ICC), the National Fire Protection Association (NFPA), and theCalifornia State Fire Marshal Chapter 7A WUI Task Group; and voluntary fire outreachprograms, such as Firewise, Fire Adapted Communities, and the Fire Learning Network.
Concepts like defensive space and the home ignition zone educate the public on how toprotect their homes. Recently, a fire hazard mitigation methodology has been developed
based on the relationships among fuel layout, fire hazard, and structure hardening [16].
For maximum effectiveness, these efforts require science-based data and guidance. Increased
understanding of the vulnerabilities of structures in WUI communities and the potential
pathways for flames and firebrands will help to enhance life safety and to improve
community resistance to these fires. The hazard may be reduced through improvements inmaterials, designs, and configurations. Homeowners and community planners can recognize
ways in which neighbors can work together to reduce the fire threat. Strategies may be
developed for both existing communities and new construction.

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The goal of this work is to improve our understanding of the mechanisms by which fences
and mulch can transport fire to a home. Better understanding of the role of these features asconduits of fire spread to structures, identification of particularly hazardous configurations,and determination of effective hazard mitigation approaches promote efforts to protect
against ignition and fire spread. Helping fire departments to identify very high hazard
situations will enhance first responder safety and effectiveness. The results of this work will
be used to improve codes and standards and to provide guidance to homeowners, community
designers, and first responders.
Background
WUI fires ignite the exteriors of structures through flame radiation and convection, direct
flame impingement, and firebrands. In contrast to the large body of knowledge on ignition
and fire growth within buildings, reflecting decades of fire research, the complexities of theinteractions between the built environment and exterior fire exposure are in the early stagesof exploration. Our understanding of WUI fire behavior is confounded by the large numberof potential fire and firebrand exposure scenarios, the wide variety of WUI fuels (vegetativeand structural), and the extensive assortment of exterior construction materials andassemblies. The research presented in this report joins other efforts to better understandstructure vulnerabilities to fires on nearby fences and mulch beds.
1.2.1. Structure Vulnerabilities
Fire may ignite a structure through numerous pathways. At close range, exposed combustible
materials may ignite through radiation/convection or direct flame contact. Ignitions may also
occur through firebrands. These burning particles break off from a larger object in a fire andare blown or lofted to a new location, where they can ignite spot fires. Firebrands may ignite
susceptible parts of the building exterior and may penetrate into interior spaces throughvulnerable openings in the building envelope. An object ignited by flames or firebrands may
itself become a fire source of firebrands and additional radiation and flame exposures tosurrounding fuels (targets).
1.2.2. Fence Studies
Fences are a linear landscaping feature. Fences frequently “link” multiple parcels, as one
fence connects to an adjacent fence. If they are combustible, they may provide a linear pathfor fire from one end of the fence to the other. They may also produce firebrands capable ofigniting spot fires downwind. Several studies have looked at how fences may ignite in a WUI
fire and how the fire may behave.
The fire performance of commercial fencing systems in common use in Australia was studied
by CSIRO [17]. A bushfire flame front simulator was constructed using a grid of liquid
propane burners. Hardwood and treated pine fences were exposed to a range of bushfire
exposures, from burning leaf litter through structural fire. Ignition and fire behavior werestudied under ambient wind conditions of approximately 5 km/h to 8 km/h (1.4 m/s to
2.2 m/s, or 3 mi/h to 5 mi/h). When burning leaf litter was distributed along the base and railsof treated pine fences, the fire spread slowly laterally along the fencing, eventually causing

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collapse. Hardwood fences (yellow stringybark eucalyptus and southern mahogany) resisted
ignition by leaf litter and radiative heat exposure typical of an approaching bushfire. They
ignited under flame immersion from a propane burner but did not support lateral flame
spread under the conditions tested. In cone calorimeter studies, the surface moisture content
was found to determine fire behavior more than average moisture content, suggesting thatweather conditions on the day of the fire will have a significant impact. Sample conditioning
at a lower temperature and higher relative humidity (RH) led to longer ignition times.
The CSIRO study [17] found that noncombustible pre-painted and metallic-coated sheet steel
fences provided significant protection against radiant heat, preventing combustible items near
the fencing from igniting and reducing the radiant heat exposure on a nearby structure. Gaps
between boards of hardwood and treated pine fences reduced their ability to shield radiantheat, and the barrier failed completely when fences burned through or collapsed.
A study of the ignition of fences and wood shields by a grass fire [18] showed that fenceswith narrow gaps between spruce boards ignited, while those with widely separated boards
did not. This was likely because the fire front attacked the tightly spaced fence for almost
twice as long as the fence with wide gaps. The experiments were carried out in ambient
winds of 1 m/s to 6 m/s (2 mi/h to 13 mi/h).
Wood fencing assemblies were exposed to wind-driven firebrand showers by NIST
researchers and colleagues at the BRI facility. With no nearby fine fuels, the fencing
assemblies experienced smoldering ignition that transitioned to flaming [19]. Flaming
ignition occurred when shredded hardwood mulch was arranged at the base of the fencing
assemblies. A study of double redwood lattice fencing assemblies on mulch beds subjected to
firebrand showers at wind speeds of 4 m/s to 9 m/s (9 mi/h to 20 mi/h) showed rapid growthof flames over the entire assembly [20].
Research was carried out by IBHS on combustible fences in wind-driven firebrand showers[21]. A Wildfire Research Fact Sheet jointly published by NFPA and IBHS [22] includes thefollowing conclusions, primarily derived from the IBHS study: (1) noncombustible fencing
should be used where the fence attaches to a building; (2) the area at the base of the fence
should be kept clear of debris, including fine vegetative fuels and mulch; (3) a fence designthat allows for greater air flow makes windblown ember accumulation and lateral flame
spread more difficult, and fence ignitions from windblown firebrands are more likely tooccur where vertical fencing planks attach to horizontal support members; (4) fences built
from lattice attached to both sides of the support posts should be avoided in wildfire-prone
areas, and (5) vinyl fencing burns when subjected to flaming exposures from burning debris
and deforms if subjected to radiant heat. Conclusion (4) is a preliminary result from this
NIST study that will be discussed in detail in Section 4.4.
A previous NIST study conducted experiments to examine the fire spread along privacy
fences relative to wind speed and angle [23]. Western redcedar (Thuja plicata) was theprimary fence material, although comparisons were made to California redwood and vinyl
fences. The presence of a combustible layer of mulch beneath the fence was significant; inthese experiments, fire did not spread without mulch. With mulch, fire was found to spreadhorizontally along privacy fences as fast as 1.44 m/min, with the fastest rate occurring whenwinds were in line with the fence. At this angle, moderately high winds of about 13.5 m/s(30 mi/h) produced the fastest spread rate, while higher winds of about 18 m/s (40 mi/h)19

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caused complex competition between sustained spread and extinguishment. No significant
differences in fire spread rate were found between cedar and redwood fence fires, and woodpreservative applied to the fence resulted in qualitative but not quantitative differences inburning behavior. The vinyl fence fire differed substantially from the wood fence fires due todifferences in the fence structure and to the melting behavior of the vinyl when heated.
Downwind mulch targets were found to be susceptible to ignition by fence firebrands, withsmoldering ignitions occurring at a distance of 18 m (59 ft) from the fence.
In summary, the following findings were obtained:
1) Combustible mulch beneath the privacy fence was necessary for significant fire
spread.
2) Fire spread was fastest when the wind was in line with the fence (versus 45° and 90°).
3) Winds of 13.5 m/s (30 mi/h) produced the fastest fire spread of 1.44 m/min(4.7 ft/min).
4) Redwood and western redcedar fire spread rates were not significantly different.
5) A wood preservative did not significantly change fire spread rate.
For these experiments, an airboat propeller was used to generate a wind field that was foundto be uniform horizontally and vertically within 2.5 m/s. A flow straightener angled
downward by about 9° improved the distribution of wind at the leading edge of the base of
the fence
1.2.3. Mulch Studies
Combustible landscaping mulch may provide a continuous pathway for fire over the ground.
Once ignited, it may also act as a source of firebrands. Several studies have looked at ignition
and flame spread over mulch materials.
In a wind field, mulch is easily ignited by single flaming firebrands and multiple glowing
firebrands [24]. Firebrand experiments showed that smoldering ignition by firebrands always
transitioned to flaming on shredded hardwood mulch, Japanese Cypress wood chips, and pine
bark nuggets [25]. For a mulch bed adjacent to a re-entrant corner with oriented strand board(OSB) walls, the fire propagated to the back side of the assembly in all cases with windspeeds from 6 m/s to 8 m/s. When vinyl siding was attached, fire propagation showeddependence on mulch type, vertical separation distance, and wind speed.
A study on the ease of ignition of landscaping mulch found differences in the ignition
response to high-temperature flames (propane torches for 15 s) and smoldering sources(cigarettes) for several combustible mulches [26]. Thirteen types of mulch were spread overcircular test areas at a thickness of 10 cm (4 in) and allowed to weather. The mulch was
exposed to lit cigarettes after settling for two weeks or nine months, or to 15 s of propane
torch flame after settling for one year. All experiments were performed in mild ambient
temperatures with low winds below 2.2 m/s (5.0 mi/h). Under brief high-temperature
ignition, the ground rubber mulch consistently ignited and was hard to extinguish, withflames spreading rapidly. Pine straw ignited and propagated easily. Shredded hardwood barkand pine bark nuggets ignited but generally self-extinguished under the experimental
conditions.

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Four landscaping mulches were used to develop test protocols to measure ignition and flame
spread over beds of mulch and forest litter [27]. These tests were performed in the absence ofwind. Pine straw mulch readily ignited with a Class C wood crib, the smallest ignition source
studied. In the flame spread tests, only pine straw mulch sustained spread to the end of thetest bed. Both the flame spread rate and the flame height decreased when the mulch depth
was reduced in half, from 76 mm to 38 mm. Pine bark nuggets were reliably ignited by theClass B wood crib, while shredded hardwood and pine bark mulches required Class A woodcribs, the largest source. For these three mulch types, with no wind, the flames selfextinguished shortly after the propane area burner used for ignition was shut off.
A study on the combustibility of landscape mulches evaluated flame height, fire spread rate,
and temperature at 10.2 cm (4 in) and 40.6 cm (16 in) heights for eight mulch types [28].
Mulches were arranged in circular plots with 2.4 m (8 ft) diameter and allowed to weather for
79 days. The plots were ignited on a hot dry day, with a fan producing winds of about 4.5 m/s
to 6.7 m/s (10 mi/h to 15 mi/h) in the middle of each plot. Of the mulches tested, shredded
rubber mulch was rated most hazardous, with the greatest flame height and temperatures.
Pine needles and shredded western redcedar bark were only slightly less hazardous, with fast
rates of flame spread. Shredded western redcedar bark produced embers that ignited mulch inadjacent beds. Medium pine bark nuggets were moderate in flame height and temperatures,and the flame spread rate was low. Composted wood chips were found to be the least
hazardous of the mulch types tested, with slow smoldering combustion, low flames that wererarely seen, and the second lowest temperature readings.
In general, studies that include wind find that fire spreads over combustible mulch, even formulch types that tend to self-extinguish without wind.
Approach
A series of field experiments on the fire spread behavior of ignited wood fences and mulch
have been conducted by NIST. The experiments examined the spread of fire along woodfences and mulch beds toward a structure in the presence of wind at various speeds. The
ability of firebrands generated by these burning materials to ignite spot fires at the base of thestructure was observed. Early descriptions of this work were presented at conferences [29,30, 31].
In each of these experiments, a portable, airboat-style fan was used to direct a wind field witha prescribed speed in the direction of a small shed. A fence panel, mulch bed, or combination
was positioned between the fan and the shed at a prescribed location, aligned with the wind.
The fence panel or mulch bed was ignited with a propane burner at the end near the fan, andthe fire spread due to flames, smoldering, and firebrands was observed. Comparing the fire
behavior of a fence with and without mulch provided insights into the contribution to WUI
fire spread made by fine fuels that accumulate at the base of fences, such as leaves, needles
and other debris. The small shed was used as a target structure for flame spread andfirebrands. For most experiments, a target mulch bed was placed along the base of the wall toobserve spot fire ignitions from firebrands.
Fence types used in the experiments included western redcedar privacy fences, vinyl privacy
fences, California redwood lattice fences, pressure treated pine lattice fences, western
redcedar good neighbor fences, and wood-plastic composite fences. Mulch types included
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shredded hardwood, mini pine bark nuggets, pine straw, and shredded rubber. Experiments
were also performed on artificial turf.
Objectives
The overall goal of the work described in this report is to assess the severity of the fire hazard
that fences and mulch pose to structures. This is accomplished by studying the rate andmechanisms of fire spread in the presence of wind, both as the fire progresses along the fenceor mulch bed and as it jumps via firebrands from the fence or mulch bed to combustible
materials at the base of the shed. The main objectives of the experiments are:•

To observe the burning behavior of wind-driven fires along fences and mulch beds;•

To understand the rate of fire spread along fences and mulch beds as related to windspeed;•

To determine the impact of fence style, fence material type, and type of mulch on thefire spread rate;•

To determine whether a fire along a fence or mulch bed poses a potential ignition
danger to an attached or adjacent structure; and•To ascertain whether a burning fence installation or mulch bed produces significant
firebrands capable of igniting downwind combustibles.

It is anticipated that this work will contribute technical knowledge to improve codes andstandards for auxiliary structures and to support efforts to address the WUI fire problem byhardening structures and creating defensible space. Future reports will describe the expansionof this study to fire spread over other home landscape features, including woodpiles, railway
ties, and landscape timbers. A device to measure firebrand flux and size will provide
quantitative data in the near future. Research is also anticipated to explore methods ofhardening, such as noncombustible components or flashing, coatings, and design options that
reduce the vulnerability.

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Experimental Description
To investigate the spread of fire through direct flame impingement or firebrand spotting, aseries of outdoor experiments was performed on fences and mulch beds arranged in front of astructure in a generated wind field. Figure 4 shows a schematic of the experimental setup forfences and mulch beds. A wind machine, consisting of a gasoline engine turning a 2.11 m(83 in) propeller mounted on a trailer, was directed toward a small structure. A flow
straightener was employed to remove large-scale swirl from the supplied wind and to direct
the wind downward slightly toward the ground. A fence section, with or without a mulch bed
beneath, was arranged perpendicular to the wall of the structure and parallel to the wind flow.
The fence and/or mulch bed were placed in contact with the wall of the structure or separated
from it by one of three fixed distances. To study the potential for firebrands to ignite thestructure, a target pan of hardwood mulch was usually positioned at the base of the structure
wall. This mulch bed served as a surrogate for any combustible material next to a structure.
The fence, mulch bed, or combination was ignited with a propane burner to simulate priorignition via one or more firebrands. Wind was directed at the fence and structure with thewind aligned with the plane of the fence in all but two cases. Three wind speeds were used in
the study, with nominal values of 6 m/s (13 mi/h) for low, 10 m/s (22 mi/h) for medium, and14 m/s (31 mi/h) for high wind speed levels. The fire spread behaviors of multiple fence
types, materials, and configurations were investigated, and several mulch types andarrangements were tested. The following sections will detail the experimental setup,
equipment, measurements, conditions, and the parameters explored. Many of theuncertainties in the experimental setup and measurements are covered in Appendix A.

Fig. 4. Major components of the experiment (not to scale).

Research Location and Site Description
The experiments were conducted in Frederick, MD at the Frederick County Public Safety
Training Facility. A large, nearly flat asphalt and concrete area near a water-supply pond was

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utilized. The pond and its wall provided a noncombustible background downwind of thefirebrand-generating experiments. Water for extinguishment was provided through a nearby
hydrant and a diesel pump, which provided a high-pressure source of pond water.
An aerial view of the site is shown in Fig. 5, marked up with locations of the target shed,equipment/conditioning building, and wind machine. For the usual configuration, as shown,
the wind flow was applied from the wind machine from the SSW direction at an angle of200° ± 1°. The long-range firebrand experiments described in Section 4.5 were performed ina pathway extended along the wall next to the pond, with wind applied from the SSE
direction at an angle of 148° ± 0.5°.

Fig. 5. Aerial view of site used for experiments. Google Earth image with NIST overlay.

Wind Field Generation
2.2.1. Wind Machine
The wind machine used to impose a wind field on the target structures, shown in theforeground of Fig. 6, was assembled and mounted on a trailer by American Airboat. The
power was provided by a 6.0 L displacement, 450 HP rated marine engine with multi-port
fuel injection. The wind machine utilized Whirlwind Propellers model AB300ex-WT79,
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which had three quiet-design, graphite composite blades with a width of 33 cm (13 in) and asweep diameter of 2.11 m (83 in). The wind machine incorporated a high performance
positive drive belt with 2.3:1 reduction. A manual “cruise control” mechanism was designed
and added to the single lever binnacle-style throttle control in order to allow maintenance of
selected engine speeds, which were monitored with a built-in tachometer.

Fig. 6. Photo of test site showing fan, flow straightener and target shed in an experiment on a mulch
bed without a fence.

2.2.2. Flow Straightener
A flow straightener was used to remove large-scale swirl from the supplied wind and adjust
the wind direction. The flow straightener consisted of two framed sections of aluminumhoneycomb with cells 19 mm (3/4 in) across and 11 cm (4.4 in) thick. The two framed
sections, each measuring 1.2 m × 2.4 m (4 ft × 8 ft), were stacked as shown in Fig. 6, withthe front plane of the flow straightener positioned 45 cm (18 in) in front of the fan at theheight of the fan center. Since the lowest sweep extent of the wind machine propellers is 1 m
above the ground, the column of air moved horizontally by the wind machine by itself would
not begin to be felt at the ground for a distance of several meters. To enable the generated
wind field to reach the base of any combustible object being tested, such as a fence and/or
mulch bed, with substantial velocity, the flow straightener was angled downward by
approximately 7°.
2.2.3. Velocity Profiles
The wind field from an axial fan, such as the wind machine used in these experiments,changes with position downwind [32]. At the outlet of an axial fan, the velocity profile shows

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a minimum in the central hub region of the propellers. The profile then smooths out with
distance from the fan. According to the Air Movement and Control Association International(AMCA), a uniform velocity profile is achieved at a distance of about 2.5 duct diametersfrom the outlet of the fan, for ducted flows with velocities up to 12.7 m/s (28 mi/h). This
provides a general guideline for the minimum distance of the wind machine from the fence ormulch bed being tested, recognizing that the free outlet condition and the presence of theflow straightener modify both the distance necessary to obtain a uniform velocity profile andthe radial extent of the high velocity region downwind.
Further discussion of the flow field is provided in Section 2.7.1 and in Appendix C.
Target Shed
Experiments were classified as Series 1, 2, or 3, depending on the nature of the firebrandtarget downwind of the fence or mulch being tested. A target shed was used for Series 1 andSeries 2 experiments. Series 1 experiments looked at the possibility of direct ignition of a
combustible shed wall by firebrands. In Series 2 experiments, a mulch bed at the base of theshed served as a surrogate for combustible materials (such as leaves or pine needles in
addition to mulch) that could ignite and carry a fire to the shed wall. Series 3 experiments
were performed without a shed, with a mulch bed located far downwind from the test subject
to examine long distance ignition by firebrands.
A shed with a 2.43 m (8 ft) square footprint and a height of 2.43 m (8 ft) along the front andrear faces can be seen in the background of Fig. 6. The shed was positioned 10.67 m (35 ft)
away from the plane of the wind machine propellers. An artificial eave was added to the shedon the windward side, extending 45 cm (18 in) outward at the same 30° angle as the roofline.
The eave was constructed from standard pressure treated pine 2×4s4 and 1.5 cm (0.59 in)thick T1-11 weather-resistant southern yellow pine plywood panel siding.
A false wall was attached to the shed on the windward side to allow replacement of burned
wall layers without damaging the original shed wall. The design of the false wall included
layers (from the shed outward) of 1.6 cm (5/8 in) gypsum board, standard pressure treatedpine 2×4s (the same type used for the eave), 1.6 cm (5/8 in) gypsum board, and 1.5 cm(0.59 in) thick southern yellow pine plywood panel siding.
For Series 1 experiments, a second layer of plywood panel siding was attached to the lower
part of the false wall, as shown in Fig. 7 (a), providing a sacrificial layer that was easily
replaced. For Series 2 experiments, James Hardie fiber cement siding was added to thebottom of the false wall, as shown in Fig. 7 (b), as a noncombustible layer that prevented thefalse wall from getting burned and requiring replacement. The plywood panel siding andfiber cement siding were each 1.22 m (4 ft) tall by 2.44 m (8 ft) wide, with thicknesses of
1.5 cm (0.59 in) and 6.4 mm (¼ in), respectively.

4

The term “2×4” is used to refer to dimensional lumber, also known as framing lumber. The cross-sections of lumber are referred to by
their nominal size, in this case 2 in by 4 in, but the actual depth and width (the “dressed” size) after cutting and smoothing are 3.8 cm by 8.9
cm (1 ½ in by 3 ½ in) [65]. Similarly, 4×4s measure 8.9 cm by 8.9 cm (3 ½ in by 3 ½ in), and 1×6s measure 1.9 cm by 14.0 cm (¾ in by
5 ½ in).

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The false wall and siding layer added approximately 15 cm to the shed depth, for a final shedfootprint of 2.43 m wide by 2.58 m deep. The vertical gray strip appearing in Fig. 7 is a metal
corner bead protecting the edge of the shed.

Fig. 7. Target shed configurations for a) Series 1 and b) Series 2 experiments.

Mulch Types and Preparation
For each experiment, a pan was placed on the ground to hold mulch and/or fences and tocollect the burned debris. A mulch bed was often placed under the fence to allow testing of
the effect of the presence of mulch on flame spread and spot fires. The mulch was considereda surrogate for any fine combustible fuel next to or beneath a fence. Mulch was also tested
alone, without a fence, to explore its fire spread and spotting behavior and differentiate itfrom that of mulch combined with fences.
2.4.1. Mulch Pans
To accommodate a bed of mulch 5 cm deep, two pans were fabricated from 26 gauge[0.454 mm (0.0179 in) thick] galvanized steel sheets. Each steel sheet pan was 87.6 cm(34.5 in) wide and 1.83 m (6 ft) long with 2.5 cm (1 in) high side walls. The two pans wereoverlapped and connected at the walls with two small C-clamps, for a total combined lengthof 3.35 m (11 ft). When mulch was placed in the pan, it was generally spread at a compressed
depth of 5 cm ± 1 cm. The mulch depth was tapered over roughly the outermost 20 cm to27

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meet the side walls at a depth of 2.5 cm, and also tapered toward the front of the pan to adepth of 2.5 cm. This slight sloping at the edges was done to decrease the step change fromground to full-depth mulch and reduce the effect of the pan and mulch on the wind field near
the ground.
After a fence was placed at the correct location in the pan, the fence panel was raised to theprescribed height above the ground and attached to the end posts. If, as in all but a few cases,
the height above the ground was 0 m, then the mulch was checked to make sure that it barely
contacted the bottom of the fence and was adjusted where necessary. Downwind edges of themulch pan and the fence post furthest from the wind machine were both located at theprescribed separation distance from the shed wall. Since the combination of fence panel andposts was typically 2.62 m (8.6 ft) or less in length, the excess pan length extended upwind
from the leading fence post and included a 30 cm to 60 cm length of mulch to allow theobservation of counterflow flame spread on the mulch upwind of the fence. Approximately
0.17 m3 (6 ft3) of uncompressed mulch was required to fill the mulch bed. The shredded
hardwood mulch was compressed by stepping on it, with pressure of up to about 34 kPa(5 psi), based on the weight and contact area of the researchers. The resulting density of themulch measured after compression was 253 kg/m3 (15.8 lb/ft3) ± 3 %.
The mulch pans tended to warp after exposure to repeated fires and handling to dispose ofdebris. Two solid steel bars with dimensions of 10.2 cm × 20.3 cm × 2.5 cm (4 in × 8 in ×
1 in) were placed in the pans toward the outside where they overlapped to keep the panscompressed to the ground. Two additional bars with dimensions of 7.6 cm × 12.7 cm ×2.5 cm (3 in × 5 in × 1 in) were used in the windward corners of the pan to prevent the windfrom lifting the edge off the ground and affecting the wind flow over the bed. During earlyexperiments, low-profile heavy rocks found at the test site were used for the same functions.
For the experiments on two fence panels connected end-to-end, three pans were attached withclamps for a combined length of about 5.33 m (17.5 ft).
An exception to the use of steel pans under the mulch or fence was made for the experiments
on artificial turf. As a synthetic groundcover that rolled out like a carpet, artificial turf did not
require containment to hold it in place. Noncombustible Durock cement board [33], with a
low thermal conductivity (0.18 W/m-K) of the same order of magnitude as soil [34], wasused for these experiments. Two cement boards measuring ½ in thick, 3 ft wide, and 5 ft long
were arranged end-to-end, with the artificial turf unrolled on top. Steel bars were used tokeep the artificial turf from curling up.
2.4.2. Mulch Types
Mulch types used in these experiments included shredded hardwood, mini pine bark nuggets,pine straw, and shredded rubber. Photographs of these mulches are presented in Fig. 8.
The predominant mulch used in these experiments was shredded hardwood, shown in Fig.
8 (a). The shredded hardwood mulch was procured in 56.6 L (2 ft3) bags. Shredded hardwood
mulch was tested at the full 5 cm depth most of the time, although an abbreviated series ofexperiments with 2.5 cm depth was also conducted to see what effect the depth of
combustible mulch might have on flame spread.

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Mini pine bark nugget mulch was also procured in 56.6 L (2 ft3) bags. A photograph isshown in Fig. 8 (b).
Pine straw mulch is commonly utilized for landscaping in the U.S. southeastern states.
Multiple needle lengths and quality are available. The pine straw mulch used in theseexperiments was slash pine (Pinus Elliottii) with nominal 23 cm (9 in) length needles and of
A-grade quality (low level of non-pine-needle debris). The mulch was obtained from twovendors in South Carolina; the products appeared to be identical. A photograph of the pine
straw mulch is shown in Fig. 8 (c). For medium and high wind conditions, a chicken-wiremesh with 2.5 cm (1 in) openings was used to hold the pine straw mulch down and prevent itfrom blowing away prior to burning. This was deemed a realistic measure given that a typicalpine straw mulch bed installation would stay in place naturally after it had been compressed
and interwoven together due to cyclical exposure to wind and rain. No other mulches weresusceptible to being blown away in significant quantities.

Fig. 8. Mulch types: a) shredded hardwood mulch, b) mini pine bark nuggets, c) pine straw, andd) shredded rubber mulch.

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Experiments were also performed on shredded rubber mulch, shown in Fig. 8 (d). No
experiments were performed on this mulch in combination with a fence. The brand used was
Rubberific Red Rubber Mulch, which was purchased in 22.7 L (0.8 ft3) bags.
In addition to the above types of mulch, two experiments looked at the fire behavior of asingle variety of artificial turf. The synthetic fibers were made from polypropylene with aurethane-coated backing. The turf was cut to a width of 0.91 m (3 ft) ± 8 mm and rolled onto
the cement board substrate described in the previous section. There are a wide variety ofmaterials and designs for artificial turf – the experimental results for the single type selected
for this study should not be considered as representative of the fire behavior for all varieties.
2.4.3. Mulch Conditioning
All natural mulches used in these experiments were dried to 6.5 % ± 1 % moisture content.
Three alternative drying processes were utilized during the study: natural heating and drying
in the sun on an outdoor raised mesh platform, mesh bags placed on wire shelving in a wooddrying kiln at NIST, and thin layers placed in an indoor space conditioned to 30 % relative
humidity (RH). A moisture content of 6.5 % was selected because it is on the order of values
seen in wood in summertime in the American Southwest [35] – a low value, yet more
realistic than the far lower moisture content that could have been achieved throughoven-drying.
Mulch moisture content was measured with an Arizona Instruments Computrac MAX 1000
moisture analyzer (see Fig. 9). After drying, the mulch was placed in plastic bins that could
hold between 56.6 L (2 ft3) and 113.3 L (4 ft3) for storage and transport. The bins were stored
either in the conditioned (30 % RH) indoor space or in sealed bins in a building at the testsite. The dehumidifying equipment at the latter site was unable to reduce the water vapor
content below 35 % RH; however, with sealed bins, large amounts of mulch, and a small
moisture gradient, the moisture content was not expected to change significantly when
moved from 30 % to 35 % relative humidity conditions. A chart of equilibrium moisture
content (EMC) of wood as a function of relative humidity and temperature shows that EMC
ranges from 5.6 % to 6.3 % at 30 % RH and 6.3 % to 7.1 % at 35 % RH, for temperaturesfrom 43.3 °C to -1.1 °C (110 °F to 30 °F) [35, 36]. Therefore, a moisture content estimate of
6.5 % ± 1 % encompasses the sets of conditions at both sites, as well as the effects ofvariations in initial drying.

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Fig. 9. Moisture analyzer used for measuring moisture content of mulch.

The rubber mulch was not carefully conditioned. Each 28 L (1 ft3) bag was emptied into twolarge bins such that the depth of mulch was about 10 cm (4 in). These were only stored in theconditioned environment for about one day, so some residual moisture remained in themulch.
2.4.4. Target Mulch Bed
To study whether the fence or mulch bed being tested was capable of generating firebrands
that could threaten a structure through spot fires, many experiments included a target bed ofshredded hardwood mulch placed along the base of the shed wall, as shown in Fig. 10. Theseexperiments were classified as Series 2 experiments. The target mulch bed was 0.46 m(18 in) wide and 2.44 m (8 ft) long. Two steel pans, each 1.37 m (4.5 ft) long, wereoverlapped in the middle to create the 2.44 m total length. The pans had 2.5 cm (1 in) walls
on the far ends and on the back edge that abutted the shed wall.

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Fig. 10. Target mulch bed and digital timer.

The target mulch bed served as a surrogate for any combustible material next to a structure.
Because of its rough texture, any firebrands landing on this surface tended to stay in place.
Shredded hardwood mulch was selected as a conservative worst-case for combustibles near
the structure: dry, consisting of easily ignited small pieces, and comprised of innumerable
crevices in which firebrands could lodge. The mulch was conditioned to 6.5 % ± 1 %moisture content, as described in Section 2.4.3.
The mulch bed was prepared by filling the pans with an even layer of mulch and compressing
the mulch by foot, as described in Section 2.4.1. The target mulch bed was 2.5 cm thick, withthe first 3 cm of the leading edge slightly tapered down to about 1.5 cm thick to decrease theseverity of the abrupt change in height from the ground and to reduce the number of sliding
firebrands that were caught at the front edge of the bed.
Fence Types, Materials, and Preparation
2.5.1. Privacy Fences
An informal survey of southern California and northern Texas fence companies performed
for the previous NIST fence study [23] found that privacy fences are the most common type
constructed in those areas. A variety of top styles are used (e.g., cap, dog-ear, lattice). The
simplest style privacy fence (plain flat top) was selected for that initial series of experiments
as well as for this more in-depth investigation. The survey also found that western redcedar,
California redwood, and vinyl were the most common fence materials, leading to theselection of those materials for the preliminary study. Since redwood and western redcedar
displayed very similar fire behavior, the current series of experiments focused on western
redcedar. A photo of a western redcedar privacy fence is shown in Fig. 11 (a).

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Fig. 11. Fence types: a) western redcedar privacy, b) aged privacy, c) vinyl privacy, d) redwood
lattice, e) pressure treated pine lattice, f) western redcedar good neighbor, g) wood-plastic
composite #1, and h) wood-plastic composite #2.

The western redcedar (WRC) privacy fence panels in this study were made primarily withvertical boards of 1×6 lumber, measuring 1.9 cm (3/4 in) thick, 14.0 cm (5.5 in) wide, and1.83 m (6 ft) tall. These boards were chosen because their usage appears to be more
predominant than the narrower 1×4 boards, whose width is 8.9 cm (3.5 in). The density of thewood boards was 360 kg/m3.
The fence panels were manufactured locally by two fence construction companies tomaintain commercial standards and consistency, with overall dimensions of 2.44 m (8 ft)
long by 1.83 m (6 ft) tall. To achieve the 2.44 m length, sixteen 1×6 boards and two 1×4
boards were arranged side by side. The boards were nailed to three 2×4 horizontal stringers
or rails made of pressure treated pine, with cross-sectional dimensions 3.8 cm (1.5 in) thickby 8.9 cm (3.5 in) high and density of 930 kg/m3. The bottom stringer was attached 21 cm(8.25 in) above the ground, the top stringer was attached either flush with the top of the fence(for tests A-48 through A-91) or 21 cm (8.25 in) below the top, and the middle stringer was
attached midway between top and bottom stringers. Spacing between the vertical boards
varied from 1 mm (0.04 in) to 6 mm (0.24 in) and averaged 3.5 mm (0.14 in). Uncertainties
for the dimensions of fence components and the assembled fence are discussed inAppendix A.1.

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In order to evaluate the effects of age on fire spread on a western redcedar privacy fence, four
fences were recovered from a dump in Colorado Springs by the Colorado Springs Fire
Department. All four panels were from the same fence assembly, which was estimated to bewell over ten years old. The type of wood was not determined conclusively, but western
redcedar is a common fence material in the area, and the fence appearance was consistent
with this type of wood after weathering. Due to broken or damaged boards, the four deficient
fence panels were combined into three complete ones. The fences also included weatheredpressure treated pine 2×4s. Much of the wood had signs of dry-rot and was relatively fragile
compared to new fences. The aged fence panels were 2.34 m (92 in) long as compared to2.44 mm (8 ft) for the new WRC panels; otherwise, the dimensions were identical.
Figure 11 (b) shows a photo of one of the aged privacy fences as it was ignited.
A few experiments were performed on polyvinyl chloride (PVC) privacy fences, commonlyreferred to as vinyl fences. Unlike the wood fences, vinyl panels were provided with theirown vinyl posts. The total length of these test subjects including posts was 2.44 m (8 ft). As
shown in Fig. 11 (c), vinyl fence assemblies consisted of the two end posts with two
horizontal rails flush with the top and bottom. The rails had continuous slots to hold the endsof the inserted vertical boards. The vertical sections nested with each other with a tongue andgroove design.
2.5.2. Lattice Fences
A second type of fence investigated was the diagonal lattice fence. Photos in Fig. 11 (d)and (e) show redwood and pressure treated pine diagonal lattice fences, respectively. Latticefence panels were included in this study as an example of a decorative fence type often used
to border gardens or patios. Redwood was the material used for most of the latticeexperiments. The dimensions of the diagonal lattice wood slats were 3.8 cm (1.5 in) wide and7.9 mm (5/16) in thick. The diagonally crossed slats produced diamonds that were 7.0 cm(2.75 in) across. The size of a lattice panel was 1.22 m (4 ft) high by 2.44 m (8 ft) long,
making them 0.609 m (2 ft) shorter in height than the privacy fences.
Lattice fences required two horizontal stringers or rails for strength. The material for theseboards was interior (untreated) pine, with the same transverse dimensions as the privacy
fence stringers (nominal 2×4 lumber) but oriented with the wider dimension horizontal to theground as shown in Fig. 11. The stringers were cut to a length of 2.34 m (92 in) to allow the
2.44 m (8 ft) long lattice panel to overlap the posts by 5 cm (2 in) at either end. The stringers
were located 8.9 cm (3.5 in) from the bottom of the fence panel and 21.6 cm (8.5 in) from the
top.
Pressure treated pine lattice fences were tested as a comparison with the more expensive
redwood, which is more readily available near the U.S. west coast. The pine lattice wood wasslightly thinner at 6.4 mm (1/4 in) thick, so any differences in fire performance could be due
to mass as well as material.
2.5.3. Good Neighbor Fences
Good neighbor fences are fences that look the same from both sides of the fence, so that bothneighbors see the finished, “pretty side” of the fence. The good neighbor fences used in this34

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study, shown in Fig. 11 (f), also go by the name board-on-board. They are characterized byboards that are attached to alternating sides of the horizontal stringers, allowing neighbors toenjoy both privacy and an identical appearance. Due to the interesting fire behavior found for
double lattice fences and parallel privacy fences (described in Section 4.4), the goodneighbor style privacy fence was investigated for three experiments. It was anticipated that
this fence might exhibit a hybrid of the fire behaviors of single and parallel privacy fences,since the boards are in view of each other’s faces for radiative heat transfer, although at anangle. This fence style used the same western redcedar boards as described for the standard
privacy fence in Section 2.5.1, but the vertical boards were mounted on alternating sides of
the horizontal stringers or rails. The good neighbor fence used the same type, arrangement,
and spacing of horizontal stringers as used for the privacy fences. On each side of the fence,
boards alternated with 8.9 cm (3.5 in) width spaces, and each board faced a space on the
opposite side of the fence. This spacing typically resulted in ten vertical boards on one side
and eleven on the other.
2.5.4. Wood-Plastic Composite Fences
Two wood-plastic composite fences were tested in this study. Both fences were designed tolook the same on both sides of the fence, similar to the good neighbor fence.
In the case of WPC1, shown in Fig. 11 (g), the extruded composite boards were designed tointerlink, leaving no gaps between boards. The assembly primarily consisted of top andbottom rails holding the interlinked vertical boards. The vertical boards, made of 5.6 mm(7/32 in) thick composite, stood in the slot running along the entire length of the bottom rail
made of 2.5 mm (0.1 in) thick aluminum, which was 15 cm (6 in) tall and wrapped with
5.6 mm (7/32 in) thick composite rail covers. The tops of the boards were also inserted into
the 15 cm (6 in) tall, 13 mm (1/2 in) thick top rail. Both rails were suspended from brackets
attached between their ends and the posts. Bare treated 4×4 pine posts were used instead of
the sleeves sold with the assembly.
The boards for the WPC2 fence were arranged horizontally, as shown in Fig. 11 (h). Hollow
aluminum posts 10 cm (4 in) wide were mounted on stands at each end of the fence. An
aluminum bottom rail 30 mm (1.2 in) high was inserted into slots on the posts, followed by aset of eight fence boards each 21 cm (8.3 in) tall and topped by an aluminum upper rail 30
mm (1.2 in) high. Each board was hollow with struts separating front and back surfaces, for a
total board thickness of 17 mm (0.7 in).
2.5.5. Fence Support
All wood fences were mounted on two pressure treated pine posts with square cross sections
8.9 cm (3.5 in) on a side (nominal 4×4 lumber). The posts at each end added 17.8 cm (7 in) to
the length of a single privacy fence panel, for a total length of 2.62 m (8 ft 7 in). Latticefences were mounted on the sides of the posts, for a total single panel length of 2.51 m(8 ft 3 in).
Fence panel and post assemblies were supported by boring a 2.4 cm (0.94 in) diameter holein the bottom of each post and inserting the 25 cm (10 in) long, 1.91 cm (0.75 in) diameter“leg” of a steel “foot” in the hole. Beneath the vertical rod, each foot was made with a cross

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of 4.8 mm (3/16 in) thick, 2.5 cm (1.0 in) wide steel bars. Sections of wood 4×4 posts 25 cm
long were inserted into the vinyl posts to allow mounting on the same legs/feet used for woodfences.
Each end of the fence was connected to two partially filled 208 L (55 gal) barrels of waterusing 15-gauge (1.45 mm or 0.0571 in diameter) steel wires. The purpose of this was toprovide stability and to prevent the wind from blowing the fence over or causing unrealistic
vibration or other motion. When the separation distance from the shed was less than 3 ft, thenearest fence post was attached to the eaves of the shed rather than to a water barrel. The
wire support system also allowed the fences to be adjusted for verticality.
2.5.6. Parallel Fences
During this study, rapid growth in fire spread and energy release was discovered for two
fence panels erected adjacent to each other. This fire behavior led to a series of experiments
to investigate two scenarios. In the double lattice fence scenario, two lattice fence panels aremounted on opposite sides of a 2×4 lumber frame, providing a more substantial boundary todelineate or partially enclose an area such as a garden. In the parallel fence scenario, twofences are erected by neighbors on either side of the shared property line. It is not uncommon
to find parallel fences in communities, where a homeowner erects a second fence for apleasing appearance or a functionality that is not provided by the neighbor’s existing fence.
Fences are often installed fractions of a meter apart, as in the example in Fig. 12. A fence
may also be constructed next to other types of vertical surfaces, such as auxiliary buildings.

Fig. 12. Parallel privacy fences. Colorado Springs Fire Department, used by permission.

Double lattice fences were fabricated with structural support similar to that used for single
lattice panels. As can be seen in Fig. 13, two 3.8 cm × 8.9 cm (nominal 2×4 lumber)
untreated pine rails were attached horizontally along the top and bottom of two lattice panels.
The rails were oriented with the shortest dimension contacting the lattice panels, providing an
8.9 cm (3.5 in) separation distance between them. The upper frame board was located21.6 cm (8.5 in) from the top of the lattice, and the lower one was located 8.9 cm (3.5 in)from the bottom. To provide more stability, keep the assembly square, and provide an

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attachment point to connect to the posts, four 10.2 cm (4 in) long corner braces made fromthe same untreated pine lumber were attached at the inside corners, extending upward or
downward from the horizontal frame boards.

Fig. 13. Double lattice fence.

For the parallel privacy fence experiments, the two fences were each connected to their own
independent posts. The fences were arranged with the smooth sides facing out and thehorizontal stringer sides facing inward. Spacing of the fences was defined as the distanceseparating the vertical boards, although distances between horizontal stringers was less.
Spacings included 20.3 cm (8 in), 30.5 cm (12 in), 45.7 cm (18 in), 61.0 cm (24 in), and91.4 cm (36 in). For spacings of 61 cm (24 in) and greater, two adjacent sets of mulch pans
were required. For stability, each fence was attached to two water barrels and to each otherwith steel wire. Parallel fences were tested similarly to single fences – with and without
mulch beneath and raised or lowered on the posts as needed.
Most parallel privacy fence experiments were conducted with western redcedar fences, but
additional experiments explored combinations of western redcedar privacy fences, vinyl
privacy fences, pine lattice fences, and noncombustible cement board sheeting.
2.5.7. Fence Materials
Physical and flammability properties were obtained for the fence materials used in this study.
Appendix B describes a cone calorimeter study comparing flammability measurements forwestern redcedar (WRC), vinyl (PVC), and wood-plastic composite (WPC) fence samples.
The samples were cut from fence boards – the WRC and WPC samples were solid, withdifferent thicknesses but comparable masses. The PVC sample consisted of two thin sheets
separated by three thin sheets serving as braces and had the lowest mass. Wood-containing
samples (WRC and WPC) ignited easily, and the duration of flaming was higher than that of
PVC by a factor of two. The WPC samples burned intensely, with peak and total heat releasevalues greatly exceeding those of the other two materials. The total heat release and theaverage heat release rates for WRC samples were significantly higher than for PVC.
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Firebrand generation was observed in WRC samples during flaming and after flame
extinction, with small wood firebrands flying away from the burning cone samples. No
smoldering combustion was observed for PVC and WPC samples.
2.5.8. Wood Conditioning
Wood fences, posts, and the boards used to assemble double lattice fences were stored in thesame conditioned spaces as the mulch (30 % to 35 % RH). Like the mulch, the moisture
content for these components was 6.5 % ± 1 %, selected as a value that would provide more
realistic conditions for fire spread than the enhanced conditions that would result from usingoven- or kiln-dried wood [35].
Ignition Source
The test subject was ignited by one of three types of propane burners applied near the end
farthest from the structure. The primary burner was a customized model that consisted ofeight Venturi-style brass torch heads (Bernzomatic brand Pencil Flame model), arranged withfour torch heads on each side of the test object. Two torches of each set pointed 45° upward
toward the fence and the other two pointed 45° downward toward the mulch, as can be seenin the overhead view in Fig. 14. The torches were wrapped with Kaowool ceramic fiber
blanket and then covered with aluminum foil as shown in Fig. 15 for protection from flames
and radiation after ignition of the fence and/or mulch.

Fig. 14. Propane burner for igniting fence and/or mulch, with torches exposed.

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Fig. 15. Propane burner protected by Kaowool blanket and aluminum foil.

An alternative ignition source was used on a small number of experiments with unusual fire
behavior. Because of the rapidity of the fire growth on shredded rubber mulch, and because
the vaporized rubber tended to clog the fence burner torches, a larger single propane torch
was implemented. The larger torch was a Magna Industries MagFire MT5500 with a 6 cm
diameter nozzle and maximum output of 150 kW. It was successfully used to ignite therubber mulch quickly and uniformly across the width of the pan. The torch burner was also
used for ignition of artificial turf.
For experiments in which the fence was raised above the mulch bed, described in
Section 4.3.2.4, a ring burner with propane as the fuel was used for ignition. The Imperial
model IMP1273 ring jet burner was 23 cm (9 in) in diameter and primarily made of cast iron.
It consisted of nine hubs of one to four brass torch heads each, distributed around theperimeter and across the diameter.
Measurements
2.7.1. Wind Speed Profiles
The experiments were performed under imposed wind speed conditions from 6 m/s to 14 m/s(13 mi/h to 31 mi/h) in line with the fence. In order to measure the wind velocity field, anarray of thirteen bidirectional probes was placed 1.22 m (4 ft) upwind of the fence postclosest to the wind machine (or, for a mulch-only experiment, where that post would be).
This location was selected to capture the wind field close to the fence/mulch that is the focusof the experiment without influencing the upwind measurement. Bidirectional pressureprobes measure the difference between the total pressure on the windward side of the probe
and the static pressure on the leeward side. The difference is the dynamic pressure caused by
the wind, which can be combined with temperature and a probe factor to calculate the windspeed [37]. The leads of the probes were connected to Setra Model 264 bidirectional pressuretransducers, which have a pressure range of ±373.6 Pa. Each transducer produced a voltage

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output from 0 V to 5 V, with 2.5 V output indicating zero pressure differential. Combining
the pressure measurement with ambient temperature gave a corresponding velocity range ofabout ±23 m/s (±52 mi/h). The transducer calibrations were checked periodically with apressure calibration system, and their sensitivities were found not to drift significantly.
Voltage outputs measured during daily pneumatic zeroing (which will be described in
Section 2.9.2) were used to account for any voltage offsets.
A photograph of the bidirectional probe array in front of a fence/mulch test combination isshown in Fig. 16, and the diagram in Fig. 17 indicates the locations of probes. The arrayconsisted of five probes arranged vertically on the centerline of the experiment at heights of
0.30 m (1 ft), 0.76 m (2.5 ft), 1.22 m (4 ft), 1.68 m (5.5 ft) and 2.13 m (7 ft) measured fromthe ground; two sets of two probes each extending out from the centerline in 0.61 m (2 ft)
intervals at both the lowest (0.30 m) and highest (2.13 m) positions; and an additional four
probes extending out from the centerline in 0.30 m (1 ft) intervals at the middle (1.22 m)
position. This allowed for collecting velocity data for a vertical velocity profile at thecenterline and a horizontal velocity profile at the center height, and added several additional,
more sparsely located, velocity measurements to provide a more complete picture of thevelocity field generated by the wind machine.

Fig. 16. Bidirectional probe array.

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Fig. 17. Diagram of the bidirectional probe array used to measure the velocity field.

Ambient temperature was required along with the differential probe pressures to calculate thewind speed. Temperature was measured with a type K thermocouple bead made from 24
AWG wire (0.51 mm diameter). The temperature measurement location was about 2.5 m
away from the probe array and shielded from thermal radiation from either the fire or sun.
The effect of wind angle had been studied in previous research on the fire behavior offences [23]. Since an angle of 0° (parallel or in line with the wind) was previously found to
produce the fastest fire spread, it was selected for the vast majority of experiments in order toprovide worst-case fire hazard conditions. Two experiments performed with the wind at a 45°
angle to the fences confirmed that the fire spread was slower compared to wind aligned withthe fences.
The wind profile measured by the bidirectional probe array depends primarily on the windspeed and the distance from the wind machine, with a contribution from the component of
ambient winds in the direction measured by the probes. Because the probe array is 1.22 m(4 ft) upwind from the fence and at least 3.84 m (12 ft 7 in) upwind from the shed, the effectsof these objects on the measured wind field are minimal.
Appendix C.2 shows velocity profiles measured during experiments at four distances fromthe wind machine. The experiments in this study were divided into twelve sets corresponding
to the three wind speeds and four probe array positions, and velocity measurements fromeach probe were averaged for each set. The resulting pseudocolor plots show that the velocity
profile is reasonably uniform over the central region of the wind field in the region occupiedby the fence. At ground level, the mulch bed sees a gradient in velocity from the centerline tothe edge of the mulch pan. The center velocity is somewhat lower than the assigned wind41

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speed and increases with distance along the mulch bed toward the shed due to the angled
flow straightener.
For a given experiment, the average of the velocities of the lower four probes along thecenterline was used to state the average wind velocity. This measure was selected after
studying the velocity profiles along the centerline shown in Fig. C.6.
2.7.2. Ambient Wind Speed and Direction
The ambient wind speed and direction were measured by an anemometer mounted on a 3.7 m(12 ft) pole about 7.9 m (26 ft) south-southeast of the wind machine propellers and 17.7 m(58 ft) south-southwest of the target shed. The instrument was a Young model 86000
Ultrasonic Anemometer with 5 V output and 0.25 s response time for both wind speed andwind direction. Wind speed was measured with 0.01 m/s resolution and ± 2 % ± 0.1 m/s
accuracy as stated by the manufacturer, and the wind direction was measured with 0.1°
resolution and ± 2° accuracy. Wind direction accuracy was degraded to about ± 5° due to theestimation of true north during installation and slight positional drift due to high winds which
was periodically corrected. The ambient wind measurement provided an approximate windenvironment near but not exactly at the location of the experiments, so some focused wind
gusts may have been located at the experiment and not the anemometer or vice versa.
Data Acquisition
2.8.1. Wind and Temperature Data
A data acquisition system was required to measure 16 channels of measurements from thebidirectional probe array located in front of the fence or mulch bed, an ambient temperaturethermocouple, and the local wind speed and direction from the sonic anemometer. Voltage
and thermocouple data from the sensors were collected using two National Instruments input
modules, NI-9205 and NI-9213, respectively inserted in a National Instruments cDAQ‑9174
CompactDAQ USB 4-slot chassis. The data were collected at 10 Hz and averaged over every
second for each channel. The program saved the averages and standard deviations of thesamples from each channel to the output file, which was stored on a laptop computer andlater uploaded to a permanent data storage repository. The Labview program used to collectthe data was also used to monitor data quality and spot check for sensor malfunctions.
2.8.2. Digital Video and Photographic Records
A minimum of four high-definition video cameras, Sony model HDR CX-350, were placedaround the fence to capture the fire and smoke behavior. Two cameras located on opposite
sides of the fence included the fence, shed wall, and shed mulch pan in their fields of view.
These cameras captured fire spread and spot fire ignition data. An additional two cameras
were located upwind of the fence next to the wind machine flow straightener, including thefence, shed, and shed mulch pans in their views. For experiments with parallel fences, a fifth
camera was usually placed under the flow straightener on the centerline of the experiment to42

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record the fire behavior between the two fences. Fig. 18 is a top view schematic of theexperimental setup showing the relative positions of the video cameras.
To track experiment time, a DC-Digital timer, model DC-25UT, was placed in view of twoor three of the video cameras (depending on line-of-sight blockage due to varying fence
position). The timer, visible in Fig. 10 and Fig. 16, was started simultaneously with the windmachine. This allowed the video records of the left view and one or two of the front views of
the test setup to also record the timer and thus synchronize with the remaining video
camera(s), the two stopwatches used, and the wind data, which was referenced to computer
time.

Fig. 18. Top view schematic of experimental setup showing placements of video cameras, timer, andbi-directional probe array. Google Earth image with NIST overlay.

Digital still photographs were taken throughout the testing period and afterward. The digitalstill camera used was Sony model SLT-A58. Periodically, the handheld camera was used tocapture close-up video of interesting phenomena.

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Experimental Procedures
2.9.1. Weather Conditions
The ambient wind speed was required to be less than 33 % of the nominal applied windspeed in order to carry out the experiment. If the ambient wind direction was forecast to beclose to perpendicular to the direction of the generated wind, then ambient winds needed tobe less than 25 % of the generated wind. Under these conditions, the impact of the ambient
winds on the wind field generated by the fan was minimal. Winds from the North andNorthwest were also avoided as they caused smoke and firebrands to overspread theexperiment control area.
Testing was usually not scheduled when rain chances were likely during a substantial part ofthe morning or afternoon. Excessively hot or cold weather conditions also precluded testing.
Generally, experiments were not scheduled if the heat index was expected to rise over 32 °C(90 °F) for a large part of the day, in order to avoid heat exhaustion. If temperatures were notexpected to surpass 10 °C (50 °F), experiments were precluded by difficulties with handling
tools, vaporizing propane, and drying the wet ground after fire extinguishment.
2.9.2. Preparation
Preparation for a typical experiment began with clearing the test area of debris. The mulchpans were connected and located at the prescribed distance from the shed and centered along
the shed/wind machine centerline axis. Heavy steel bars were placed in the assembled pan at
the leading edge and in the overlap region to weigh it down. If a fence was to be erected,
holes were bored into one end of each post and the posts were placed on the post support legs
close to their final location. The fence was then positioned for attachment to the posts. Shims
were used to raise the fence if a mulch layer was to be laid beneath. The fence and posts werepredrilled and then screwed together at each horizontal stringer. After the fence was secured
to the posts, the shims were removed, and a mulch layer was laid evenly and compressed byfoot throughout the pan. The posts were set perpendicular to the ground and secured withwire to two water-containing barrels located off to the side and also to the shed or shed eaves
if located at 0 m or 0.30 m (1 ft) separation distance. After the fence was secured, the support
wires were marked for safety with strips of tape or other material to prevent personnel fromrunning into them. Mulch was then laid, spread evenly, and compressed in the target mulch
pan at the base of the shed, if prescribed by the test plan.
Preparations for instrumentation included positioning of the four or five video cameras withframing of the appropriate views. The bidirectional probe array was positioned in front of theleading fence post. For fence experiments, a burner was connected to the propane gas
cylinder and was positioned on both sides of the fence, with its leading torch head located
2.5 cm (1 in) beyond the trailing side of the leading fence post. Half of the torches on theburner were aimed upward to the face of the fence and the rest aimed downward toward themulch (if mulch was present). If no fence was being tested, a layout table was checked tofind out where on the mulch bed the burner should be positioned relative to the shed, as if afence were there. The propane cylinder was opened and the burner line charged to check theburner for leaks, and then the gas was shut off with a valve. Before the first test on a given

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day, pneumatic zeroing of the pressure transducers was performed by connecting a shortlength of rubber tubing to each side of the bidirectional probes and recording the data. This
also enabled observation of the pressure transducer voltages being read by the dataacquisition system for troubleshooting problems. Voltages that were drifting indicated a poorconnection, and voltages offset significantly from 2.5 V indicated a plumbing leak.
After a safety check of the surrounding area, the wind machine was warmed up for
approximately 5 min prior to each experiment. A garden hose was attached to a fire hose,which was in turn connected to a hydrant. The hydrant was opened to charge the line, and adiesel pump was started up to pressurize the hydrant with water from the nearby pond.
Finally, a safety briefing was conducted to communicate the test procedure, participant roles,
and safety reminders. Zeroing tubes were removed from the probe array, and the test
description and filename were detailed in the logbook.
2.9.3. Operations
The following procedure was typical, although some minor aspects varied for some tests. The
data acquisition system was initiated with the selected filename and description. Between
15 s to 60 s of background data were obtained before two stopwatches were started
simultaneously and the program time was recorded. At that time, all video cameras were put
into recording mode. After 50 s of stopwatch time, a small propane torch was ignited, and at
55 s the propane cylinder was opened. At 1 min, the ignition torch was held near the burner
torches until they were all ignited. The times for initiating and completing burner ignition
were recorded. For most experiments, the burner was sustained for 90 s on the fence and/or
mulch in order to produce a self-sustaining fire that would not self-extinguish or go out in thewind. Some conditions such as cold (which diminished the propane flow) called for longer
burner duration up to 3 min. Photographs of the fence or mulch fire were usually taken
shortly after ignition.
A countdown to generating wind was performed, and a digital timer located next to the shed
pan and visible in two or three of the video camera views was initiated at the same time as
the wind machine was started. The wind level was adjusted by setting the tachometer to
950 rpm (revolutions/min) for low wind [6 m/s (13 mi/h)], 1500 rpm for medium wind
[10 m/s (22 mi/h)], and 2000 rpm for high wind [14 m/s (31 mi/h)]. The times for initiating
the wind and completing its adjustment were recorded in the log. As soon as the wind started,
the burner was removed to protect it from the fire, and the propane valve was closed.
In addition to the continuous videos recorded on fixed cameras, photographs were taken frommany angles and field of view during the experiment. The photos included the overall views
encompassing the entire fence and shed, the linear extent of the fire along the fence/mulch,
spot fires in the target mulch bed, and unusual or interesting phenomena. Some interesting
phenomena were captured using the video mode of the handheld digital camera. The
experiment ended when a spot fire in the mulch bed at the base of the structure reached thewall and after fire had also reached the end of the fence or mulch bed. Flames at the wallfrom spot fires were extinguished if the fire had not yet spread over the entire length of thefence/mulch bed in order to capture the fire spread rate over the entire fence. At the end of
the test, the fires were extinguished with a water hose and post-fire photographs wereobtained.
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Parameter Summary
From April 2016 through September 2021, 187 field experiments were carried out on fences
and mulch. The experiments in this study were performed on a variety of combinations offence panels and mulch under various conditions of wind and separation distance from thetarget shed or mulch bed. The distribution of the parameters in these experiments is shown in
Fig. 19 and summarized in this section.

Fig. 19. Distribution of 187 experiments by subject of experiment, firebrand target, fence type, mulch
type, separation distance from the wall, and wind speed.

2.10.1.

Subject of Experiment

The experiments fell into four types based on the test subject: fence plus mulch, mulch alone,fence alone, and parallel fences (with and without mulch).
Approximately half of the experiments were performed on some combination of fence andmulch. This configuration was the main focus for this study, addressing the question whether
fences could act as a wick to carry fire from one property to another within a community.
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Combining the fence with mulch showed the effect of fine combustibles in contact with thebase of the fence. Under the high wind conditions that may accompany a WUI fire, leaves orpine needles may collect next to the fence even when it is maintained free from combustible
materials such as mulch and shrubbery. For most experiments the fence was erected as
described in Section 2.5.5 at a height touching the surface of the mulch, which was arranged
according to Section 2.4.1. A small number of experiments tested the effect of raising thefence panel above the mulch bed as a potential avenue for mitigation.
Mulch beds accounted for about a quarter of the experiments. The purpose of theseexperiments was to demonstrate the fire behavior of various types of mulch placed near astructure. Mulch beds were arranged as described in Section 2.4.1.
Experiments on fences in the absence of mulch addressed the question of how fences behave
under perfectly clean conditions, with no contact with fine combustibles. These experiments
acted as a control group to examine the effect of mulch on the fire behavior of fences. Under
realistic fire conditions, it would be difficult to achieve a fence free of fine combustibles,
since WUI fires are often accompanied by winds that blow loose materials such as leaves orpine needles into corners and crevices. The methods in Section 2.5.5 were used to erect thesefences.
Parallel fences were separated from other fence categories after it became clear that their fire
behavior was significantly more hazardous than that of other configurations. Combustible
surfaces parallel to each other are subject to radiative and convective heat transfer
mechanisms that produce rapid fire growth. Configurations that were considered in theexperiments included double lattice fences, which may be used for a garden; parallel wood
privacy fences, which may be erected by neighbors on either side of a property line; andwood privacy fences parallel to surfaces that represent a combustible or noncombustible shedwall. The preparation of parallel fences for these experiments is explained in Section 2.5.6.
Because of similarities in fire and firebrand behavior, the results for this report are organizedby these four types of experimental subjects, as are the appendices that present the details ofeach experiment. The full set of experimental matrices is provided in Appendix D.
2.10.2.

Firebrand Target

The experiments involved three configurations with respect to the target for flame spread andfirebrands. A shed was arranged at right angles to the fan at a distance of 10.7 m (35 ft) for
Series 1 and 2 experiments, as described in Section 2.3 and illustrated in Fig. 4. A small
handful of experiments (Series 3) were performed without a shed in order to study theignition of mulch by firebrands at a distance.
In Series 1 experiments, a sacrificial layer of southern yellow pine plywood panel siding witha thickness of 1.5 cm (0.59 in) was attached to the shed wall, as discussed in Section 2.3,
extending to the ground with a crevice less than 2 mm. For these experiments, ignitions at thebase of the wall were taken as a measure of vulnerability to firebrands. With a smoothsurface at the base of the shed, the flow field at the base of the wall directed the firebrands
toward the right or left of the shed, and few firebrands lodged in the small space between the
plywood wall and the concrete.

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About 80 % of the experiments were Series 2, with a narrow mulch bed positioned along thebase of the shed and a noncombustible false wall to protect the shed. The mulch bed wasused here as a surrogate for building ignition vulnerability or hazard. This configuration
represented a worst case condition of fine combustible material in contact with the structure.
The firebrands that landed in the target mulch bed tended to stay in the place they landed. If
conditions were favorable, the firebrands ignited fires that then worked their way backtoward the shed, at which time they were extinguished with water. This event usually marked
the end of the experiment, unless the fire over the test subject had not yet reached the end
closest to the shed or was spreading very slowly.
For Series 3 experiments the structure was removed, and the target mulch bed was placed
23 m to 47 m (75 ft to 153 ft) from the end of the fence and/or mulch that was ignited. The
purpose of these experiments was to test the ability of firebrands generated by the mulch toignite a fuel bed at a long distance from the source. These experiments are discussed in
Section 4.5.
2.10.3.

Fence Material, Style, and Configuration

As described in Section 2.5, privacy, lattice, and good neighbor fence styles were tested inthis study. The boards for the privacy fences were made of western redcedar, vinyl, or woodplastic composite, and the lattice fences were constructed of redwood or pine. Western
redcedar boards were used to assemble the good neighbor fences. Aged western redcedar
fences that were believed to have spent at least ten years outside in Colorado were also tested
to see whether the fire behavior is significantly worse after long exposure to the elements.
Over half of the experiments with fences were performed with western redcedar privacy
fences. These fences were studied at all values of wind speed and separation distance andprovided a basis for comparing the fire behavior of different fences and mulches.
In two experiments, the length of the fences was doubled by arranging two panels end-to-end
with a post between the panels and at each end. The purpose of these experiments was todetermine whether the fire behavior had reached a steady-state flame condition in a single
panel length.
Five experiments were performed with the wood privacy fence panel raised above a bed ofshredded hardwood mulch. Vertical separation of the fence from the mulch was considered apossible mitigation technique.
2.10.4.

Mulch TypeThe mulches used in this study include shredded hardwood mulch, mini pine bark nuggets,pine straw, and synthetic (shredded rubber) mulch. In addition, two experiments were carriedout on artificial turf. The majority of experiments were performed using shredded hardwood
mulch, which also served as a common base for the comparison of different fence types andconfigurations. Details on each mulch type and its preparation for testing are presented in
Section 2.4, with photos in Fig. 8. The standard mulch thickness was 5.0 cm, which wascompressed by foot. The effects of a thinner mulch layer were investigated with a set ofexperiments on shredded hardwood mulch at half thickness, or 2.5 cm.

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2.10.5.

Separation Distance from Structure

When the shed was present, experiments were performed at distances between the shed walland the nearest end of the fence or mulch bed ranging from 0 m to 1.83 m (0 ft to 6 ft). The
four separation distances used in this study were 0 m, 0.30 m (1 ft), 0.91 m (3 ft), and 1.83 m(6 ft). Figure 20 shows a diagram of the four separation distances (SD) for Series 1
experiments, with a combustible wall attached to the shed as a target. Figure 21 shows adiagram for Series 2 experiments, in which a bed of shredded hardwood mulch served as thetarget for spot fires.
Because parallel fence experiments tended to produce large flames, they were all performed
at a separation distance of 1.83 m (6 ft).

Fig. 20. Experimental configurations with combustible wall as target.

Fig. 21. Experimental configurations with shredded hardwood mulch bed at base of structure astarget.

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For Series 3 experiments, the shed was removed, and distances were measured from the endof the fence or mulch bed farthest from the fan to the leading edge of the target mulch bed.
The long separation distances for these experiments were intended to test the distance overwhich firebrands were able to ignite spot fires in a mulch bed.
2.10.6.

Wind Speed and Direction

Wind speeds through a community during a WUI event may range from nearly stagnant toprevailing wind speeds and possibly higher, depending on local shielding and channeling due
to structures, vegetation, and terrain. The experiments in this study were performed undernominal imposed wind speed conditions from 6 m/s to 14 m/s (13 mi/h to 31 mi/h), generated
by a large fan with tilted flow straightener as described in Section 2.2. Low, medium, andhigh wind speeds corresponded to average values along the centerline in the ranges 5 m/s to
9 m/s (11 mi/h to 20 mi/h), 10 m/s to 13 m/s (22 mi/h to 29 mi/h), and 14 m/s to 18 m/s(30 mi/h to 40 mi/h), respectively. The low wind speed category was selected to overlap withthe conditions for experiments on fences performed by Manzello and colleagues inJapan [20].
Because a previous NIST fence study [23] concluded that the worst case fire behavior
occurred with the wind flow in line with the fence, almost all experiments were performed inthis configuration. The wind was at a 45° angle to the fence in two experiments. All
experiments were performed with the shed perpendicular to the fence and/or mulch bed.

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Analytical Tools
The data acquisition systems described in Section 2.8 provided raw data that needed to beanalyzed and visualized to develop an understanding of the fire behavior in each experiment.
This section describes the tools that were developed to analyze the video evidence and winddata collected from each experiment and the use of a computational fluid dynamics model tounderstand the flow field.
Video Analysis
The primary data to be collected from each experiment were flame spread and firebrandspotting. Every experiment employed four video cameras, with views from the left, right, left
front, and right front of the object being tested, from the point of view of the fan (facing theshed). In some experiments, a fifth camera was used – to record a parallel fence experimentdirectly down the center between fences, for example, or to record a closeup of an interesting
phenomenon. The videos recorded the progress of flames and charring and the ignition of
spot fires, as well as events such as burner ignition, fan engine startup and shutdown, and thestart of suppression. The frame rate was 29.97 frames/s.
Timing, flame spread, and spotting analyses were performed on videos from the left and rightcameras. The MATLAB computing environment [38] was used to build tools for tracking theflame fronts over mulch beds and fences. The graphical user interfaces (GUIs) for selecting
points on the video were developed using GUIDE in MATLAB to lay out the GUI
components (such as push buttons, pop-up menus, and plots) and to set up the framework forevent-driven programming. Several GUI applications were later migrated to MATLAB’s
App Designer interactive development environment.
3.1.1. Event Timing
All four video cameras monitoring the experiment were turned on shortly before the propane
burner was applied to the test subject and turned off as the fire was being extinguished withwater from a hose. Each camera view was fixed in place during the experiment after
adjustment to capture the field of interest. To compare the views from multiple cameras,
usually the right and left views, the timing was synchronized. The five events listed below
were selected as the primary timing markers because they could be determined within a few
video frames. Each event is illustrated in Fig. 22 under the matching letter.
a) End of Gas Burner Ignition
The End of Gas Burner Ignition event was the last moment that the small propane
ignition torch was applied to the gas burner. The torch was often applied multiple
times to each side of the burner, so this marker records the end of the final
application. Although the fence sometimes blocked the view of the propane torch
itself, the movement of the researcher’s arm while removing the torch was usuallyapparent.

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Fig. 22. Illustration of timing markers for Test A-29, showing a) end of ignition process,b) start to removal of propane burner, c) audio track for fan on, d) audio track for fan off,e) beginning of water application, and f) 5 s after the water is applied.

b) Start to Remove Gas Igniters
The Start to Remove Gas Igniters event was the moment when the gas
igniters/burners began to be moved away from the point of ignition. This typicallytook place about 1 ½ minutes after the End of Gas Burner Ignition.
c) Fan On
The Fan On event marked the time at which the fan reached the initial maximum
audio amplitude, as viewed in the audio track display. This point is indicated by thearrow in Fig. 22 (c). A precise frame number could be obtained by zooming in on theaudio timeline and identifying the frame at which the audio signal stabilized to aperiodic waveform. To the ear, this corresponded to the time at which the engine
caught after turning over. Further adjustments of the fan rpm to reach the required set
point (seen in the figure as variations in the amplitude to the right of the event arrow),which typically took between 15 s and 45 s, were not considered. Fan On marked thezero time for all analyses, and typically occurred 1 s to 5 s after the gas igniters were
removed.
d) Fan Off
The Fan Off event was the moment at which there was a detectable decrease in theaudio amplitude produced by the fan, as viewed in the audio track display. This point

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near the end of the experiment is indicated by the arrow in Fig. 22 (d). A frame
number could be determined by zooming in on the audio timeline and identifying theframe at which the audio track changed from a periodic waveform to a rougher signal.
To the ear, this corresponded to a change in pitch as the fan was turned off.
e) Water First Applied
The Water First Applied event marked the first moment when water from the hose
was observed to reach the burning object to extinguish the fire. In some videos thewater could be seen leaving the hose and landing on the fence or mulch. In othercases, the application of water could be detected by the initiation of a cloud of steam
mixed with smoke, as can be seen by comparing images (e) and (f) in Fig. 22. Early
applications of water to contain spot fires near the target shed were not included indetermination of the Water First Applied event.
In addition to the five events listed above, the initiation of the digital timer described in
Section 2.9.3 was visible from the camera on the left side. The timer was started within a few
seconds of the two events Start to Remove Gas Igniters and Fan On – all three eventsoccurred in response to a countdown given by the team leader. In Fig. 22 (b), the person
preparing to start the timer is visible near the orange traffic cone. The timer was useful for asmall number of experiments in which one or more video recordings were interrupted in the
middle of the test, resulting in two videos from those cameras. From the left camera, thetimer could be used to determine the amount of time elapsed between the end of the firstvideo and the beginning of the second. For interruptions of the right camera, the timings of
Fan Off and Water First Applied events were calculated by comparison with event times
from the left camera.
For Series 2 experiments, with a mulch bed target at the base of the shed, the timing of spot
fires was measured. Three simple timing measures were recorded for each experiment:(1) the time at which the first spot fire ignited within the target mulch bed, (2) the time ofignition for the first spot fire to put flames against the wall, and (3) the time at which flames
were first observed at the wall. Ignition was detected by the first visible sign of smoke. The
right and/or left video recording was used to identify both the first spot fire ignited and thefirst spot fire resulting in flames on the wall. These two spot fires were then tracked back intime to determine the first time at which a puff of smoke from that location was
distinguishable from the surroundings. The time when the first splash of orange was observedat the base of the wall or along its surface was recorded as the time of flames on the wall.
Uncertainties in the timing of these events are discussed in Appendix A.3.
Other times obtained from the videos included the times for flames or charring to reach boththe halfway point on the fence and the end of the fence, which were used for preliminary
flame spread rate comparisons. These were determined using the flame spread analysis
discussed in Section 3.1.3 and Section 3.1.4.
Timing markers were obtained using VirtualDub [39, 40], VEGAS Pro [41], or AVS Video
Converter [42] video processing software. Frame numbers or times of events were recorded
in an Excel file. Some discrepancies in timing were observed among video handling tools,
but markers obtained from these three software packages were found to be consistent within±0.5 s. For the most accurate results, the same tool (VirtualDub) was recommended for bothtiming the videos and extracting images from them.

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3.1.2. Conversion of Videos to Image Sequences
Videos from right and left cameras were used to determine flame spread as a function oftime. The tools developed for this analysis required the videos to be converted into sequencesof images. Each image needed to be as sharp as possible in order to estimate the location of
the flame front in the mulch bed or on the surface of the fence. Other considerations included
the digital size of the stored images and the length of time required to create them.
The software selected for this task was VirtualDub [39] equipped with Ut Video Codec
Suite [43] to provide lossless compression and Smart Deinterlacer Filter [44] to eliminate
interlacing artifacts. VirtualDub allowed the extraction of every 30 frames, resulting in a setof images spaced apart by 1.001001 s (corresponding to the video frame rate of
29.97 frames/s). This provided better image quality than other video handling tools that usedaveraging to create a sequence with one image per second. Ut Video Codec Suite, with fast
lossless compression capabilities, was used to first convert the original digital video in .m2ts
format to .avi format, using only one out of every 30 frames. For the final image sequence,
every frame was then extracted from the .avi video. This procedure reduced the time required
to extract images from the original video by more than an order of magnitude. Smart
Deinterlacer Filter eliminated lines where there was frame-to-frame motion, such as in areas
with flame or smoke, leaving the motionless parts at high resolution.
The initial conversion from .m2ts to .avi took approximately 1/5 the time of the originalvideo duration, and the extraction of frames from the .avi video took approximately 1/3 ofthe time of the duration. In cases where videos from both left and right cameras wereconverted into image sequences, images from both sequences were compared to ensure thatthe timing matched.
3.1.3. Mulch Experiment Flame Front Tracking
After the sequence of images was extracted from a video recorded by camera to the right orleft of the object being tested, the images were ready be analyzed to determine the position of
the flame front as a function of time. A MATLAB GUI tool, mulch_test_analysis.m, wasdeveloped to track the char front for mulch experiments.
Figure 23 shows an example of the GUI for a mulch test. To begin the procedure, the user
first clicked on Get Image File on the upper right of the interface. This prompted the user fora single image from the sequence for the experiment to be displayed and analyzed. Open
Excel File allowed the user to either open a new file to contain the flame front data or to add
to an existing file.
In order to measure the position of the flame front in a given image, a physical scale that took
the perspective of the camera into account was required to define distances along the length
and width of the mulch bed. The Set Perspective Lines button prompted the user for a seriesof points that defined the perspective lines on each end of the mulch bed, selected by placing
the cursor on the screen and clicking the mouse. The user was asked to identify two points on
the top of the mulch bed at the base of the shed – one close to the camera and one farther
away – and two points on the top of the mulch bed at the opposite end from the shed. The
user was also asked to identify points along the top edges of the mulch bed closest to andfurthest from the camera. Finally, the user was asked for a point at a known physical distance54

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Fig. 23. Example of GUI with perspective lines for analyzing flame spread over a mulch bed withtime.

from the shed wall. In the case shown in Fig. 23, the corner of the white square marking on
the asphalt in the foreground was known to be 1.07 m (3.5 ft) from the shed wall. Other cases
used the distance of the end of the mulch bed to the shed wall or other known lengths. The
image in Fig. 23 shows the mulch bed delineated by the set of perspective lines, spaced at
10.2 cm (4 in) intervals, and the near and far edges of the mulch pan.
In later experiments, markings added to the pavement at 0.30 m (1 ft) intervals made it easier
to determine both the physical scale and the angle of perspective lines along the length of themulch bed. For camera views in which the upwind end of the mulch pan was not in theframe, as was the case for some of the experiments with 0.91 m (3 ft) and 1.83 m (6 ft)
separation distances, the perspective line away from the structure was established using otherinformation. On a sunny day, such as that shown in Fig. 23, the shadow cast by thebidirectional probe array could be used, assuming that the array had been positioned parallelto the shed wall. The location of the perspective line along the wall at mulch height wasestimated in cases for which the mulch pan was separated from the wall.
Selecting the Flame Front button allowed the user to move among the video frames and use
the mouse to click on the location of the flame front in each image. The frame time and the
distances of the flame front from the shed wall and the centerline were automaticallyrecorded in the Excel file for later plotting. Buttons on the lower right allowed the user to55

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move forward and backward among the frames and to specify the number of frames to skip
for experiments with slower flame spread.
The boundary between burned and unburned mulch was considered to be the location where
the color of the mulch changed from brown to black. Flames attached to the mulch wereobserved in some video frames but were too sporadic to serve as an indicator of flame front
location. The flame front was typically not a straight line. The leading edge of the flame front
was therefore defined as the point of the continuous burn pattern closest to the structure. On
occasion, burn spots appeared in the mulch bed downwind of the flame front due tofirebrands. When a spot became connected to the main burned region, the location of theflame front jumped to the leading edge of the spot.
Uncertainties in the mulch flame front analysis are described in Appendix A.4.1.
3.1.4. Fence Experiment Flame Front Tracking
A second MATLAB GUI tool, fence_test_analysis.m, was developed to track the burned areaof the fence as a function of time.
The procedure for defining the geometry for fence experiments was similar to that describedin the previous section for the mulch tool, with flame front data collected over the surface ofthe fence rather than that of the mulch bed. Figure 24 shows the GUI for the fence flame
front analysis. The Select Image button prompted the user for an image (preferably fromearly in the image sequence) that was displayed in the active window. The Open Excel Filebutton allowed the user to either open a new file to contain the perspective line and flame
front data or to overwrite an existing file. The Set Fence Perspective button prompted theuser for information about the experiment, including four points defining a set of corners of
the fence, the height and length between these markers, the time of the current image, and thetime interval between images in the sequence. Each corner point was located using crosshairs
and selected with the mouse button. As each point was selected, its 2-D location in the image
was entered into a sheet of the Excel file, along with user-provided data defining the physicaldimensions. The final fence outline was displayed as shown in Fig. 24.
After the fence was defined, the Get Points button allowed the user to define the flame front
location at specific time intervals using the crosshairs and mouse button. The time interval
could be changed using the Skip button. A suggested time interval for collecting 50 data
points was provided by the Frame Skip Calculator function. Navigation Tools allowed theuser to move forward and backward among the video frames.
Three points on the fence were expected for each image. The first point identified thelocation closest to the shed where the fence was charred, the second marked the height of thecharred region in the ignition area, and the third marked the highest point of the char on thefence outside of the ignition region, defined as two boards or more downwind from the post
farthest from the shed. The two definitions of char height enabled the separation of ignition
effects, which included fire development before the fan was turned on, from the effects offlame spread downwind. After the three points were selected, the MATLAB code
automatically moved to the next image in the sequence. The time and flame front locations
were automatically recorded in the Excel file for later plotting.

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Fig. 24. GUI for selection of location points for fence video.

The MATLAB program also allowed the capture of two-dimensional char profiles on thefence. The Choose Key Frame button presented crosshairs to select points outlining the charregion on the fence. Char profiles were typically captured at five times evenly distributed
through the experiment.
Obscuration by flames and smoke, lighting, discontinuities in the char location where theboards met, and placement of the cursor all contributed to the uncertainty in the location of
the char front on the fence. Uncertainties in the fence flame front analysis are described ingreater detail in Appendix A.4.
Wind Analysis and Visualization
An interactive graphic user interface (GUI) program, windplots.mlapp, was written toconvert the voltage files from the pressure transducers into wind velocities and display the
results. This program was based on the MATLAB App Designer tool. As described in
Section 2.7, the wind field just upwind of the fence or mulch bed was measured by an arrayof bidirectional probes, and ambient wind and temperature data were collected from a nearby
sonic anemometer and a thermocouple. The process for zeroing the probes is explained in
Section 2.9.2, while Section 2.8.1 describes the collection and initial processing of the wind
and ambient weather data by using a Labview program to write the data to an Excel file.
Figure 25 and Fig. 26 show examples of the data visualization from windplots.mlapp. The
first step in the wind analysis procedure was to obtain the zero voltage level for each probe,
using the Step 1: Zeros button in the upper left corner. This prompted the selection of an
Excel file containing measurements taken while the zeroing tubes were applied and before
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the fan was turned on. The data columns and plots populated by the data in one such file areshown in Fig. 25, with the following annotations:A.

Voltage data as a function of time. The time extended from the lower bound to theupper bound defined in the boxes below the plot, which could be changed todefine a time range in which readings were stable. All data presented in the GUI
plots and tables were from the specified time range. The voltage data typically lay
between 2.4 V and 2.6 V. In the example shown, the data from one probe
fluctuated significantly within this range, indicating that there may have beenconnection issues.

B.

The probe array, with a number and radio button for each probe. Probes that wereclearly faulty (i.e., with values well outside the expected range) could be turned
off by clicking on the radio button.

C.

Average value for each probe in the array. Black dots at the center of each color
square indicate the location of the probe in the array by height and distance fromthe centerline. Probes that were faulty and had been turned off were assigned thecolor white.

D.

Average values for the five probes along the centerline. Values were not plotted
for faulty probes.

E.

Average values for the five probes extending horizontally from the centerline tothe outer edge of the array at a height of 1.22 m (4 ft) from the ground. Values
were not plotted for faulty probes.

F.

Table of probes, showing location, average value, and standard deviation of the
data

G.

Ambient temperature as a function of time.

H.

Wind rose plot of wind speed and wind direction during the specified time range.
Statistical calculations were performed with the CircStat MATLAB toolbox [45].
The green line indicates the orientation of the experiment, plotting the direction
from the fan to the shed. The red line indicates the mean direction of the ambient
wind.

I.

Ambient wind speed measurements as a function of time.

J.

Average values and standard deviations for temperature, wind speed, and winddirection.

Outliers could be filtered out by setting Max and Min values for the data set. Data for
missing zero probe values could be downloaded from a previous analysis.

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Fig. 25. Visualization of zeroing data for 29 July 2016, used for tests A-57 to A-60.

After the probe data from the zeroing analysis were available, the Step 2: Wind button was
selected to determine the wind speeds during an experiment performed on the same day.
Figure 26 shows an example of the data visualization of wind speed from Test A-57, which
was carried out on the same day as the zero readings in Fig. 25. The plots and tables are thesame, but the values from the probes are wind speeds rather than voltage. When the winddata from an experiment were read from the Excel file, the plot of wind speed as a functionof time showed a step increase when the fan was turned on and a step decrease when it was
turned off. The lower and upper bounds were then selected to encompass only the time range
when the fan was on.
Finally, selecting Step 3: Save stored the statistical data in a separate Excel file and the plots
in a dedicated subdirectory with the rest of the data for that experiment.

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Fig. 26. Visualization of wind speed and ambient data for Test A-57.

Flow Simulations
Several observations, including the movement of mulch particles and firebrands and theslowing of flame spread velocity as the flame front approached the shed, suggested that theflow field could be playing an important role in these field experiments. To obtain insights
into how the flow field might affect firebrand behavior and flame spread, the experimental
setup was modelled using the NIST Fire Dynamic Simulator (FDS) [46]. FDS is acomputational fluid dynamics software program that is typically used for calculating firedriven fluid flow. Although the heat from a fire will modify the flow field due to buoyancy
effects, these calculations considered only the wind flow in the absence of fire, sometimes
referred to as “cold flow” calculations.
In addition to illustrating the basic flow field for the experimental setup, FDS simulations
helped to establish the acceptable ambient wind speed limits for running experiments asgiven in Section 2.9.1. This was accomplished by adding a cross-flow wind to the model.
3.3.1. FDS Model of Mulch Experiments
As an initial exploration of the flow behavior in these field experiments, FDS was used for
calculating fluid flow only (without fire) in an exterior computational space with open
boundaries [29]. Subgrid mixing due to turbulence was represented by a Smagorinsky large
eddy simulation (LES) model. Validation work [47] has shown that FDS is capable of goodresults when compared with experiments for similar problems, including building ventilation,

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wind impinging on the exterior of a structure, and transport of pollutants in an urban
environment.
In the model of the mulch test, illustrated in Fig. 27, the fan and shed structure (in brown)
were represented as obstacles. Since the intent of the model was to provide general insightrather than a quantitative comparison, the propeller-driven flow field and the geometries ofthe structure and the fan/flow straightener system were not represented in detail. The fan was
simulated by a square block 1.8 m (6 ft) on a side and 0.5 m (1.6 ft) thick with a steadyvelocity boundary condition directed toward the structure at an angle of 7° to the horizontal.
The distance of the fan from the structure was 10.7 m (35 ft) in agreement with theexperimental setup. Boundary conditions at the exterior of the domain were open. Resolutionin x, y, and z directions was 0.1 m (4 in).

Fig. 27. FDS model results showing instantaneous velocity vectors, colored by wind speed. Side view(a) shows velocities along the center plane, and top view (b) shows velocities in a plane 0.1 m (4 in)above the ground.

3.3.2. Flow Field, including Vortex at Base of Shed
Instantaneous velocity vectors in Fig. 27 illustrate some of the features of the wind flow,which was highly turbulent. Figure 27 (a) and (b) show instantaneous velocity vectors along
the center plane of the experimental geometry (side view) and in a plane 0.1 m (4 in) above
the ground (top view), respectively. The vectors are colored by wind speed, with red

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corresponding to the wind speed of 9 m/s (20 mi/h) applied by the fan, as indicated in thecolor scale.
The velocity flow field shows that a vortex formed near the ground in front of the structure
and wrapped around the sides, consistent with the horseshoe vortex observed for a surfacemounted cube at right angles to a flow stream that is described in the scientific literature [48].
The behavior of the wind near the ground is of particular interest for the mulch experiments.
The side view in Fig. 27 (a) shows the vortex near the ground in front of the structure. From
the top view in Fig. 27 (b), the wind velocity slightly above the ground was directed toward
the structure at distances between one and two structure heights (about 2 m to 7 m) from thestructure. Within one structure height distance (about 2 m) from the structure, the windvelocity was lower and directed away from the structure and toward the fan.
Figure 28 shows time averages for the component of velocity in the direction toward or awayfrom the shed, vx. Between the fan and the structure, the wind was directed toward the shed(positive values of vx, red) except for a region close to the ground just in front of thestructure, where vx was strongly negative (blue). This marks the counterflow region of thevortex. The vortex extended toward the fan a distance of 1.7 m (5.6 ft), approximately
0.7 times the dimensions of the shed, which is roughly consistent with the literature on flow
around surface-mounted cubes at high Reynolds number [49, 50].

Fig. 28. FDS model results showing time-averaged contours of wind speeds in the x-direction. Side
view (a) shows contours along the center plane, and top view (b) shows contours in a plane 0.1 m(4 in) above the ground.

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Simulations at imposed wind speeds from 6 m/s (13 mi/h) to 14 m/s (31 mi/h) indicated thatthe vortex size was relatively independent of wind speed in this range. Vortex intensity andconcurrent and opposed flow velocities were strongly affected by wind speed.
FDS simulations were also performed for fence experiments. The geometry was the samewith the addition of a thin obstacle representing a single fence panel at a separation distanceof either 0 m or 0.9 m (3 ft) from the structure. The results were qualitatively the same as forthe mulch simulations, with a vortex forming in front of the structure in all cases.
3.3.3. Wind Effects on Firebrands and Fire Spread
The flow field strongly influences the behavior of firebrands generated in the mulch bed.
When the fan was on during the experiments, larger pieces of mulch from the target bed
could be observed rolling on the pavement away from the shed wall or towards the sides ofthe shed. The vortex motion could also be observed from the directionality of the smoke andflames. For the spot fires in the target mulch bed at the base of the shed, the smoke andflames generally extended away from the shed, while for the burning object (fence and/or
mulch bed) the smoke and flames extended toward the shed.
Firebrands produced near the ground experience the opposing flow from the vortex, whichacts to keep them away from the structure and directs them into the high velocity flow aroundthe left or right side of the structure shown in Fig. 27 (b). These firebrands may ignite
combustible materials at a distance from the structure. In these field experiments, spot fires
were occasionally observed to ignite at the outer edge of the target mulch bed.
Firebrands that are either produced at or lofted to an intermediate height of 0.1 m (4 in) to
1 m (3.3 ft) may be caught in the vortex flow and deposited close to the wall of the structure,
where they can ignite combustible wall material or mulch adjacent to the wall. Firebrands
that approach the structure at a height greater than a meter may enter an updraft thattransports them over the top of the structure; open vents could allow these firebrands to enter
the structure.
The flow field also affects the flame spread over the fence or mulch bed. Depending on how
far away from the shed the mulch bed is ignited, the flames spreading toward the structure
may initially experience concurrent flow, with wind velocity in the same direction as theflame spread. As the flame front enters the vortex region, however, the flame spread changes
to opposed flow mode and may slow considerably. If a firebrand ignites a spot fire in themulch within the vortex region, the flame spread from the point of ignition back to the main
flame front is concurrent with the vortex flow field and is thus rapid. An example of thisbehavior will be shown in Section 4.1.2.3.

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Experimental Results
The set of 187 experiments described in this report represents a survey of the effects ofcertain fences and mulch on the spread of fire to a structure, given various wind conditions
and separation distances. Both direct flame spread and spot fires ignited by firebrands werestudied. A variety of fences and mulches were tested under a range of conditions; theparameters were described in Section 2.10. Because of the large number of combinations andlimitations on performing the experiments, only a few of the experiments were replicated.
The comparison of quantitative data was made more difficult by the fact that many
phenomena involved in firebrand spotting, such as generation of firebrands and ignition
processes, are stochastic in nature. The analysis of the data from this set of experiments wastherefore focused on uncovering trends and on discovering different modes of behavior,rather than on quantitative results.
Appendix D shows the experimental matrix, divided into categories of Fence Only (without
mulch below the fence), Mulch Only (without a fence), Fence Plus Mulch, and Parallel
Fences. The number of each experiment corresponds to cases whose details are presented inAppendix F through Appendix I. Appendix E explains the contents of each case writeup,
which include a description of the experiment, photographs from before and during theexperiment, flame spread plots, critical times, and ambient and applied winds.
Mulch Only Experiments
This section reports on the fire behavior observed in mulch bed experiments. A variety ofmulch types were studied, including shredded hardwood mulch, pine bark mulch, pine straw
mulch, and rubber mulch. The experiments were performed at nominal imposed wind speed
conditions from 6 m/s to 14 m/s (13 mi/h to 31 mi/h), categorized as low, medium, and highas described in Section 2.10.6. Separation distances from the nearby structure were between
0 m and 1.83 m (6 ft), as shown in Section 2.10.5.
The test matrix for Mulch Only experiments is shown in Table D.1 in Appendix D. The
details of each experiment, including parameter values, images, flame spread data, windplots, and summary values, can be found in Appendix F. In each case, timing was measured
from the point after ignition when the fan was turned on.
4.1.1. Example
Some aspects of fire behavior over a combustible mulch bed are demonstrated in Fig. 29. In
this experiment (Test A-83), a fire was ignited in a bed of shredded hardwood mulch
separated from the structure wall by 0.91 m (3 ft). A target mulch bed was placed along thebase of the structure to catch firebrands generated by the burning mulch bed. At t = 0 min,the fan was turned on to generate a wind field in the medium range with an average windspeed along the centerline of 9.3 m/s (20.8 mi/h), and the gas burners were removed. Whitelines mark off distance from the wall at 0.30 m (1 ft) intervals.
The first three frames in the sequence in Fig. 29 show that the fire progressed at a steady rate,with the mulch darkening as it charred and with orange flames appearing sporadically. As themulch burned, ash accumulated and changed the surface color to gray. After 4 min, the flame

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front slowed as it encountered the vortex at the base of the shed described in Section 3.3.2.
A firebrand from the burning mulch ignited a fire in the middle of the target mulch bed
shortly before t = 10 min, as indicated by the smoke rising from this site. The smoke at thislocation was observed to blow away from the wall, showing the direction of the wind (andthe presence of the vortex). By t = 12 min, the spot fire had increased in size, reaching both
the wall of the structure and the front edge of the target mulch bed.

Fig. 29. Time sequence of shredded hardwood mulch bed at medium wind speed separated fromshed wall by 0.91 m (3 ft) [Test A-83].

The flame spread plot in Fig. 30 shows the flame front location as a function of time (greenline) for this case. In order to keep track of the location of the flame front within the mulch
bed, the plot includes black horizontal lines to mark the distance of each end of the mulch

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bed from the shed wall. The line at 0.91 m (3 ft) represents the closest proximity of the mulch
bed to the wall, and the line at 4.27 m (14 ft) marks the approximate location of the far end.
The initial location of the flame front (at time t = 0 min when the fan was turned on) was
3.35 m (11 ft) from the shed wall, or 2.44 m (8 ft) from the end of the mulch bed closest to
the shed wall.
The green line showing flame front location with time illustrates the decrease in the rate of
spread as the flame in this experiment approached the shed wall. During the first threeminutes of the experiment, the flame front progressed from 3.35 m (11 ft) to 1.83 m (6 ft)
from the shed wall, for a flame spread rate of 0.54 m/min. In the time period from 5 min to
10 min, the flame front progressed from 1.44 m (4.7 ft) to 1.23 m (4.0 ft), with a flame
spread rate of 0.044 m/min. The distance from the wall at which flame spread started to slow
significantly is consistent with the extent of the vortex described in Section 3.3.2.
The black error bars in Fig. 30 show the expanded uncertainty for each of these points, basedon the uncertainty analysis for mulch in Appendix A.4.1 and the uncertainties from Table
A.5. For Test A-83, the separation distance was greater than zero and markers on the groundhelped to set the distance scale, so the expanded uncertainty (95 % confidence level) was±6.9 cm.
The main flame front united with a spot fire at about 10.5 min into the experiment, resulting
in a sudden jump to the end of the mulch bed.

Fig. 30. Flame front location vs. time for Test A-83, with HW mulch at medium wind speed and0.91 m (3 ft) separation distance from structure, showing expanded uncertainty.

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4.1.2. Fire Spread Behavior
This section describes observed effects of spot fires, wind speed, and separation distance onfire spread rate. Shredded hardwood mulch beds were used to study these effects.
4.1.2.1. Effects of spotting along mulch bed
The fire spread behavior along mulch beds containing shredded hardwood mulch showedvariability even under seemingly identical conditions. One significant factor was the spotting
behavior along the length of the mulch bed. In some experiments, the flame front moved
approximately evenly across the entire width of the bed. Figure 29 displayed one example ofthis mode of fire spread behavior; other examples are shown in Fig. 31 (a) and (c) under
medium and high wind speed conditions, respectively. In other experiments, such as thoseshown in Fig. 31 (b) and (d), the fire front did not widen to the sides of the mulch bed but
remained highly irregular in shape. These experiments typically started with a narrowburning profile leading from the point of ignition by the burners. Firebrands ignited themulch downwind of the main flame front, creating spot fires that expanded with time until
the individual flame fronts merged. Curved patterns within the burned areas of (b) and (d)indicate the irregular burning in these two experiments, in which spotting played a large rolein flame spread.

Fig. 31. Two modes of fire spread behavior in HW mulch beds: uniform flame fronts for a) medium[Test A-42] and c) high [Test A-27] wind speeds, and flame spread through spotting for b) medium[Test A-8] and d) high [Test A-10] wind speeds.

The experiments portrayed in Fig. 31 (a) and (b) were run under similar medium wind speed
conditions, and (c) and (d) were both run under high wind speed conditions. The reasons
behind the differences in fire spread behavior are unknown. Figure 32 shows plots of flame
front location as a function of time for the experiments displayed in Fig. 31, along with anadditional experiment performed at low wind speed (Test B-72) that was dominated by
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spotting behavior along the length of the mulch bed. Higher wind speeds resulted in fasterfire spread rates and shorter experimental time durations. However, for each wind speed
there was significant variability. The experiments that were dominated by firebrand spotting
progressed more slowly than those that maintained a more uniform flame front. The vertical
jumps in the plots in Fig. 32 indicate the times at which the main flame front merged with aspot fire downwind, instantly moving the flame front location to the maximum downwind
extent of the spot fire. The uncertainties for these plots can be determined from theuncertainty analysis for mulch in Appendix A.4.1.

Fig. 32. Effects of dominant spread mode and wind speed on flame front location vs. time for HW
mulch beds at zero separation distance from structure.

For the experiments in Fig. 32, the mulch bed was against the shed wall, so the black lines at
0 m and 3.35 m (0 ft and 11 ft) mark the nearest and farthest distances of the mulch bed to
the wall respectively. The initial location of the flame front (at time t = 0 min when the fan
was turned on) was between 2.2 m and 2.4 m (7.2 ft to 7.9 ft) from the end of the mulch bed
closest to the shed wall. This variation reflected both the uncertainty in placement of the
propane burner and the variation in flame spread toward the wall between propane burner
ignition and the time at which the fan was turned on.
The scientific literature on fire behavior may be able to provide some basic insight into thedifferences in mulch flame spread when either fronts or spot fires are dominant. In a
concurrent wind-driven fire, with the fire spreading downwind, a uniform flame front allows
the flames to be close to the fuel for effective preheating. Higher wind speeds increase theflame spread rate by driving the flame closer to the unburnt fuel in its path, increasing therate of heat transfer [51]. The slower flame spread for experiments with an irregular flame
front may reflect the higher fireline curvature in these cases. Fireline curvature is a factor in
spread rate over grasslands, where fires with a narrow, pointed head burn more slowly thanthose with a broad parabolic shape [52, 53]. In addition, the large downwind spot fires inthese experiments may increase the turbulence of the air flow across the surface of the mulch
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bed. An increase in turbulence intensity decreases the spread rate by enhancing the mixing of
air and fuel gases, which decreases the flame length, or by lifting the flame higher from thefuel surface [54, 55].
4.1.2.2. Effects of wind speed
Figure 32 shows that increasing wind speed was associated with higher fire spread rates andlower experiment durations for the two dominant flame spread modes: uniform spread andspotting. This plot gathered data from experiments at zero separation from the shed wall.
Figure 33 (a) and (b) show that the same trend with wind speed was observed for separation
distances of 0.91 m (3 ft) and 1.83 m (6 ft) respectively. For each separation distance, theexperiments performed at high wind speeds were the fastest. Experiments performed at low
wind speeds required a considerably longer time for the flame front to reach the end of themulch bed closest to the shed wall. The uncertainties for these plots can be determined fromthe uncertainty analysis for mulch in Appendix A.4.1.
The flame spread plots from multiple experiments carried out under basically identicalconditions show the range of behavior encountered when the field experiments wererepeated.

Fig. 33. Effects of wind speed on flame spread for HW mulch at separation distances of a) 0.91 m(3 ft) and b) 1.83 m (6 ft).

4.1.2.3. Effects of wind vortex
As discussed in Section 3.3.3, the wind vortex at the base of the shed has a strong influenceon the behavior of flame spread and firebrands. For many mulch experiments, the flame
spread was observed to slow as the flame front approached the structure wall. In every
Series 1 experiment with zero separation distance, the first ignition of the plywood layer on
the shed wall resulted from a firebrand landing in and igniting the mulch at a point muchcloser to the wall than the fire front. Figure 34 shows a particularly clear example of thisphenomenon. The first photo, labelled as time tspot, shows a flame front whose forward
movement stalled near the wall of the structure. At this time a firebrand had just landed very
close to the wall and ignited the mulch there. From this second point of ignition, flames

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spread toward the wall and eventually ignited the plywood. In the opposite direction, theflames from this second ignition point rapidly (within 15 s) burned in a line back toward theoriginal flame front, as shown in the top right photograph. This is consistent with thedirection of the wind in the vortex near the ground as shown in Fig. 28. The flame front thenspread laterally from the burned line, while the original flame front continued to move
forward very slowly.

Fig. 34. Time evolution of firebrand spot fire ignited at wall of structure, beginning with firstsign of spot fire at tspot [Test A-2].
4.1.2.4. Effects of separation distance
The example in Section 4.1.1 showed the flame spread slowing as the flame front approached
the opposing flow caused by the vortex at the base of the shed, which begins at about 1.83 m(6 ft) from the shed, as shown in Fig. 28. This distance depends only on the height of theshed and not on the wind speed. A change in flame spread rate should therefore be observed
for mulch experiments in general as the flame front crosses this boundary.
In Fig. 35, the flame front position as a function of time is plotted for several HW mulch
experiments by separation distance. In this plot, the black horizontal lines mark only the endof the mulch bed closest to the shed wall for the four separation distances, with the horizontal
axis marking a separation distance of 0 m. Ignoring the vertical jumps resulting from theflame front catching up to spot fires, a change in slope is indeed observed when the flame
front reaches between 1.5 m and 1.8 m (5 ft to 6 ft) from the shed wall. The uncertainties forthese plots can be determined from the uncertainty analysis for mulch in Appendix A.4.1.

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Note that the mulch bed for experiments with a separation distance of 1.83 m (6 ft) is fully
located outside of the vortex. For these cases, the flame spread rate does not decrease overthe length of the mulch bed.

Fig. 35. Effects of separation distance on flame spread for HW mulch at medium wind speed.
4.1.3. Type of Mulch
The fire behavior was studied for four mulch types: shredded hardwood mulch, mini pine
bark nuggets, pine straw mulch, and shredded rubber mulch. The effects of halving the
thickness for shredded hardwood mulch were observed. Each mulch type was described in
Section 2.4.2 and illustrated in Fig. 8.
More data from mulch experiments can be found in Appendix F.
4.1.3.1. Shredded hardwood mulch
Almost half of the mulch experiments (22 out of 45) were performed on shredded hardwood
mulch, as can be seen in Table D.1. The previous section showed that the fire spread
behavior of hardwood mulch was affected by spotting along the mulch bed, wind speed, andthe flow field near the shed. Higher wind speed results in faster flame spread, and thecounter-flow on the ground next to the shed slows the flame spread considerably. A firebrandthat is lofted can be carried by the flow at the top of the vortex and deposited close to theshed wall, where it can ignite combustible materials and immediately threaten the structure.
These wind flow effects were described more fully in Section 3.3.3.
For shredded hardwood mulch, spotting to the target mulch bed occurred in almost every
experiment, with spot fires igniting both near the wall and on the outer edge of the target

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mulch bed. The exception was Test A-97, performed at low wind speed and 0.91 m (3 ft)
separation distance, for which no spot fires ignited during the 103 min test duration.
Two experiments were performed at half of the usual mulch thickness. Figure 36 comparesplots of flame front location as a function of time for hardwood mulch beds 2.5 cm and 5 cm
thick under the same conditions of wind speed and separation distance. The flame spreadcharacteristics are very similar. Note that the flame front for Test A-38 (half thickness) was
the farthest back from the shed when the fan was turned on (t = 0), which accounts for someof the difference in the time needed to reach the end of the mulch bed. The uncertainties forthese plots can be determined from the uncertainty analysis for mulch in Appendix A.4.1.

Fig. 36. Effects of HW mulch thickness on flame spread for high wind speed and separation distanceof 1.83 m (6 ft). Black lines mark distance of each end of the mulch bed from the shed.

The second experiment performed with the HW mulch bed at half thickness is plotted in Fig.
37 with two full-depth experiments performed under the same conditions. The flame spread
for Test A-43 progresses along a nearly identical path to Test A-42, until the former
experiment was ended when a spot fire in the target mulch bed reached the wall. The flame
fronts for both of these experiments were even across the width of the mulch bed, as
compared to Test A-8, which experienced significant spotting as the front progressed. As was
discussed in Section 4.1.2.1, spot fires along the length of the mulch bed were found to
significantly affect the rate of flame spread. The uncertainties for these plots can be
determined from the uncertainty analysis for mulch in Appendix A.4.1.

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Fig. 37. Effects of HW mulch thickness on flame spread for medium wind speed and zero separation
distance from shed.

4.1.3.2. Pine bark mulch
The fire behavior of mini pine bark nugget mulch was similar to that of shredded hardwood
mulch, although fire spread occurred somewhat more slowly. Figure 38 shows a time
sequence of a typical pine bark mulch experiment. After the fan was started with medium
wind speed at time t = 0, the flame front moved toward the shed wall and expanded laterally
to the edges of the mulch pan. By t = 8 min, a firebrand had ignited a spot fire farther along
the pine bark mulch bed, which grew as a separate fire until it merged with the main flame
front at about t = 15 min. A spot fire ignited directly next to the shed wall at aboutt = 16 min, apparently from a firebrand deposited at the rear of the target mulch bed.
To give an idea of the range of fire behavior observed, Fig. 39 shows examples from fourother pine bark mulch experiments. The flame front in some experiments was elongated as inFig. 39 (a), while for others the fire quickly spread to the walls and the flame front moved
forward as a nearly straight line, as in (b) and (c). In the case shown in (d), the burning was
somewhat spotty near the flame front.
As seen in Fig. 39 (c), the pine bark mulch left little residue after lightweight firebrands andash blew away in the wind field. Spotting along the mulch bed was not uncommon; both Fig.
38 and Fig. 39 (a) give examples. Spotting to the target mulch bed occurred in every pine
bark experiment, with spot fires igniting both near the wall as in Fig. 39 (b) and on the outeredge of the target mulch bed as in Fig. 39 (d).

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Fig. 38. Time sequence of pine bark mulch bed in medium wind speed with zero separation fromshed wall [Test A-99].

Fig. 39. Fire behavior examples for pine bark mulch beds with a) medium [Test A-100], b) low[Test C-4], c) medium [Test A-72], and d) high [Test A-86] wind speeds.

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Figure 40 shows the flame spread for mini pine bark mulch experiments by separation
distance. All three plots demonstrate that flame spread rate increases with wind speed.
Comparison with Fig. 33 for shredded hardwood mulch shows that fire spread more slowly
over the pine bark mulch, generally doubling or more the time for the fire to reach the end of
the mulch bed. This likely results from the difference in texture – as shown in Fig. 8, theshredded hardwood mulch contains long, thin particles that easily ignite and transport fire, ascompared to the chunks of mini pine bark mulch. The uncertainties for these plots can be
determined from the uncertainty analysis for mulch in Appendix A.4.1.
The effect of the vortex at the base of the shed on flame spread rate was not as apparent forsome of these plots as for the shredded hardwood mulch plots discussed in Section 4.1.2.3.
The flame spread rate decreased at about 1.83 m (6 ft) from the wall for some experiments(Tests A-72 and A-55), and the flame front moved more slowly toward the wall after
merging with a downwind spot fire for some (Tests A-99 and A-51) However, the plots forother experiments (Tests B-83, C-4, and C-31) did not change significantly in slope as theflame front entered the vortex.

Fig. 40. Effects of wind speed on flame spread for PB mulch at separation distances of a) 0 m and0.30 m (1 ft), b) 0.91 m (3 ft), and c) 1.83 m (6 ft).

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4.1.3.3. Pine straw mulch
Pine straw mulch was found to burn intensely and rapidly. Figure 41 shows the progressionof flames over a pine straw mulch bed in contact with the target hardwood mulch bed at thebase of the structure. Because of the rapid fire spread, the propane burner was removed after
1 min, 30 s earlier than the normal ignition protocol. The flames reached the target mulch bed
90 s after the fan was turned on at medium wind speed. No ignition took place within theshredded hardwood mulch bed for any pine straw mulch experiment, even though the pine
straw mulch was in direct contact with the shredded hardwood mulch. In addition, no spot
fires were observed ahead of the flame front along the length of the mulch bed. The
firebrands produced by pine straw mulch were apparently too fine and lacking in energy
content to ignite spot fires.
Later in this report (in Section 4.3.4.3), experiments on the combination of pine straw mulch
and western redcedar privacy fences will be presented. Although pine straw mulch by itself
was unable to ignite fuels through lateral contact and firebrands, it was found to be effective
in supporting ignition of the fence along its length.

Fig. 41. Time sequence for pine straw mulch bed in medium wind speed [Test A-98].

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Figure 42 shows that flame spread over pine straw mulch was rapid for all wind speeds
studied in this set of experiments. The flame spread rate did not show a strong relationshipwith wind speed. Wind is not necessary for a fire to spread rapidly over pine straw mulch, as
was demonstrated in experiments by Beyler et al. [27] on a test protocol for flame spreadover mulch in the absence of wind. The flame spread does appear to slow in the opposing
flow of the wind vortex near the shed wall, as evident in the change in slope for all threewind speeds when the flame front reaches about 1.8 m (6 ft) from the wall. This is similar tothe observation for hardwood mulch discussed in Section 4.1.2.4. The uncertainties for theseplots can be determined from the uncertainty analysis for mulch in Appendix A.4.1.

Fig. 42. Effects of wind speed on flame spread for PS mulch at 0.91 m (3 ft) separation distance.

4.1.3.4. Shredded rubber mulch
Synthetic rubber mulch is a product made from recycled tires for use in landscaping. Its fire
behavior is illustrated in Fig. 43. The rubber mulch was easy to ignite, and flames spreadquickly. Figure 43 (a) shows the conditions at the time that the igniter was removed and thewind machine was turned on. Due to the rapid fire spread, the igniter was removed 15 s
before the completion of the 90 s ignition protocol. (In the two other rubber mulch
experiments in this study, the single propane torch described in Section 2.6 was used to ignite
the mulch rapidly across the full width of the bed.) The flames were considerably higher thanwith the plant-based mulches tested in this study. The ease of ignition and the high flames
and black smoke shown in Fig. 43 (b) are consistent with findings from the University of
Nevada Cooperative Extension [56].
Following the initial fire spread over the rubber mulch bed, which lasted less than one minuteafter the wind machine was turned on (at t = 0 min), the flames subsided as shown in Fig.
43 (c) and (d). The burning pattern of flames interspersed with blackened areas continued
until the fire was extinguished with a water spray at t = 6 min. Examination of the mulch bed
after the experiment revealed an upper layer of crumbly black residue above a layer of virgin

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rubber mulch, as shown in Fig. 44. This was consistent with the rapid decrease in height of
the intense flames observed as the top layer of mulch burned. The residual noncombustible
surface layer interfered with the transport of oxygen to the combustible mulch beneath,slowing the subsequent burning process and resulting in a high temperature area that may
expel toxic gases over a long time period. In a low wind speed case, the sporadic burning wasobserved for over an hour until the fire was extinguished. Disturbing the burning mulch bed
can introduce oxygen to unburnt fuel, renewing the flaming in that location.
The rubber mulch ignited spot fires in the target mulch bed next to the shed wall in less than
4 min for medium and high wind speed cases. In the low wind speed case, the rubber mulch
was allowed to burn for over 70 min without ignition of a spot fire in the target mulch.
Smoke and low flames were observed throughout the experiment. Before turning off the fan,
the wind speed was increased to high, and multiple spot fires were ignited within 3 min.

Fig. 43. Time sequence for rubber mulch bed in medium wind speed [Test C-27].

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Fig. 44. Shredded rubber mulch extinguished after 6 min, showing upper layer of crumbly residue andlower layer of virgin material.

Although this study did not examine the contents of the smoke emitted by rubber mulch, thetoxicity of smoke from tire fires is well-known [57, 58, 59]. Under these test conditions, thefire in this mulch was not found to be more difficult to suppress than in the other mulches
tested.
The flame spread plots for the rubber mulch experiments in Fig. 45 show that the flame front
reached the end of the mulch pan within about 2 min in each case. Because the medium windspeed experiment used a different ignition source than both low and high wind speed
experiments (propane burner and single propane torch respectively), no conclusion can bereached on the relationship of flame spread to wind speed for this mulch type.

Fig. 45. Effects of wind speed on flame spread for rubber mulch at 1.83 m (6 ft) separation distance.

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The uncertainties for the plots in Fig. 45 can be determined from the uncertainty analysis formulch in Appendix A.4.1.
4.1.3.5. Artificial turf
A single variety of artificial turf was selected for a preliminary comparison of its fire
behavior to that of other groundcovers. The selected artificial turf was described in
Section 2.4.2, and the cement board substrate was described in Section 2.4.1. Note that there
are many formulations of artificial turf comprising different materials and physical designs.
The experimental results for this single product should not be considered as representative offire behavior for artificial turf in general.
The fire behavior of the selected artificial turf sample was observed in two experiments, at
low and medium wind speeds. The experiments were performed one after the other, with the
fan on an idle setting for a brief period between experiments. Time sequences for Tests D-22
(low wind speed) and D-23 (medium wind speed) are presented in Fig. 46 and Fig. 47,
respectively. The shed is to the left of the image, and the wind machine is to the right.
The propane torch described in Section 2.6 ignited the artificial turf in the center far from theshed, as shown in the top image of Fig. 46. The groundcover did not ignite readily – flaming
ignition was achieved after three applications of direct flame from the torch. After flaming
ignition, the artificial turf continued to burn throughout the two experiments.
Figure 46 shows the progress of the fire in low wind speed. The flames proceeded slowly
down the length of the artificial turf for Test D-22, traveling about 0.7 m (2.3 ft) during the
24 min experiment. The fire spread asymmetrically, moving fastest along one edge of theartificial turf bed. The wind machine was then allowed to idle for about 2 min while theinstruments were reset.
The fan was brought back up to medium wind speed for Test D-23, which was again allowed
to run for about 24 min. Figure 47 shows that the flame front became more linear during thesecond experiment, with the backing rolling up in lines as the flame spread evened out fromside to side. A closeup of the final condition of the artificial turf in Fig. 48 shows (left toright) the charred remains near the ignition area, the rolled-up segments with turquoisebacking, the lighter areas where the synthetic fibers had melted, and the unburned artificial
turf beyond the final extent of the fire.
The flame spread plots of the two experiments in Fig. 49 show the extent of the melted fibers
as a function of time. Because of the asymmetry of the flame spread and the rolling up of theartificial turf as it burned, no conclusion can be drawn from these two experiments regarding
the effect of wind speed on the flame spread rate. The uncertainties for these plots can be
determined from the uncertainty analysis for mulch in Appendix A.4.1.
No spot fires were observed from burning artificial turf. This study did not examine thecontents of the smoke emitted by artificial turf.

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Fig. 46. Time sequence for artificial turf in low wind speed [D-22].

Fig. 47. Time sequence for artificial turf in medium wind speed [D-23].

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Fig. 48. Curled segments and char residue after burning of artificial turf [Test D-23].

Fig. 49. Effects of wind speed on flame spread for artificial turf at 1.83 m (6 ft) separation distance.

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4.1.4. Firebrand Spotting
The target mulch bed positioned along the base of the shed in Series 2 experiments served asa surrogate for building ignition vulnerability, providing a worst case condition of finecombustible material in direct contact with the structure. Ignition of a spot fire that thenprogressed to the wall demonstrated the ability of the burning mulch bed to generate
firebrands capable of threatening the structure. Note that spot fire ignition depends on manyfactors, including the firebrand (e.g., fuel type, size, shape, glowing or flaming combustion
state), the fuel bed (e.g., fuel type, density, porosity, temperature, moisture content), thelanding characteristics (e.g., fully or partially embedded, bouncing), and environmental
conditions (e.g., temperature, humidity, wind speed) [60]. The firebrands generated byshredded hardwood mulch were captured in water pans in previous field experiments andfound to be irregular in shape and less than 10 mm in the largest dimension [23].
Characteristics of the firebrand spotting caused by a burning mulch bed, including thelocation, timing, and frequency of spot fires, illustrated the hazard to the structure. Some
examples of spot fires from experiments on shredded hardwood mulch beds are shown in
Fig. 50. In each of these cases, a spot fire ignited in the middle of the target mulch bed ornear the shed wall, similar to the experiment shown in Fig. 34. These ignitions wereconsistent with mulch firebrands that were lofted upward from their initial positions near the
ground, carried toward the shed by the wind vortex, and deposited somewhere within thetarget mulch bed.

Fig. 50. Spot fires in target mulch bed resulting from mulch experiments a) HW mulch at half
thickness in medium wind [Test A-43], b) HW mulch in high wind [Test A-80], c) HW mulch in medium
wind [Test A-42], d) HW mulch in low wind [Test A-104].

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Ignitions were also observed along the outer edge of the target mulch bed, as in Fig. 50 (c).
The firebrands creating these spot fires may have been transported on the ground before
being caught in the mulch. These fires progressed slowly toward the wall, with smoke andflame patterns extending away from the wall as visual indicators of the flow field. The flame
spread toward the wall was clearly in opposed flow mode.
In most cases, the fire that first reached the structure was a spot fire ignited close to the shedwall by a lofted firebrand, even for mulch beds in intimate contact with the target mulch bed.
The experiments did not provide insight into the size or energy content of firebrands thatwere capable of igniting spot fires in the target mulch bed. Individual firebrands in flight
were small and difficult to observe directly under the daylight conditions of the field
experiments. Firebrands large enough for visual tracking usually traveled on the ground andseldom reached the target bed. The first sign of an ignition was smoke; it was not possible todistinguish the firebrand responsible for the spot fire from the target mulch bed. The
roughness of the target mulch bed prevented firebrands from moving after they had landed,
so that firebrand accumulation was possible only by chance. Spot fires occurred infrequently.
For the 156 experiments in this study using a target mulch bed (Series 2), only 11 resulted in
more than ten spot fires before the experiment was ended. The largest number of spot fires
for the Mulch Only experiments discussed in this section was seven. Some spot fires, but notall, transitioned from smoldering to flaming combustion during the experiment.
Spot fire ignitions often occurred after the flame front had reached the end of the mulch bed,
as in the cases shown in Fig. 50 (b) and (d). The times to ignition of the first spot fire areplotted as a function of wind speed in Fig. 51 and as a function of separation distance in Fig.
52. As mentioned in Section 2.7.1 and discussed in Appendix C.2, the wind speed value wasthe average of the velocities measured by the lower four bidirectional probes along thecenterline of the probe array.
The data indicates that the time to ignition of the first spot fire was related to wind speed.
Spot fires ignited in less than 20 min in all medium wind speed cases and in less than 10 min
in all high wind speed cases. For shredded hardwood mulch and pine bark mulch in low windconditions, spot fires that threatened the structure took up to an hour to ignite. Figure 52
shows that ignition times did not depend on separation distance.
Timing data was also collected for the ignition of the first spot fire to put flames against thewall (Fig. 53) and the first observation of flames on the wall (Fig. 54). The first spot fire toreach the wall was influenced by the location at which the spot fire ignited within the mulch
bed. Firebrands that were lofted by the wind vortex and deposited close to the wall weremore likely to reach the wall quickly. Transition from smoldering to flaming had to occur
before flames could be observed on the wall.
With the exception of one experiment on pine bark mulch (Test C-31), for which early spot
fires that ignited at the front of the target mulch bed failed to reach the shed wall, Fig. 53 andFig. 51 are very similar. In many cases, the first spot fire to ignite was also the first to reach
the wall. Comparing Fig. 54 with Fig. 53 demonstrates that the time between spot fire
ignition and flames on the wall was generally a few minutes or less.

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Fig. 51. Time to ignition of first spot fire vs. wind speed for mulch experiments.

Fig. 52. Time to ignition of first spot fire vs. separation distance for mulch experiments.

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Fig. 53. Time to ignition of first spot fire to put flames against the wall vs. wind speed for mulch
experiments.

Fig. 54. Time to flames reaching the wall vs. wind speed for mulch experiments.

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Table 1 lists the number of Series 2 experiments of each mulch type by whether or not spot
fires were ignited in the target mulch bed. This table summarizes the data from Table D.1,
where the color key identifies experiments in which the fire spread to the shed throughspotting (in green) and experiments in which the fire did not reach the shed (in pink). The
three shredded hardwood mulch and rubber mulch experiments that did not produce spot
fires were at low wind speeds. Neither the bed of pine straw mulch nor artificial turf
generated spot fires in any experiment.
Table 1. Number of experiments producing spot fires for each mulch type.
Type of Mulch

Number of Experiments With
Spot Fires

Number of Experiments
Without Spot Fires

Shredded hardwood (HW)

102

Pine Bark (PB)

100

Pine Straw (PS)05

Rubber2

1

Artificial Turf (AT)02

4.1.5. Summary
The mulch experiments demonstrated the dependence of fire behavior on the wind field, typeof mulch, and firebrand spotting. Key findings from this set of experiments include thefollowing.
Fire spread behavior. The fire spread behavior in a mulch bed is affected by the flow field,
ignition of spot fires downwind, and the geometry of the mulch bed and structure.
•

Flame spread is strongly affected by the wind flow field. The flame front progresses
faster as wind speed increases.

•

The flow field is highly turbulent and is modified by obstacles.

•

A vortex is generated when the wind flow is perpendicular to the wall of a structure.
The vortex slows the flame spread over a mulch bed as it approaches the structure dueto the change in flow direction. On a smooth ground surface, the surface winds tend
to move firebrands toward a line parallel to the wall where the vortex meets the flowfrom the fan and to the sides of the structure. Firebrands that are lofted may be carriedclose to the wall by the vortex and deposited at the base. Such spot fires threaten the
structure immediately and also spread rapidly outward from the wall.

•

Spot fires along the mulch bed allow the fire to jump in the direction of the wind.
This makes the flame spread more unpredictable, even as it slows the progress of theflame front.

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Type of mulch. Fire behavior is a strong function of the type of mulch. The material
properties and physical characteristics of the mulch influence the ease of ignition, fire
intensity, rate of fire spread, production and size of firebrands, and smoke toxicity.
•

Shredded hardwood mulch and pine bark mulch can burn and emit firebrands forlonger than an hour.

•

Pine straw mulch burns rapidly with high intensity. The fire did not spread laterally
into an adjacent bed of shredded hardwood mulch. Firebrands did not ignite spot fires
in the target mulch bed.

•

Rubber mulch burns with high initial intensity and significant smoke output.
Formation of a crumbly residue layer reduces the flame height and allows the fire toburn for a long time as virgin material beneath is ignited. Larger flames can breakthrough when the residue layer is disturbed.

•

The single type of artificial turf tested in this study was difficult to ignite andexhibited slow flame spread.

•

Reducing the mulch thickness for shredded hardwood mulch from 5 cm to 2.5 cm
does not have a large effect.

Firebrand spotting. Firebrands are generated by all mulch types tested.
•

Spot fires often ignite close to the wall of the structure, due to the deposition of lofted
firebrands by the vortex at the base of the wall.

•

Deposition of firebrands is unpredictable due to variety in shapes and sizes and theturbulence in the wind field.

•

The time to spot fire ignition decreases as wind speed increases. Spot fires ignited inless than 20 min in all medium wind speed cases and less than 10 min in all high windspeed cases.

•

Spot fires may occur after the flame front has reached the end of the mulch bed andthe initial flaming combustion has subsided.

•

Firebrands from pine straw mulch did not ignite spot fires in the target mulch bed.

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Fence Only Experiments
The experiments in this section investigate the fire behavior for a fence in the absence ofmulch at its base. This represents the best-case fire scenario for combustible fences, in which
the area underneath and along the sides of the fence is completely clear of fine fuels,
including grass or accumulated leaves. This condition was explored for western redcedar
privacy fences and redwood lattice fences at a variety of wind speeds and separation
distances from the nearby structure. A vinyl fence was tested under high wind speed
conditions, and a wood-plastic composite fence was tested at low wind speeds.
The test matrix for Fence Only experiments is shown in Table D.2 in Appendix D. The
details of each experiment, including parameter values, images, flame spread data, windplots, and summary values, can be found in Appendix G. In each case, timing was measured
from the point after ignition when the wind machine was turned on.
4.2.1. Example
The slow progress of fire over a combustible fence in the absence of fine fuels is shown in
Fig. 55. In this experiment (Test A-26), a western redcedar privacy fence was separated fromthe shed by 1.83 m (6 ft) and exposed to high wind speeds averaging 12.7 m/s (28.4 mi/h)along the centerline. Figure 55 (a) shows the ignition of the end of the fence farthest from theshed using the propane burner. Image (b) shows the glowing combustion of the area aroundthe ignition at about 2.5 min after the fan had been turned on, including the stringer, post, andthe first two boards near the ground. At about t = 4.5 min, image (c) shows that a piece of thefirst board had fallen to the ground. In image (d), at about t = 7.5 min, the wind had blown
this firebrand along the ground and back into contact with the fence, where it caused glowing
combustion in a new location. Image (e) shows that the combustion has not progressedbeyond the first two boards by t = 11.5 min. This is about one minute after the fan was turnedoff, marking the end of the experiment.
A smaller, more easily transported firebrand was deposited in the target mulch bed at thebase of the shed at about 8 min after the fan was turned on. Figure 56 shows the resulting
spot fire at approximately one minute after it was ignited. The firebrand ignited the mulch
bed at its intersection with the shed wall. This indicates that the firebrand was likely to havebeen lofted, striking the wall of the structure and falling into the mulch bed or following thepath of the vortex adjacent to the wall that was discussed in Section 3.3.2.

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Fig. 55. Progression of glowing combustion over western redcedar privacy fence in high wind speed[Test A-26], showing a) ignition, b) erosion of board at t = 2.5 min, c) large firebrand at t = 4.5 min,d) secondary ignition by firebrand at t = 7.5 min, and e) burn pattern at t = 11.5 min.

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Fig. 56. Firebrand spotting at 9 min after fan on for Test A-26.

4.2.2. Fire Spread Behavior
In the absence of fine fuels adjacent to and beneath a combustible fence, the fire spread froma single point of ignition was found to be slow. Glowing combustion was the primary mode
observed during the burning process, with the occasional appearance of small flames. The
gap between boards delayed horizontal flame spread for privacy fences, although it was notwide enough to arrest the fire. For diagonal lattice fences, the lack of alignment between slats
and wind slowed the flame spread. For both fence types, the stringer near the ground made ofpressure treated pine contributed to flame spread by providing a continuous path aligned withthe wind. Its thickness may have deterred rapid burning in some cases, although in othercases a transition to flaming combustion allowed the stringer to accelerate fire spread.
As the combustion progressed over the fence boards, it was typical for pieces of board to
break off. If ignited, these pieces became large firebrands that were capable of spreading thefire to other combustibles, depending on their aerodynamic properties and the wind. Under
the conditions in this study, large pieces usually remained near where they fell.
Only two experiments, both with WRC privacy fences, showed the fire spreading asignificant distance downwind, beyond the immediate region of ignition. In these cases, thestringer transitioned to flaming combustion for an extended period of time, providing acontinuous pathway for carrying the fire downwind at a more rapid pace than glowing
combustion. The separation distance between the end of the fence and the wall of thestructure in these experiments was 1.83 m (6 ft). Both experiments were over an hour induration. In Test A-101, shown in Fig. 57, a fire ignited at the base of the fence farthest fromthe shed took over an hour to progress from the ignition point to a point halfway down thelength of the panel. As can be observed in the bottom two images, pieces of the fence boards
broke off as the fire on the stringer progressed. In this case, no spot fire was observed over91

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the entire test duration. In the other case (Test A-91) a spot fire ignited in the target mulch
bed after 35 min. The wind speed level was medium for Test A-101 and high for Test A-91.

Fig. 57. Time sequence for Test A-101, western redcedar privacy fence only (no mulch), with medium
wind speed at 1.83 m (6 ft) separation from shed.

In Fig. 58 the location of the flame front is plotted as a function of time for all fence-only
experiments performed on western redcedar privacy fences and redwood lattice fences. The
initial location of the flame front reflects the separation distance of the fence from the shedwall, which was found to affect the rate of spreading of the fire toward the structure. The
flame front progressed toward the wall for experiments with 1.83 m (6 ft) separation but
remained near the point of ignition for smaller separation distances. This is consistent withthe effects of wind near the ground, as discussed in Section 3.3.2. Near the ground, the vortex
generated near the wall of the structure creates a stagnation point at about 1.83 m (6 ft) fromthe wall, as illustrated in the contours of the average wind field in Fig. 28. In these fence-only
experiments, the burning remained close to the ground, and none of the flame fronts
approached within 2 m of the wall of the structure, even when the experiment ran for over anhour. The uncertainties for these plots are described in the uncertainty analysis for fences inAppendix A.4.2 and listed in Table A.6.
Similar fire behavior was observed in fence experiments performed by IBHS [21, 22]. In the
IBHS experiments, firebrands were expelled from a firebrand generator into a wind field thatwas oriented perpendicular to the fence panels. Some of these firebrands lodged in cracks
and interstices in the fence, igniting the fence and resulting in holes that expanded slowly
through glowing combustion.

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Fig. 58. Location of the flame front on the fence as a function of time. Blue lines indicate experiments
on western redcedar privacy fences, and red lines are from redwood lattice fence experiments. Linestyle indicates separation distance between the fence and the shed: 0 m (dotted), 0.30 m (1 ft)(dashed), 0.91 m (3 ft) (dot-dash), and 1.83 m (6 ft) (solid).

4.2.3. Type of Fence
The fire behavior of four fence types (western redcedar privacy, redwood lattice, and vinyl
and wood-plastic composite privacy fences) was studied without fine fuels beneath. Eachfence type was described in Section 2.5 and illustrated in Fig. 11.
More data from experiments with fences only (without mulch) can be found in Appendix G.
4.2.3.1. Western redcedar privacy fence
Figure 59 shows photos from experiments with western redcedar privacy fences at medium
wind speeds, for the four separation distances studied. Slow fire spread through glowing
combustion and occasional flaming were observed in all cases. Figure 59 (a) shows a fence
board shortly before a piece broke off during combustion, and in images (c) and (d) pieces offence boards are lying on the ground. Results from experiments performed at high windspeeds were very similar, as shown in Fig. 60.

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Fig. 59. WRC privacy fence without mulch, in medium wind speed at separation distances of a) 0 m[Test A-18], b) 0.30 m (1 ft) [Test A-21], c) 0.91 m (3 ft) [Test A-65], and d) 1.83 m (6 ft) [Test A-30].

Fig. 60. WRC privacy fence without mulch, in high wind speed at separation distances of a) 0 m[Test A-28], b) 0.30 m (1 ft) [Test A-32], c) 0.91 m (3 ft) [Test A-25], and d) 1.83 m (6 ft) [Test A-91].

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4.2.3.2. Redwood lattice fence
Burn patterns resulting from the combustion of redwood lattice fences are shown in Fig. 61
for medium wind speeds and various separation distances. All lattice fences in this study had
a diagonal grid. The wood slats were therefore at an angle to the wind, so that flame spreadalong the slats was not aligned with the wind. The horizontal stringer provided a path for thefire aligned with the wind but was much thicker than the slats, and progress along the stringer
was also slow. As was observed for WRC privacy fences, pieces of the fence occasionallybroke off. If the pieces continued to smolder, they became firebrands that could spread thefire farther if transported by sufficiently high winds.

Fig. 61. Redwood lattice fence only, in medium wind speed at separation distances ofa) 0 m [Test A-82], b) 0.30 m (1ft) [Test A-113], c) 0.91 m (3 ft) [Test A-77], and d) 1.83 m (6 ft)[Test-A-75].

4.2.3.3. Vinyl privacy fence
A single test was performed on a vinyl privacy fence without mulch. It was difficult to ignite
the fence using the propane burner, and little change had been observed in the char patternwhen the experiment was ended after 4 min. The final condition of the fence is shown in Fig.
62.

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Fig. 62. Vinyl privacy fence in high wind conditions at 0 m separation distance [Test A-34].

4.2.3.4. Wood-plastic composite fence
A wood-plastic composite fence (WPC1) was studied both with and without mulch. In theabsence of more data, the following is considered limited preliminary data. Moreexperiments to study the fire behavior of this category of fence are planned.
Very different fire behavior was observed for the wood-plastic composite #1 (WPC1) fence
in the absence of mulch. As shown in Fig. 63, a fire ignited at the upwind base of the fence
under low wind conditions developed into a large fire with flames extending well above thefence and licking the shed from 1.83 m (6 ft) away. The fire was expanded by the boards
falling out of the top and bottom frames as the composite material softened; boards 1.83 m(6 ft) in length fell to either side of the fence and created a fire zone up to 3.6 m (12 ft) wide.
Some softened boards near the post leaned forward as they fell and increased the fire
exposure of the shed. The final configuration of the boards after extinguishment is shown in
Fig. 64.
Fires in the target mulch bed next to the shed can be seen in the final frame of the sequencein Fig. 63, at t = 14.1 min. Fires ignited along a large part of the front edge of the mulch bed
in the final 10 s before the fire was extinguished, likely due to direct flame contact rather
than firebrands.

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Fig. 63. Time sequence for Test E-2, wood-plastic composite fence WPC1 alone (without mulch),with low wind speed at 1.83 m (6 ft) separation from shed.

Fig. 64. Final configuration of wood-plastic composite boards after Test E-2.

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As occurred for many of the cases in which the fence was fully involved, there was evidence
that the shed might have been in danger of igniting if the fire had been allowed to continue.
Figure 65 shows water being applied to the underside of the eaves of the shed as part of theextinguishment of the fire after the experiment. Smoke can be seen rising from the shed roof
in this image – water was also applied to cool the roof. It was clear from this and otherexperiments with large flames that 1.83 m (6 ft) separation distance was insufficient toprevent the structure from ignition.

Fig. 65. Water being applied to the eaves of the shed after Test E-2. Smoke rising from the roof isvisible.

Flame spread over the WPC1 fence as a function of time will be discussed with the otherwood-plastic composite fence experiments in Section 4.3.3.7.
4.2.4. Firebrand Spotting
Firebrand spotting to the target mulch bed was an uncommon occurrence in the experiments
carried out on fences in the absence of fine fuels. This is illustrated in Table D.2, where thecolor key identifies experiments in which the fire spread to the shed through spotting (ingreen) and other cases in which little spread occurred (in red) and summarized in Table 2.
Out of 18 bare fence experiments with a target mulch bed present, 16 of which were withwood fences, spotting occurred in only six cases. The four cases for WRC privacy fences all
occurred under high wind conditions.
The large pieces of wood that broke off the fence boards primarily stayed on the ground nearwhere they fell. None of these large firebrands were observed igniting spot fires in the target
mulch bed at the base of the shed.

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Table 2. Number of experiments producing spot fires for each fence type.
Type of Fence(No Mulch)

Number of Experiments
With Spot Fires

Number of Experiments
Without Spot Fires

Western redcedar privacy (WRC)4

6

Redwood lattice (RWL)

1

5

Vinyl privacy (Vinyl)

01Wood-plastic composite (WPC1)

1

0

For those fence-only experiments in which firebrand spotting occurred, the times to ignitionof the first spot fire are plotted as a function of wind speed in Fig. 66. Data was too sparse todetect a relationship with either wind speed or separation distance (not plotted). Under highwind speed conditions, spot fire ignition occurred as early as 3 min after the wind machine
was turned on.

Fig. 66. Time to ignition of first spot fire vs. wind speed for experiments with fence only.

Figure 67 and Fig. 68 plot the time to ignition for the first spot fire to put flames against thewall and the time to flames on the wall for fence experiments without mulch. The spot fires
produced by wood fences all reached the wall. One spot fire took almost 20 min to put flames
against the wall after ignition, while the others reached the wall within 3 min. The burning
wood-plastic composite fence resulted in flames along the entire front of the target mulch
bed, possibly resulting from direct flame contact rather than from firebrands. None of thesefires reached the wall, but in this case that was of secondary concern to the large flames fromthe burning fence itself.

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Fig. 67. Time to ignition of first spot fire to put flames against the wall vs. wind speed for experimentswith fence only.

Fig. 68. Time to flames reaching the wall vs. wind speed for experiments with fence only.

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4.2.5. Summary
Because the fire behavior is so different, this summary will discuss wood/vinyl fences andwood-plastic composite fences separately.
It may be difficult to keep a wood fence sufficiently clear of fine combustible materials toachieve the slow-growth fire behavior described in this section. Even without mulch,
windblown debris such as leaves and pine needles may accumulate during a WUI event.
For the set of experiments performed in this study, the absence of fine combustibles resulted
in the following findings for wood fences:
Fire spread behavior. The fire spread over wood fences in the absence of fine combustibles
is generally slow.
•

The fence combusts slowly from the point of ignition.

•

The burning process is characterized by glowing combustion and occasional brief
flaming.

•

The stringer provides a continuous path for the fire aligned with the wind. If thestringer transitions to flaming combustion, fire spread along the fence is accelerated.

•

The fire spread behavior does not depend on the wind flow field or on separation
distance from the shed.

Type of fence. The glowing combustion typical of wood fences without mulch is affected byphysical connection and gaps.
•

Western redcedar privacy fences burn slowly with glowing combustion andoccasional small flames near the point of ignition. The gap between boards may slow
but not arrest horizontal flame spread. Stringers provide a continuous pathway for the
fire.

•

Redwood lattice fences burn slowly with glowing combustion and occasional small
flames near the point of ignition. Diagonal slats direct the flame spread. Stringers
provide a continuous pathway for the fire in the direction of wind flow.

•

Vinyl privacy fences result in limited fire exposures and low or no fire spread under
the tested wind conditions.

Firebrand spotting. Wood fences produce large firebrands (up to 14 cm × 20 cm) frompieces of the fence breaking off and small firebrands (on the order of millimeters) fromglowing combustion.
•

Large firebrands may be generated by detachment of pieces of the fence. Under theconditions of these experiments, the large firebrands remained on the ground close towhere they fell. They were able to ignite other combustible objects that contacted
them.

•

Although uncommon, spot fires ignited by small firebrands from wood fences canoccur, especially in high winds. A spot fire was seen to ignite close to the structure in
as little as 3 min after the wind machine was turned on.

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Wood-plastic composite fences. Even in the absence of fine combustibles, limited
preliminary data indicates that ignition of wood-plastic composite fences can result in highintensity fire behavior. The fire behavior of this category of fence needs to be studied further;upcoming experiments are planned at NIST.
•

The entire fence can become engulfed in flames.

•

The top and bottom frames can distort and release burning boards, which fall and can
extend the flaming region 1.83 m (6 ft) to each side of the fence. Softened boards canfall forward in the wind, extending the flaming region beyond the fence as well. Thisis a life safety hazard for people attempting to egress near the fence.

•

Fires in the target mulch bed next to the shed appear to have been ignited by radiationand direct flame contact rather than by firebrands.

•

Smoke coming from the roof of the shed after the experiment indicates that aseparation distance of 1.83 m (6 ft) between a fence and a structure is inadequate toprevent ignition. A long fence will increase the fire exposure by adding to theavailable fuel and linear extent, and other factors such as sloping terrain will also add
to the hazard.

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Fence and Mulch Experiments
A fence is not typically isolated from fine combustible materials. At its base there may begrass, mulch, vegetative plantings, or accumulated leaves or needles. Dry leaves and otherlightweight materials may be moved around by the wind. Accumulated debris can become
compressed by the action of wind and rain, fixing the potential fuel in place as a target for
ignition. When a fence is in close contact with the ground, it can be difficult to keep the area
near the bottom of the fence clear of combustible materials.
In this set of experiments, fences were erected over a bed of mulch in a variety of
combinations. The results not only demonstrate the fire behavior under the specificconditions tested but also provide an understanding of the general case in which finecombustible materials lie along the base of a combustible fence. As listed in Table D.3, Table
D.4, and Table D.5 of Appendix D, a variety of fences, including western redcedar privacyand good neighbor fences, redwood and pine lattice fences, and vinyl privacy fences were
tested in combination with shredded hardwood mulch. Western redcedar privacy fences werecombined with other mulches as well, including pine bark mulch and pine straw mulch.
Some experiments were performed with shredded hardwood mulch at half the usual thickness(2.5 cm compared to the usual 5 cm) to observe the effects of mulch thickness on fire
behavior.
The details of each experiment, including parameter values, images, flame spread data, windplots, and summary values, are presented in Appendix H. The data analysis focuses on thefire spread along the fence, which was assisted by the progress of the fire along the mulch
bed.
4.3.1. Example
Figure 69 shows a sequence of video images from an experiment (Test A-29) in which awestern redcedar fence was erected above a bed of shredded hardwood mulch. In thisexperiment, the wind speed was in the medium range, averaging 9.6 m/s (21.5 mi/h) along
the centerline, and the separation distance between the end of the fence and the shed was
1.83 m (6 ft). After the fan was turned on, marking the start of the experiment at t = 0 min,the fire rapidly (within 1 min) reached the end of the fence closest to the shed. The burnpattern on the fence downwind from the ignition area at t = 1 min shows that the initialprogress of the fire remained below the bottom stringer. At later times, the charred area
extended above the bottom stringer in the part of the fence panel closest to the shed, reachinga maximum height a few boards away from the end of the fence. This pattern will be related
to the wind field in Section 4.3.2.2. The fire remained well below the middle stringer until
the spot fires reached the wall, ending the experiment.
On a smaller scale, the upward fire spread was fastest in the narrow gaps between boards, as
indicated by the char marks and by the increasing gap widths in the images at t = 3 min andt = 5 min. Within the gaps, the progression of the fire was aided by radiative and convective
heat transfer between the edges of the boards. The fire was also shielded from the horizontal
winds, which otherwise interfere with upward fire spread.
The first sign of smoke in the target mulch bed next to the shed occurred shortly aftert = 2 min. By t = 3 min, four spot fires were visible along the outer edge of the target mulch

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bed. By t = 4 min, at least two more spot fires had ignited closer to the shed wall, including
the spot fire that was the first to reach the wall at about t = 4 min 45 s. The final image in Fig.
69 shows the spot fires at t = 5 min, shortly before the experiment ended and the fan was
turned off.
This experiment may be compared with the experiment with the same fence in the absence ofmulch in Fig. 57, which was performed under similar conditions. The most obvious change
in the fire behavior when mulch has been added to the base of the fence is the much fasterprogress of the fire. Contributing factors to this result include the continuous pathway
provided by the mulch and the high temperatures of the fire at the base of the fence.

Fig. 69. Image sequence from video of western redcedar fence combined with shredded hardwood
mulch, at medium wind speed and 1.83 m (6 ft) separation distance from structure [Test A-29].

Figure 70 plots the location of the charred point on the fence that is closest to the shed wall
as a function of time. As an aid to orientation, the horizontal lines mark the inner and outeredges of the posts at each end of the fence. This plot shows a rapid progress of the flame
front during the first minute of the experiment, with a maximum flame spread rate of
3.6 m/min ± 0.2 m/min.
The plot can be compared with medium wind speed plots in Fig. 33 (b), which show
experiments under the same conditions with shredded hardwood mulch alone. The interaction
with the fence more than halved the time for the fire to reach the end of the mulch bed.
Combining a fence with mulch led to a fire that is more intense and moves more rapidly thanfor either fuel on its own.
The black error bars in Fig. 70 show the expanded uncertainty (95 % confidence level) for
each of these points. As shown in the uncertainty analysis for fences in Appendix A.4.2 andlisted in Table A.6, the expanded uncertainty for horizontal flame front location was ±4.8 cm.

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Fig. 70. Flame front location vs. time for Test A-29, showing expanded uncertainty.

The time progression of the burning privacy fence is shown in Fig. 71, where the outlines ofcharred regions on the fence are plotted at five equal time intervals after the fan was turnedon and ending at the end of the test. The light brown lines on this plot outline the posts on
both ends of the fence and the three stringers. This plot shows the rapid progress of theflames from the ignition area on the lower right to the end of the fence closest to the shed. It
also shows how the lower and middle stringer slowed the upward spread of the fire. Similar
profiles are given in Appendix H for all experiments involving fences.

Fig. 71. Char patterns on western redcedar privacy fence during Test A-29, from the left camera view(ignition at the lower right).

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The uncertainties for the plots in Fig. 71 are described in the uncertainty analysis for fencesin Appendix A.4.2 and listed in Table A.6.
4.3.2. Fire Spread Behavior
This section describes the observed effects of wind speed and separation distance on fire
behavior for the combinations of fence and mulch. In other experiments, raising the height ofa privacy fence over the mulch bed was studied as a possible way to mitigate the fire spread.
Experiments that added a second fence panel to the end of the fence setup were conducted todetermine whether a longer fence would make a difference in the results.
4.3.2.1. Effects of wind speed
Experiments at all three wind speed levels (low, medium, and high) and all four separation
distances (0 m, 0.30 m, 0.91 m, and 1.83 m) were performed on western redcedar privacy
fences and on redwood lattice fences, with shredded hardwood mulch at the base of the fencein each case. Characteristic fire behaviors were sought by looking for common features offences and mulch under similar conditions and trends with changes in key variables.
Figure 72 compares images at the three wind speed levels from experiments performed on
privacy and lattice fences at 1.83 m (6 ft) separation distance from the shed. Similarities in
the progress of the charred area on the fence were apparent between the two fence types. In
every case, the fire burned rapidly to the end of the mulch bed and fence. The char patternnear the end of the experiment was highest for the lowest wind speed, reflecting both thetendency of higher wind speeds to keep flames closer to the ground and the longer time
duration for the experiment. As will be discussed in Section 4.3.5, ignition of spot fires was afunction of wind speed, so that a spot fire under low wind speed conditions generally
required more time to reach the wall and end the experiment.
In these experiments, the stringers tended to slow the upward progress of the char by limiting
the flame height on one side of the fence. The fence often burned below the lowest stringer
for a while before the fire continued to spread upwards. In an actual WUI fire, stringers may
also enhance the fire spread, since they provide locations for firebrands to lodge and ignite
new fires on the fence.
For this study, none of the experiments were performed in the presence of vegetation. Latticefences in particular are typically used in a garden setting, with plants nearby or growing on
them. This vegetation would provide another source of fuel to ignite the fence and encourage
flame spread.

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Fig. 72. Burn patterns for low, medium and high wind levels for WRC privacy fences: a) Test A-102,b) Test A-29, and c) Test A-31 and RW lattice fences: d) Test A-107, e) Test B-79, and f) Test B-85.

The positions of the char front on the fence closest to the shed wall for these experiments areplotted as a function of time in Fig. 73. The plots show a general trend of faster flame spreadwith increasing wind speed, with the flame front taking considerably longer to reach the endof the fence and mulch bed for low wind speeds. For medium and high wind speeds, therelationship is less clear, with little distinction between these plots for privacy fences andflame spread slightly faster for medium than high wind for the lattice fence experiments.
Figure 73 also indicates that for each wind speed the flame front proceeded more rapidly
along the western redcedar privacy fence than the redwood lattice fence, at least for aseparation distance of 1.83 m (6 ft). This was not generally the case, however, as will beshown in the next section. The uncertainties for these plots are described in the uncertainty
analysis for fences in Appendix A.4.2.

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Fig. 73. Effects of wind speed on flame front location vs. time for WRC privacy fences and RW latticefences shown in Fig. 72.

More detailed information for each of these experiments, including char distance, height, andprofiles vs. time, can be found in Appendix H.
4.3.2.2. Effects of separation distance
Assembling images from experiments performed under similar wind conditions but different
separation distances can provide insights on how a fence with mulch beneath behaves in fireas a function of distance from a nearby structure. In Fig. 74, the burn patterns at four
separation distances are shown for western redcedar privacy and redwood lattice fences
combined with shredded hardwood mulch. All experiments in this figure were carried out
under low wind conditions. The wind flow is directed at the structure – note that the results
will be different for other orientations of the structure with respect to the wind.
The effects of the vortex at the base of the shed that was described in Section 3.3.2 areapparent in the burn patterns for both privacy and lattice fences in Fig. 74. The fences that
abut the shed wall, with zero separation distance, are char-free within about 0.5 m (1.6 ft) of
the wall. Just upwind from the unburned region, the char is at its maximum height away fromthe ignition area. This shows the effects of the wind profile, with the flow from the fan rising
as it approaches the shed wall and then separating into winds that form the vortex at the baseof the shed and winds that go over the roof.
As the fence was positioned farther from the shed wall, the char pattern demonstrating theinfluence of the wind field moved downwind along the fence. At separation distances of
0.91 m and 1.83 m (3 ft and 6 ft respectively), the effects of the vortex were no longer seen,
but the shape of the char continued to reflect the upward tilt of the flow. Similar effects fromthe vortex and local wind direction were seen for medium and high wind speeds (not shown),
although the maximum height on the fence was lower under these conditions.
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Fig. 74. Burn patterns at low wind speed for separation distances of 0 m, 0.30 m (1 ft), 0.91 m (3 ft),and 1.83 m (6 ft) for WRC privacy fences: a) Test A-106, b) Test A-105, c) Test A-95, andd) Test A-102 and RW lattice fences: e) Test B-66, f) Test B-74, g) Test B-59, and h) Test A-107.

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Figure 75 compares the plots of char progress as a function of time for all experiments shownin Fig. 74. The horizontal lines in this plot mark the end of the fence and mulch bed closest to
the shed wall for the four separation distances (the line for zero separation is the x-axis).
Looking at the privacy fence (solid lines) and lattice fence (dashed lines) results separately,
flame spread down the fence was generally faster for larger separation distances and slower
for separation distances of 0 m and 0.30 m (1 ft). The lattice fence experiment at 1.83 m (6 ft)
separation was an exception to this, with a flame spread rate comparable to that for small
separation distances. Part of the increase in flame spread rate over the fence with increasing
separation distance may be related to the distance from the wind machine; wind profiles are
discussed in Appendix C. The uncertainties for these plots are described in the uncertainty
analysis for fences in Appendix A.4.2.
For separation distances of 0.91 m (3 ft) and smaller, the flame spread over the lattice fence
was faster than that over the privacy fence. This is the opposite trend from that found in Fig.
73, showing the difficulty of reaching conclusions based on a small number of experimentsin the presence of variables that could not be controlled in the test environment (including
ambient winds, temperature, and humidity). In all cases, placing mulch under the fence panel
resulted in steady progression of the fire along the fence, reaching the end of the panel inminutes.

Fig. 75. Effects of separation distance on flame front location vs. time for WRC privacy fences andRW lattice fences at low wind speed.

Note that the wind pattern will change for a different wind direction or geometry of the fencewith relationship to the structure. The pattern of charring is likely to be different under
different conditions.
More detailed information for each of these experiments, including char distance, height, andprofiles vs. time, can be found in Appendix H.

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4.3.2.3. Effects of fence length
The standard fence arrangement for this study involved a single fence panel between two
posts. To determine how fence length affects the fire behavior, two experiments were carriedout with a set of two western redcedar privacy fence panels placed end-to-end, with shredded
hardwood mulch at the base. The first experiment, Test C-19, was arranged in the usual
manner, with a 1.83 m (6 ft) separation distance from the shed downwind. In the second
experiment, Test D-3, the shed was removed completely. Figure 76 shows the two
configurations. Both experiments were performed at low wind speeds.

Fig. 76. Set-up for two-panel length western redcedar fence combined with shredded hardwood
mulch a) with and b) without a structure downwind [Tests C-19 and D-3, respectively].

Figure 77 shows a video image of Test C-19 at 45 min after the fan was turned on, shortly
before the experiment was ended. From the viewpoint of this camera, the wind machine wasto the right of the fence and the shed was to the left. The fence was ignited at the lower rightcorner. This image can be compared to Fig. 78, which shows a single panel length
experiment under the same conditions. Note that the latter fence also appeared in theprevious section in Fig. 74 (d), where it was used to illustrate char patterns at different
separation distances. The progress of the char front toward the shed for these two
experiments is plotted as a function of time in Fig. 79.
The char pattern for the downwind panel of the two-panel length fence was consistent withthat in the single panel experiment, indicating that a single fence panel was sufficient forlearning about fire behavior close to the structure. As seen in Fig. 77, the fire stayed below
the lowest stringer as it progressed along the first panel of the double panel fence from thepoint of ignition. Comparing Fig. 77 and Fig. 78, the flame spread over the panel closest to
the shed is similar for single and double panel lengths. The char pattern rises as it approaches
the post to the left, reflecting the rise in the air flow approaching the shed. The char neverexceeds the lowest stringer on the upwind panel and remains below the center stringer on the111

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downwind panel, although singe marks are observed along the length of this stringer. The
upward spread of fire between boards is especially noticeable on the lee side of the center
post of the double panel fence, where the height of the char pattern exceeds that at any otherpoint along the fence.

Fig. 77. Two-panel length western redcedar fence combined with shredded hardwood mulch, at low
wind speed and 1.83 m (6 ft) separation distance [Test C-19], at t = 45 min.

Fig. 78. Single panel western redcedar fence combined with shredded hardwood mulch, at low windspeed and 1.83 m (6 ft) separation distance [Test A-102], at t = 20 min.

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Figure 79 shows that it took about 25 min after the fan was turned on for the flame front toreach the center post. The fire slowed as it approached this point. After passing the center
post, the fire took about 12 min to reach the post closest to the shed. This compares to about
9 min for the fire on the single panel fence to move from the point of ignition to the same
post. The uncertainties for these plots are described in the uncertainty analysis for fences inAppendix A.4.2.

Fig. 79. Effects of fence length on flame front location vs. time at low wind speed.

An image from Test D-3, in which the shed was removed, is shown in Fig. 80. Compared toFig. 77 showing the case with the shed in place, the char pattern is generally higher along thelength of the two panels, although it remains below the center stringer. The char reaches its
highest point at the same place for both experiments – on the lee side of the center post.
Figure 81 compares the progress of the flame front downwind as a function of time for both
two-panel length experiments. The flame front progressed considerably slower down thefence when a structure was present downwind, possibly due to local recirculation effects. The
uncertainties for these plots are described in the uncertainty analysis for fences inAppendix A.4.2.

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Fig. 80. Two-panel length western redcedar fence combined with shredded hardwood mulch withouta structure downwind, at about t = 12 min [Test D-3].

Fig. 81. Comparison of two-panel length fences with and without a structure downwind.

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4.3.2.4. Effects of height above mulch bed
Mulch beneath the fence significantly changes the fire behavior. Without a fine combustible
fuel at the base, fences burn very slowly beyond the point of ignition, as discussed in
Section 4.2. With a mulch bed below, the fire progresses from one end of a fence panel to theother in minutes. This suggests that one potential mitigation technique might be raising thefence panel to a height that allows the fire on the mulch bed to decouple from the fence.
A series of experiments was carried out to look at this option. Photographs in Fig. 82 show
the configurations before ignition, in which western redcedar privacy fence panels wereattached to the end posts with clearances from the shredded hardwood mulch bed ranging
from 0 cm to 15.2 cm. The fence/mulch combinations in cases (a) through (d) were ignitedusing gas burners on each side of the fence. In case (e), a ring burner (described in
Section 2.6) was positioned on its side to the left of the post. This method placed more fire
directly on the fence panel during ignition. Experiments were performed under low windspeeds (5 m/s to 9 m/s) at a separation distance of 1.83 m (6 ft) from the shed wall.

Fig. 82. Setups for western redcedar privacy fence panels at a height above the shredded hardwood
mulch pan of a) 0 cm [Test A-102], b) 5.1 cm (2 in) [Test C-10], c) 7.6 cm (3 in) [Test C-11],d) 10.2 cm (4 in) [Test C-14], and e) 15.2 cm (6 in) [Test C-28].

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Figure 83 shows the char pattern late in each experiment, and Fig. 84 plots the location of theflame front with time. With the privacy fence in direct contact with the top of the mulch bed
in case (a), the mulch and fence interacted to generate a burn pattern typical of thiscombination. (Images from Test A-102 were previously displayed in Fig. 74 (d) and Fig. 78.)
As shown in Fig. 84, raising the fence panel more than 5.1 cm (2 in) above the mulch bed
slowed the involvement of the fence in the fire. For cases (b) and (c), the fire did not risesignificantly above the lowest stringer during the experiment. In case (d), with the privacy
fence lifted by 10.2 cm (4 in) above the mulch, only the bottom surfaces of the panel boards
and the area around the burner ignition were observed to char until 24 min into the test. At
this time the bottom stringer ignited, followed by the board segments below the stringer. In
case (e) the fence burned only slowly away from the point of ignition, duplicating the fire
behavior that was observed in Section 4.2 in the absence of mulch. This indicated that thefence panel was indeed decoupled from the mulch along its length when it was raised
15.2 cm (6 in) above the mulch bed.

Fig. 83. Comparison of results from privacy fences raised above the hardwood mulch pan by a) 0 cm
[Test A-102], b) 5.1 cm (2 in) [Test C-10], c) 7.6 cm (3 in) [Test C-11], d) 10.2 cm (4 in) [Test C-14],and e) 15.2 cm (6 in) [Test C-28].

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The uncertainties for the plots in Fig. 84 are described in the uncertainty analysis for fencesin Appendix A.4.2 and listed in Table A.6.
These results demonstrate a trend of fire behavior with fence height above a mulch bed under
specific conditions of wind and ignition and are not intended to recommend a specific fence
height. Different types of fence and mulch may behave differently. It should also be notedthat attaching a wire screen below a raised fence to deter animals from going in or out of theyard would reduce the value of this approach, since leaves and other combustible materials
could then collect at the base of the fence.

Fig. 84. Effects of height above hardwood mulch bed on flame front location vs. time for WRC privacy
fences shown in Fig. 83.

4.3.3. Type of Fence, combined with HW Mulch
This section compares the fire behavior for three types of privacy fences (new and aged
western redcedar and vinyl), two types of lattice fences (redwood and pine), western redcedar
good neighbor fences, and a wood-plastic composite fence. All were combined with shredded
hardwood mulch beneath the fence. Comparison with the fence experiments without mulch
from Section 4.2 shows the impact of having fine combustible materials beneath the fence.
4.3.3.1. Western redcedar privacy fence
The fire behavior of western redcedar privacy fences combined with shredded hardwood
mulch was studied under a variety of conditions. A number of these experiments have beenpresented already. A video image from the same experiment used as an example in
Section 4.3.1 is shown in Fig. 85. At this time, 4.5 min after the fan was turned on, the flames
have covered the full length of the fence panel and several spot fires have ignited in themulch bed next to the shed. This image shows the typical char pattern shape at a 1.83 m (6 ft)

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separation distance, with increasing height toward the end of the fence near the shed thatlikely reflects the wind field, as discussed in Section 4.3.2.1. The charred area remains below
the center stringer. Char marks and increased gap widths show that fire spreads upwards
between the boards as well as over their surfaces. The stringers provide a continuous
connection from board to board, and the bottom of the lowest stringer is subjected to the heatfrom the fire in the mulch bed.

Fig. 85. WRC privacy fence with HW mulch at medium wind speed, at about t = 4 ½ min [Test A-29].

In Fig. 86, plots (a) through (d) show the flame spread as a function of time for experiments
at the four separation distances studied. As described previously, the flame front position wasmeasured as the closest distance to the shed of the char pattern on the fence. Comparing theexperiments performed under the same nominal conditions (same wind speed and separation
distance), it’s clear that there was considerable variation among experiments. However, two
trends can be observed. First, increasing the wind speed generally resulted in faster flame
spread along the fence. This was most apparent for low wind speed conditions, under which
the flame front could take twice as long to reach the end of the fence as under medium windspeed conditions. Second, the flame spread was generally faster (shorter time to reach the endof the fence) at longer separation distances from the shed. These two trends have been
discussed at greater length in Sections 4.3.2.1 and 4.3.2.2, respectively. The uncertainties forthese plots are described in the uncertainty analysis for fences in Appendix A.4.2.
The addition of a western redcedar privacy fence to a shredded hardwood mulch bed
significantly increased the flame spread rate, as can be seen by comparing Fig. 86 to the plots
for hardwood mulch alone in Fig. 32 and Fig. 33. In many of the experiments for fence plus
mulch at separation distances of 0 m and 0.91 m (3 ft), the fire reached the end of the mulch
bed in half the time taken for the mulch alone. With the fence present, the fire did not slow asmuch as it approached the wall of the structure.

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Fig. 86. Effects of wind speed on flame spread for WRC privacy fence combined with HW mulch atseparation distances of a) 0 m, b) 0.30 m (1 ft), c) 0.91 m (3 ft), d) 1.83 m (6 ft).

4.3.3.2. Aged privacy fence
Experiments with mulch at all three wind speeds were performed on the aged western
redcedar (presumed) privacy fences described in Section 2.5.1, at a separation distance of
1.83 m (6 ft). The results were similar to those for new western redcedar privacy fences, as
can be seen by comparing Fig. 87 to Fig. 85. The fires on both new and aged WRC fences
remained well below the center stringer. By the time of this photo just after t = 5 min, all of
the fence boards had burned below the first stringer, releasing large and small firebrands inaddition to firebrands from the mulch. One of the large broken-off pieces of fence can beseen on the ground between the fence and the target mulch bed. Several spot fires ignited in
the target mulch bed between t = 4 min and t = 5 min, as can be observed from the smoke.
Flame spread over the aged privacy fences is plotted in Fig. 88. For these three experiments,
the flame spread rate increases with increasing wind speed. At a separation distance of
1.83 m (6 ft) from the shed, the time for the fire to travel from the ignition point to the far
end of the fence panel is 2 min for high wind speeds and less than 8 min for low wind speeds.
This agrees well with the results for new WRC privacy fences plotted in Fig. 86 (d). The
uncertainties for these plots are described in the uncertainty analysis for fences inAppendix A.4.2.

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Fig. 87. Aged privacy fence, at medium wind speed, at t = 5.2 min [Test C-25].

Fig. 88. Effects of wind speed on flame spread for aged privacy fence/HW mulch at separation
distance of 1.83 m (6 ft).

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4.3.3.3. Vinyl privacy fence
The fire behavior of a vinyl privacy fence with shredded hardwood mulch at its base differedstrongly from that of the same fence without mulch. With mulch, the fire reached the shedwall within minutes.
Figure 89 provides a sequence of images showing phenomena observed during thisexperiment, in which there was zero separation distance between the fence and the shed. The
gas burner ignited the vinyl fence/mulch combination readily at the base of the fence far fromthe shed, and at t = 1 min the fire was well-established. The image at t = 1.5 min shows thefire engaging the vinyl post, which resulted in intensification of the flames, a significant
amount of black smoke, and thin blackened pieces of the fence skittering over the ground orlofted in the air. In this image a large fragment can be seen in mid-air and two smaller pieces
are on the ground. By t = 3.5 min, spot fires had ignited both along the fence downwind of
the main flame front and in the target mulch bed close to the wall. The vinyl fence post,
panel, and bottom frame were blackened and distorted. The final image at t = 6.2 min shows
the conditions just before water was applied. By this time, the bottom frame was no longer
confining the fence panel, which swung back and forth in the wind. The mulch had burned
from the point of ignition to the shed, and several vinyl fence fragments can be seen on the
ground next to the mulch bed.
This fire behavior can be compared to that of the vinyl fence in the absence of mulch, which
was difficult to ignite and supported little fire spread, as described in Section 4.2.3.3.
Note that the charring in the upper right corner of the vinyl fence in Fig. 89 resulted from
Test A-34 without mulch, in which so little damage took place that the same fence wasturned upside down and reused.

Fig. 89. Image sequence for vinyl privacy fence attached to wall with medium wind speed [Test A-35].

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4.3.3.4. Redwood lattice fence
Experiments on a redwood lattice fence combined with shredded hardwood mulch were
performed at all wind speeds and separation distances. An image from an experiment at
1.83 m (6 ft) separation distance and medium wind speed is shown in Fig. 90. The fire
behavior trends with wind speed and separation distance were previously discussed in
Sections 4.3.2.1 and 4.3.2.2, respectively. As a summary, in all cases the fire burned rapidly(within 12 min) from the point of ignition to the end of the fence and mulch bed close to theshed. The char pattern on the fence was highest for the lowest wind speed but did not exceed
the half-height of the fence during the experiment. The burning was most intense below thelower stringer, with chunks of the fence occasionally breaking off. These large firebrands
added to the flux of smaller firebrands from the fence and mulch. The shape of the charpattern on the fence reflected the wind vortex near the shed. The target mulch bed ignited inevery case, and spot fires generally reached the shed wall more quickly at higher windspeeds.
These fire behavior trends for fences combined with shredded hardwood mulch were thesame for redwood lattice fences and for western redcedar privacy fences.

Fig. 90. Redwood lattice fence combined with shredded hardwood mulch, 3½ min after fan on, at
medium wind speed [Test B-79].

The four plots in Fig. 91 show flame front location as a function of time for redwood latticefence/HW mulch experiments at the four separation distances. Although the highest windspeed generally resulted in the fastest flame spread for these experiments, the relationshipbetween flame spread rate and wind speed was inconsistent. A relationship between flame
spread rate and separation distance was not clear. The uncertainties for these plots aredescribed in the uncertainty analysis for fences in Appendix A.4.2.

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Fig. 91. Effects of wind speed on flame spread for RW lattice fence/HW mulch at separation
distances of a) 0 m, b) 0.30 m (1 ft), c) 0.91 m (3 ft), d) 1.83 m (6 ft).

4.3.3.5. Pine lattice fence
Experiments at the three wind speed levels were performed on pine lattice fences withshredded hardwood mulch at the base. All used a separation distance of 1.83 m (6 ft) fromthe shed. Fig. 92, an image from a pine lattice fence experiment at medium wind speed
captured 3.5 min after the fan was turned on, can be compared directly to Fig. 90, whichshows a redwood lattice fence experiment under the same conditions and at the same time.
The plot of flame front location as a function of time in Fig. 93 confirms the similarity in fire
behavior between pine and redwood lattice fences combined with shredded hardwood mulch.
At low wind speeds, the charring took about 11 min to reach the end of the fence closest to
the shed, while at medium and high wind speeds the time to reach the end was less than
5 min. The flame front reached the end of the fence sooner for the medium wind speed case(green solid line) than for the high (red solid line). This is partially because the initial
location of the flame front for the medium wind speed case was closer to the far end of thefence. The maximum flame spread rates for these two cases were not appreciably different.
The uncertainties for these plots are described in the uncertainty analysis for fences inAppendix A.4.2.

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Fig. 92. Pine lattice fence combined with shredded hardwood mulch, 3 ½ min after fan on, at medium
wind speed [Test B-81].

Fig. 93. Effects of wind speed on flame spread comparing pine lattice and redwood lattice fences at
1.83 m (6 ft) separation distance. Shredded hardware mulch was arranged at the base of bothfences.

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4.3.3.6. Good neighbor fence
The fire behavior of a good neighbor fence differed qualitatively from that for the privacy
fences and lattice fences observed in this study. The time sequence in Fig. 94 shows the
progress of fire over a good neighbor fence under low wind speed conditions at 1.83 m (6 ft)
separation distance. For the first 6 min of the experiment, the fire remained below the firststringer. By t = 8 min, flames extended upward to the center stringer along the inner surfacesof the boards, on the side facing the stringers. The fire above the first stringer then moved
downwind and attacked the second stringer. By t = 10 min, the fire extended into the space
above the second stringer. The fire exceeded the height of the third stringer at t = 12 min, andby the time the fire was extinguished at t = 13.7 min the fence was in flames along its entire
height. The stepping-stone nature of the upward fire growth downwind is illustrated clearlyin the final frame of the sequence.

Fig. 94. Time sequence for good neighbor fence at low wind speed [Test C-18].

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This fire behavior can be compared with the discussion and images in Section 4.3.2 forprivacy and lattice fences, which show fire spread (from a single ignition near the ground)remaining below the center stringer and manifesting as charring rather than flaming.
The reasons behind the differences in fire behavior can be found in the alternating board(board on board) construction of the good neighbor fence. Attachment of the boards toalternate sides of the stringers places combustible material on two vertical planes separated
by the 3.8 cm (1.5 in) stringer thickness. This creates corridors between the stringers forconvection of heat and combustible gases. In addition, heat is transferred between innerboard surfaces through thermal radiation, even if the boards are not overlapping. The
importance of these physical phenomena will become even more apparent in Section 4.4 onparallel fences.
Figure 95 shows the fire behavior of the good neighbor fence at higher wind speeds. Bothvideo images are just prior to water application that ended each experiment after spot fires
reached the shed wall. These images show the higher intensity of the fire over the goodneighbor fence as compared to the privacy and lattice fences shown in Section 4.3.2. They
also show the same relationship between wind speed and the height of the fire on the fence
shown in Fig. 72. These experiments don’t answer the question of how the fire would evolve
over the fence at later times or downwind over multiple panels.

Fig. 95. Good neighbor fence experiments at a) t = 4.5 min for medium wind speed [Test C-22] andb) t = 2.6 min for high wind speed [Test C-23].

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The downwind progress of the flame front for good neighbor fences combined with shredded
hardwood mulch is plotted in Fig. 96 for the three wind speeds. The flame spread rateincreases with increasing wind speed, and the time to reach the end of the fence near the shedis similar to that found for the other combinations of wood fence and shredded hardwood
mulch in this section. The uncertainties for these plots are described in the uncertainty
analysis for fences in Appendix A.4.2.

Fig. 96. Effects of wind speed on flame spread for good neighbor fence/HW mulch at separation
distance of 1.83 m (6 ft).

4.3.3.7. Wood-plastic composite fence
In this study, limited testing was carried out on two types of wood-plastic composite fences.
The two experiments performed on wood-plastic composite fences with shredded hardwood
mulch illustrated the effects of both fence design and material formulation. Both wereprivacy fences, with interlocking vertical boards for WPC1 and horizontally-oriented boards
held in place by a frame for WPC2. Material properties for both fences are discussed inAppendix B, and an experiment on the WPC1 without mulch has already been discussed in
Section 4.2.3.4. Both experiments were carried out at low wind speed and 1.8 m (6 ft)
separation distance.
Because only three experiments were performed on WPC fences, the following is consideredlimited preliminary data. More experiments with different designs and materials undervarious conditions are needed to fully understand the fire behavior of this category of fence.
The fire behavior for the WPC1 fence with shredded hardwood mulch beneath was similar tothat observed in Section 4.2.3.4 for the same fence in the absence of mulch. Figure 97 showsa fire ignited at the upwind base of the fence developing into a large fire with flames
extending well above the fence within 6 min of turning on the fan. As before, the boards fell

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to the sides as the top and bottom frames softened and distorted, expanding the fire on bothsides and toward the downwind side of the fence and creating a fire zone up to 3.6 m (12 ft)
wide. The final configuration of the boards after extinguishment is shown in Fig. 98, with thefinal positions of boards to either side and downwind of the fence panel highlighted witharrows.
As in the case without mulch in Section 4.2.3.4, fires ignited in the target mulch bed next tothe shed only after the flames had become intense. The first fire in the target mulch bed next
to the shed ignited just before t = 7 min. Because it ignited immediately adjacent to the shedwall, this may have been a spot fire, possibly ignited by a firebrand originating in the mulch
bed beneath the fence. The set of fires in the target mulch bed just before extinguishment canbe seen in the final frame of the sequence in Fig. 97, at t = 7.8 min.

Fig. 97. Time sequence for wood-plastic composite fence #1 at low wind speed and 1.83 m (6 ft)
separation distance [Test E-1].

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Fig. 98. Final configuration of wood-plastic composite boards after Test E-1. Yellow arrows highlightthe final positions of fallen fence boards.

The fire behavior for the WPC2 fence with shredded hardwood mulch beneath is quite
different, as shown in the sequence in Fig. 99. As the fire consumed each horizontal board inturn, the boards above it slipped downward, still confined within the black aluminum frame
defining the panel. Fig. 100 gives an example of the boards collapsing downward over a
period of one second as the support from the bottommost two boards gave way. As theremaining segment of each board fell flat against the ground, it tended to disrupt the flames,
slowing the burning process until the flames reestablished themselves on the edges of theboard. The flame height never reached above the halfway point on the fence, and the fire
diminished on its own as it ran low on fuel. With the frame holding the boards in line withthe fence during the fire, the fuel stayed within a couple of feet from the centerline. The final
configuration of the boards shortly before extinguishment is shown in Fig. 101.
A spot fire ignited at t = 19 min near the wall. It can be seen in the final frame of Fig. 99 as adark spot with smoke at the far end of the wall near the clock.

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Fig. 99. Time sequence for wood-plastic composite fence #2 at low wind speed and 1.83 m (6 ft)
separation distance [Test F-1].

Fig. 100. Collapse of horizontal planks shortly after t = 12 min [Test F-1].

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Fig. 101. Final configuration of wood-plastic composite boards after Test F-1.

The location of the flame front as a function of time for the wood-plastic composite fences
tested in this study is plotted in Fig. 102. This includes WPC1 without mulch (discussed in
Section 4.2.3.4) in addition to the two fence types WPC1 and WPC2 in combination withshredded hardwood mulch from this section. All three experiments were performed at low
wind speed and at 1.83 m (6 ft) separation distance from the shed. Note that the lines
denoting the fence posts are in different locations because WPC2 fence panels are shorter
than WPC1. The uncertainties for these plots are described in the uncertainty analysis forfences in Appendix A.4.2.
Experiments on WPC1 with and without HW mulch (Tests E-1 and E-2, respectively)
indicated that the presence of mulch below the fence acted to accelerate the progress of theflame down the fence. Comparing WPC2 with WPC1 under the same conditions (Tests F-1
and E-1, respectively), the flame front for WPC2 is considerably slower, reaching the end of
the fence in almost twice the time of WPC1 despite being ignited closer to the shed. At 6 minfrom the point of ignition to the end of the panel, the flame front progresses faster over the
WPC1 fence/HW mulch combination than for any other fence-mulch combination in thisstudy.

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Fig. 102. Flame spread as a function of time for wood-plastic composite fence experiments.

4.3.4. Type of Mulch, combined with WRC Privacy Fence
This section compares the fire behavior for western redcedar privacy fences in combination
with different types of mulch, including shredded hardwood mulch at two thicknesses, pine
bark mulch, and pine straw mulch. Comparison with the experiments of mulch alone from
Section 4.1 illustrates how combustible objects can interact to intensify fire behavior,including flame spread and firebrand generation.
4.3.4.1. Shredded hardwood mulch with half thickness
A series of experiments to investigate the effects of mulch thickness on fire behavior was
performed on a western redcedar privacy fence combined with shredded hardwood mulch.
For these experiments, the thickness of the mulch bed was reduced by half to 2.5 cm.
Experiments were performed at all three wind speed levels and four separation distances.
Mulch thickness was not found to have a large effect on fire behavior. Figure 103 comparessnapshots at t = 5 min from experiments with full and half mulch thickness (images (a) and(b) respectively) that were carried out under the same conditions – medium wind speed and1.83 m (6 ft) separation distance. Both show similar char pattern shapes, reflecting the windfield near the structure as described in Section 4.3.2.1. Multiple spot fires are visible in thetarget mulch beds in both images.

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Fig. 103. WRC fence with HW mulch in medium wind speed with mulch thickness equal to a) 5 cm
[Test A-29] and b) 2.5 cm [Test A-64], at t = 5 min.

Figure 104 shows the location of the flame front on the fence as a function of time forexperiments with half mulch thickness. The general trend of increasing flame spread ratewith increasing wind speed is demonstrated in these plots. Comparing these plots withmatching plots in Fig. 86 from experiments with full mulch thickness, mulch thickness did
not appear to significantly affect flame spread rate. A possible exception was found in low
wind speed experiments, for which the flame front was generally observed to reach the endof the fence 2 min to 5 min faster for the thinner mulch bed. The uncertainties for these plots
are described in the uncertainty analysis for fences in Appendix A.4.2.

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Fig. 104. Effects of wind speed on flame spread for WRC privacy fence combined with half thickness(2.5 cm thick) HW mulch at separation distances of a) 0 m, b) 0.30 m (1 ft), c) 0.91 m (3 ft), d) 1.83 m(6 ft).

4.3.4.2. Pine bark mulch
A full set of experiments, at all wind speeds and separation distances, was performed with a
bed of mini pine bark mulch at the foot of a western redcedar privacy fence.
Figure 105 and Fig. 106 show sequences for experiments performed under the same
conditions (medium wind speed and 1.83 m (6 ft) separation distance). The flame spread
progressed at about the same rate for both experiments, although the char pattern was higher
for Test A-74 than for Test A-90. The charring over the single panel did not reach higher
than the center stringer, consistent with the results for other privacy fences (and latticefences) with hardwood mulch discussed in Section 4.3.2.1.

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Fig. 105. Time sequence for WRC privacy fence and PB mulch in medium wind speed [Test A-74].

Fig. 106. Time sequence for WRC privacy fence and PB mulch in medium wind speed [Test A-90].

The plots of flame spread as a function of time for the four separation distances are given in
Fig. 107. Experiments with pine bark mulch generally progressed somewhat more slowly
than those with shredded hardwood mulch, as can be seen by comparing these plots to Fig.
86. However, the difference in time for the fire to reach the end of the fence panel was
usually 5 min or less.
The key finding that combining combustible objects increases the hazard disproportionately
is illustrated by comparing Fig. 107 to Fig. 40, showing fire spread plots for a mini pine barkmulch bed. Adding a privacy fence to the pine bark mulch bed resulted in flames reaching
the end of the mulch pan two to four times faster. The uncertainties for these plots aredescribed in the uncertainty analysis for fences in Appendix A.4.2.
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Fig. 107. Effects of wind speed on flame spread for WRC privacy fence combined with PB mulch atseparation distances of a) 0 m, b) 0.30 m (1 ft), c) 0.91 m (3 ft), d) 1.83 m (6 ft).

4.3.4.3. Pine straw mulch
In Section 4.1.3.3, a bed of pine straw mulch was found to burn intensely and quickly, but
without igniting the target hardwood mulch bed through flames or firebrands, even when in
direct contact. The addition of a western redcedar privacy fence was found to make thesituation more hazardous. As can be seen in Fig. 108, the pine straw mulch ignited the fence
panel along its entire length within 30 s as the fire rapidly spread downwind. The fence thengenerated firebrands, which ignited the target mulch bed at t = 4 min. A flaming section of
the target mulch bed is visible in the image at t = 5 min, with more spot fires visible at
t = 10 min.
After the pine straw mulch was consumed, the privacy fence continued to undergo glowing
combustion and occasional flaming, which slowly consumed the boards near the ground.
This is evident in the differences between images at t = 5 min and t = 10 min. The difference
between the fence burning in this case and in the experiments without mulch discussed in
Section 4.2.3.1 is that here the pine straw mulch rapidly ignited the entire fence at its base,allowing the fence to become a source of firebrands along its entire length. This fire behavior
can also be expected if fine combustible debris such as needles or leaves are allowed toaccumulate along the fence.

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Fig. 108. Time sequence for WRC privacy fence in PS mulch at medium wind speed [Test A-109].

Seven experiments were performed on western redcedar privacy fences combined with pine
straw mulch, at medium and low wind speeds as listed in Table B.3. In all cases, thiscombination resulted in the ignition of spot fires in the target mulch bed by firebrands,
presumably from the fence.
Plots of flame front location as a function of time in Fig. 109 show the rapid spread of fire as
the pine straw mulch burned and ignited the privacy fence. The fastest flame spread was for aseparation distance of 1.83 m (6 ft), shown in plot (c), for which the fence panel was awayfrom the flow effects of the shed. In these experiments, the flame front reached the end of thefence/mulch bed in less than a minute. At 0.91 m (3 ft) separation, shown in plot (b), the firecrossed the length of the panel in 1.5 min or less. This matched the spread rate over pine
straw mulch in the absence of the fence, which was plotted in Fig. 42. For a separation
distance of 0 m, the pine straw mulch bed abutted the target hardwood mulch bed at a
distance of 0.46 m (18 in) from the shed wall. In plot (a), the flames on the fence reached thetarget mulch bed at about t = 3 min, after which the final approach to the wall was slow, withspot fires reaching the wall first. The uncertainties for these plots are described in theuncertainty analysis for fences in Appendix A.4.2.
In all cases, the fire spread much faster on the western redcedar privacy fence for pine straw
mulch than for shredded hardwood mulch, as can be seen by comparison of Fig. 109 with
Fig. 86. Unlike the latter, flame spread with pine straw mulch did not vary with wind speed.

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Fig. 109. Effects of wind speed on flame spread for WRC privacy fence combined with PS mulch atseparation distances of (a) 0 m, (b) 0.91 m (3 ft), and (c) 1.83 m (6 ft).

As pointed out in Section 4.1.3.3, a study by Beyler et al. [27] showed that fire spreads
quickly over pine straw mulch even with no wind, suggesting that wind is not a dominantfactor.
4.3.5. Firebrand Spotting
All combinations of fence and mulch tested in this study ignited spot fires in the target mulch
bed next to the shed, as shown in Table 3. The presence of both fence and mulch generated
firebrands, even when one of the elements had not demonstrated the capacity to ignite spot
fires on its own (e.g., vinyl fence or pine straw mulch). Table 3 summarizes the data from
Table D.3, Table D.4, and Table D.5, where the color key identifies in green thoseexperiments in which the fire spread to the shed. The only fence plus mulch experiments inwhich the fire did not reach the shed (in pink) were Series 1 experiments, which did notinclude a target mulch bed at the base of the shed wall. Only Series 2 experiments were
included in Table 3.

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Table 3. Number of experiments producing spot fires for each fence plus mulch type.

Fence and Mulch Combination

Number ofExperiments With
Spot Fires

Number ofExperiments
Without Spot Fires

Western redcedar privacy fence (WRC) /Shredded hardwood mulch (HWM)

220

Aged western redcedar privacy fence (aged WRC) /Shredded hardwood mulch (HWM)

30Vinyl privacy fence (Vinyl) /Shredded hardwood mulch (HWM)

1

0

Redwood lattice fence (RWL) /Shredded hardwood mulch (HWM)

120

Pine lattice fence (PL) /Shredded hardwood mulch (HWM)

30Good neighbor fence (GN) /Shredded hardwood mulch (HWM)

30

Wood-plastic composite fence (WPC) /Shredded hardwood mulch (HWM)

20

Western redcedar privacy fence (WRC) /Shredded hardwood mulch at half thickness (half HWM)

130

Western redcedar privacy fence (WRC) /Pine bark mulch (PBM)

13

0

Redwood lattice fence (RWL) /Pine bark mulch (PBM)

10Western redcedar privacy fence (WRC /Pine straw mulch (PSM)

70

The times to ignition of the first spot fire as a function of average wind speed of the bottom
four probes along the centerline are displayed in Fig. 110 for the combinations of fence andmulch listed above. In general, the time to ignition decreased with increasing wind speed.
The longest times to spot fire ignition occurred for western redcedar privacy fences withshredded hardwood mulch, although some experiments with this combination spotted very
quickly. Spot fires ignited in less than 15 min for all medium wind speed cases and in lessthan 6 min at high wind speeds, with one exception. The scatter was widest at low windspeeds, with ignition times ranging from 2 min to 34 min.
Figure 111 shows that spot fire ignition times did not depend on separation distance.

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Fig. 110. Time to ignition of first spot fire vs. wind speed for fence plus mulch experiments.

Fig. 111. Time to ignition of first spot fire vs. separation distance for fence plus mulch experiments.

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Times for the ignition of the first spot fire to put flames against the wall and the firstobservation of flames on the wall are plotted in Fig. 112 and Fig. 113, respectively. Bothmeasures show a tendency to decrease with increasing wind speed, in agreement with thetimes to first spot fire ignition. Although spot fires can take 20 min or more to reach the wallwith flames after ignition, most do so within a few minutes. These are typically the spot fires
that are ignited by firebrands deposited near the wall by the wind field.
Finally, Fig. 114 compares the first spot fire ignition times for the three sets of experiments
discussed thus far: fence plus mulch, mulch only, and fence only experiments. This plotshows that the combination of fence and mulch considerably shortens the time to spread thefire through firebrands over mulch alone.

Fig. 112. Time to ignition of first spot fire to put flames against the wall vs. wind speed for fence plus
mulch experiments.

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Fig. 113. Time to flames reaching the wall vs. wind speed for fence plus mulch experiments.

Fig. 114. Time to first spot fire as a function of nominal wind speed for experiments for fences withmulch (red), mulch beds only (blue), and fences only (gold).

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4.3.6. Summary
A fence with mulch at its base is more hazardous than either the fence or the mulch bed
separately. The mulch and fence interact to promote rapid flame spread downwind plus thegeneration of firebrands that can ignite other fuels in the vicinity. The fire behavior depends
on fuel, geometry, and wind. More specifically, in these experiments the fire behavior
depends on the types of fence and mulch, wind speed, and distance from the structure.
These experiments are also informative for a fence with vegetation planted close by or for afence with windblown leaves, needles, and other debris at its base. Any fine fuels adjacent toa fence are likely to interact with it during a fire.
Key findings from this set of experiments include the following.
Fire spread behavior. Adding fine combustible materials to the base of a fence enables fire
spread along the base of the fence, allowing the combination of fence and mulch to act as awick transporting fire along the entire length of the fence.
•

Combining a fence with mulch leads to a fire that is more intense and moves more
rapidly than for either fuel on its own. The mulch carries fire continuously along thelength of the fence. Adding a fence may reduce the time for the fire to reach the endof the mulch bed by a factor of two or more.

•

Flame spread rate varies from one experiment to the next. In general, the flame front
progresses faster with higher wind speed. This is most evident at the lowest windspeed, for which the flame front moves considerably slower than for medium andhigh wind speeds. In all cases, fire continued to spread toward the structure.

•

When wood fences in a single plane (privacy or lattice fences) are ignited at groundlevel only, as in this study, flames typically remain below half of the fence height.
Char patterns reflect the wind field. Flames spread higher on the fence with lower
wind speeds.

•

Stringers slowed the upward fire spread in these experiments by limiting the flame
height on one side of the fence. In a WUI fire, however, they may enhance the fire
spread by providing locations for firebrands to lodge and ignite new fires on thefence.

•

Upward fire spread was fastest in the gaps between boards, aided by radiative andconvective heat transfer between the edges of the boards.

Type of fence. Fire behavior depends to varying degrees on the style (privacy, lattice, goodneighbor) and material (wood, vinyl, wood-plastic composites) of the fence with shredded
hardwood mulch at its base. Because of the intensity of their fire behavior, wood-plastic
composite fences are separated here from the description of other fence types.
•

Wood privacy and lattice fences (western redcedar, aged, redwood, pine) exhibit
similar fire behavior, including fire spread timing, spot fire ignitions throughfirebrands, and char patterns.

•

The flame spread rate in the horizontal direction is similar for all wood fences,including privacy, lattice, and good neighbor fences. Away from the wind field near
the structure, the fire spread from the ignition point to the end of the fence panel in
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2 min to 5 min for high and medium wind speeds and 7 min to 12 min for low windspeeds. All fences carried fire.
•

For wood good neighbor (board on board) fences, radiative and convective heattransfer between the boards connected to alternating sides of the stringer cause theflame to rise to the top of the fence downwind. Flaming combustion was moreprevalent for good neighbor fences than for other wood fences combined with thesame mulch type.

•

With mulch at its base, vinyl privacy fences, including panel, bottom frame, andfence post, blacken and distort along the entire length of the fence. The bottom frame
fails to confine the panel within a few minutes of ignition.

Wood-plastic composite fences. Limited preliminary data indicate that ignition of certain
wood-plastic composite fences can result in high intensity fire behavior. Upcoming
experiments are planned to study further the fire behavior of this category of fence.
•

For one WPC fence type:o The entire fence became engulfed in flames.
o The top and bottom frames distorted and released burning boards, which felland extended the flaming region a distance equal to the fence height of 1.83 m(6 ft) to each side of the fence. Some softened boards fell forward in the wind,extending the flaming region beyond the fence as well.
o Firebrands landing close to the wall of the shed, possibly from the mulch,
ignited spot fires. Many fires in the target mulch bed were ignited by radiationand direct flame contact.
o Smoke coming from the roof of the shed after the experiment indicated that aseparation distance of 1.83 m (6 ft) between a fence and a structure wasinadequate to prevent ignition from this burning fence.

•

For a second WPC fence type:o The fire remained below the halfway point of the fence height.
o Horizontal boards fell out of the frame and burned in line with the fence.

•

More experiments are needed to uncover the reasons (materials, design) for the fire
behavior of WPC fences. The significant difference in energy release between the twocomposite fences tested in this study highlights the need for a fence test method thatcan be used to assess the hazard of the material/design configuration.

Type of mulch. Fire behavior of a fence with mulch along its base varies with the type ofmulch.
•

Reducing the thickness of shredded hardwood mulch from 5 cm to 2.5 cm does not
significantly affect flame spread rate or char patterns on the fence, except at low windspeeds.

•Mini pine bark mulch somewhat slows the flame spread over a western redcedar
privacy fence as compared to shredded hardwood mulch, although the difference intime for the fire to reach the end of the fence panel may be less than 5 min.
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Adding a privacy fence to a pine bark mulch bed results in flames reaching the end of
the mulch pan two to four times faster than for mulch only.

•

Pine straw mulch burns rapidly with high intensity and ignites the fence quickly.
When combined with a western redcedar privacy fence, the fire can reach the end of apanel within 1 min. After the pine straw mulch is consumed, the wood fence
generates firebrands capable of igniting spot fires.

Mitigation. This set of experiments included an exploration of the effects of lifting a fence
above a mulch bed below, as a potential way to decouple the mulch from the fence in a WUI
fire.
•

For shredded hardwood mulch, lifting a fence 15 cm (6 in) above the mulch
decouples the burning behavior of the fence from the mulch between posts. Thisconclusion may not hold for mulches that burn with higher flames, such as pine straw
or rubber mulch, or in the presence of vegetation.

•

The benefits of raising the fence above the mulch may not be realized if a barrier isplaced between them to keep wildlife out or pets in. Combustible debris such asleaves or needles that collect along the barrier will reduce the advantages of thisdesign in a fire.

Firebrand spotting. Firebrands capable of igniting spot fires are generated by all
combinations of fence and mulch tested.
•

Spot fires are consistently ignited from firebrands within a few minutes of mulch andfence ignition.

•

The time to first spot fire ignition decreases as wind speed increases. Spot fires
occurred in less than 15 min in all medium wind speed cases and in less than 6 min inalmost all high wind speed cases.

•

Firebrands that are deposited by the wind field into fine combustible materials next toa structure may result on flames on the wall in a few minutes or less.

•Spot fires may occur after the flame front has reached the end of the fence and mulch
bed combination and the initial flaming combustion has subsided.

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Parallel Fence Experiments
The hazardous fire behavior of two fences parallel to each other was discovered through an
attempt to reconcile observations from three research efforts. Diagonal lattice fence
assemblies exposed to firebrand showers by Manzello et al. [61] became fully involved inflaming combustion after being ignited from the mulch bed at their base. This contrasted withthe fire spread observed for combinations of lattice fence and mulch bed in this study and ina study of fences by IBHS [21, 22], in which the fire remained within the lower half of thefence for ground-level ignitions.
The discrepancy was resolved by changing the configuration of the lattice fence, from
redwood lattice slats attached to two 2×4 stringers on a single side to the double-sided
assembly used in the Manzello experiments. The difference was remarkable, as is shown inthe comparison in Fig. 115. While the fire stayed low to the ground for the single latticefence in video image (a), reaching the end of the fence nearest the structure in about 3 min,the double fence assembly in image (b) was fully engulfed in flame in half that time.

Fig. 115. Comparison of a) single (Test B-79 at t = 3 min) and b) double (Test B-75 at t = 1.5 min)
RW lattice fences in medium wind speeds.

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The existence of two very different burning modes for single and double lattice fences raised
the more general question of whether fire behavior is enhanced for combustible fences thatare parallel to each other. This arrangement may be common within residential communities.
A homeowner may erect a fence close to the property line and adjacent to that of theirneighbor due to aesthetic considerations or to meet differing needs for privacy orconfinement. An example of this is shown in Fig. 12. In addition, fences are often built close
to sheds or other auxiliary buildings. If these configurations are particularly hazardous in a
WUI environment, then it is important to address them as part of a mitigation strategy for
WUI fire reduction.
The selection of parallel fence experiments was based on the exploration of real-world
scenarios that may be of concern. The effects of spacing were studied for parallel western
redcedar privacy fences. WRC privacy fences were also paired with a cement board, a vinyl
privacy fence, and a pine lattice fence to look at the effects of combinations of different
materials and design. Most experiments, including a comparison of redwood and pine latticefences, were carried out under low wind speed conditions. The effects of wind speed were
tested on redwood lattice fences. Several tests were run without mulch to see whether the
mitigating effects of removing all fine combustibles, discussed in Section 4.1, also held forparallel fences. All experiments were performed with separation distances of 1.83 m (6 ft)
from the structure. This placed the parallel fences outside of the effects of wind that dominate
the region close to the structure.
The test matrix for Parallel Fence experiments is shown in Table D.6 and Table D.7 inAppendix D. The details of each experiment, including parameter values, images, flame
spread data, wind plots, and summary values, can be found in Appendix I. In each case,timing was measured from the point after ignition when the fan was turned on.
4.4.1. Example
Figure 116 and Fig. 117 display the time sequence for the development of fire on parallel
western redcedar privacy fences in a bed of shredded hardwood mulch. The faces of theboards were separated by 46 cm (18 in), and the average wind speed along the centerline forthis experiment (Test C-6) was 6.6 m/s (14.9 mi/h). At each displayed time, the experiment isshown concurrently in images from videos recorded by the left and left front cameras.
Insights on the processes contributing to the rapid growth of fire on parallel fences can beobtained by comparing the fire behavior between the panels with the observations on theoutside.
As shown in Fig. 116, for the first two minutes after the upwind end of the fence and mulch
bed were ignited, the flames propagated along the base of the fence in a similar manner as
observed for the single panel WRC fence/HW mulch combination in Fig. 69. The charpattern and flames between the two fence panels and on the outside remained below the firststringer. At 2 ½ min, however, there were signs that the space between the panels washeating up quickly. Along the base of the fence ahead of the flames, there was a line of whitesmoke that indicated that water was being driven out as water vapor or steam, and the left
front view shows that the fires along the base of each panel had merged and increased inintensity. Within another 15 s, white smoke was emerging from the end of the parallel fences,followed shortly by flames, as shown in the image on the left at t = 3 min.

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Fig. 116. Time sequence for parallel WRC privacy fences with shredded hardwood mulch at 46 cm(18 in) spacing and low wind speed [Test C-6], showing first three minutes.

Subsequent development of the parallel fence fire is shown in Fig. 117. After the flamesreached the downwind end of the fence panel, they climbed the inner and outer surfaces,

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exceeding the height of the fence 3 ½ min after the fan had been turned on. The flames thenworked their way upwind between the panels until by t = 4 min the entire fence wasengulfed.

Fig. 117. Time sequence for parallel WRC privacy fences with shredded hardwood mulch at 46 cm(18 in) spacing and low wind speed [Test C-6], for times from 3 ½ min to 6 min.

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4.4.2. Type of Fence
This section compares the fire behavior for parallel western redcedar privacy fences, double
redwood, and double pine lattice fences, in which both fences were identical. These fence
configurations and their motivations were described in Section 2.5.6. Shredded hardwood
mulch was arranged below the fences in each of these cases.
The effects of parallel fence spacing, wind speed, lack of mulch below the fences, anddissimilar fences are described in Sections 4.4.3 through 4.4.5.
4.4.2.1. Parallel western redcedar privacy fence
As observed in the example at the beginning of this section, the space between fence panels
plays a key role in the enhanced fire behavior of parallel WRC privacy fences in HW mulch
beds. After ignition of the mulch bed and fences at the upwind end, the temperature rises inthis space, and radiative feedback and convective heat transfer cause highly nonlinear fire
growth that engulfs the entire fence within minutes. Figure 118 shows the condition ofparallel WRC privacy fences spaced 20 cm (8 in) apart at a time five minutes after the fan
was turned on. As indicated by the arrow, flames extended more than 0.6 m (2 ft) above thefence.

Fig. 118. Parallel western redcedar privacy fences with shredded hardwood mulch at 20 cm (8 in)spacing and low wind speed, at t = 5 min [Test B-82].

4.4.2.2. Double redwood lattice fence
Although the ignition process was different (gas burner rather than firebrand shower), theseexperiments agreed with the observations in Manzello et al. [61] that double lattice fences ina mulch bed become fully involved in a fire. Figure 119 displays a video image of a double

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redwood lattice fence near peak fire intensity, three minutes after the fan was set to a low
wind speed. By its nature, the lattice fence is significantly more porous than a privacy fence.
This may alter the balance of heat transfer and combustion processes, with both enhanced
entrainment through the sides of the fence assembly and reduced isolation of the space
between fences from the surrounding environment affecting the convective heat transfer. For
the double lattice fence, a lattice fence is attached to each side of the stringers. At 9 cm (thewidth of the stringers), the spacing between the lattice fences is considerably smaller than thespacing of 20 cm to 91 cm in the parallel privacy fence experiments in this study, whichchanges the radiative feedback. More research, including analysis and modeling, is needed to
fully understand the physical processes responsible for this fire behavior.

Fig. 119. Double redwood lattice fence with pine bark mulch at low wind speed, at t = 3 min[Test A-103].

4.4.2.3. Double pine lattice fence
Changing the type of wood used for the diagonal lattice from redwood to pine did not make adifference in the rapid fire growth behavior for the double lattice fence. Figure 120 shows apine lattice fence experiment carried out under the same conditions as the redwood latticeexperiment in Fig. 119. At the moment pictured, at 3 min 15 s after the fan had been turned
on, the fire continues to exceed the height of the fence, having already consumed a large
portion of the lattice upwind. Spot fires caused by firebrands have spread across the target
mulch bed at the base of the structure from front to back.

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Fig. 120. Double pine lattice fence with shredded hardwood mulch at low wind speed, at aboutt = 3:15 [Test C-29]. Note spot fires in the target mulch bed at the base of the structure caused by
firebrands.

4.4.3. Fire Spread Behavior
Parallel WRC privacy fences and double RW and pine lattice fences in HW mulch beds were
found to be significantly more hazardous than single fences of the same type with mulch.
Under the tested conditions, the entire fence was engulfed in flames within a few minutes of
imposition of wind. Our understanding of situations that could result in this dangerous fire
behavior was expanded by studying the effects of spacing between the parallel privacy fences
and of doubling the fence length, and the effects of wind speed on fire spread for a double
lattice fence.
4.4.3.1. Effects of fence spacing and length
The first experiment on parallel WRC privacy fences in a bed of HW mulch was designed torepresent a scenario in which two neighbors erected separate fences very close to theproperty line because both neighbors wanted to view the “good side” (i.e., the smooth side)of their fence. The fences were thus arranged with the stringers on the inside and the innersurfaces of the boards spaced 20 cm (8 in) apart. After finding that this configuration led tosevere fire behavior, the next question was how far apart these fences would need to be for
their fire behavior to decouple into that of two individual privacy fences.
Figure 121 shows the fire behavior near maximum flame height for parallel privacy fences atincreasing spacings. For spacings from 20 cm (8 in) to 46 cm (18 in), the flames exceeded
the height of the fence within 3 min to 5 min after ignition. At 61 cm (24 in), in Fig. 121 (d),the time for rapid flame growth to occur increased to 14.5 min. The outer char marks in this152

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final case suggest that the flaming remained below the central stringer until rapid growthoccurred.
In all of these cases, flames extended above the fence by a meter (3 ft) or more. Flames also
extended downstream of the fence panel by at least 0.3 m to 0.6 m (1 ft to 2 ft).
End views for each of these parallel privacy fence cases are shown in Fig. 122. During theperiod of most intense burning shown in these video images, flames attached to the two fence
panels and the mulch burned on the ground throughout the space between. As in the example,
the fire between the two fence panels in each case started spreading low along the fence as it

Fig. 121. Flames extending above and downwind of parallel WRC privacy fences at fence panel
spacings of a) 20 cm at t = 5:00 [Test B-82], b) 30 cm at t = 5:00 [Test C-5], c) 46 cm at t = 4:00
[Test C-6], and d) 61 cm at t = 14:30 [Test C-7].

Fig. 122. Fire behavior of parallel WRC privacy fences at fence panel spacings of a) 20 cm[Test B-82], b) 30 cm [Test C-5], c) 46 cm [Test C-6], and d) 61 cm [Test C-7].

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moved downwind from the ignition point, then moved up the panels at the end nearest thestructure, and then spread back upwind toward the fan along the full height of the panels.
The increased time to reach rapid flame growth for experiment C-7 suggested that a criticalpoint was approaching that might be reached by increasing the panel spacing beyond 61 cm(24 in). The expectation for that critical spacing was that the interactions between the twofence panels would drop below the threshold for severe fire behavior. With a spacing of
91 cm (36 in), the fire in experiment C-8 was indeed confined to the individual fence panels,
as shown in Fig. 123.

Fig. 123. Fire behavior for parallel privacy fences spaced apart by 91 cm [Test C-8] showing a) end
view and b) left side view at t = 18:00.

However, a closer look at the results revealed some differences between the parallel fence
experiment in Fig. 123 and the experiments on single privacy fences under similar
conditions. The final char pattern on the exterior of the parallel fences rose linearly in heightnear the structure, as illustrated by the red line in Fig. 123 (b). In addition, the flames on the
inner surfaces of the fence panels moved backward against the applied wind, generating thediscrete blackened char marks to the right of the red line and above the continuous charpattern closer to the mulch. These observations indicated that the critical panel spacing forseparating the fire behavior for the parallel fences had not yet been achieved, and that the fire
growth for this case may have been limited by the length of the fence.
To test the latter hypothesis, a final experiment was performed with an extra set of panels todouble the fence length at the same parallel fence spacing of 91 cm. In this experiment, theflames over the first (upwind) set of panels were limited in height by the lowest stringer, as
shown in the end and left side views in Fig. 124 from t = 5 min after the fan was turned on.
After another 30 s, however, large flames were visible through the gaps between the boardsof the second (downwind) panel, and by t = 6 min the downwind panel was fully engulfed inflames. The resulting large fire is shown in Fig. 125, with flames extending more than ameter (3 ft) above the fence.
It was not possible to use this experimental setup to investigate spacings between parallel
fences wider than 91 cm because of the limited width of the wind field provided by the single
large fan.

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Fig. 124. Fire behavior at t = 5:00 for two-panel length parallel privacy fences at 91 cm spacing[Test C-13].

Fig. 125. Fire behavior at t = 6:15 for two-panel length parallel privacy fences at 91 cm spacing[Test C-13].

The conclusion from this set of experiments was that fences should not be built parallel toeach other in a WUI environment, even when they are spaced apart by a meter (3 ft). In
addition to providing a conduit for fire, this design introduces a severe flame and radiation
hazard to the community.
A quantitative comparison of the flame spread for parallel WRC privacy fence experiments is
displayed in Fig. 126. The plot shows the closest distance of char on the fence from the wallof the structure as a function of time. The horizontal black lines mark the locations of thefence posts. For experiments with spacings from 20 cm through 46 cm, an initial warmup
period was followed by the fast flame spread rates (on the order of 5 cm/s) that mark the
rapid development of severe fire conditions, with flames extending above the height of thefence. The flame spread was slower for Tests C-7 and C-8, with panel spacings of 61 cm and91 cm respectively. The transition to flame engulfment of the parallel fence in Test C-7 did
not occur until after the flames had traveled the full length of the fence panels, and thus does

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not appear as a steeper slope on this plot. Test C-8, with the largest panel spacing, nevertransitioned to rapid fire growth, although the progress of the fire along the base of the fence
was faster than for Test C-7. The cause of timing differences between Tests C-7 and C-8 was
not clear.
The uncertainties for the plots in Fig. 126 are described in the uncertainty analysis for fencesin Appendix A.4.2.

Fig. 126. Extent of charring as a function of time for parallel WRC privacy fences with spacings
between fence panels from 20 cm to 91 cm.

The experiment for parallel WRC privacy fences with a spacing of 30 cm (12 in) was
repeated at a different time of the year. Fig. 126 shows that the time period before rapid fire
growth was significantly longer for Test C-32, which was carried out under significantly
colder conditions than Test C-5. The average ambient temperature for Test C-32 was 13.6 °C(56.5 °F), compared to 28.8 °C (83.8 °F) for Test C-5. The large difference in initial fire
spread demonstrates the potential effects of the ambient environment, as well as the inherent
variability in these experiments. After the fire started to grow rapidly between the fences,
however, both experiments proceeded in similar ways, with flames engulfing both fences.
Flame spread for the one-panel and two-panel length parallel WRC fence experiments(Tests C-8 and C-13 respectively) are compared in Fig. 127. The panel spacing for bothexperiments was 91 cm (36 in). As can be observed from the plot, the flame spread over the
first panel of the two-panel length experiment (Test C-13) was somewhat faster than the
flame spread over the one-panel set of parallel fences (Test C-8). In Test C-13, the fire tookslightly over 5 min to travel from the ignition point to the center post. Although the plot thenshows a pause in the progress of the charred area on the exterior of the boards, flames in thespace between fences were visible between the boards during this time. After traversing thecenter post, the conditions in the space between the panels led to rapid flame engulfment overthe second set of fence panels, shown in the plot by a steep drop in the flame spread curve. It

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can be speculated that the farther progress of flames along an extended length of parallel
fences would occur in mere seconds. The uncertainties for these plots are described in theuncertainty analysis for fences in Appendix A.4.2.

Fig. 127. Location of char front closest to the wall as a function of time for one- and two-panel length
parallel WRC privacy fences with 91 cm spacing. Black horizontal lines indicate positions of end andcenter posts.

4.4.3.2. Effects of wind speed
The effects of wind speed on parallel fences were studied using double redwood latticefences with shredded hardwood mulch beds. Video images from experiments at low,
medium, and high wind speeds respectively are shown in Fig. 128.
The video images are from times shortly after each fence is engulfed in flames. As shown
here, low wind speeds were observed to result in the highest flames and the longest time to
rapid fire growth. As wind speed increased, the flames were lower on the fence and firedevelopment was faster. At the highest wind speed, Fig. 128 (c) shows that the top few
inches of the fence were not yet involved at this point in the development of the fire. The
high temperature plume may have extended farther downwind than for lower wind speeds,
but this was not measured.

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Fig. 128. Double redwood lattice fences with hardwood mulch as a function of wind speed: a) low
wind (Test A-103 at t = 3:00), b) medium wind (Test B-75 at t = 1:30), and c) high wind (Test B-62
at t = 1:30).

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The flame spread data associated with these experiments are presented in Fig. 129. This setof plots confirms that the fire took the longest time to reach the end of the fence for the lowwind speed, while flame spread in high and medium wind speeds took the shortest time.
Little difference was observed between flame spread plots for double pine lattice fences(Test C-29) and double redwood lattice fences (Test A-103) under low wind speed
conditions. The uncertainties for these plots are described in the uncertainty analysis forfences in Appendix A.4.2.
All double lattice fences combined with mulch beds were fully engulfed in fire within 5 minin these experiments. The speed and high hazard of fire development on double wood latticefences is in agreement with those of parallel WRC privacy fences.

Fig. 129. Location of char front closest to the wall as a function of time for double parallel redwood
and pine lattice fences with HW mulch at various wind speeds.

4.4.4. Parallel Fences Without a Mulch Bed
It is difficult to keep parallel privacy fences free from fine combustible materials. The space
between fences erected along neighboring property lines is subject to the growth of weeds
and the accumulation of leaves, needles, and other debris. In addition, materials may blow
directly into this space from the corner where the fences enclosing each property separate.
Despite the improbability of encountering parallel fences free of debris, looking at thissituation allows us to bound the problem and better understand the mechanisms. The study
included a small number of parallel fence experiments without a mulch bed at the base. In theabsence of the feedback from fine combustibles, a large fire was indeed more difficult toestablish. In all but one case, removing the mulch beneath the parallel fences resulted in slow
charring starting at the point of ignition, similar to the fire behavior for single fences without
mulch described in Section 4.1.

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Closeups near the point of ignition are shown in Fig. 130. The only case in which the firemoved beyond the corner where ignition took place was the double redwood lattice fence at
low wind speed, shown in Fig. 131.
The color key for the matrix of parallel fence experiments in Tables B.5 and B.6 reflects
these findings.

Fig. 130. Parallel fences without mulch: a) western redcedar privacy fence with 15 cm (6 in) spacing[Test B-84], b) redwood lattice fence at medium wind speed [Test B-80], and c) pine lattice fence at
low wind speed [Test C-20].

Fig. 131. Double redwood lattice fences without mulch at t = 4 min with low wind speed [Test A-110].

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4.4.5. Mixed Parallel Fences
Having established that wood privacy and lattice fences erected next to each other exhibit
dangerous fire behavior, the next question was whether a fence built in close lateralproximity to a shed, garage or other type of fence might also present a risk. For an initial lookat this issue, experiments were performed on a WRC privacy fence parallel to a cement
board, a vinyl privacy fence, and a pine lattice fence. All experiments were performed with a
bed of shredded hardwood mulch at the base, under low wind speed conditions, and at
1.83 m (6 ft) separation distance from the shed. Spacings between fence surfaces were either
31 cm (12 in) or 46 cm (18 in).
4.4.5.1. Cement board parallel to WRC privacy fence
Experiments at two different spacings were performed for a noncombustible cement board
placed parallel to a western redcedar privacy fence. (The cement board was the same type asthat attached to the shed wall, described in Section 2.3.) Figure 132 shows three images taken
at the same time from different points of view for an experiment (Test D-7) with 31 cm(12 in) spacing between the fence panel and the cement board. Figure 132 (a) shows thespace between the fence and cement board, and (b) and (c) provide side views of the WRC
privacy fence and the cement board, with gold arrows pointing in the direction of the applied
wind. The cement board was discolored near the burning mulch without further damage. The
video images were captured at t = 6 min, shortly before the experiment was ended when aspot fire igniting close to the shed reached the shed wall.
The char pattern in Fig. 132 (b) rises from the area of ignition toward the end of the privacy
fence and then decreases slightly in height, remaining below the center stringer. This issimilar to the humped char profile shape observed previously for WRC privacy fences
combined with HW mulch. In the discussion in Section 4.3.2, the humped shape of the charprofile was attributed to the wind field, which separates into a lower vortex and a flow streamthat passes over the roof as the air flow approaches the shed. In this case, however, thenearby cement board is expected to cause an asymmetry in the flow field and thermal
environment around the burning WRC fence. In the presence of the cement board, the flame
spread was considerably faster, with flames reaching the end of the fence in 3 min. This
raised the question of whether a larger spacing between the WRC privacy fence and thecement board would give results intermediate between narrow spacing and a single panel
WRC fence, with a similar char pattern and a slower flame spread.

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Fig. 132. Cement board (noncombustible) parallel to WRC privacy fence with 31 cm (12 in) spacing,near end of experiment (t = 6 min), in camera views from a) near the fan, b) left side, and c) right side
[Test D-7]. Arrows indicate wind direction.

In fact, the fire behavior changed when the spacing between privacy fence and cement board
was increased to 46 cm (18 in). Figure 133 shows images from late in this experiment(Test C-15), which ran for 32 minutes before a spot fire reached the shed wall. For thisspacing, the fire did not extend above the level of the first stringer on the WRC privacy
fence, as seen in Fig. 133 (a) and (b). The shape of the charred profile did not display thehump seen for fence and mulch experiments in Section 4.3.2, possibly indicating disruptionof the wind field by the cement board. Figure 133 (c) again showed discoloration of thecement board near the burning mulch on the ground. The wood post, bottom stringer, andHW mulch continued to burn throughout the test.

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Fig. 133. Cement board (noncombustible) parallel to WRC privacy fence with 46 cm (18 in) spacing,near end of experiment, in camera views from a) near the fan, b) right side, and c) left side
[Test C-15]. Arrows indicate wind direction.

The effects of placing the noncombustible cement board parallel to the WRC privacy fence
and changing the spacing are summarized in Fig. 134 and Fig. 135. The similarities in the
humped char pattern for the WRC privacy fence/cement board combination with a spacing of
31 cm (12 in) (Test D-7) and a single panel WRC fence (Test A-102) are apparent bycomparing Fig. 134 (a) and (c), respectively. All experiments were performed under the same
conditions: with hardwood mulch below the fences, in low wind flow, and with 1.83 m (6 ft)
separation distance. The increase in flame spread rate by the addition of the cement board isshown in the plot of char front vs. time in Fig. 135. With the cement board in close proximity
to the WRC privacy fence, the flames reached the end of the fence in 3 min, about a third of
the time for the single panel privacy fence.
Increasing the spacing between WRC fence and cement board to 46 cm (18 in) in Test C-15
limited the char pattern over the wood fence to the lowest stringer, as seen in Fig. 134 (b) andslowed the flame spread. For this test, the fire reached the end of the western redcedar
privacy fence about 11 min after the fan was turned on. As shown in Fig. 135, this was on thesame order as the single panel WRC fence, although the char pattern suggests differences in
the mechanisms of spread. The complexities of flame spread for a combustible fence next toa noncombustible fence or wall, with asymmetries in the thermal and wind environments on
the free and confined sides of the combusting fence, deserve further investigation.

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Fig. 134. Comparison of char patterns near the end of experiments with parallel WRC fence / cement
board at a) 31 cm (12 in) spacing [Test D-7] and b) 46 cm (18 in) spacing [Test C-15] and c) single
WRC panel [Test A-102]. Arrows indicate wind direction.

Fig. 135. Extent of charring as a function of time comparing parallel WRC fence/cement board
combination at 31 cm (12 in) and 46 cm (18 in) spacings to single panel WRC fence.

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The uncertainties for the plots in Fig. 135 are described in the uncertainty analysis for fencesin Appendix A.4.2.
4.4.5.2. Vinyl privacy fence parallel to WRC privacy fence
Figure 136 shows the burning characteristics for a WRC privacy fence arranged parallel to avinyl privacy fence with a spacing of 46 cm (18 in) (Test C-16). Photo (a) was taken about
21 min after the fan was turned on, and photos (b) and (c) were taken near the end of theexperiment, which was allowed to run for 78 min before it was ended. A spot fire separately
ignited the WRC fence near the downwind post at about t = 16 min, but no spot fires weredetected in the target mulch bed. Like the WRC privacy fence/cement board combination at
the same spacing described in the previous section, the char marks on the WRC privacy fence
panel were unusual compared to other burn patterns seen throughout this study, in that thefire did not extend above the bottom stringer except close to the point of ignition. The
burning mulch caused the vinyl fence to distort at its base, leaving the boards without supportand eventually causing them to fall out of the frame, as shown in Fig. 136 (c).

Fig. 136. Vinyl privacy fence parallel to western redcedar privacy fence near end of test in camera
views from a) near the fan, b) right side, and c) left side [Test C-16].

The continuous flame spread was slow for this combination of fences, taking 27 min to reachthe end of the western redcedar privacy fence. This was more than double the flame spreadtime for a single WRC privacy fence combined with hardwood mulch under the same
conditions, as shown in Fig. 137. This was also double the time for the flames to reach theend of the WRC privacy fence/cement board combination discussed in the previous section.
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Fig. 137. Flame spread comparison of parallel WRC fence/vinyl fence combination at 46 cm (18 in)spacing to single panel WRC fence.

The uncertainties for the plots in Fig. 137 are described in the uncertainty analysis for fencesin Appendix A.4.2.
4.4.5.3. Pine lattice fence parallel to WRC privacy fence
A western redcedar privacy fence was paired with a pine lattice fence at spacings of 31 cm(12 in) and 46 cm (18 in). With 31 cm spacing (Test D-8), this combination of fences
behaved similarly to parallel WRC fences, as shown in Fig. 138. The fire grew rapidly overboth fences until flames extended above the top of the privacy fence and beyond the end of
the fences. The images in Fig. 138 show the state of both fences at t = 2:15, when the flames
were longest and filled the space between the two fences.

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Fig. 138. Pine lattice fence parallel to western redcedar privacy fence with 31 cm (12 in) spacing, at
maximum flame length (t = 2:15), in camera views from a) near the fan, b) left side, and c) right side
[Test D-8].

When the spacing was increased to 46 cm (18 in) (Test C-17), the fire behavior for theparallel WRC privacy fence / pine lattice fence agreed with the observations for each fence
burning individually under similar conditions. Figure 139 shows the burning characteristicsof the parallel fences shortly before the experiment ended at t = 15 min. The burn patterns forboth fences remained below the halfway point during the experiment and rose at the end
closest to the structure. The patterns can be compared with Fig. 74 (d) and (h) for WRC
privacy and redwood lattice fences respectively, under the same conditions of a HW mulch
bed at low wind speed and 1.83 m (6 ft) separation.

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Fig. 139. Pine lattice fence parallel to western redcedar privacy fence with 46 cm (18 in) spacing,near end of test, in camera views from a) near the fan, b) right side, and c) left side [Test C-17].

For a more concise look at the effects of spacing with mixed privacy and lattice fences,
Figure 140 compares images from the WRC fence side of these two experiments with animage from a single panel WRC experiment performed under the same conditions. Similarly,
Fig. 141 compares images from the pine lattice fence side with a single panel pine latticeexperiment. In each case, the similarity of the parallel fence case with 46 cm (18 in) spacing
compared with the single panel case is evident. The plots of maximum char extent as afunction of time in Fig. 142 show similar flame spread behavior for the 46 cm (18 in) spacingcase compared with single panels, while the flame spread is much more rapid for the 31 cm(12 in) case. The uncertainties for these plots are described in the uncertainty analysis forfences in Appendix A.4.2.
For this combination of fences, therefore, the evidence supports the conclusion that a spacingof 46 cm (18 in) was sufficiently large to decouple the fire behavior.

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Fig. 140. Comparison of flame spread patterns for WRC privacy fence parallel to pine lattice fence at
spacings of a) 31 cm (12 in) [D-8] and b) 46 (18 in) [C-17] with c) single panel WRC privacy fence[A-102]. Arrows indicate wind direction.

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Fig. 141. Comparison of flame spread patterns for WRC privacy fence parallel to pine lattice fence at
spacings of a) 31 cm (12 in) [D-8] and b) 46 (18 in) [C-17] with c) single panel PL fence [C-21].
Arrows indicate wind direction.

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Fig. 142. Flame spread comparison of parallel WRC privacy fence/pine lattice fence combination at
31 cm (12 in) and 46 cm (18 in) spacing to single panel WRC privacy fence and pine lattice fence.

4.4.6. Firebrand Spotting
Almost all parallel fences combined with mulch resulted in spot fires in the target mulch bed
next to the shed, as listed in Table 4. The single exception was the vinyl fence paired with awestern redcedar privacy fence, which burned in low wind speeds for over 75 min without aspot fire ignition. This table summarizes the information about fire spread to the shedthrough firebrands from parallel fence experiments with mulch in Table D.6 and Table D.7,
where a dark green background indicates flames above the fence, light green indicates spreadto the shed through spot fires, and pink indicates no spot fires. All experiments with flames
extending above the fence generated spot fires.

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Table 4. Number of experiments producing spot fires for each combination of parallel fences withmulch.

Parallel Fence and Mulch Combination

Number ofExperiments With
Spot Fires

Number ofExperiments Without
Spot Fires

2 × Western redcedar privacy fence (WRC) /Shredded hardwood mulch (HWM)

80

2 × Redwood lattice fence (RWL) /Shredded hardwood mulch (HWM)

20

2 × Redwood lattice fence (RWL) /Pine bark mulch (PBM)

10

2 × Pine lattice fence (PL) /Shredded hardwood mulch (HWM)

10

Western redcedar privacy fence (WRC) /Cement board /Shredded hardwood mulch (HWM)

20

Western redcedar privacy fence (WRC) /Vinyl privacy fence /Shredded hardwood mulch (HWM)

01

Western redcedar privacy fence (WRC) /Pine lattice fence (PL) /Shredded hardwood mulch (HWM)

20

The times to ignition of the first spot fire are plotted as a function of wind speed in Fig. 143.
Only two experiments were performed at medium and high wind speeds; although the times
to ignition for both were short (less than 2 min), the relationship with wind speed for parallel
fences cannot be determined from these experiments. Spot fires ignited in less than 12 min in
all cases.
All parallel fence experiments were performed at a separation distance from the shed of
1.83 m (6 ft), so no data exists to determine the effects of separation distance.
Times for the ignition of the first spot fire to put flames against the wall and the first flames
to reach the wall are plotted in Fig. 144 and Fig. 145, respectively. With a single exception,
flames reached the wall within a few minutes of first spot fire ignition.
Figure 146 compares spot fire ignition times for the parallel fences with mulch to the single
fences with mulch discussed in Section 4.3.5. As a category, firebrands from parallel fences
ignited spot fires in the target mulch bed more quickly. This is reasonable, since the large
area on fire in these cases may increase the rate of firebrand generation over that of the single
fence that is burning only low to the ground, and the higher fire intensity may loft more
firebrands from both mulch and fence into the vortex that carries them toward the shed wall.

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Fig. 143. Time to ignition of first spot fire vs. wind speed for parallel fence plus mulch experiments.

Fig. 144. Time to ignition of first spot fire to put flames against the wall vs. wind speed for parallel
fence plus mulch experiments.

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Fig. 145. Time to flames reaching wall vs. wind speed for parallel fence plus mulch experiments.

Fig. 146. Comparison of times to ignition of first spot fire between single fences and parallel fenceswith mulch as a function of wind speed.

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In the absence of mulch beneath the fence, all three double redwood lattice fence
experiments resulted in firebrand spotting. This included the case at low wind speeds thatdeveloped large flames and experiments at medium and high wind speeds that showed little
flame spread beyond the area of ignition. There was little spread over either the parallel
WRC privacy fence or double pine lattice fence, and no spot fires were observed in theseexperiments. Table 5 summarizes this information, which relates to the parallel fence
experiments without mulch listed in Table D.6 and Table D.7.
Table 5. Number of experiments producing spot fires for parallel fences without mulch.

Parallel Fence Combination

Number ofExperiments With
Spot Fires

Number ofExperiments Without
Spot Fires

2 × Western redcedar privacy fence (WRC)01

2 × Redwood lattice fence (RWL)

30

2 × Pine lattice fence (PL)01

The times to first spot fire ignition for all experiments without mulch that produced spot fires
are plotted in Fig. 147 as a function of wind speed. The plot includes the three double
redwood lattice fences discussed in this section as well as the fence only experiments
discussed in Section 4.2.4. When spot fires occurred in cases without mulch present, they
took anywhere between 2 min and an hour to ignite. No relationship with wind speed wasobserved.

Fig. 147. Time to first spot fire as a function of nominal wind speed for experiments on single (blue)
and parallel (green) fences only (without mulch) in which spotting occurs.

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4.4.7. Discussion
This set of experiments showed that wood fences erected parallel to each other result in amode of fire behavior that is much more dangerous than that for single fences. In this mode,the flame spread suddenly accelerates, leading to large flames that engulf the fence andthreaten nearby structures through direct flame, radiation, and enhanced convection, as wellas firebrands that are lofted into the air. Similar behavior was found for western redcedar
privacy fences spaced up to 91 cm (3 ft) apart and for double redwood and pine latticefences, with a lattice attached to both sides of a two-by-four wood beam.
Parallel fences exhibit eruptive fire behavior, which is described as a sudden change in therate of fire spread and in energy release within a very short time period [62]. The fences
exchange thermal radiation between the surfaces, and the heat feedback is enhanced byconvective heat transfer through the interior space between the fences. The applied windtransports the hot air downwind from the point of ignition, and buoyancy carries the hot air
upward to preheat the upper parts of the fences. After the entire downwind end of the fence
has ignited, the fire spreads backwards within the interior space.
It was initially postulated that the entrainment of air through the highly porous sides of thelattice fence may be an important factor in the fire behavior of parallel fences (i.e., a chimneyeffect). However, the experiments on WRC privacy fences showed that this was not the case.
The gaps between the boards of the privacy fences may allow some air flow, but it seems
more likely that the fire behavior results from a combination of radiation between the parallel
planes and convection of gases from the hot plume downwind and rising (through buoyancy)
lengthwise through the space between the fences. Improved understanding of this fire
behavior will shed light on other fence designs, such as good neighbor fences, that may also
demonstrate inherently hazardous fire behavior.
In addition to the privacy and lattice fences, with sides that face each other directly, it shouldbe mentioned again that the good neighbor fence, whose boards are offset, demonstrated
some of the characteristics of the fire behavior of parallel fences. The discussion in
Section 4.3.3.6 observed that the flames spread upward on the good neighbor fence andreached the top of the fence within 14 min. This differed from the flame spread over othersingle wood fence panels in a mulch bed, which was generally confined to the lower half ofthe fence after ignition at the base.
The rapid progress of flames along a set of parallel fences can be expected to be much fasteronce the fire has been established. Multiple ignitions by firebrands along the base of thefence or igniting debris between the fences would make this an even faster more hazardous
feature.
4.4.8. Summary
Findings from this set of experiments include:
Fire spread behavior. Parallel combustible fences with fine combustible material at the baseare a highly hazardous configuration for WUI fires, allowing rapid development of large
fires.

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Fire behavior of parallel wood fences is greatly enhanced compared to that for asingle line of fencing. The behavior is eruptive, undergoing large changes in fire
spread and energy release in a short time.

•

Parallel wood privacy fences and double wood lattice fences can be engulfed in flame
within a few minutes of ignition.

•

A large fire occurs even when wood privacy fences are separated by 91 cm (3 ft).
A safe spacing distance between parallel combustible privacy fences was notdetermined from this set of experiments.

•

The hazard of parallel fences increases with length. Two-panel-long parallel privacy
fences spaced apart by 91 cm (3 ft) showed eruptive fire behavior that was notobserved for one-panel-long parallel fences under the same conditions. After thermal
conditions between the fences have intensified due to the initial ground fire between
the panels, flames engulf the parallel fences in 30 s to 1 min. It can be speculated thatthe farther progression of flames along an extended length of parallel fences would
occur in mere seconds, and that multiple ignitions along or between the fences in a
WUI fire would make this an even faster and more hazardous event.

•

Low wind speeds result in the highest flames. The time to rapid fire growth was
longer by about a minute for low compared to medium or high wind speeds.

•

The difference between the fire behavior of single and parallel fences is a good
example of the need to test not only materials but assemblies for accepted use in WUIenvironments.

Type of fence. Similar fire behavior was observed for parallel wood privacy fences anddouble wood lattice fences.
•

Parallel WRC privacy fences and double redwood and pine lattice fences showedsimilar fire behavior, with a fire ignited at the base moving downwind along the
ground for a meter or so and then quickly rising to envelop the entire height of thefence panel, with large flames extending above and beyond the fence.

•

The time for parallel WRC privacy fence panels at a spacing of 46 cm (18 in) or lessto become fully engulfed by flames was as little as 4 min. The time for flames toengulf redwood and pine lattice fences separated by stringers 8.9 cm (3.5 in) thick
was even less at 1.3 min to 2.5 min. It is unclear whether this difference is influenced
more by the design (lattice vs boards) or the spacing between fence panels.

•

Changing the type of wood used for the diagonal lattice from redwood to pine did not
make a difference in the rapid fire growth behavior for the double lattice fence.

Mixed parallel fences. A WRC privacy fence mounted next to a noncombustible cement
board or a vinyl privacy fence in a HW mulch bed did not display the eruptive fire behaviorof parallel wood fences. The severity of the fire behavior for a WRC privacy fence next to apine lattice fence depended on spacing.
•

Eruptive fire behavior was not encountered for a WRC privacy fence mounted next toa cement board or a vinyl privacy fence at spacings of either 31 cm (12 in) or 46 cm(18 in).

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A WRC privacy fence separated from a noncombustible cement board by 31 cm(12 in) resulted in a char pattern similar to that of a single panel WRC privacy fence,
although flame spread was about three times faster.

•For a spacing of 46 cm (18 in), the fire behavior was less intense for a WRC privacy
fence in combination with either a cement board or a vinyl privacy fence, ascompared to a single panel WRC fence. The char pattern remained below the bottom
stringer and the flame spread was at the same rate or slower.

•

A WRC privacy fence next to a pine lattice fence showed eruptive behavior for 31 cm(12 in) spacing. For 46 cm (18 in) spacing, the char patterns on each fence weresimilar to those for the individual fences on their own.

•

Further research is needed to fully investigate the possible fire hazards of combustible
fences built next to auxiliary structures or to other fences.

Parallel fences without mulch. Keeping the base of parallel fences and the space between
them free of fine combustibles would be a challenging maintenance task, and this is notconsidered to be a viable mitigation approach.
•Without fine combustibles at the base, a large fire is less likely to develop overparallel or double fences.

•

At low wind speeds, large flames developed over the double redwood lattice fenceeven in the absence of mulch.

Firebrand spotting. Firebrands capable of igniting spot fires are generated by all
combinations of fence and mulch tested.
•

Spot fires are ignited within a few minutes of mulch and fence ignition.

•

No parallel fence experiment (other than the vinyl/WRC one) took longer than 12 min
to ignite the target mulch bed. This is not surprising, given that the fences were
engulfed in flame. This could have both increased the rate of firebrand generation
over the single fence that is burning only low to the ground and lofted the firebrands
higher due to the energy in the plume. Higher firebrands are expected to travel farther
in the same wind field and thus ignite more fires downwind.

Opportunities for model development: There is a need to better understand the physics
behind the fire behavior of parallel fences in order to identify certain high hazard designs to
be avoided in WUI areas. The results in this section may be helpful in validating a physicsbased fire model, including:•Fire behavior of parallel fences compared to single fences.

•

Dependence on parallel fence spacing for time at which flames engulf the fences.

•

Dependence of fire behavior on parallel fence length, including the spacing for which
a second panel length results in explosive fire growth, and the time for fire to spreaddown a long fence.

•

Lack of hump in char pattern for a wood privacy fence parallel to a noncombustible
or vinyl fence, and its relationship to the wind field.

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Long-Range Firebrand Experiments
A final question addressed by this work was the ability of firebrands from landscaping
elements to ignite combustible objects far from the burning source. This was considered inthree experiments in which the shed was removed and the burning source placed far from thetarget mulch bed.
4.5.1. Experimental Design
The long-range firebrand experiments were performed at the Frederick facility described in
Section 2.1, in an orientation along the pond that allowed for maximum distance downwind.
The applied wind field was directed from the SSE at an angle of 148° ± 1°. The structure was
not included in this test setup. The target mulch bed was situated at a distance 50 m (164.5 ft)
from the upwind edge of the burning object (firebrand source), with the wind machine
propeller 3.7 m (12 ft) farther upwind. The surface between the source and the target wasasphalt and concrete, representing a worst case scenario due to the ease of transport offirebrands over the ground. Roads and driveways make this a realistic condition for WUIneighborhoods. Since the purpose of the burning object in these experiments was to serve asa source of firebrands, the wind speed was varied, with lower wind speeds to promote flame
growth at the burning source and higher wind speeds (up to about 15 m/s) to promote
firebrand transport toward the target mulch bed. The wind machine was shut off or set to idle
at times during each experiment to permit replacement of burned mulch in the target bed.
The bidirectional probe array was used to monitor wind speeds upwind of the burning source.
Over the long distance from the wind machine to the target mulch bed, the applied wind field
expands and diffuses, decreasing the wind speed at the target. The wind speed levels reported
in this section were measured near the fan. No measurements were taken to quantify the windspeed near the target mulch bed.
4.5.2. Firebrand Spotting
Firebrand sources for the three long-range experiments were a double redwood lattice fence
and two beds of shredded hardwood mulch. Figure 148 and Fig. 149 provide views from the
double lattice fence and one of the hardwood mulch beds, respectively, looking toward thetarget mulch bed in the distance.
In both cases, spot fires ignited in the target mulch bed located about 47 m (155 ft) from thefirebrand source within 3 min after the wind machine was set to a high wind speed.

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Fig. 148. Double lattice fence experiment without a structure and with a target mulch bed situated47.6 m (156 ft) from the downwind end of the fence [Test C-3].

Fig. 149. Shredded hardwood mulch experiment without a structure and with a target mulch bed
situated 47.0 m (154.5 ft) from the downwind end of the mulch bed [Test C-2].

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Images from Test C-3 are presented in Fig. 150. In Fig. 150 (b) and (c), the double redwood
lattice fence is shown at t = 7 min and t = 10 min after the fan was started. For thisexperiment, the fan was alternated between low and medium wind speeds for about 7 ½ min,during which time no ignitions were observed. The fan was then increased to a high windspeed. A little more than 2 min later, a spot fire ignited in the target mulch bed 47.6 m(156 ft) downwind from the double fence. Figure 150 (a) shows this spot fire at t = 10 min, at
which time the double lattice fence had fully collapsed, as seen in (c).

Fig. 150. Spot fire ignition resulting from double redwood lattice fence, showing a) the target mulch
bed at t = 10 min, after 2 min at high wind speed, and the double lattice fence at b) t = 7 min andc) t = 10 min after the fan was turned on [Test C-3].

Figure 151 and Fig. 152 show the source and target mulch beds during Test C-2. For thisexperiment, the fan was turned on to a high wind speed and held there for 6 ½ min. Two spot
fire ignitions were identified during this period, with smoke appearing at about t = 5:40 and
t = 6:15. Figure 151 shows the state of the source mulch bed and the spot fire in the target
mulch bed during this period, at t = 6 min. The spot fires were then extinguished andreplaced with fresh mulch, and the fan was set to a medium wind speed for 4 min. During
this time, no ignitions were observed. At t = 13:40, the fan speed was increased again to
produce a high wind speed, and several spot fires ignited between 1 min and 2 min later.
Photos of the source and target mulch beds at t = 16 min are shown in Fig. 152.

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Fig. 151. Spot fire ignition in a) target mulch bed resulting from b) burning shredded hardwood mulch
at t = 6 min, with high wind speed [Test C-2].

Fig. 152. Spot fire ignitions in a) target mulch bed resulting from b) burning shredded hardwood
mulch at t = 16 min, after 2 min at high wind speed [Test C-2].

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Many of the spot fires occurred in the middle of the target mulch, indicating that some
firebrands were lofted rather than simply moving across the ground. To test this idea, an
experiment was carried out in which the target mulch bed was elevated above the ground by
20 cm (8 in). Figure 153 shows the setup for this experiment from the point of view of theraised target, with the source mulch bed and the wind machine in the background. The target
mulch bed was placed 38.1 m (125 ft) from the downwind end of the mulch bed. The fan was
initiated at a high wind speed, and the spot fire seen in Fig. 154 appeared 4 min later.

Fig. 153. Setup for shredded hardwood mulch bed without a structure and with an elevated target
mulch bed situated 38.1 m (125 ft) from the downwind end of the mulch bed [Test D-1].

Fig. 154. Spot fire ignition in a) target mulch bed resulting from b) burning hardwood mulch att = 6 min [Test D-1].

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As an example of the universality of these findings, ignitions in a target mulch bed were also
obtained from a stacked woodpile of maple firewood. In this case, the target mulch bed was
26.8 m (88 ft) from the upwind end of the woodpile, and the wind machine propeller was
3.4 m (11 ft) farther upwind. The woodpile was 1.2 m (4 ft) long. The images in Fig. 155 (a)and (b) are from target mulch bed and woodpile videos respectively, at about 20 min after
ignition of the woodpile and 3 min after the wind machine has been set to a high wind speed.
At this time, two spot fires are well-established in the target mulch bed, the first having
ignited within 2 min from the start of high wind speeds. When the wind speed was reduced tomedium levels, no further ignitions occurred.

Fig. 155. Woodpile experiment without a structure and with a) a target mulch bed situated 25.6 m(84 ft) from the downwind end of b) the woodpile [Test C-1].

4.5.3. Summary
Findings from this set of experiments include:
Firebrand spotting. Ignition of spot fires was demonstrated from firebrands transported by
the wind over long distances.
•

Firebrands from fences and mulch are capable of igniting spot fires in combustible
material located at least 47.6 m (156 ft) from the burning item under high windconditions and over a paved surface. Spotting may have occurred at greater
downwind distances, but this was the maximum distance evaluated at the test site.
Even at this distance, these experiments demonstrate the spotting potential of theselandscape fuels both within and beyond residential parcels.

•

Some firebrands capable of igniting spot fires are lofted in high wind conditions.

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Discussion
Hazardous Scenarios
The fire behavior found for combinations of fences and mulch in this study can be applied tofire codes/standards and best practices for communities.
Highly hazardous fire behavior was found for parallel fences and one type of wood-plastic
composite fence. Fire spread rates varied with fence design, wind speed and the presence orabsence of mulch; however, all combustible fences spread fire and burned downwind. In
large WUI incidents, first responders may not be available to suppress these fence fires formany hours. While rates varied, during WUI events even relatively small fire spread rates
can bring the fire to the structure and/or other combustibles before defensive actions by first
responders can stop possible structure ignitions. Fences can add as a ladder fuel to carryflames from the ground to combustible items at higher levels such as eaves, roofing, or tree
branches. Multiple ignition points along the fence and the presence of nearby vegetation will
worsen the hazard.
Fences may impact egress. High fire hazard fences create a line of flames that can renderadjacent pathways impassable. This study identified one high hazard fence whose boards fellout of a frame and continued to burn on the ground, creating a linear flaming obstacle whose
width was twice the height of the fence. The fences tested were 1.8 m (6 ft) tall. Taller fences
than those tested will likely result in wider impassable paths.
A more detailed discussion on implementing guidance based on the relationships among fuellayout, fire hazard, and structure hardening can be found in the recently published NIST
report entitled WUI Parcel/Structure/Community Fire Hazard Mitigation Methodology(HMM) [16].
Some sample ignition scenarios are shown in Fig. 156, which is reproduced from the HMM
report. This figure illustrates some ways in which combustible fences and mulch beds may
transport fire to multiple residences in a neighborhood. The illustrated scenarios combine thisexperimental study with data collected by NIST from the Camp Fire study [14]. These are
only a few examples; many other pathways involving combustible fences and mulch may beconstructed and are seen in the field.
Four different ignition pathways are illustrated in this figure. The wind is blowing from rightto left.
•

In scenario A, a firebrand ignition occurs in windblown leaf debris against a
combustible privacy fence. The fire spreads downwind and gains intensity (A1),
acting as a ladder fuel to carry the fire into the branches of a nearby tree. Once thetree is ignited, the fire quickly spreads to the eaves of Residence 1 (A3), even though
this home is separated from adjacent parcels by a noncombustible steel chain linkfence. This scenario was observed during the Camp Fire [14] and resulted in thedestruction of the residence, highlighting how fire hazards from a neighboring parcel
can impact the adjacent property and result in residential loss.

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Fig. 156. Illustration of numerous fire spread pathways among neighboring parcels via linear featuresand other combustibles. Inset photographs are from field observations and experiments.
Reproduced from [16], Fig. 10.

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In scenario B, ignition of accumulated leaf litter supports a fire spreading along the
combustible privacy fence outlining adjacent properties. The fire spreads along
path B2, as discussed in Section 4.3, toward a combustible shed. When the fire
reaches the shed, the narrow space between the fence and shed results in rapid fire
growth of large flames (B3), similar to the parallel fence experiments described in
Section 4.4. The resulting shed fire then ignites Residence 3 along path B4, throughdirect flame and radiation. This shed-to-structure ignition path is currently being
studied at NIST [63, 64].
The fire along the fence from ignition B continues to spread toward Residence 3 onpath B6 and Residence 2 on path B7. This results in a second point of ignition for
Residence 3, which is already on fire due to the burning shed. Luckily, Residence 2 isdefended by first responders, who remove the fence at the point where it connectswith the house. The associated photo shows a documented example from the CampFire in which such an intervention prevented fire from reaching the residence.

•

In scenario C, two neighbors have erected combustible privacy fences on theirrespective properties, with mulch beds or other ground litter on each side. Leaf litterand other debris have also accumulated in the narrow space between these parallel
fences. As we know from Section 4.4, this is a rapid growth scenario that quickly
results in large flames. The fire continues along path C3 and provides a third ignition
point on Residence 3.

•

Finally, scenario D describes the structure-to-structure ignition of Residence 4 bynearby Residence 3, which is now fully involved in fire. This scenario has previously
been studied at NIST [65].

These scenarios describe structural ignitions from direct fire exposures in moderate to highdensity WUI residential communities and illustrate the critical role combustible fences canplay in carrying fire between and within parcels. If the combustible fences in this illustration
are made out of wood, they will also generate firebrands, causing further ignitions and likely
threatening additional structures far downwind. Noncombustible fences, such as thosecomposed of masonry, concrete or metal, will not generate firebrands or spread fire between
and within parcels. However, maintenance is necessary to keep all fences free of leaf litterand other combustible debris that can accumulate next to any obstacle and itself create a fire
hazard.
Field observations from NIST WUI case studies and the NIST experiments have identified
the need to restrict the presence of high fire hazard fences in high density WUI, to spatiallysegregate fuels to prevent direct flame propagation, and to limit the nonlinear effects ofmultiple fuels in contact with each other. Solutions to reduce the structure ignition hazards
posed by high fire hazard fences include but are not limited to:‒Best practices guidance for homeowners‒

Homeowners Association (HOA) requirements on placement of high fire
hazard fences‒

County or State regulations on the placement of high fire hazard fences‒

National or international codes and standards

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More information on these approaches may be found in the HMM report [16].
Limitations
This study was a survey of the fire behavior in wind of a variety of combustible fences andmulches near a structure. It illuminated the differences in behavior for selected materials andfence designs and demonstrated certain trends. An understanding of the limitations of thiswork will help to direct its applications and to plan additional research to improve ourknowledge. The scenarios studied in this report did not include multiple ignition points along
the fence or mulch bed, the presence of nearby vegetation, sloping terrain, or other factorsthat can increase the ignition propensity, rate of fire spread, and fire intensity. Theseexperiments may therefore underpredict the fire hazard associated with fences in actual WUI
fires.
A small number of common fence types and mulches were considered in this study. In
communities, there are infinite combinations of fences, mulches, landscape designs,
orientations, proximity to other fuel sources, and ambient conditions. Continuing attention to
fire pathways observed in WUI fires and development of a standard fire test focused on
fences will help to identify other situations that should be addressed.
Some other limitations of this research include:
Combinations of fuels were limited: The experiments looked at the hazards associated withcertain combinations of fences and mulch that may be found in the WUI. Many materials andfence designs were not included in the study. The experiments also did not capture the hazard
from fences placed near other fuels like woodpiles or vegetation or fences running parallel to
a combustible shed. Some experimental configurations were not tested because of concerns
for extensive flaming that could threaten the survivability of the shed and personnel safety –in some cases, the shed had to be actively cooled to prevent ignition.
Few experiments were repeated: Most experiments were performed once. The ability toquantify the effects of the parameters in this study was severely limited by the sparsity of
replications, the stochastic nature of fire phenomena, and the variables that could not be wellcontrolled, such as ambient conditions. The analysis of these data was therefore focused on
uncovering trends and on discovering different modes of behavior, rather than on quantitative
results.
Distance downwind was limited for long-range spotting study: At high wind speeds around
14 m/s (31 mi/h), firebrands from burning fences and mulch caused spot fire ignitions in amulch bed up to 47.6 m (156 ft) downwind. This was the longest distance that could be tested
at the test site and therefore likely underrepresents the spotting potential.
Fuels were ignited at a single location on or near the ground: The test protocol provided
repeatable conditions to characterize fire performance and identify both dangerous andpotentially desirable attributes of landscape features. In WUI fires, however, firebrands canignite fences, mulch, and leaf or needle debris at multiple locations on the ground and at
higher locations along fence stringers. It is likely that this would result in a faster spreading
fire. Burning vegetation can ignite fences at locations above the ground.

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Ignition was by gas burner rather than a natural source: In these experiments, the fence
and/or mulch bed were ignited at a single point at ground level by a gas burner. The method
differs from natural ignition in WUI fires in multiple ways. A gas burner is a severe ignition
source, igniting by continuous flame contact and differing in heating rate and geometricextent from most natural ignition sources in a WUI fire.
The orientation of wind to the structure wall was limited: The experiments documented inthis report were conducted with the shed wall perpendicular to the fence and mulch bed.
Alignment of the fence with the wind was found to maximize fire spread rate in a previous
fence study that did not include a structure [23]. The addition of a structure wall at rightangles to the imposed wind produced a wind field with a recirculation zone at the base of thewall, which influenced both firebrand trajectories and flames. Reorienting the structure at anangle to the wind is expected to reduce the recirculation zone and enhance the flow aroundthe structure. This would increase the flame spread rate nearing the structure. It could also
shorten the time for firebrands to ignite spot fires at the base of the wall, change the spread ofspot fires toward the wall, and affect the flame length and plume flow near the structure forlarge fires.
The mulch was preheated by heat conduction through the steel pan: The use of steel pans tohold the mulch and/or fence being tested introduced a heat transfer mechanism that does notcontribute to fire spread under real conditions. The thermally conductive metal pans
evaporated water and preheated the mulch ahead of the fire front. Soil has a much lower
thermal conductivity, for which preheating is negligible. White smoke or steam downwind of
the flame front was often observed and was particularly noticeable in pine bark mulch
experiments.
Accumulation of windblown debris was not considered in Fence Only experiments: This
study considered the fire behavior of fences free of fine combustible mulch at their base, as
an ideal. These fence experiments likely underestimate the hazard associated withcombustible fences. Catastrophic WUI events most frequently occur during windy
conditions. During such an event, a fence that extends to the ground, even if kept clear andwell-maintained, can accumulate windblown debris at its base. If this occurs, thecombination of fence and combustible materials at its base may ignite and generatesignificant flame and firebrand exposures.
Effects of terrain were not studied: The experiments documented in this report wereconducted on level ground. Terrain may significantly impact fire development. In general,
fire spreads upslope faster than downslope. The interaction of terrain with wind can greatly
affect fire spread, flame lengths and ember generation, transport, and ignitions.
Smoke toxicity was not included: This study was focused on fire behavior and did not include
smoke toxicity as a factor in the total hazard from burning fences or mulch.

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Conclusions
Combustible fences and mulch provide pathways by which wildland-urban interface (WUI)
fires may reach and potentially ignite structures in a community. Once ignited, these fuels
become sources that may ignite nearby objects through radiation, direct flame contact, andfirebrands. It is important to understand the mechanisms by which these combustible
landscaping elements can transport fire to a home in order to find ways to address the risk.
Such knowledge helps with proactive design within the community. It informs homeowners
on what they can do to protect themselves and their properties. It also helps fire departments
to plan defensive strategies, placing resources and assigning tasks where they will be themost effective. The goal is to enhance the safety of members of the public and first
responders.
This section pulls together the main findings of this report and presents the primary
recommendations. General findings on fire spread for fence/mulch combinations arefollowed by findings categorized by very high, high, medium, and low hazard. Organizing
the findings in this way suggests how mitigation may be prioritized in order to reduce thepotential fire exposure of nearby structures.
Key Findings
The experiments in this study demonstrated a range of fire spread hazards from various types(materials/design) and configurations of fences and mulch ignited close to a structure in awind field, as well as the importance of maintenance. General findings are listed first andfollowed by very high, high, medium, and low hazard configurations. The findings arelabelled according to the following categories:
FH

Fire Hazard

LSLife Safety

HR

Hazard Reduction – materials, assemblies, implementation/housekeeping

IC

Improved Characterization – recommended future work to characterize thesefuels more fully

6.1.1. General Findings
The results from these experiments on fire spread demonstrated that:
F1. As combustible materials are combined, the hazard increases disproportionately.
(FH)
Fuel agglomeration provides significant increase in energy release and increases fire andember exposures. For a single combustible fence panel by itself, the fire behavior was limited
to glowing combustion near the area of ignition, with firebrands generating spot fires only onrare occasions. When a combination of a wood fence and shredded hardwood mulch was
ignited at the base, the flames progressed steadily in the direction of the wind, although they
did not rise above the lower half of the fence. Adding a second wood fence parallel to thefirst resulted in flames engulfing the fences within a few minutes of ignition.

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F2. Fences may impact egress. (LS)In a WUI fire, high and very high hazard fence configurations may result in a line of flamesclose to egress paths from a house or auxiliary dwelling. In one set of experiments on awood-plastic composite fence, the top and bottom frames distorted and allowed burning
boards to fall to either side. This created a 4 m (12 ft) wide zone of flames along the fenceline.
F3. Fire spread rates vary with fence materials and design, wind speed, and fuelconfiguration, including the presence or absence of mulch. (FH)This report provides data on a variety of fence and mulch materials, designs, andconfigurations.
F4. Spot fires due to firebrands may ignite within a few minutes, even over a
distance of 47.6 m (156 ft) or more from the burning item, and may continue toignite long after the initial flaming combustion has subsided. (FH)
Firebrands capable of igniting spot fires downwind were generated by nearly all
combinations of fence and mulch tested in this study. All wood fences with mulch at the basecaused spot fires in the target mulch bed. Spot fires were often ignited within a few minutesof mulch and fence ignition. Shredded hardwood mulch and pine bark mulch burned andemitted firebrands for longer than an hour. Ignition of spot fires was also demonstrated fromfirebrands transported by the wind over distances as far as 47.6 m (156 ft) from the burning
item under high wind conditions and over a paved surface. The wind field may deposit
firebrands close to the wall of the structure. If a home is undefended during a WUI fire andnot properly hardened, these firebrands may pose a serious threat to the home.
F5. A standard test method is needed to evaluate the fire characteristics of fences.
(IC)
A standard test method is needed to assess the fire performance of fences. The method should
consider not only materials but assemblies and be carried out in a vertical orientation. Itshould be able to distinguish the fire behavior of various materials, including wood-plastic
composites, wood, and vinyl, and designs, including privacy, lattice, and good neighbor.
6.1.2. Very High Hazard Configurations
Certain fence and mulch combinations were found to result in rapid fire spread and large
flames. In a region subject to WUI fires it’s advisable to remove these fuel sources if
possible. A standard test should be developed to evaluate the fire characteristics of fences.
F6. Rapid fire growth and large flames were found for parallel fences and one typeof wood-plastic composite fence. (FH)‒

Parallel wood privacy fences and double wood lattice fences were engulfed in flames
within a few minutes of ignition. Radiative exchange between the parallel burning
surfaces and convective uplift (buoyancy) of the hot gases trapped in the bounded
space caused rapid intensification of combustion and eruptive fire behavior. A large
fire occurred even when wood privacy fences were separated by 91 cm (3 ft). A

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parallel fence configuration can arise when neighbors erect fences along both sides ofa property line.
‒

For a western redcedar privacy fence next to a pine lattice fence, the fire behavior
depended on spacing. Rapid fire growth and intense flames were found for a spacingof 31 cm (12 in). The char patterns on each fence were similar to those for the fencesindividually when the spacing between them was 46 cm (18 in).

‒

Limited testing indicates that ignition of certain wood-plastic composite fences canresult in high intensity fire behavior. For one of the two types tested, the fence burned
intensely, with large flames extending above the fence. The warped frame allowed
vertical boards 1.8 m (6 ft) tall to fall to both sides, creating a 3.7 m (12 ft) wide zoneof flames that could block egress and threaten property.

F7. Good neighbor fences serve as a ladder fuel to carry flames from the ground to
the top of the fence. (FH)‒

For good neighbor fences at low wind speed, the flames reached the top of the fence
downwind from the ignition point. This is due to radiative and convective heattransfer between the boards connected to alternating sides of the stringer, the samemechanisms that caused rapid flame growth between parallel fences. At higher windspeeds the maximum height of the fire stayed below the center stringer of the fence.

F8. Rubber mulch generates large flames initially and when disturbed. (FH)‒

Rubber mulch burned with black smoke and large initial flames, followed by a long
period of sporadic flaming as a top layer of crumbly solid residue slowed the flow ofoxygen to the unburned fuel beneath. Disturbing the mulch bed renewed the flamingas the unburned fuel was exposed to air.

6.1.3. High Hazard Configurations
Many fence and mulch combinations exhibited fire behavior in the high hazard range,
supporting fire spread and generating firebrands but not progressing to full involvement withlarge flames. This section describes the behavior of some configurations that fall into thiscategory.
F9. A fence with mulch at its base transports fire through the community andprovides a steady source of firebrands to ignite combustible material downwind.
(FH)‒

Wood privacy or lattice fences combined with mulch were more hazardous than either
the fence or the mulch bed separately. Adding fine combustible materials to the baseof a fence promoted fire spread along the base of the fence, allowing the combinationof fence and mulch to act as a wick transporting fire along the entire length of thefence. With ignition at the base of the fence, flames remained below half of the fence
height. In every case, firebrands ignited spot fires in the target mulch bed.

‒

Stringers slowed the upward fire spread in these experiments by limiting the flame
height on one side of the fence. In a WUI fire, however, they could provide locations
for firebrands to lodge and ignite new fires on the fence.

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The flame spread rate in the horizontal direction was similar for all wood fences,including privacy, lattice, and good neighbor fences. Away from the wind field near
the structure, the fire spread from the ignition point to the end of the single fence
panel was 2 min to 5 min for high and medium wind speeds and 7 min to 12 min for
low wind speeds.

‒

For one type of wood-composite fence, the fire remained below the halfway point of
the fence height. Horizontal boards fell out of the frame and burned in line with thefence.

‒

Lifting a fence 15 cm (6 in) above shredded hardwood mulch decoupled the burning
behavior of the fence from the mulch between posts. This conclusion may not holdfor mulches that burn with higher flames, such as pine straw or rubber mulch. The
benefits of raising the fence above the mulch may not be realized if a barrier is placed
between them to keep wildlife out or pets in. Combustible debris such as leaves orneedles that collect along the barrier will reduce the advantages of this design in a
fire.

F10. Fire spreads easily across the fine overlapping particulates of a mulch bed. The
fire intensity, rate of fire spread, and production and size of firebrands depend on
the material properties and physical characteristics of the mulch. (FH)‒

Rubber mulch burned with black smoke and large flame initially and when disturbed.
See description under Very High Hazard.

‒

By itself, the pine straw mulch was consumed without igniting spot fires. However,embedded combustible objects were easily ignited by the intense flames. When
combined with a western redcedar privacy fence, the fire in the pine straw mulch
ignited the fence quickly and spread to the end of the panel within one minute. The
burning wood fence then generated firebrands capable of igniting spot fires.

‒

The fire spread behavior in a shredded hardwood mulch bed was affected by the flow
field, ignition of spot fires downwind, and the geometry of the mulch bed andstructure. Fire progressed in hardwood mulch beds through both continuous flame
spread and firebrand spotting. Spot fires allowed the fire to jump in the direction of
the wind.

‒

Fire spread more slowly over the mini pine bark mulch than over the shredded
hardwood mulch, often taking at least twice as long to reach the end of the mulch bed.
This likely results from the difference in texture – the chunks of mini pine bark mulch
do not ignite as easily as the long, thin particles that characterize the shredded
hardwood mulch.

F11. More information is needed on the fire behavior of a combustible fence next toan auxiliary structure. (IC)‒A WRC privacy fence separated from a noncombustible cement board by 31 cm(12 in) resulted in a char pattern similar to that of a single panel WRC privacy fence,with flame spread about three times faster. For a spacing of 46 cm (18 in), the fire
behavior was less intense for a WRC privacy fence in combination with either acement board or a vinyl privacy fence, as compared to a single panel WRC fence. The

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char pattern remained below the bottom stringer, and the flame spread was at thesame rate or slower.
6.1.4. Medium Hazard Configurations
Some fences and mulch experiments demonstrated very slow fire spread without flaming andlittle or no generation of firebrands.
F12. Without nearby fine combustible materials, the fire spread over a single
combustible fence is slow and dominated by glowing combustion. (HR)The fire spread over wood fences in the absence of fine combustibles was generally slow anddominated by glowing combustion with occasional small flames. Wood fences produced
large firebrands from pieces of the fence breaking off and small firebrands from glowing
combustion. However, spotting in the target mulch bed was rare in these experiments. It
should be noted that it may be difficult to keep a wood fence sufficiently clear of finecombustible materials to achieve the slow-growth fire behavior. Windblown debris such asleaves and pine needles may accumulate during a WUI event.
F13. Fence and ground coverings with added or inherent fire resistance reduce theflame spread rate and the hazard due to flames and firebrands. (HR)‒Vinyl privacy fences did not support significant burning under the tested windconditions. With mulch at its base, vinyl privacy fences, including panel, bottom
frame, and fence post, blackened and distorted along the entire length of the fence.
Distortion allowed the boards to fall out of the bottom frame. No firebrands weregenerated.

‒

The single type of artificial turf tested in this study was difficult to ignite andexhibited slow flame spread.

6.1.5. Low Hazard Configurations
Although fences made of noncombustible materials such as stone, brick, or steel were notincluded in this study, they can be classified as low fire hazard. They do not burn on theirown and have been shown provide protection against radiant heat [17]. However, any fence
can trap and accumulate windblown debris along their length. If not removed, combustible
materials at the base of even noncombustible fences can allow fire to spread.
F14. Noncombustible fences free of leaf litter and other combustible debris will not
spread fire. (HR)
Maintenance is required to reduce the accumulation of fine combustible materials along a
fence. This minimizes the amount of windblown debris such as leaves and pine needles that
can ignite during a WUI event.
Primary Recommendations
The following recommendations for members of a residential community are intended toaddress both ember and fire (radiation/convection) exposure hazards generated by
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combustible fences. Although the recommendations are intended primarily for moderate tovery high hazard WUI locations, they are expected to reduce local fire hazards in any
community.
For more detailed recommendations on spacings of combustible elements and hardening ofstructures and parcels, refer to the WUI Structure/Parcel/Community Fire Hazard Mitigation
Methodology report [16].
R1.

Avoid parallel fences, to reduce exposure to large flames. Parallel fences can resultin highly hazardous fuel accumulation corridors that are difficult to access andmaintain. Spacing of 0.9 m (3 ft) between fences is not sufficient.

R2.

Avoid combustible fences where they can impact egress, to protect life safety.

R3.

Avoid proximity to other combustible fuels, to reduce fire intensity and limit fire
spread. This includes fuels above the fence and fuels across parcel boundaries. Avoid
mulch at base of fence.

R4.

Avoid proximity of combustible fences to residence, including neighboring
residence, to prevent direct ignition. The relationship between spacing and structure
to prevent structure ignition is a function of structure construction materials/assemblyand fence materials/design.

R5.

Replace combustible landscape features with noncombustible or low fire hazard
features when possible. Fire spread is more likely with wood and wood-plastic
composite fences than with fences made of vinyl or noncombustible materials such asstone, brick, or steel.

R6.

Keep fence and yard clear of debris, to reduce the amount of fuel and potential
pathways for fire.

R7.

Harden structures against firebrands, to prevent structure ignition from embers
produced by fences or other combustible sources.
Recommendations for Future Work

More research is needed to determine the vulnerabilities of structures to fence fires relative tofence types and materials, the proximity and connection of the fence to the structure, and thedesign and exterior materials of the structure itself. Once the vulnerabilities are better
understood, mitigation techniques such as material treatments and coatings can be exploredbeyond the simple solutions of increased separation and replacement of materials with
noncombustible options.
S1. Study the effects on fire behavior of closely spaced parallel wall surfaces incommunities.
The same radiation exchange and convective transport of hot gases between burning parallel
surfaces that led to eruptive behavior for parallel wood fences can potentially result in highly
hazardous situations for other closely spaced parallel surfaces. As one approach, theStructure Separation Experiments project at NIST [63, 64] is addressing the question of how
far apart residences should be from other nearby structures.

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S2. Continue to study the fire behavior of landscape features and potential
mitigation methods.
There are many combustible landscape features that may contribute to fire hazards in a parcel
or community. Work is ongoing at NIST to understand the interactions of fires on woodpiles,
landscape timbers, creosote-treated timber, and sheds. The work will include strategies for
mitigation. Together, these studies will inform existing and new codes and standards withquantitative fire spread mitigation and structure protection strategies based on experimental
data.
S3. Improve data collection methods.
The range of fire behaviors found for a variety of materials, designs, and configurations
suggests improvements in data collection methods that would be useful for future targetedstudies. For fence and mulch configurations resulting in large flames and very high hazard
conditions, the radiative and convective heat flux received at vulnerable locations could be
evaluated by heat flux sensors and/or infrared (IR) imaging. Firebrand fluxes, sizes, andenergy content could be assessed in future studies by new measurement technology,
including a three-dimensional firebrand tracking system under development at NIST [66].
S4. Use fire modeling to better understand the physics behind the fire behavior.
Modeling can be used to extend the understanding from this study to other configurations offences, mulch beds, structures, and other fuels, in order to identify other high hazard
configurations. The results in this report may be helpful in validating a physics-based firemodel, including:‒Fire behavior of fences as a function of material properties and design.

‒

Fire behavior of parallel fences compared to single fences.

‒

Dependence on parallel fence spacing for time at which flames engulf the fences.

‒

Dependence of fire behavior on parallel fence length, including the spacing for which
a second panel length results in explosive fire growth, and the time for fire to spreaddown a long fence.

‒

Char patterns for wood privacy fences, both individually and in parallel withcombustible and noncombustible fences and walls.

S5. Revisit fire tests for evaluating fences and fence materials to ensure that they
represent the actual fire hazard.
A standard test method is needed to assess the fire performance of fences. The method should
consider not only materials but assemblies and be carried out in a vertical orientation. Itshould be able to distinguish the fire behavior of various materials, including wood-plastic
composites, wood, and vinyl, and designs, including privacy, lattice, and good neighbor. The
need for such a test method is highlighted within this report by the significant difference inenergy release between the two wood-plastic composite fences included in the study. A fence
test method will inform AHJs and the public about implementation options with fences andallow AHJs not only to assess the performance/hazard of composite fences but to compare
and assess all combustible fencing options under identical conditions.

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References[1]

V. C. Radeloff, D. P. Helmers, H. A. Kramer, M. H. Mockrin, P. M. Alexandre, A.
Bar-Massada, V. Butsic, T. J. Hawbaker, S. Martinuzzi, A. D. Syphard and S. I.
Stewart, "Rapid growth of the US wildland-urban interface raises wildfire risk,"
Proceedings of the National Academy of Sciences, vol. 115, no. 13, pp. 3314-3319,
2018.
[2] R. B. Hammer, S. I. Stewart and V. C. Radeloff, "Demographic trends, the wildlandurban interface, and wildfire management," Society and Natural Resources, vol. 22,
pp. 777-782, 2009.
[3] J. T. Abatzoglou and A. P. Williams, "Impact of anthropogenic climate change on
wildfire across western US forests," Proceedings of the National Academy of
Sciences, vol. 113, no. 42, pp. 11770-11775, 2016.
[4] J. K. Balch, B. A. Bradley, J. T. Abatzoglou, R. C. Nagy, E. J. Fusco and A. L.
Mahood, "Human-started wildfires expand the fire niche across the United States,"
Proceedings of the National Academy of Sciences, vol. 114, no. 11, pp. 2946-2951,
2017.
[5] J. R. Haas, D. E. Calkin and M. P. Thompson, "A national approach for integrating
wildfire simulation modeling into Wildland Urban Interface risk assessments within
the United States," Landscape and Urban Planning, vol. 119, pp. 44-53, 2013.
[6] CAL FIRE, "Top 20 Most Destructive California Wildfires," 13 Jan 2022. [Online].
Available: https://www.fire.ca.gov/media/t1rdhizr/top20_destruction.pdf. [Accessed 4
July 2022].
[7] Münchener Rückversicherungs-Gesellschaf, NatCatSERVICE, "Munich Re
NatCatSERVICE, Natural catastrophes in 2018," January 2019. [Online]. Available:
https://www.munichre.com/content/dam/munichre/contentlounge/websitepieces/documents/munichre-natural-catastrophes-in-2018.pdf. [Accessed 4 July 2022].
[8] A. Maranghides, D. McNamara, W. Mell, J. Trook and B. Toman, "A case study of acommunity affected by the Witch and Guejito Fires: Report #2 - Evaluating the effectsof hazard mitigation actions on structure ignitions," NIST Technical Note 1796,
National Institute of Standards and Technology, Gaithersburg, MD, 2013.
[9] A. Maranghides and W. Mell, "A case study of a community affected by the Witch
and Guejito fires," NIST Technical Note 1635, National Institute of Standards andTechnology, Gaithersburg, MD, 2009.
[10] A. Maranghides and D. McNamara, "2011 Wildland Urban Interface Amarillo Fires
Report #2 - Assessment of fire behavior and WUI measurement science," NISTTechnical Note 1909, National Institute of Standards and Technology, Gaithersburg,
MD, 2016.
[11] A. Maranghides, W. Mell, K. Ridenour and D. McNamara, "Initial reconnaissance ofthe 2011 wildland-urban interface fires in Amarillo, Texas," NIST Technical Note
1708, National Institute of Standards and Technology, Gaithersburg, MD, 2011.
[12] A. Maranghides, D. McNamara, R. Vihnanek, J. Restaino and C. Leland, "A casestudy of a community affected by the Waldo Fire - Event timeline and defensive

197

NIST TN 2228
August 2022

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]
[22]

[23]

[24]

actions," NIST Technical Note 1910, National Institute of Standards and Technology,
Gaithersburg, MD, 2015.
B. Gabbert, "Looking back at the fatal wildfire that burned into Gatlinburg," Wildfire
Today, 23 November 2017. [Online]. Available:
https://wildfiretoday.com/2017/11/23/ looking-back-at-the-fatal-wildfire-that-burnedinto-gatlinburg/. [Accessed 12 August 2019].
A. Maranghides, E. Link, W. Mell, S. Hawks, M. Wilson, W. Brewer, C. Brown, B.
Vihnaneck and W. D. Walton, "A Case Study of the Camp Fire - Fire Progression
Timeline," NIST TN 2135, National Institute of Standards and Technology,
Gaithersburg, MD, 2021.
D. D. Evans, L. R. Scott and W. D. Walton, "Structures ignited by Virginia WUI fires
2/2015 - 2/2017," NIST GCR 18-018, National Institute of Standards and Technology,
Gaithersburg, MD, 2018.
A. Maranghides, E. Link, S. Hawks, J. McDougald, S. Quarles, D. Gorham and S.
Nazare, "WUI Structure/Parcel/Community Fire Hazard Mitigation Methodology,"
NIST Technical Note 2205, National Institute of Standards and Technology,
Gaithersburg, MD, 2022.
J. Leonard, R. Blanchi, N. White, A. Bicknell, A. Sargeant, F. Reisen and M. Cheng,"Research and investigation into the performance of residential boundary fencing
systems in bushfires," CMIT-2206-186, CSIRO Manufacturing & Infrastructure
Technology, Clayton South, Victoria, Australia, 2006.
A. M. Grishin, A. I. Filkov, E. L. Loboda, V. V. Reyno, A. V. Kozlov, V. T.
Kuznetsov, D. P. Kasymov, S. M. Andreyuk, A. I. Ivanov and N. D. Stolyarchuk, "Afield experiment on grass fire effects on wooden constructions and peat layer
ignition," International Journal of Wildland Fire, vol. 23, no. 2, pp. 445-449, 2014.
S. Suzuki, E. Johnsson, A. Maranghides and S. L. Manzello, "Ignition of wood
fencing assemblies exposed to continuous wind-driven firebrand showers," Fire
Technology, vol. 52, pp. 1051-1067, 2016.
S. Suzuki and S. L. Manzello, "Understanding structure ignition vulnerabilities using
mock-up sections of attached wood fencing assemblies," Fire and Materials, vol.
TBA, pp. 1-10, 2019.
Insurance Institute for Business & Home Safety, "IBHS Wildfire Fence Research,"
2020. [Online]. Available: https://vimeo.com/433770802. [Accessed 9 June 2022].
Insurance Institute for Business & Home Safety and National Fire Protection
Association, "Wildfire Research Fact Sheet: Fencing," 2017. [Online]. Available:
https://www.nfpa.org/-/media/Files/Firewise/Factsheets/FirewiseFactSheetsFencing.ashx. [Accessed 9 June 2022].
E. L. Johnsson and A. Maranghides, "Effects of wind speed and angle on fire spreadalong privacy fences," NIST Technical Note 1894, National Institute of Standards andTechnology, Gaithersburg, MD, 2016.
S. L. Manzello, T. G. Cleary, J. R. Shields and J. C. Yang, "Ignition of mulch andgrasses by firebrands in wildland-urban interface fires," International Journal of
Wildland Fire, vol. 15, pp. 427-431, 2006.

198

NIST TN 2228
August 2022

[25] S. L. Manzello, S. Suzuki and D. Nii, "Full-scale experimental investigation toquantify building component ignition vulnerability from mulch beds attacked by
firebrand showers," Fire Technology, vol. 53, no. 2, pp. 535-551, 2017.
[26] L. G. Steward, T. D. Sydnor and B. Bishop, "The ease of ignition of 13 landscapemulches," Journal of Arboriculture, vol. 29, no. 6, pp. 317-320, 2003.
[27] C. Beyler, J. Dinaburg and C. Mealy, "Development of test methods for assessing thefire hazards of landscaping mulch," Fire Technology, vol. 50, pp. 39-60, 2014.
[28] S. Quarles and E. Smith, "The Combustibility of Landscape Mulches," University of
Nevada Cooperative Extension, 2011.
[29] K. M. Butler, E. L. Johnsson, M. G. Fernandez, M. Zarzecki, G. P. Forney and E.
Auth, "Wind effects on flame spread and ember spotting near a structure," inProceedings of the 2017 Fire and Materials Conference, San Francisco, CA, February
6-8, 2017.
[30] K. M. Butler, E. L. Johnsson and W. Tang, "Structure vulnerability to firebrands fromfences and mulch," in The Fire Continuum Conference: Preparing for the Future of
Wildland Fire, Missoula, MT, May 21-24, 2018.
[31] K. M. Butler, E. L. Johnsson, M. G. Fernandez, M. Zarzecki and E. Auth, "Flame
spread along fences near a structure in a wind field," in Fire Safety 2017 International Conference on Research and Advanced Technology in Fire Safety,
Santander, Spain, October 20-21, 2017.
[32] AMCA International, "Fans and Systems," AMCA Publication 201-02 (R2007), Air
Movement and Control Association International, Inc., Arlington Heights, IL, 2007.
[33] United States Gypsum Company, "DUROCK Brand Cement Board Systems:
Technical Notes, SA932 09305," 2012. [Online]. Available:
https://www.buildsite.com/pdf/usg/USG-DUROCK-Brand-Cement-Board-SystemsTechnical-Notes-B26750.pdf. [Accessed 23 12 2021].
[34] O. T. Farouki, "Thermal Properties of Soils, CRREL Monograph 81-1," U.S. Army
Corps of Engineers, Hanover, NH, 1981.
[35] W. T. Simpson, "Equilibrium moisture content of wood in outdoor locations in the
United States and worldwide," Research Note FPL-RN-0268, United States
Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI,
1998.
[36] S. V. Glass and S. L. Zelinka, "Chapter 4: Moisture Relations and Physical Properties
of Wood," in Wood Handbook, Wood as an Engineering Material, General Technical
Report FPL-GTR-190, Madison, WI, U.S. Department of Agriculture, Forest Service,
Forest Products Laboratory, 2010, pp. 4-1 to 4-19.
[37] B. J. McCaffrey and G. Heskestad, "A robust didirectional low-velocity probe forflame and fire application," Combustion and Flame, vol. 26, pp. 125-127, 1976.
[38] The Mathworks Inc., "MATLAB: 2019, 9.6.0.1072779 (R2019a)," The Mathworks
Inc., Natick, Massachusetts, 2019.
[39] Avery Lee, "VirtualDub 1.10.4 (Build 35491)," GNU General Public License, 2013.

199

NIST TN 2228
August 2022

[40] G. Diamantopoulos, S. Salehi and J. Buechler, Learning VirtualDub: The complete
guide to capturing, processing, and encoding digital video, Birmingham, UK: Packt
Publishing, 2005.
[41] VEGAS Creative Software, "VEGAS Pro 15.0 (Build 416)," MAGIX Software
GmbH, Berlin, Germany, 2018.
[42] AVS4YOU.com, "AVS Video Converter 12.0.3.654," Online Media Technologies
Ltd, London, UK, 2020.
[43] T. Umezawa, "Ut Video Codec Suite 21.3.0," GNU General Public License, 2020.
[44] D. Graft, "Smart Deinterlacer Filter 2.8," 2020.
[45] P. Berens, "CircStat: A MATLAB toolbox for circular statistics," Journal of
Statistical Software, vol. 31, no. 10, pp. 1-21, 2009.
[46] K. B. McGrattan, R. J. McDermott and C. G. Weinschenk, "Fire Dynamics Simulator
Users Guide," NIST Special Publication 1019, 6th ed., National Institute of Standards
and Technology, Gaithersburg, MD, 2013.
[47] K. McGrattan, R. McDermott, S. Hostikka, J. Floyd, C. Weinschenk and K. Overhalt,"Fire Dynamics Simulator Technical Reference Guide, Volume 3: Validation," NIST
Special Publication 1018-3, 6th ed., National Institute of Standards and Technology,
Gathersburg, MD, 2017.
[48] J. C. R. Hunt, C. J. Abell, J. A. Peterka and H. Woo, "Kinematical studies of the flows
around free or surface-mounted obstacles; applying topology to flow visualization,"
Journal of Fluid Mechanics, vol. 86, no. 1, pp. 179-200, 1978.
[49] R. Martinuzzi and C. Tropea, "The flow around surface-mounted, prismatic obstacles
placed in a fully developed channel flow," Journal of Fluids Engineering, vol. 115,
pp. 85-92, 1993.
[50] K. Sedighi and M. Farhadi, "Three-dimensional study of vortical structure around a
cubic bluff body in a channel," Facta Universitatis: Mechanical Engineering, vol. 4,
no. 1, pp. 1-16, 2006.
[51] H. -T. Loh and A. C. Fernandez-Pello, "A study of the controlling mechanisms offlow assisted flame spread," Symposium (International) on Combustion, vol. 20, no. 1,
pp. 1575-1582, 1985.
[52] N. P. Cheney, J. S. Gould and W. R. Catchpole, "The influence of fuel, weather andfire shape variables on fire-spread in grasslands," International Journal of Wildland
Fire, vol. 3, no. 1, pp. 31-44, 1993.
[53] J. M. Canfield, R. R. Linn, J. A. Sauer, M. Finney and J. Forthofer, "A numericalinvestigation of the interplay between fireline length, geometry, and rate of spread,"
Agricultural and Forest Meteorology, Vols. 189-190, pp. 48-59, 2014.
[54] M. J. Gollner, C. H. Miller, W. Tang and A. V. Singh, "The effect of flow andgeometry on concurrent flame spread," Fire Safety Journal, vol. 91, pp. 68-78, 2017.
[55] V. B. Apte, R. W. Bilger, A. R. Green and J. G. Quintiere, "Wind-aided turbulent
flame spread and burning over large-scale horizontal PMMA surfaces," Combustion
and Flame, vol. 85, no. 1-2, pp. 169-184, 1991.
[56] S. Quarles and E. Smith, "The combustibility of landscape mulches," SP-11-04,
University of Nevada Cooperative Extension, Reno, NV, 2011.

200

NIST TN 2228
August 2022

[57] CalEPA, "Tire Fire Smoke: Major constituents and potential for public healthimpacts," Office of Environmental Health Hazard Assessment, California
Environmental Protection Agency, Sacramento, California, 2002.
[58] A. Singh, S. N. Spak, E. A. Stone, J. Downard, R. L. Bullard, M. Pooley, P. A. Kostle,
M. W. Mainprize, M. D. Wichman, T. M. Peters, D. Beardsley and C. O. Stanier,"Uncontrolled combustion of shredded tires in a landfill - Part 2: Population exposure,public health response, and an air quality index for urban fires," Atmospheric
Environment, vol. 104, pp. 273-283, 2015.
[59] D. M. DeMarini, P. M. Lemieux, J. V. Ryan, L. R. Brooks and R. W. Williams,"Mutagenicity and chemical analysis of emissions from the open burning of scrap
rubber tires," Environmental Science & Technology, vol. 28, pp. 136-141, 1994.
[60] A. C. Fernandez-Pello, "Wildland fire spot ignition by sparks and firebrands," Fire
Safety Journal, vol. 91, pp. 2-10, 2017.
[61] S. L. Manzello, S. Suzuki and I. Hagiwara, "Exposing fencing assemblies to firebrandshowers characteristic of burning structures," in Proceedings of the 2017 Fire andMaterials Conference, San Francisco, CA, February 6-8, 2017.
[62] D. X. Viegas and A. Simeoni, "Eruptive Behaviour of Forest Fires," Fire Technology,
vol. 47, pp. 303-320, 2011.
[63] A. Maranghides, S. Nazare, E. Link, K. Prasad, M. Hoehler, M. Bundy, S. Hawks, F.
Bigelow, W. Mell, A. Bova, D. McNamara, T. Milac, D. Gorham, F. Hedayati, B.
Raymer, F. Frievalt and W. Walton, "Structure Separation Experiments: Phase 1
Preliminary Test Plan," NIST TN 2161, National Institute of Standards andTechnology, Gaithersburg, MD, 2021.
[64] A. Maranghides, S. Nazare, E. Link, M. Bundy, A. Chernovsky, E. Johnsson, K.
Butler, S. Hawks, F. Bigelow, W. Mell, A. Bova, D. McNamara, T. Milac, D.
Gorham, F. Hedayati, B. Raymer, F. Frievalt and W. Walton, "NIST Outdoor
Structure Separation Experiments (NOSSE): Preliminary Test Plan," NIST TN 2199,
National Institute of Standards and Technology, Gaithersburg, MD, 2022.
[65] A. Maranghides and E. L. Johnsson, "Residential Structure Separation Fire
Experiments," NIST TN 1600, National Institute of Standards and Technology,
Gaithersburg, MD, 2008.
[66] N. Bouvet, E. D. Link and S. A. Fink, "Development of a New Approach toCharacterize Firebrand Showers During Wildland-Urban Interface (WUI) Fires: a
Step Towards High-Fidelity Measurements in Three Dimensions," NIST TN 2093,
National Institute of Standards and Technology, Gaithersburg, MD, 2020.
[67] B. N. Taylor and C. E. Kuyatt, "Guidelines for evaluating and expressing theuncertainty of NIST measurement results," NIST Technical Note 1297, National
Institute of Standards and Technology, Gaithersburg, MD, 1994.
[68] "American Softwood Lumber Standard, Voluntary Product Standard PS 20-20,
Revision 1," National Institute of Standards and Technology, Gaithersburg, MD,
2021.
[69] R. A. Bryant, "A comparison of gas velocity measurements in a full-scale enclosure
fire," Fire Safety Journal, vol. 44, pp. 793-800, 2009.

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[70] H. Seefeldt and U. Braun, "Butning behaviour of wood-plastic composite decking
boards in end-use conditions: The effects of geometry, material composition, andmoisture," Journal of Fire Sciences, vol. 30, no. 1, pp. 41-54, 2011.
[71] L. Shi, M. Yit and L. Chew, "Influence of moisture on autoignition of woods in cone
calorimeter," Journal of Fire Sciences, vol. 30, no. 2, pp. 158-169, 2012.
[72] M. J. Spearpoint and J. G. Quintiere, "Predicting the piloted ignition of wood in thecone calorimeter using an integral model - effect of species, grain orientation and heat
flux," Fire Safety Journal, vol. 36, no. 4, pp. 391-415, 2001.
[73] P. A. Enright and C. M. Fleischmann, "Uncertainty of heat release rate calculation of
the ISO5660-1 cone calorimeter standard test method," Fire Technology, vol. 35, no.
2, pp. 152-169, 1999.
[74] A. Lukosius and V. Vekteris, "Precision of heat release rate measurement results,"
Measurement Science Review, vol. 3, no. 3, pp. 13-16, 2003.
[75] L. Dubrulle, M. Zammarano and R. D. Davis, "Effect of fire-retardant coatings andaccelerated-weathering on the flammability of wood-based materials in wildlandurban interface (WUI) communities," NIST Technical Note 2094, National Institute ofStandards and Technology, Gaithersburg, MD, 2020.
[76] R. H. White, M. A. Dietenberger and N. M. Stark, "Cone calorimeter tests of woodbased decking materials," in Proceedings of the 18th Annual Conference on Recent
Advances in Flame Retardancy of Polymeric Materials, Wellesley, MA, 2007.
[77] V. Babrauskas and W. J. Parker, "Ignitability measurements with the cone
calorimeter," Fire and Materials, vol. 11, no. 1, pp. 31-43, 1987.
[78] K.-C. Tsai, "Orientation effect on cone calorimeter test results to assess fire hazard ofmaterials," Journal of Hazardous Materials, vol. 172, pp. 763-772, 2009.
[79] L. Cheng, W. Wu, W. Meng, S. Xu, H. Han, Y. Yu, H. Qu and J. Xu, "Application of
metallic phytates to poly(vinyl chloride) as efficient biobased phosphorous flame
retardants," Journal of Applied Polymer Science, vol. 135, no. 33, pp. 46601-46610,
2018.
[80] M. A. Lackner, A. L. Rogers and J. F. Manwell, "Uncertainty analysis in windresource assessment and wind energy production estimation," in AIAA 2007-1222,
45th AIAA Aerospace Sciences Meeting and Exhibit,, Reno, NV, 2012.
[81] G. H. Weinberg, J. A. Schumaker and D. Oltman, Statistics: An Intuitive Approach,
4th ed., Monterey, CA: Brooks/Cole Publishing Company, 1981.
[82] ICC and NARCD Councils, "Communities dealing with wildland/urban interface fire;
Wildfire Safe, Sound & Code Smart; WUI Fact Sheet," 1 August 2013. [Online].
Available: http://sectionb10.org/yahoo_site_admin/assets/docs/
WUI_Fact_Sheet_080120131.347133404.pdf. [Accessed 15 October 2018].
[83] S. L. Manzello and S. Suzuki, "Experimentally simulating wind driven firebrandshowers in wildland-urban interface (WUI) fires: Overview of the NIST firebrandgenerator (NIST dragon) technology," Procedia Engineering, vol. 62, pp. 91-102,
2013.
[84] J. Skelly, "Is rubber mulch a better alternative for landscaping?," Nevada Appeal,
Northern Nevada, 2010.

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[85] S. L. Quarles, "Ignition Potential of Fencing Subjected to an Ember Exposure andFlame Spread to a Building," Insurance Institute for Business & Home Safety (IBHS),
in progress, Richburg, SC, 2017.
[86] V. C. Radeloff, R. B. Hammer, S. I. Stewart, J. S. Fried, S. S. Holcomb and J. F.
McKeefry, "The wildland-urban interface in the United States," EcologicalApplications, vol. 15, no. 3, pp. 799-805, 2005.
[87] S. I. Stewart, V. C. Radeloff, R. B. Hammer and T. J. Hawbaker, "Defining thewildland-urban interface," Journal of Forestry, vol. 105, no. 4, pp. 201-207, 2007.
[88] USDA and USDI, "Urban wildland interface communities within vicinity of Federal
lands that are at high risk from wildfire," Federal Register, vol. 66, no. 3, pp. 751-777,
2001.
[89] A. Bar-Massada, S. I. Stewart, R. B. Hammer, M. H. Mockrin and V. C. Radeloff,"Using structure locations as a basis for mapping the wildland urban interface,"
Journal of Environmental Management, vol. 128, pp. 540-547, 2013.
[90] SILVIS Lab, "Wildland-Urban Interface (WUI) Change 1990-2010," University of
Wisconsin-Madison, Dept. of Forest & Wildlife Ecology, Madison, WI, 2019.
[Online]. Available: http://silvis.forest.wisc.edu/data/wui-change/. [Accessed 2
August 2019].
[91] D. M. Theobald and W. H. Romme, "Expansion of the US wildland-urban interface,"
Landscape and Urban Planning, vol. 83, pp. 340-354, 2007.
[92] P. M. Alexandre, M. H. Mockrin, S. I. Stewart, R. B. Hammer and V. C. Radeloff,"Rebuilding and new housing development after wildfire," International Journal of
Wildland Fire, vol. 24, pp. 138-149, 2015.
[93] G. K. Dillon, J. Menakis and F. Fay, "Wildland fire potential: A tool for assessing
wildfire risk and fuels management needs," in Proceedings of the Large Wildland
Fires Conference, 19-23 May 2014, Missoula, MT, Fort Collins, CO, 2015.
[94] S. Suzuki, D. Nii and S. L. Manzello, "The performance of wood and tile roofing
assemblies exposed to continuous firebrand assault," Fire and Materials, vol. 41, pp.
84-96, 2017.
[95] S. L. Manzello, S.-H. Park, S. Suzuki, J. R. Shields and Y. Hayashi, "Experimental
investigation of structure vulnerabilities to firebrand showers," Fire Safety Journal,
vol. 46, pp. 568-578, 2011.
[96] S. L. Manzello and S. Suzuki, "Experimental investigation of wood decking
assemblies exposed to firebrand showers," Fire Safety Journal, vol. 92, pp. 122-131,
2017.
[97] S. L. Manzello, S. Suzuki and Y. Hayashi, "Exposing siding treatments, walls fittedwith eaves, and glazing assemblies to firebrand showers," Fire Safety Journal, vol. 50,
pp. 25-34, 2012.
[98] S. L. Quarles and C. D. Standohar-Alfano, "Wildfire Research: Ignition Potential of
Decks Subjected to an Ember Exposure," Technical Report, Insurance Institute for
Business & Home Safety, 2017, updated 2018.

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[99] E. Koo, P. J. Pagni, D. R. Weise and J. P. Woycheese, "Firebrands and spotting
ignition in large-scale fires," International Journal of Wildland Fire, vol. 19, pp. 818843, 2010.
[100] A. Lee, "What is VirtualDub?," [Online]. Available: http://www.virtualdub.org/.
[Accessed 21 2 2020].

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Appendix A. Uncertainties
Each measurement of wind speed, distance, time, or other variable discussed in this report isassociated with an uncertainty. Uncertainties generally consist of several components that aregrouped into two categories according to the method used to estimate their value [67].
Type A uncertainties are quantified by statistical methods, including the calculation ofstandard deviation, curve fitting using the method of least squares, or analysis of variance(ANOVA). Type B uncertainties are evaluated by other means, usually using scientificjudgment based on all available relevant information. This may include previous
measurement data, experience with relevant materials and instruments, manufacturer’s
specifications, or data from literature sources and handbooks.
For an output quantity 𝑌 that is not measured directly but is determined from 𝑁 input
quantities 𝑋1 , 𝑋2 , … , 𝑋𝑁 through a functional relation 𝑓:
𝑌 = 𝑓(𝑋1 , 𝑋2 , … , 𝑋𝑁 ) ,(A-1)

the estimate 𝑦 of the output quantity is given by applying the same function to estimates
𝑥1 , 𝑥, … , 𝑥𝑁 of the input quantities:
𝑦 = 𝑓(𝑥1 , 𝑥2 , … , 𝑥𝑁 ) .

(A-2)

For Type A evaluation of uncertainties, in which 𝑛 repeated independent observations are
made under the same conditions, the input quantity xi can be estimated by the sample mean,𝑛1𝑥𝑖 = 𝑋̅𝑖 = ∑ 𝑋𝑖,𝑘
𝑛

(A-3)

𝑘=1

and the standard uncertainty 𝑢(𝑥𝑖 ) associated with xi is the estimated standard deviation of
the mean,
𝑛

1/2

1
2
𝑢(𝑥𝑖 ) = 𝑠(𝑋̅𝑖 ) = (∑(𝑋𝑖,𝑘 − 𝑋̅𝑖 ) )𝑛(𝑛 − 1)

(A-4)

𝑘=1

This is equivalent to the estimated standard deviation of the input quantity 𝑠𝑖 divided by thesquare root of the size of the sample:𝑠𝑖𝑠(𝑋̅𝑖 ) =(A-5)√𝑛
For Type B evaluations, estimates of the input quantity xi and standard uncertainty u(xi)depend on the assumed form of the probability distribution and the range of values. If thequantity is modeled by a normal distribution with essentially all (approximately 99.73 %) of
its values contained within a lower limit 𝑎− and an upper limit 𝑎+ , and with a mean value(𝑎+ + 𝑎− )2then the standard uncertainty is given by𝑎𝑢(𝑥𝑖 ) =
3
𝑥𝑖 =

205

(A-6)

(A-7)

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where 𝑎 = (𝑎+ − 𝑎− )/2. In this case, 𝑎 is equivalent to the 3𝜎 value.
Another alternative is to assume a rectangular probability distribution., with a lower limit 𝑎−and upper limit 𝑎+ and a value that is equally probable to lie anywhere within the interval. In
this case, the input quantity can be estimated by Equation (A-6) and the standard uncertaintyis𝑎𝑢(𝑥𝑖 ) =(A-8)√3
The combined standard uncertainty of the measurement result 𝑦, representing its estimatedstandard deviation, is the positive square root of the variance obtained from the law ofpropagation of uncertainty:𝑁

𝑢𝑐2 (𝑦) = ∑ (
𝑖=1

𝑁−1

𝑁

𝜕𝑓 2 2𝜕𝑓 𝜕𝑓) 𝑢 (𝑥𝑖 ) + 2 ∑ ∑𝑢(𝑥𝑖 , 𝑥𝑗 )𝜕𝑥𝑖
𝜕𝑥𝑖 𝜕𝑥𝑗(A-9)

𝑖=1 𝑗=𝑖+1

where 𝑢(𝑥𝑖 ) is the standard uncertainty associated with the estimate of input quantity 𝑥𝑖 and𝑢(𝑥𝑖 , 𝑥𝑗 ) is the estimated covariance associated with 𝑥𝑖 and 𝑥𝑗 . When the input quantities areuncorrelated, the equation reduces to the RSS (root sum square) method of combining
uncertainties,𝑁𝑢𝑐 (𝑦) = √∑[𝑐𝑖 𝑢(𝑥𝑖 )]2(A-10)

𝑖=1

where 𝑐𝑖 = 𝜕𝑓/𝜕𝑥𝑖 .
Finally, an expanded uncertainty 𝑈 is determined by multiplying the combined standard
uncertainty 𝑢𝑐 by a coverage factor 𝑘,
𝑈 = 𝑘𝑢𝑐 .

(A-11)

This provides an interval 𝑦 − 𝑈 to 𝑦 + 𝑈 within which the value of 𝑌 can be asserted to lie
with a high level of confidence. A coverage factor of 𝑘 = 2 defines a level of confidence ofapproximately 95 %.
Relative standard uncertainty 𝑢𝑟 (𝑥𝑖 ), relative combined standard uncertainty 𝑢𝑐,𝑟 (𝑦), andrelative expanded uncertainty 𝑈𝑟 are calculated by dividing the uncertainty measure by theabsolute value of the associated quantity, as in:𝑢𝑟 (𝑥𝑖 ) =𝑢(𝑥𝑖 )|𝑥𝑖 |

𝑢𝑐,𝑟 (𝑦) =𝑢𝑐 (𝑦)
|𝑦|

𝑈𝑟 =

𝑈
|𝑦|

Equation (A-10) can be written in terms of relative standard uncertainties as𝑁

2𝑢𝑐 (𝑦)𝑢(𝑥𝑖 )= √∑ 𝑠𝑖2 (
)
𝑦
𝑥𝑖
𝑖=1

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where 𝑠𝑖 is the nondimensional sensitivity coefficient for input quantity 𝑥𝑖 ,
𝑠𝑖 =

𝜕𝑦 𝑥𝑖
𝜕𝑥𝑖 𝑦

(A-13)

This appendix does not address the matter of reproducibility of the measurement results inthis study; that is, the closeness of the agreement of measurements under different conditions.
Most experiments in the study were unique; those that were repeated were performed on
different days under different ambient conditions and in the presence of other uncontrolled
variables.
The uncertainties examined in this section are associated with four categories of variables.
Section A.1 looks at the uncertainties for the experimental setup, including dimensions of test
objects, setup distances, and material preparation. Section A.2 discusses the Type A
uncertainties in the wind data measured by bidirectional probes. Section A.3 looks at theuncertainties inherent in assessing the timing of events, and Section A.4 considers theuncertainties in the flame spread analyses.
A.1.

Experimental Setup

Setting up the experiments included preparing the materials for testing, assembling the mulch
beds and fences, and arranging the test setup. The setup and procedures were described in
Section 2 of this report. The uncertainties associated with the experimental setup are Type B,
either estimated through scientific judgment or obtained from source literature. Table A.1
provides a list of the variable types and their uncertainties. All of the uncertainties listed inthis table are Type B, determined from sources in the literature or by scientific judgment.
The moisture content (MC) of the wood mulch and fences at the beginning of eachexperiment was controlled by allowing the materials to come to equilibrium in conditioned
spaces, as described in Section 2.4.3. The relative humidity for these spaces was suitable forbringing the wood to a nominal MC of 6.5 %, a value that is reasonable for the summertimein the American Southwest [35, 36]. The variability of MC for wood over the expected rangeof ambient temperature and relative humidity is ± 0.8 %. This was consistent withmeasurements of mulch samples as described in Section 2.4.3.
Estimates of uncertainties for the dimensions of wood components used to construct thefences cam be derived from the American Softwood Lumber Standard [68]. This voluntarystandard includes the western redcedar, pine, and redwood fences tested in this study, as wellas the pressure treated pine posts and stringers. Nominal sizes for lumber are larger than theminimum dressed sizes (e.g., a 2×4 rail corresponds to 3.8 cm by 8.9 cm (1 ½ in by 3 ½ in)).
For thicknesses of 1 in or more, the actual dressed size must exceed the specified minimum
value by at least 3.2 mm (1/8 in). Uncertainties can be extracted from the voluntary standard
by assuming that the lumber used in this study satisfied the definition of slight variation insawing. This is defined as variations not exceeding 1.6 mm (1/16 in) over or under for
nominal 1-inch lumber, 3.2 mm (1/8 in) for nominal 2-inch, 5 mm (3/16 in) for nominal
3-inch to 7-inch, and 6 mm (1/4 in) for nominal 8-inch or greater. The dimensions for boards
used in constructing fences is specifically called out in the standard as satisfying the 1.6 mm(1/16 in) tolerance for thickness.

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Sizes may shrink or expand with moisture. Tolerance allowances are 0.7 % shrinkage orexpansion for each four percentage points of MC change below 30 % for redwood andwestern redcedar, and 1 % for other wood types [68]. As discussed in Section 2.4.3, thefences in this experiment are held to MC of 6.5 % ± 1 %. The standard relative uncertainty(standard uncertainty divided by the mean value) of dimensions due to shrinkage andexpansion for fences in this study is therefore 0.2 %, or 0.002. These tolerances are fortransverse shrinkage, in directions tangential to or across the tree growth rings. Longitudinal
shrinkage, parallel to the grain, is much smaller [36], and can be neglected relative to thevariations due to sawing.
The calculation of uncertainty for fence dimensions includes the components of variation insawing the lumber and in shrinking and expansion due to changes in moisture content. Since
the American Softwood Lumber Standard does not translate the variability due to sawing to anormal distribution, we will assume that the range defines the 3𝜎 value of a normal
distribution. For nominal 4-inch lumber, for example, the standard deviation from sawing istaken as 𝑢1 = (5 mm)/3 = 1.7 mm. The standard deviation from shrinkage/expansion is
𝑢2 = 0.002 × 89 mm ≅ 0.2 mm. An additional uncertainty of 2 mm is added to thecalculation for the height of the lattice fence to account for variations in the assembly
process.
As stated in Section 2.5.1, spacing between the vertical boards varied from 1 mm (0.04 in) to
6 mm (0.24 in) and averaged 3.5 mm (0.14 in).
To calculate the uncertainty for the length of both privacy and lattice fences, the standard
uncertainties for the length of the stringer and the widths of two posts were combined using
the RSS method. The result was then multiplied by 2 to calculate the total expanded
uncertainty (95 % confidence level).

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Table A.1. Uncertainty in wood fence dimensions
Measurement

ComponentStandard
Uncertainty,𝒖𝒊

Board thickness (19 mm):
SawingExpansion/shrinkage

0.5 mm0.04 mmBoard width (140 mm):
SawingExpansion/shrinkage

1.7 mm
0.3 mmBoard/Fence height (1.83 m):
SawingExpansion/shrinkage

2.0 mm
< 0.1 mm

Spacing between boards (3.5 mm)

1.0 mmPost width (89 mm):
SawingExpansion/shrinkage

1.7 mm
0.2 mmStringer height (89 mm):
SawingExpansion/shrinkage

1.7 mm
0.1 mmStringer thickness (38 mm):
SawingExpansion/shrinkage

1.1 mm0.08 mm

Stringer length (2.44 m):
SawingExpansion/shrinkage

2.0 mm
< 0.1 mm

Lattice fence height (1.22 m):
Assembly
SawingExpansion/shrinkage

2.0 mm
2.0 mm
< 0.1 mm

Privacy fence length (2.62 m):
Post width
Stringer length
Post width

1.7 mm
2.0 mm
1.7 mm

Lattice fence length (2.51 m):
Post width
Stringer length
Post width

1.7 mm
2.0 mm
1.7 mm

CombinedStandard
Uncertainty𝒖𝒄

Expanded
Uncertainty
𝟐𝒖𝒄

0.5 mm

1.0 mm

1.7 mm

3.4 mm

2.0 mm

4.0 mm

1.0 mm

2.0 mm

1.7 mm

3.4 mm

1.7 mm

3.3 mm

1.1 m

2.1 mm

2.0 mm

4.0 mm

2.8 mm

5.7 mm

3.1 mm

6.2 mm

3.1 mm

6.2 mmThe uncertainties for mulch pan dimensions and the locations relative to the shed andcenterline of mulch pan, bidirectional probe array, and wind machine are presented in Table
A.2. All uncertainties listed in this table are Type B, determined by scientific judgment.
The mulch pan is described in Section 2.4.1 and the target mulch bed at the base of the shedis described in Section 2.4.4. The mulch pans were 87.6 cm (34.5 in) wide with 2.5 cm (1 in)high side walls, and two pans were overlapped and clamped for a total length of 3.35 m(11 ft). A few sets of pans were made during the study, and the pan walls tended to warp over209

NIST TN 2228
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time. The expanded uncertainties for both pan width and length are estimated as ±1 cm. The
target mulch pans along the shed were 46 cm (18 in) wide and 1.37 m (4.5 ft) long, with twopans overlapped to give a total length matching the shed wall dimension at 2.44 m (8 ft) long.
The target mulch bed was not confined by a lip on the outside edge facing the fence, whichallowed firebrands to land on the target mulch without needing to clear a height. The widthof the target mulch bed thus varied over its length, by an expanded uncertainty of ±2.0 cm.
The mulch to be ignited was prepared by spreading it over the mulch pans and compressing itby foot to a depth of 5 cm, double the height of the walls of the mulch pan, as discussed in
Sections 2.4.1 and 2.9.2. The mulch was tapered at the edges of the pan to a depth of 2.5 cm.
For cases with a fence, the depth was designed to cover the gap between the pan top surface
and the bottom of the fence, which was set (with a pair of measured shims) at 5 cm. The
mulch bed thickness varied over its surface due to the nature of the mulch as overlapping
particles whose individual thicknesses are an appreciable fraction of the thickness of themulch layer. The mulch bed thickness also depended on the evenness of the spreading overthe mulch bed and the uniformity of the compaction. The expanded uncertainty of the mulch
bed depth was ±1 cm. The same uncertainty was estimated for the depth of the mulch at half
thickness and for the target mulch bed, both of which were compressed at a nominal heightof 2.5 cm
After the shed and wind machine were placed into position, they usually remained there overmultiple test days and several tests. For every setup, a long tape measure that covered the full
10.7 m (35 ft) distance between them was used to adjust the equipment to the desired
position. This procedure was estimated to result in an expanded uncertainty (95 % confidence
level) of ±3 cm. In the transverse direction, the center of the fan hub was within ±1 cm of thecenterline of the experiment.
The separation distance, from the shed to the mulch pan, was measured by tape measure foreach experiment. The expanded uncertainty for a confidence level of 95 % was estimated as±1 cm. A longer tape measure used for distances greater than 11 m (36 ft) had an expanded
uncertainty of ±2.5 cm (±1 in).
The bidirectional probe array was typically placed 4 ft from the end of the fence. For cases
without a fence, the measurement was made from the shed to the probe array, with the
distance based on the separation distance of each experiment. The expanded uncertainty wasestimated for the distance of the array from the shed as ±2 cm and for the distance of thecenterline probes to the actual centerline as ±1 cm. There was an additional uncertainty forthe position of each probe within the array, which was estimated as ±1 cm in the verticaldirection and ±0.5 cm perpendicular to the plane of the probe array.

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Table A.2. Uncertainty in dimensions for experimental setup
Measurement

ComponentStandard
Uncertainty
𝒖𝒊

CombinedStandard
Uncertainty𝒖𝒄

Expanded
Uncertainty
𝟐𝒖𝒄

Mulch bed width (0.88 m)

5 mm

5 mm

10 mm

Mulch bed length (3.35 m)

5 mm

5 mm

10 mm

Target mulch bed width (0.46 m)

10 mm

10 mm

20 mm

Mulch thickness (50 mm or 25 mm)

5 mm

5 mm

10 mm

Wind machine to shed (10.67 m)

15 mm

15 mm

30 mm

Wind machine to centerline

5 mm

5 mm

10 mm

Mulch pan to shed:
Separation distance (0-1.83 m)

5 mm

5 mm

10 mm

Bidirectional probe array to shed

10 mm

10 mm

20 mm

Bidirectional probe array to centerline

5 mm

5 mm

10 mm

Probe position, x (toward shed)
Probe array
Position within array

10.3 mm

20.6 mm

10 mm
2.5 mmProbe position, y (toward center)
Probe array

5 mm

5 mm

10 mm

Probe position, z (vertical)
Position within array

5 mm

5 mm

10 mm

Distances > 11 m (36 ft)

25 mm

25 mm

51 mm

A.2.

Wind DataWind speed uncertainties involve the bidirectional probe design and the measurement
statistics from the wind field. Bidirectional probes are simple tubular devices that calculate
velocities from the pressure differential between front and back openings. They are similar topitot static tubes, with added advantages of robustness in a fire environment and insensitivity
to flow angle. Velocity is calculated using the equation:
𝑉=

1
2𝑅𝑢
√
∆𝑃 𝑇
𝐶 𝑃𝑠 𝑀𝑎𝑖𝑟(A-14)

where 𝐶 is the probe constant, 𝑅𝑢 is the universal gas constant, 𝑃𝑠 is standard atmosphericpressure, 𝑀𝑎𝑖𝑟 is the molecular mass of dry air, ∆𝑃 is dynamic pressure, and 𝑇 is absolutetemperature.
McCaffrey and Heskestad [37] measured the probe constant for the bidirectional probe as
𝐶 = 1.08 for Reynolds numbers greater than 1000. A study of the angular sensitivity showed
the value remaining within ±10 % for flow directions within 50° of the probe axis. For flows
generally parallel to the probe axis and a probe Reynolds number above 500, a relative
standard uncertainty of 0.07 can be used for the probe constant [69]. For the fence and mulch
study, the Reynolds number was above this limit for all wind velocities.

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The dynamic pressure in Equation (A-14) was the difference in pressure between windward
and leeward sides of the bidirectional probe. As described in Section 2.7.1, the probe signals
were converted to a pressure differential through transducers, which the manufacturer rated
with an accuracy of 1 %. Because the probe array was located upwind of the fire and not
exposed to its heat, the ambient temperature measurement was used to calculate velocity. The
standard limit of error for the Type K thermocouples used in this study was 0.75 % or 2.2 °C,
whichever is greater.
Table A.3 lists the relative standard uncertainty for each variable and uses Equation (A-12) todetermine the contribution of each variable to the relative standard uncertainty of the windspeed calculated from the bidirectional probe 𝑢𝑟,𝐵𝑃 = 0.0703. In Appendix C, this Type B
uncertainty of the calculated wind speed from the probe will be combined with the Type A
evaluation of the uncertainty of unsteady wind speed measurements (the standard error of themean) to determine a total uncertainty of the mean wind speed for each probe.
Table A.3. Uncertainty budget for point velocity measurementsabMeasurement
Component

Value
𝒙𝒊

Relative standard
uncertainty
𝒖𝒓,𝒊 = 𝒖(𝒙𝒊 )/𝒙𝒊

Nondimensional
sensitivity coefficient𝒔𝒊Percent
contribution,%

Probe constant, 𝑪

1.08

0.07

-1.0

99.2 %

Probe differential
pressure, ∆𝑷 (Pa)

56

0.01

0.5

0.5 %

Ambient
temperature, 𝑻 (K)293

0.0075(not less than 2.2 °C)

0.5

0.3 %

Standard atmosphericpressure, 𝑷𝒔 (Pa)1013250.0003-0.5

0%Molecular weight of dry
air, 𝑴𝒂𝒊𝒓 (kg/kmol)

28.97

0.0001-0.5

0%Velocity, 𝑽

8.93

0.0703 a (0.1406 b)

Total relative standard uncertainty
Total relative expanded uncertainty (coverage factor 𝑘 = 2 for a confidence level of about 95 %)212

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A.3.

Timing Data

A common system of timing was required in order to synchronize multiple videos recorded
during each experiment and relate the experiments to each other. Five events that could be
easily identified in the test videos were selected: End of Gas Burner Ignition, Start to
Remove Gas Igniters, Fan On, Fan Off, and Water First Applied. The events were defined in
Section 3.1.1. Time t = 0 for each experiment was defined as the Fan On event, when theengine of the wind machine engaged after turning over, and the end of the experiment was
defined as Fan Off, when the engine was turned off and the audio signal (including the soundpitch and amplitude) started to change.
The Fan On and Fan Off events were detected through the audio signal from the video. The
image in Fig. A.1 shows a screen shot from the analysis of the left camera video from
Test A-29. One second of audio signal (29 frames) is shown for the two channel feeds. The
tracks are displayed in red against a black background, except for the center frame that is
highlighted in blue. This is the frame that was selected as the Fan On event. The audio signal
to the left and right of this frame illustrates the difference between the engine turning overand engaged. It is not difficult to locate the frame in which the change in signal occurs plus
or minus a few frames. Accounting for variations in the signal from one experiment to thenext and for differences in the decisions of multiple analysts, the standard uncertainty for
Fan On is conservatively estimated as ±0.5 s.
A similar screen shot in Fig. A.2 was taken from the one-second period during which thewind machine was turned off. The dark blue highlight in the center marks the frame that has
been selected as Fan Off, and the audio signal is seen to be qualitatively different to the left
and the right of this frame. The differences are not as apparent as for the Fan On event,
however, and a standard uncertainty estimated at ±1 s reflects this.
The test duration is defined as the time difference between Fan On and Fan Off. As such, thestandard uncertainty for this measurement combines the uncertainty from these two events.
Using the RSS method gives a combined standard uncertainty for test duration of ±1.1 s. As acheck, the differences in test duration measured from the right and left videos for eachexperiment (which were analyzed independently) were calculated. Considering the entire setof experiments, the mean of this difference was -0.27 s and the standard deviation was 1.01 s,
comparable to the estimated combined standard uncertainty of the test duration.
The three timing events that relied on visual evidence were usually easily determined thoughexamination of the video. For the End of Gas Burner Ignition event, the analysis must
identify the final application of the ignition torch to the gas burner. If the torch is visible, andthe video is carefully examined, the end of the final application can easily be defined within±1 s. This is estimated as the standard uncertainty. If the torch movement is hidden by thefence, the only way to set this time may be to calculate it using the timing from the video on
the other side of the fence.
The Start to Remove Gas Igniters event is easily detected from both sides of the fence as themoment when the gas igniters start to be lifted from the mulch bed after ignition has been
established. This event can easily be identified within ±0.5 s.

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Fig. A.1. Audio feed from left camera showing frame selected for Fan On (centered, in blue). Total
time shown is 1 s, or 29.97 frames.

Fig. A.2. Audio feed from left camera showing frame selected for Fan Off (centered, in blue). Total
time shown is 1 s, or 29.97 frames.

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The Water First Applied event is similar to the End of Gas Burner Ignition event in that thewater striking the burning object may be blocked by the fence. Judgment is involved indeciding when the water first reaches its target. When the water application is visible, thestandard uncertainty is estimated as ±1 s.
Three time events were identified to evaluate how fast spot fires occurred in the target mulch
bed and how quickly they threatened the structure. The first was the time at which the first
spot fire ignited. Even if this spot fire did not reach the shed wall, it could threaten othercombustible objects, such as vegetation, near the structure. The second event was ignition of
the first spot fire to result in flames against the wall. The third was the first time at which
flames were observed on the wall. Spot fire ignition times were determined by using video
processing software to first identify the spot fire in the mulch bed and then to track it
backwards in time to the point at which the first wisp of smoke could be detected. The
indication of ignition was often subtle, and there were very few cases in which the depositionof the igniting ember was observed. The timing uncertainty was therefore estimated as ±10 s.
The orange color of the flame made it easy to identify flames nearing the wall, but the 3D
nature of the flames and their often transient nature added to the uncertainty, estimated at±3 s.
Table A.4 summarizes the timing uncertainties, which are all Type B and are determined byscientific judgment.
Table A.4. Uncertainty in timing data
Measurement

Component Standard
Uncertainty
𝒖𝒋

Combined Standard
Uncertainty𝒖𝒄

Expanded
Uncertainty
𝟐𝒖𝒄

End of Gas Burner Ignition (s)Start to Remove Gas Igniters (s)Fan On (s)Fan Off (s)Water First Applied (s)Test duration (s)Fan onFan off
Time to spotting (s)Fan onSmoke detected
Time to flames on wall (s)Fan onFlame detected

1.0
0.5
0.5
1.0
1.0

1.0
0.5
0.5
1.0
1.0
1.1

2.0
1.0
1.0
2.0
2.0
2.2

10

20

3.0

6.1

0.5
1
0.5
10
0.5
3

Other approaches to determining timing were considered. A digital timer was visible in mostvideos recorded from the left side, but variations of a few seconds in when the timer was set
by one of the test operators made this a less accurate measure of the test timing. A consistent
discrepancy of about 14 frames, or 0.5 s, between two of the video processing software
packages resulted in a decision to use a single tool (VirtualDub) for both timing the videos
and extracting images from them.

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A.4.

Flame Spread Analysis

The measurement of the flame front location, described in Section 3.1.3 for mulch
experiments and Section 3.1.4 for experiments with fences, was subject to uncertainty frommultiple sources. These included the physical uncertainties of the experimental setup as
described in Appendix A.1, the process of determining the perspective lines, the uncertainty
in setting the physical scale, and the determination of the leading edge of the flame front.
Uncertainties in timing, covered in Appendix A.4, also contributed to the uncertainty of theplotting of flame front location as a function of time, but the contribution was small relative
to the uncertainties in the location of the flame front.
Sources of uncertainty were Type B, with standard uncertainty based on scientific judgment.
An estimate of the component standard uncertainty is given for each source.
A.4.1. Mulch
For mulch experiments, the location of the flame front was expressed as the distance from theshed wall to the closest boundary of the charred mulch. The definition required the charred
region to be continuous with the area of ignition; spot fires along the mulch bed were ignored
until the main burning region joined them. Section 3.1.3 describes the analysis of video
images to track the progress of the flame front over time. Sources of uncertainty included thelocation of the mulch pan (the separation distance, covered in Appendix A.1), the selection ofpoints to define perspective lines at the shed wall and away from the shed wall, thedimensional scale, and the ability to locate the flame front visually.
The perspective line at mulch height along the wall could be off by ±0.5 cm for zeroseparation cases, due to uneven piling of mulch against the wall and distortions in the mulchpan, and as much as ±1 cm for experiments with nonzero separation distances, for which theline along the top of the target mulch pan was estimated.
The accuracy of the perspective line away from the shed wall depended on the availability
within the camera’s field of view of a good physical line parallel to the shed wall. In earlyexperiments up to Test A-50, the far end of the mulch pan could be used when it wasavailable. When it was not, other lines were found, such as the shadow of the probe array on
a sunny day or the near end of the mulch pan when the separation distance was large. The
standard uncertainty in flame front location due to the far perspective line was estimated as±3 cm for these experiments. Painted lines added to mark the pavement at 0.30 m (1 ft)
intervals starting with Test A-51 decreased the uncertainty by providing a series of lines
parallel to the shed wall that could be used to identify perspective lines. The standard
uncertainty for experiments with markers was estimated as ±1 cm.
Distances from the shed wall were calibrated by selecting a point at a known distance. Up to
Test A-50, distances were usually calibrated using a mark on the pavement at 1.0 m (39 in)from the shed wall. For later experiments, a distance marker near the ignition area wasselected. Setting the dimensional scale was estimated to contribute ±0.5 cm to theuncertainty.
For the video image analysis, the surface of the mulch was assumed to be in a plane along the
top of the mulch pan. Perspective lines at and far from the shed wall were projected to the topof the mulch pan at 2.5 cm. In actuality, the depth of the mulch varied from 2.5 cm along the216

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sides to 5 cm at the center. The warping of the mulch pans over time could increase the levelof the mulch by as much as 1 cm in affected areas. The effect of the mulch depth uncertainty
and bias on flame front location was minimal for straight line-of-sight from the camera but
could be ±0.5 cm near the ignition area.
After identifying points to define the perspective lines, the selection of each point using themouse was dependent on the patience and care of the analyst. An additional uncertainty of±0.5 cm was included for this factor.
Camera lens distortions were neglected in the assessment of uncertainty for perspective lines.
An accurate visual determination of the flame front location was inherently challenging. The
inhomogeneity of the various mulch types, with gaps and shadows between overlapping
particles, made it difficult to mark the boundary of the char with precision. The contrast
between burned and unburned areas in the mulch depended on the lighting on sunny or
cloudy days and on the presence of smoke and flames in the image. Frames in which thesmoke, flames, or an obstruction completely obscured the flame front were skipped. This
source of uncertainty was estimated as ±3 cm.
The combined standard uncertainty for the flame front location for mulch experiments,
calculated as the square root of the sum of squares from all of these sources, ranged from
±25 mm to ±39 mm, as listed in Table A.5. The expanded uncertainty with coverage factor
𝑘 = 2, for a confidence level of 95 %, ranges from ±50 mm to ±77 mm (±2.0 in to ±3.0 in).
Table A.5. Mulch experiments: Uncertainty in flame spread analysis

Measurement
Flame front location
Separation distance (Sep)(from App. A.1)
Perspective line at wall
Zero Sep/Nonzero Sep
Perspective line far from wall
Markers/No Markers
Distance scale
Mulch depth
Point selection
Visual char boundary
TimingFan On (from App. A.3)

ComponentStandard
Uncertainty
𝒖𝒋

CombinedStandard
Uncertainty𝒖𝒄

Expanded
Uncertainty
𝟐𝒖𝒄

34 mm/45 mm

67 mm/89 mm

0.5 s

1.0 s

5 mm
5 mm/10 mm
10 mm/30 mm
5 mm
5 mm
5 mm
30 mm
0.5 s

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A.4.2. Fences
For fence experiments, the location of the flame front was expressed in two dimensions
extending over the surface of the fence and including the posts at both ends. Section 3.1.4
describes the analysis of video images to track the progress of the flame front over time. The
horizontal location of the flame front was considered to be the furthest point the char had
reached downwind, and the vertical location was the highest point reached by the char. The
height of the char was measured both in the ignition region and in the area downwind from
ignition. In addition to flame front location as a function of time, 2-D profiles of the charregion were captured at specific times.
Sources of uncertainty in defining the boundaries of the char front in horizontal and verticaldirections included the dimensions and setup of the fence, the selection of points identifying
fence corners, and the ability to locate the flame front visually.
Uncertainties for the lengths and heights of the two types of fences most commonly used in
the study (western redcedar privacy and redwood or pine lattice) were calculated inAppendix A.1. The maximum standard uncertainty values for these measures are repeated inTable A.6.
Identification of the corners of the fence in the video images was subject to some
imprecision. For the two lower corners, the mulch may obscure the exact location,
introducing an uncertainty estimated as ±5 mm in both directions. For the upper corners, theheight of the post sometimes differed slightly from that of the fence panel, introducing an
uncertainty of ±3 mm in the vertical direction. The uncertainty of the position of each upper
point in the horizontal direction was also estimated as ±3 mm. Combining these values
separately in vertical and horizontal directions resulted in a standard uncertainty of ±8 mm
for this factor.
The selection of each point using the mouse was dependent on the accuracy of the pointerand the patience and care of the analyst. An uncertainty of ±2 cm was included for thisfactor.
Camera lens distortions were neglected in the assessment of uncertainty.
The visual determination of the char boundary provided some challenges. Although easier todistinguish than for the mulch bed, dark spots in the boards and shadows sometimes masked
the location of the char front, as did the presence of flames and smoke. The char front was
not sharp – the wood color changed from light to dark over a distance of a few millimeters.
Error was introduced when selecting points on the surfaces of the stringers and posts because
they were not in the plane of the fence panel. This source of uncertainty was estimated as±1 cm.
The combined standard uncertainty for the flame front location for fence experiments,
calculated as the square root of the sum of squares from all of these sources, was 1.4 cm inboth horizontal and vertical directions, as listed in Table A.6. The expanded uncertainty withcoverage factor 𝑘 = 2, for a confidence level of 95 %, was ±2.8 cm (±1.1 in).

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Table A.6. Fence experiments: Uncertainty in flame spread analysis

Measurement

ComponentStandard
Uncertainty
𝒖𝒋

Horizontal flame front
Fence length (from App. A.1)
Four corner points
Point selection
Visual char boundary

3.1 mm
8 mm
20 mm
10 mm

Vertical flame front
Fence height (from App. A.1)
Four corner points
Point selection
Visual char boundary

2.8 mm
8 mm
20 mm
10 mm

TimingFan On (from App. A.3)

0.5 s

CombinedStandard
Uncertainty𝒖𝒄

Expanded
Uncertainty
𝟐𝒖𝒄

24 mm

48 mm

24 mm

48 mm

0.5 s

1.0 s

As a final note, some situations required judgment calls. One of these was the discontinuityof the flame front at some fence board edges due to the spread of fire in the thin space
between the boards. In these cases, following every detail with a large number of selected
points would result in a 2-D char with many sharp spikes. Instead, selecting a midpoint
between char boundaries on adjacent boards emphasized the general shape of the charprofile.
In some experiments, including many with parallel fences, smoke emerged from under thefence ahead of the char, making it difficult to decide on the flame front location. Fortunately,
the fire spread in these experiments was rapid, making the precision of each point lessimportant.

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Appendix B. Flammability Comparison of Fence Materials
Flammability measurements to compare three types of fence materials used in this study,
rigid polyvinyl chloride (PVC), western redcedar (WRC) wood, and wood-plastic
composite #1 (WPC1), were obtained using the cone calorimeter. While the flammability
performance of bulk PVC and wood materials is well known, the knowledge about theburning behavior of wood-plastic composites is limited. The intent of this materials study
was to compare flammability performance of samples taken from the actual fence panels
tested in this study. The flammability performance of fence materials was tested using
standard cone calorimetry protocols as described in ASTM E-1354. The cone calorimeter test
results reported here include ignitability, heat release rate, and mass loss of samples at
specified external heat flux. The comparison did not include measures of smoke toxicity.
B.1.

Test Sample

Specimens with nominal dimensions of 100 mm × 100 mm were cut from PVC, WRC, and
WPC1 fence panels. Cross-sectional and top views of the specimens are shown in Fig. B.1.
The WRC and WPC1 samples were solid, with nominal thicknesses of 20 mm and 6 mm
respectively. The PVC sample consisted of two sheets 1.23 mm (0.048 in) thick separated bya gap of 18 mm that was supported by three 1.23 mm thick braces. Although PVC and WRC
samples were nominally the same total thickness, the geometry was very different. The PVC
samples can be characterized as hollow structures with thin elements separated by large gaps
and studs as seen in Fig. B.1 (a). Previous studies [70] have shown that hollow-shaped
geometries remove combustible material and reduce the fire load but simultaneously increase
fire propagation. Nominal masses for PVC, WRC, and WPC1 samples were 44.3 g ± 0.1 g,
66.7 g ± 2.0 g, and 59.1 g ± 1.1 g, respectively.

Fig. B.1. Specimens of a) PVC, b) WRC, and c) WPC1 fence boards showing cross-sectional and top
views.

Samples were conditioned for 48 hours in the laboratory, with nominal temperatures of 20 °C
and variable relative humidity not exceeding 50 %. The samples were allowed to come toequilibrium under laboratory conditions. Although moisture content in cellulosic fuels isknown to have a significant effect on burning behavior, no attempt was made to control the
moisture content of wood samples. The influence of moisture on the average mass loss rateand hence the heat release rate can be ignored when moisture content in wood is lower than
11 % [71]. The thermal conductivity, however, is known to increase with increasing moisture220

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content. Thermal conductivity increases approximately 1.3 times when moisture content
increases from 10 % to 30 % [72].
B.2.

Experimental Procedure

Cone calorimeter measurements were performed using the protocols described in
ASTM E-1354. Three samples of each fence material were tested in a horizontal
configuration at an incident heat flux of 50 kW/m². Uncertainties in heat release ratemeasurements are typically within 5 % and 10 % for HRRs larger than 50 kW/m² [73, 74].
Cone calorimeter samples were placed in an aluminum foil pan with nominal thickness of
0.035 mm ± 0.01 mm (specified by ASTM E-1474), as shown in Fig. B.2. The sample
surface was placed at a distance of 25 mm from the cone heater base. A spark igniter wasused to ignite pyrolysis gases generated by heating the sample. Digital still images weretaken from various locations to provide additional visual characterization of test samples.
(a)

(b)

(c)

Fig. B.2. Cone calorimeter samples of a) PVC, b) WRC, and c) WPC1 fence materials.

B.3.

Test Results

Cone calorimetry data for PVC, WRC, and WPC1 fence materials are given in Table B.1,
Table B.2, and Table B.3, respectively. Data include the original sample mass, time toignition (TTI), flame out (FO) time, peak heat release rate (PHRR), average HRR integrated
over 600 s, total heat release (THR) obtained by integrating HRR over duration of flaming
combustion, effective heats of combustion (EHOC), and percentage of original sample mass
lost over the experiment.
Table B.1. Cone calorimetry data for PVC fence material at 50 kW/m2.
SampleMass, g

TTI, s

FO, s

PHRR,
kW/m2

1
2
3
Avg.

44.2
44.4
44.2
44.3 ± 0.1

37
37
39
38 ± 1443449
430
441 ± 10

153
157
168
159 ± 8221

Avg.
HRR_600s, kW/m2
51
54
52
52 ± 2

THR,
MJ/m2

EHOC,
MJ/m2

Mass lost,
%

29
29
30
29 ± 18.7
8.9
9.1
8.9 ± 0.2

74.9
74.5
75.1
74.8 ± 0.3

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Table B.2. Cone calorimetry data for WRC fence material at 50 kW/m2.
SampleMass, g

TTI, s

FO, s

PHRR,
kW/m2

1
2
3
Avg.

65.8
65.3
69.0
66.7 ± 2.0

18
15
18
17 ± 2864885
929
893 ± 33

184
197
190
190 ± 7

Avg.
HRR_600s, kW/m2
81
84
81
82 ± 2

THR,
MJ/m2

EHOC,
MJ/m2

Mass lost,
%

67
69
72
69 ± 2

12.3
12.4
12.6
12.5 ± 0.1

82.5
84.6
82.2
83.1 ± 1.3

Table B.3. Cone calorimetry data for WPC1 fence material at 50 kW/m2.
SampleMass, g

TTI, s

FO, s

PHRR,
kW/m2

1
2
3
Avg.

59.8
57.9
59.7
59.1 ± 1.1

24
23
31
26 ± 4

1004
1040
1060
1035 ± 28

417
421
429
422 ± 6

Avg.
HRR_600s, kW/m2
234
229
239
234 ±5

THR,
MJ/m2

EHOC,
MJ/m2

Mass lost,
%

169
173
174
172 ± 3

28.0
29.4
29.7
29.0 ± 0.8

88.6
89.7
82.4
86.9 ± 4.0

Heat release rate measured using the cone calorimeter is the most important property, since
the fire hazard is dominated by the rate of heat released as the material is consumed in a fire.
A burning material will spread fire to nearby products only if it gives off enough heat toignite them. Heat release rate is largely influenced by the chemical stability of a material.
Several parameters are derived from heat release rate data, including the peak value in the
HRR curve (termed as the peak heat release rate), the average heat release rate over time, thetotal heat release rate, and the effective heat of combustion (defined as the instantaneous
HRR divided by the instantaneous mass loss rate). The EHOC is used to provide timeresolved insights into the materials undergoing combustion and the completeness of the
combustion process.
The values for WRC and WPC1 samples are in good general agreement with cone
calorimeter results for similar materials published in previous studies [75, 76].
The temporal profiles of HRR for PVC, WRC, and WPC1 samples are shown in Fig. B.3.
The WRC samples exhibit two characteristic peaks of wood burning, PVC samples exhibit asingle peak, and the WPC1 samples have three distinct peaks in their respective temporalprofiles.

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500
PVC-1
PVC-2
PVC-3
WRC-1
WRC-2
WRC-3
WPC-1
WPC-2
WPC-3

450

HRR, kW/m2

400

350
300
250
200
150
100
50

0
0

200

400

600

800

1000

1200

1400

Time, sFig. B.3. Heat release data for PVC, WRC, and WPC1 fence materials exposed to 50 kW/m2
heat flux.

B.4.

Ignitability and Extinction

Ignitability of materials has great importance in fire safety research, since it is associatedwith the initiation and subsequent growth of fire. Sample orientation has a significant effecton the ignitability of the samples. Earlier studies have shown that ignition times forhorizontal orientation are shorter than for vertical orientation for plastic and wood
samples [77]. This is primarily due to lower convective cooling in the horizontal orientationcompared to the vertical orientation [78].
At an external heat flux of 50 kW/m2, the times for sustained flaming ignition weresignificantly less for wood (17 s ± 2 s) as compared to the PVC samples (38 s ± 1 s), andignition times for WPC1 samples (26 s ± 4 s) were intermediate between WRC and PVC
samples. This indicates that WRC wood samples were easiest to ignite. The flame out (FO)times for PVC samples were lower than those for WRC samples by nearly a factor of two.
Once ignited, WRC exhibited flaming combustion for 893 s ± 33 s, as compared to
441 s ± 10 s for PVC samples. Longer ignition times and shorter duration of flaming for PVC
samples suggest that the risk of contributing to the fire hazard is lower for PVC than for
WRC wood. The WPC1 samples continued to burn for a longer duration (1035 s ± 28 s),
indicating greater fire hazard compared to WRC and PVC samples.
B.5.

Heat Release Rate

Many studies report PHRR values and use this value for screening materials. However, the
PHRR value may not always be an appropriate indicator of fire hazard, depending on the end
use application of the products tested. For example, in this study, the PHRRs for WRC and
PVC samples vary by only 20 %. However, the average HRR over 600 s and the total heat223

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released during the flaming combustion of these two samples vary by 60 % and 135 %respectively. The PHRR value (422 kW/m2 ± 6 kW/m2) and the average HRR over 600 s(234 kW/m2 ± 5 kW/m2) for the WPC1 samples are significantly higher than for WRC and
PVC samples.
The THR is related to the total amount of fuel present and the fraction consumed. Hence, the
THR is an indicator of the fire load and fire hazard associated with the material. Higher THR(172 MJ/m2 ± 3 MJ/m2) and higher average HRR (234 kW/m2 ± 5 kW/m2) for WPC1
samples reveal that this material has greater flammability compared to PVC and WRC
samples, whose corresponding values are significantly lower.
Another derived parameter that can be used for comparing burning intensities of samples isthe effective heat of combustion. The lower EHOC for PVC samples(8.9 MJ/m2 ± 0.2 MJ/m2) suggests incomplete combustion of fuel and corroborates the lower
burning intensity of these materials compared to wood (12.5 MJ/m2 ± 0.1 MJ/m2) and woodplastic composites (29.10 MJ/m2 ± 0.8 MJ/m2).
B.6.

Firebrand Generation and Char Residue

Significantly different burning behavior related to firebrand generation was visually observed
and digitally recorded for PVC, WRC, and WPC1 samples. Images of PVC, WRC, and
WPC1 samples after 800 s of exposure to 50 kW/m2 heat flux are shown in Fig. B.4.
Formation of firebrands in the wood sample is clearly shown in Fig. B.4 (a). Firebrands wereobserved following sustained flaming combustion in the wood specimen. Small glowing
firebrands separated from the test specimen and flew away from the sample holder,suggesting generation of potential secondary ignition sources. Flaky ash residue remained
after glowing combustion ceased, as shown in image (b).
After the same exposure period of 800 s, Fig. B.4 (c) shows that the PVC sample has formed
a dome-shape hard carbonaceous residue with a layer of white particles. The white particles
are understood to be the residue of metal elements incorporated to improve the flame
retardancy and smoke suppression of the polymer [79].
The WPC1 sample burned intensely. However, no evidence of firebrand generation wasnoted. The residue of the WPC1 sample was woody dust that exhibited no glowing
combustion after the flaming combustion ceased, as seen in Fig. B.4 (d).

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Fig. B.4. Digital images of (a) firebrand formation in WRC wood sample after flame extinction,(b) flaky, ash residue of wood, (c) hard char residue of PVC sample, and d) flaky, woody residue of
WPC1 sample after 800 s of exposure to 50 kW/m2 heat flux.

B.7.

Concluding Remarks

In this limited experimental test series, the burning behaviors of selected WRC, PVC, and
WPC1 fence materials have been characterized and compared using cone calorimetry data.
Cone samples 100 mm × 100 mm in size were cut from fence boards. The WRC and WPC1
samples were solid, with thicknesses of 20 mm and 6 mm respectively. The PVC sample
consisted of two thin sheets separated by 20 mm with three thin sheets serving as braces.
Because of differences in the densities of these materials, the mass of the WRC wood sample
was 50 % greater than the PVC sample, and the total masses of WRC and WPC1 samples
were comparable.
Cone testing revealed distinct differences in both quantitative and qualitative burning
behaviors of WRC, PVC, and WPC1 fence materials. Wood-containing samples (WRC and
WPC1) ignited easily, and the duration of flaming was higher compared to PVC by a factorof two. The WPC1 samples burned intensely, with peak and total heat release values greatly
exceeding those of the other two materials. While the PHRR value for wood was marginally

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(20 %) higher than that for PVC samples, the total heat release and the average heat release
rates for wood were significantly higher, indicating that wood has higher flammability than
PVC samples.
Visual observations suggested that WRC samples continued to smolder for a longer duration
after the flames extinguished. Firebrand generation was observed in wood samples during
flaming and after flame extinction. No smoldering combustion or firebrand generation wasobserved for PVC and WPC1 samples.
With this limited data set, this study has demonstrated that WPC1 fence materials are highly
flammable and can contribute significantly towards the development and growth of the fire
hazard, particularly in the WUI fire environment. The WRC wood fence material burns lessintensely compared to WPC1 samples; however, WRC has a tendency to smolder and form
firebrands that can serve as potential secondary ignition sources. The burning behavior of
PVC materials suggests that they have little or no tendency to form firebrands. From a
flammability viewpoint, their contribution to the fire hazard is significantly less than that ofmaterials used for WRC or WPC1 fences. However, it is important to note here that PVC is
not completely benign in a fire environment, and smoke toxicity effects should be weighedagainst limited burning and flame spread in a WUI fire.

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Appendix C. Wind Characterization
Measurements from the bidirectional probe array described in Sections 2.7.1 and 2.8.1
provided wind speed data in a plane orthogonal to the direction of the wind flow from thefan. For each individual experiment, the data collected from each probe were averaged overtime to observe the wind speed as a function of horizontal distance from the centerline andheight above the ground. This appendix describes how the data from multiple experiments
were combined to provide insights into wind speed contours as a function of nominal applied
wind speed and probe array location between the fan and the shed. The uniformity of thewind field in the region of the burning fence and/or mulch bed was of particular interest.
C.1.

Measurement Distance from Wind Machine

The first step to organizing the wind data was to identify the distance of the measurement
probe array from the wind machine for each experiment. Since the position of the probe array
moved with that of the burning fence and/or mulch bed, this distance varied as a function of
the separation distance.
The geometry of the experimental setup is shown in Fig. C.1. This diagram focuses on thelocation of the probe array relative to the wind machine, fence, and target shed. The
horizontal distance 𝑑𝑤𝑝 from the wind machine to the bidirectional probe array is equal tothe distance 𝑑𝑤𝑠 from the wind machine to the shed minus the distance 𝑑𝑝𝑠 from the probearray to the shed, 𝑑𝑤𝑝 = 𝑑𝑤𝑠 − 𝑑𝑝𝑠 . The distance of the probe array from the target shed isequal to the distance 𝑑𝑝𝑓 of the probe array from the fence plus the total length 𝑑𝑓𝑒𝑛𝑐𝑒 of a
privacy fence plus the separation distance 𝑑𝑠𝑒𝑝 of the fence from the target shed, or
𝑑𝑝𝑠 = 𝑑𝑝𝑓 + 𝑑𝑓𝑒𝑛𝑐𝑒 + 𝑑𝑠𝑒𝑝 . Combining these equations results in 𝑑𝑤𝑝 as a function of
separation distance, 𝑑𝑤𝑝 = [𝑑𝑤𝑠 − (𝑑𝑝𝑓 + 𝑑𝑓𝑒𝑛𝑐𝑒 )] − 𝑑𝑠𝑒𝑝 .

Fig. C.1. Distances between experimental elements. Distances to scale.

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For all experiments, 𝑑𝑤𝑠 = 10.67 m (35 ft) and 𝑑𝑝𝑓 = 1.22 m (4 ft). The length of privacy
fences was equal to the panel length plus the width of the two posts at each end of the panel,
for a total length of 2.62 m (8 ft 7 in). (For lattice fences, the panel overlapped each post by
5 cm (2 in), for a total length of 2.51 m (8 ft 3 in). This difference in length will not be
considered in this analysis.) The distance from the wind machine to the probe array as afunction of separation distance is therefore 𝑑𝑤𝑝 = 6.83 m − 𝑑𝑠𝑒𝑝 , with all measurements inmeters.
In Fig. C.2, the locations of the probe array are highlighted for the four separation distances
between fence/mulch and target shed used in this study. The position of the fence that
corresponds to the longest separation distance, 𝑑𝑠𝑒𝑝,𝐴 = 1.83 m (6 ft), is shown in gray. For
cases at this separation distance, the fence and/or mulch bed overlaps probe array locations C
and D. The velocity profiles at these locations are therefore directly relevant to the flow fieldover the test object in these cases. As the location moves closer to the target shed, the flow
field is expected to be more diffuse, lower to the ground, and increasingly affected by thetarget shed.

Fig. C.2. Locations of bidirectional probe array for separation distances of (A) 1.83 m, (B) 0.91 m,(C) 0.30 m, and (D) 0 m. Distances to scale.

The distance from the fan to the probe array is listed in Table C.1 for each probe array
location. With this information, the wind data from the full set of experiments can beorganized to look at how velocity patterns change at increasing distances from the fan.
An expectation can be established for the uniformity of the velocity profiles by referring to
the literature on ducted axial fans [32]. As discussed in Section 2.2.3, a uniform air velocity
across the duct area of a ducted axial fan is expected to be achieved at a minimum of 2 ½
equivalent duct diameters. The rightmost column of Table C.1 provides the distance from the
fan to the probe array divided by the fan diameter D = 2.11 m (83 in) and shows that the 2 ½
diameter threshold is exceeded for every probe array location except for A.

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Table C.1. Distances from fan for bidirectional probe array at various separation distances.

Array location

Separation
distance 𝒅𝒔𝒆𝒑 (m)Distance fromprobe array toshed 𝒅𝒑𝒔 (m)Distance fromfan to probearray 𝒅𝒘𝒑 (m)Distance from fan toprobe array / Fandiameter𝒅𝒘𝒑 / D

A

1.83

5.67

5.00

2.4

B

0.91

4.75

5.92

2.8

C

0.30

4.14

6.53

3.1

D

0

3.84

6.83

3.2

C.2.

Wind Profiles

To calculate mean wind field profiles, the experiments were divided into 12 categories
according to the three wind speed levels (Low, Medium, High) and four separation distances(corresponding to array locations A, B, C, D) used in the study. Experiments that were twofence panels long, experiments oriented at an angle of 45° to the wind, and experiments
without a shed were not included in the calculations. The number of experiments used foreach of these categories is shown in Table C.2.
Table C.2. Number of experiments in each category of wind speed and array location.
Array Location

Low Wind Speed

Medium Wind
Speed

High Wind Speed

A

36

22

19

B

9

14

11

C

5

12

12

D

10

17

12

A statistical analysis using standard methods [67, 80, 81] was performed on the dataset
collected from the bidirectional probes. Three indices were used to keep track of variables
during the calculation: the experiments were indexed by 𝑘 = 1, … , 𝑀, the probes wereindexed by 𝑗 = 1, … ,13, using the numbering system from Fig. C.3 (repeated from Fig. 17),and the individual values in a dataset from experiment 𝑘 were indexed by 𝑖 = 1, … , 𝑁𝑘 .

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Fig. C.3. Diagram of the bidirectional probe array used to measure the velocity field.
(Repeated from Fig. 17).

The calculation for wind speed embedded in the Labview program described in Section 2.8.1
for collecting data from the bidirectional probes is
𝑉𝑖,𝑗,𝑘 = 0.0698

1√∆𝑃𝑖,𝑗,𝑘 𝑇𝑖,𝑘
𝐶

(C-1)

where 𝐶 is the probe constant, ∆𝑃 is the probe differential pressure in Pa, and 𝑇 is theambient temperature in K. This equation is derived from Equation A.14, with the universal
gas constant 𝑅𝑢 , standard atmospheric pressure 𝑃𝑠 , and molecular mass of dry air 𝑀𝑎𝑖𝑟
incorporated into the value 0.0698. The indices indicate that this is the 𝑖th data point from the𝑗th probe in experiment 𝑘.
The mean value 𝑉̅𝑗,𝑘 of the wind speed for each probe 𝑗 during experiment 𝑘 is given by𝑁𝑘

1𝑉̅𝑗,𝑘 =
∑ 𝑉𝑖,𝑗,𝑘
𝑁𝑘

(C-2)

𝑖=1

with a standard deviation of𝑁𝑘1
2
𝜎 𝑗,𝑘 = √ ∑(𝑉𝑖,𝑗,𝑘 − 𝑉̅𝑗,𝑘 )
𝑁𝑘

(C-3)

𝑖=1

that expresses the variability of the unsteady process under study. This Type A analysis
quantifies the variation in wind speed due to fluctuations caused by the fan, ambient wind,

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and any other influences during an experiment. The uncertainty of the mean value of thewind speed for probe 𝑗 during experiment 𝑘 is equal to the standard error of the mean,𝜎𝑗,𝑘𝜎𝑉̅𝑗,𝑘 =(C-4)√𝑁𝑘
The Type A uncertainty of the unsteady wind speed measurements can now be combinedwith the Type B relative standard uncertainty derived in Table A.3 for the bidirectional probe
wind speed calculation. The RSS method is used to arrive at the total standard uncertainty of
the wind speed value for probe 𝑗 during experiment 𝑘,
2𝑢(𝑉̅𝑗,𝑘 ) = √𝜎𝑉̅2𝑗,𝑘 + (𝑢𝑟,𝐵𝑃 𝑉̅𝑗,𝑘 )(C-5)

with 𝑢𝑟,𝐵𝑃 = 0.0703 multipled by the mean wind speed to obtain the absolute contribution of
the probe calculation.
With these calculated values, the measurements from experiments that fell into each of thetwelve categories of nominal wind speed and separation distance displayed in Table C.2 canbe combined to estimate the means and uncertainties of data from each of the 13 bidirectional
probes. The calculation requires the assumption that measurements from all experiments inthe same category were taken from the same data distribution. This assumption may be
weakened by the variability from test to test in wind machine operation and ambient windconditions, as well as variations in probe array positioning. The measurements were also
assumed to be taken from a normal distribution.
Combining the mean values for each probe 𝑗 over all of the experiments at the same nominal
wind speed and separation distance from the shed can be accomplished through a weighted
mean. This calculation accounts for the differences in the duration, and thus the number ofdata points, for each experiment. With 𝑁𝑘 equal to the number of data points in experiment 𝑘
and 𝑀 the number of experiments in the set being considered, the weighted mean of the windspeed for probe 𝑗 is̅∑𝑀
𝑘=1 𝑁𝑘 𝑉𝑗,𝑘𝑉𝑗 =∑𝑀
𝑘=1 𝑁𝑘(C-6)

for the set of experiments being considered. This calculation was done for each of the twelve
cases.
The uncertainty of the weighted mean for each probe can be calculated as𝑢𝑐 (𝑉𝑗 ) =1∑𝑀
𝑘=1 𝑁𝑘𝑀√∑ 𝑁𝑘2 𝑢2 (𝑉̅𝑗,𝑘 )

(C-7)

𝑘=1

Multiplying 𝑢𝑐 (𝑉𝑗 ) by coverage factor 𝑘 = 2 results in the expanded uncertainty for theweighted mean with a confidence level of 95 %.
A MATLAB program was written to calculate the means and uncertainties for thebidirectional probe velocities from individual experiments and in combination, and to display
the results. In Fig. C.4 the weighted mean values of the probe velocities are shown for the231

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three nominal wind speeds and four separation distances in the study. The data are displayedin pseudocolor plots, in which a matrix of colored cells on a gray background represents thewind speed value at each probe. Dots indicate the location of bidirectional probes as shownin Fig. C.3. Velocity scales are identical within each family of nominal wind speeds. To get aclearer look at the variation of the mean wind speeds across the center of the flow field,
weighted mean values of probes along the horizontal line 1.22 m (4 ft) above the ground andalong the vertical centerline are plotted in Fig. C.5 and Fig. C.6, respectively.
The plots in Fig. C.4 show the spatial extent of the wind field generated by the fan undereach condition. The winds from the fan were felt over a region with a diameter of about
1.2 m (4 ft) approximately centered on probe 9, at 1.2 m (4 ft) above the ground. With theprobe array at position A, at a distance of 5 m (16.4 ft) from the fan, there was a strong
minimum at the center that corresponded to the hub of the fan. As the distance from the fan
increased, this minimum washed out, until the velocity profile became reasonably uniform
over this entire region at positions C and D. The uniform region extended from the ground toover 1.7 m (5.5 ft) above the ground, spanning both the mulch bed and the fence along
essentially its full height. In the streamwise direction, Fig. C.2 shows that positions C and D
overlapped with the position of the burning fence or mulch bed at the largest separation
distance from the shed. For all of the experiments in the study, therefore, the burning object
was subjected to a wind field that is reasonably uniform.
The plots in Fig. C.4 also illustrate the tendency of the wind speed at the centerline point at
ground level (at probe #3) to increase with increasing distance from the fan. This is inaccordance with the angled flow straightener that tilts the flow field toward the ground.
The maximum Type A relative standard uncertainty value for the weighted mean windspeeds over all probes and all conditions of wind speed and location of the probe array was
0.0067. This is an order of magnitude smaller than the relative standard uncertainty for thebidirectional probe measurement of 𝑢𝑟,𝐵𝑃 = 0.0703 from Table A.3. The uncertainty in the
instrument measurement thus dominated the total uncertainty for the measured values ofwind speed by the probe array.
The profiles in Fig. C.5 and Fig. C.6 provide a more quantitative look at the wind speed data
across the center of the profile horizontally and vertically. For each of the three wind speed
categories, high, medium, and low, the plots demonstrate the trend toward a more uniform
flow field with increasing distance downwind from the fan. In successive plots from position
A (red) to position D (blue), the wind speed increases at 1.22 m (4 ft) above the centerline,
near the center of the flow field, and the maximum wind speeds in a ring around this point
decrease.
Note that the achievement of a uniform air velocity across the width of the region influenced
by the fan at positions C and D is consistent with the guideline from the literature on ducted
axial fans [32] discussed in Section 2.2.3. The discussion of Table C.1 points out that the 2 ½
diameter threshold for a uniform flow is easily exceeded at those two locations.
The profiles in Fig. C.6 support the selection of the average of the velocities of the lower four
probes along the centerline as a measure of the average wind velocity for a given experiment.

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Fig. C.4. Mean wind speed pseudocolor plots by wind speed and probe array location, over all
experiments. Dimensions to scale.

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Fig. C.5. Horizontal weighted mean wind speed profiles 1.22 m (4 ft) above the ground.

Fig. C.6. Vertical weighted mean wind speed profiles along the centerline.

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C.3.

Measure of Wind Speed for Each Experiment

In order to compare experimental results, such as the time to ignite a spot fire, a measure wasneeded to quantify the wind speed for each experiment. The measure needed to be
representative of the actual wind speed felt by the burning fence or mulch bed and its
immediate surroundings. It also needed to account for bad probes, since not every probe wasproperly collecting data for every experiment.
The measure that was chosen to represent the wind field for each experiment was the mean
wind speed for the lower four probe values along the centerline. Using the probe numbers in
Fig. C.3, the characteristic wind speed for a specified experiment 𝑘 is given by(𝑉3,𝑘 + 𝑉4,𝑘 + 𝑉9,𝑘 + 𝑉10,𝑘 )(C-8)
4This measure uses data from probes that extend from 0.30 m (1 ft) to 1.68 m (5.5 ft) in thevertical direction along the centerline. It measures wind speeds starting a short distanceabove a mulch bed and encompassing nearly the full height of a fence.
𝑉̅𝑘 =The standard uncertainty for this wind speed measure for experiment 𝑘 is given by
1
𝑢𝑐,𝑘 = √𝑢𝑐2 (𝑉3,𝑘 ) + 𝑢𝑐2 (𝑉4,𝑘 ) + 𝑢𝑐2 (𝑉9,𝑘 ) + 𝑢𝑐2 (𝑉10,𝑘 )
4

(C-9)

Both calculations can account for bad probes by dropping their wind speed or uncertainty
values from the equation and averaging over the number of good probes.
The plots of weighted mean wind speeds for probes along the centerline in Fig. C.6 indicate
the degree of uniformity of the wind field over the lower four probes for each of the four
array locations, A through D. Table C.3 lists the value of 𝑉̅ using the weighted mean values
from this plot.
Table C.3. Weighted mean velocity for lower four probes along centerline in each category of windspeed and array location, in m/s.
Array Location

Low Wind Speed

Medium Wind
Speed

High Wind Speed

A

6.18

10.61

14.92

B

6.27

10.37

14.57

C

6.18

10.21

14.28

D

6.16

10.44

14.17

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Appendix D. Experimental Matrices
The tables in this appendix show the test numbers for the combinations of fence and mulch,wind speed, and separation distance used in this study. Detailed data for each experiment canbe found by type (i.e., fence only, mulch only, fence plus mulch, and parallel fences) and by
test number in Appendix F through Appendix I.
Abbreviations used in the tables in this appendix are:CW

Combustible wall (Series 1 experiments)

MaS

Mulch as Surrogate (Series II experiments)

WRC

Western redcedar

RW

Redwood

WPC

Wood-plastic composite

HWM

Shredded hardwood mulch½ HWM

Half thickness (2.5 cm) shredded hardwood mulch

PBM

Pine bark mulch

PSM

Pine straw mulch

RM

Rubber (synthetic) mulch

AT

Artificial turf

Wind speed ranges are:Low

5 m/s to 9 m/s (11 mi/h to 20 mi/h)Medium

10 m/s to 13 m/s (22 mi/h to 29 mi/h)High

14 m/s to 18 m/s (30 mi/h to 40 mi/h)236

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Table D.1. Mulch only.
Wind
Speed
Level

Separation
Distance(m)

Low

Series 1:
Combustible Wall

Series 2:
Mulch as Surrogate

No
Fence

No
Fence

No
Fence

No
Fence

No
Fence

No
Fence

No
Fence

No
Fence

HWM

PBM

HWM

½
HWM

PBM

PSM

RM

AT

C-33 b

D-22 cC-27 aD-23 c

B-72 b0

B-83 a,C-4 a

0.3

MediumHigh

0.9

A-97 b

1.8

A-104 a

0.3

A-2 a,
A-8 a
A-50 b

A-72 b

0.9

A-45 b

A-55 b

1.8

0A-42 aC-9 a

C-31 aA-96 b

B-42 aA-43 aA-99 aA-98 bA-83 aA-93 aA-112 bA-44 b

A-76 aA-100 aA-87 b0

A-10 aA-27 aA-88 a0.3

A-51 b0.9

A-47 b,A-60 b
A-46 b

A-52 bA-80 aA-85 a1.8

A-14 b

A-62 b

A-78 aaSpread to shed (directly or via spotting)Spread to end of mulch bed, but not to shedcLittle spreadb237

A-38 aA-86 aA-84 b
C-34 aNIST TN 2228
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Table D.2. Fence only.
Wind
Speed
Level

Separation
Distance(m)

Low

Series 1:
Combustible
Wall
WRC Privacy
Fence

Series 2: Mulch as Surrogate

WRC Privacy
Fence

RW Lattice
Fence

Vinyl Privacy
Fence

WPC Fence

No Mulch

No Mulch

No Mulch

No Mulch

No Mulch

0
0.3
0.9
E-2 a

1.8
MediumHigh

A-18 cA-82 c0.3

A-21 cA-113 c0.9

A-65 cA-77 c1.8

A-30 c, A-101 c

A-75 b

A-28 b

A-81 c0.3

A-32 cA-79 c0.9

A-25 b

1.8

A-26 b, A-91 b

0

0

A-5 c

A-1 c

a

Flames extend above fence
Spread to shed (via spotting)cLittle spreadb

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Table D.3. Fence plus mulch – Western redcedar (WRC) privacy fences.
Wind
Speed
Level

Separation
Distance(m)

Series 1:
Combustible
Wall

Series 2: Mulch as Surrogate

WRC
PrivacyFence

WRC Privacy
Fence

WRC
Privacy
Fence

WRC
PrivacyFence

WRC Privacy
Fence

WRC
Privacy
Fence

HWM

HWM

½ HWM

PBM

PSM

HWM
45 ° Wind
Angle

Low

A-106 aB-70 aB-71 a0.3

A-105 aB-63 aB-73 a0.9

A-95 aB-58 aB-60 aA-94 a1.8

A-102 a,C-19 a (twopanels long)

B-56 aB-57 aA-108 aA-114 a0

Medium.

Highab

A-69 aA-111 a0

A-3 a, A-4 a,
A-7 a

A-19 aA-39 a,A-48 aA-57 a0.3

A-6 b

A-20 a, A-41 a

A-40 aA-73 a0.9

A-11 b

A-17 aA-54 aA-66 aA-92 a, A-89 a

1.8

A-12 b

A-29 aA-64 aA-74 a,A-90 aA-109 a0

A-9 a

A-22 aA-49 aA-58 a0.3

A-16 b,A-61 b

A-23 a, A-33 a

A-56 aA-59 a0.9

A-15 b

A-24 aA-36 aA-53 a1.8

A-13 b

A-31 aA-37 aA-63 aSpread to shed (directly or via spotting)Spread to end of mulch bed, but not to shed

239

A-115 aA-116 a

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Table D.4. Fence raised above mulch bed

All experiments with fence raised above mulch bed were performed at a separation distanceof 1.83 m (6 m) between the end of the mulch bed under the fences and the wall of the shed.
Wind Speed
Level

Height of base offence panel above
surface of mulch(cm)

Series 2: Mulch as Surrogate
WRC Privacy Fence
HWM

Low

Mediuma

5.1

C-10 a7.6

C-11 a10.2

C-14 a15.4

C-28 a5.1

C-12 aSpread to shed (directly or via spotting)240

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Table D.5. Fence plus mulch – Other fences.
Wind
Speed
Level

Separation
Distance(m)

Low

MediumHigh

Series 1:
Combustible
Wall

Series 2: Mulch as Surrogate

RW Lattice
Fence

RW
Lattice
Fence

Pine
Lattice
Fence

Vinyl
Privacy
Fence

Good
Neighbor
Fence

WPC
Fence

Aged
WRC
Privacy
Fence

HWM

HWM

HWM

HWM

HWM

HWM

HWM

0

A-70 b

B-66 b0.3

A-71 c

B-74 b

C-18 b

E-1 a,
F-1 a

C-26 b

0.9

B-59 b

1.8

A-107 b0

B-64 b0.3

A-68 b

0.9

A-67 b

1.8

B-79 b0B-65 b

0.3

B-69 b

0.9

B-61 b

1.8

B-67 b,B-85 b

C-21 b
A-35 b

B-81 b

C-22 bC-25 b

B-86 b

C-23 bC-24 ba

Flames extend above fence
Spread to shed (via spotting)cSpread to end of mulch bed, but not to shedb241

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Table D.6. Parallel fences involving western redcedar (WRC) privacy fence.

All parallel fence experiments were performed at a separation distance of 1.83 m (6 m)
between the end of the mulch bed under the fences and the wall of the shed.
Wind
Speed
Level

Fence
Spacing(m)

Low

Series 2: Mulch as Surrogate
2×WRC
Privacy
Fence

2×
WRC
Privacy
Fence

2×
WRC Privacy
Fence– Two panels
long

WRC
Privacy
Fence /
CementBoard

WRC
Privacy
Fence /
Vinyl
Privacy
Fence

WRC
Privacy
Fence /
Pine
LatticeFence

HWM

No Mulch

HWM

HWM

HWM

HWM

B-84 d

0.152
0.203

B-82 a0.305

C-5 a,
C-32 a,
D-9 a

D-7 b

0.457

C-6 a

C-15 b

0.610

C-7 a

0.914

C-8 a

C-13 aa

Flames extend above fence
Spread to shed (via spotting)cSpread to end of mulch bed, but not to sheddLittle spread
b

242

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Table D.7. Parallel lattice fences.

All parallel fence experiments were performed at a separation distance of 1.83 m (6 ft)
between the end of the mulch bed under the fences and the wall of the shed.
Wind
Speed
Level

Fence
Spacing(m)

Series 2: Mulch as Surrogate
2×
RW Lattice
Fence

2×
RW Lattice
Fence

2×
RW Lattice
Fence

2×
Pine Lattice
Fence

2×
Pine Lattice
Fence

HWM

PBM

No Mulch

HWM

No Mulch

A-103 aA-110 aC-29 aC-20 cLow

0.089

Medium0.089

B-75 aB-80 b

High

0.089

B-62 aB-68 ba

Flames extend above fence
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Appendix E. Reading the Case Descriptions
This appendix explains how to read the data from the experimental cases listed in Appendix
F though Appendix I. The data for each experiment includes a brief description, photographsfrom before and during the experiment, flame spread plots, summary data, and plots showing
applied winds and ambient weather conditions. An example of the data provided for eachexperiment is shown in Fig. E.1, and explanations of data boxes A through F follow.

Fig. E.1. Experimental case description.

E.1.

Data Box A: Experimental Configuration

The top left corner of each experimental case description, shown in Fig. E.2, provides thebasic configuration for the experiment. This includes the test number, the category of theexperiment (Fence Only, Mulch Only, Fence + Mulch, Parallel Fence with/without Mulch, or
No Shed), and the date on which the experiment was performed. The material and type offence and/or mulch, the applied wind speed level (Low, Medium, or High), and theseparation distance of the end of the fence or mulch bed from the wall of the structure arealso listed. Low applied wind speeds indicate a range of 5 m/s to 9 m/s as measured by thebidirectional probes near the centerline of the flow. Medium indicates a range of 10 m/s to
13 m/s, and High indicates 14 m/s to 18 m/s.

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Fig. E.2. Data Box A – Experimental configuration.

E.2.

Data Box B: Photographs Taken Before and During the Experiment

In the top right corner of each case description are two photos, as shown in Fig. E.3. The
photo on the left is the initial setup of the experiment as observed by the camera on the left
side as viewed from the fan. In this photo, the shed appears to the left of the fence or mulch
bed. For cases with single fences, the fence was typically mounted with the stringers facing
this side.
The photo on the right was taken during the experiment and may show the testbed from any
angle. This photo was selected to show an interesting feature of this experiment. In somecases, this photo shows the pattern of fire damage near the end of the experiment; in othercases, flames are shown from a period when the fence was burning strongly. For experimentsin the No Shed category, the right-hand photo shows the target mulch bed (with any ignited
spot fires) at a time near the end of the experiment.

Fig. E.3. Data Box B – Photographs taken before and during the experiment.

E.3.

Data Box C: Flame Spread as a Function of Time

The center left box in the case description contains plots that illustrate the progress of theflame spread over the fence or mulch bed. The plots used data from the mulch and fence
analysis procedures described in Section 3.1.3 and Section 3.1.4, respectively.
E.3.1. Fences
For experiments involving fences, presented in Appendix G through Appendix I, three flame
spread plots are shown, as in Fig. E.4. Distance from the shed is always plotted along the245

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horizontal axis and height is plotted along the vertical axis. For plots as a function of time,
therefore, the axis representing time will vary.
The upper left plot shows the distance of the char front from the end of the fence nearest theshed as a function of time. Time is plotted on the vertical axis, and the locations of the end
posts are marked with vertical lines as a visual aid. The char front begins at t = 0 s close to
the outer end post at the lower right of the plot. With time, the front advances toward the endof the fence closest to the shed at the upper left.
The lower left plot shows the maximum height of the char front downwind of the ignitionzone as a function of time. To look at the flame spread characteristics independent of theignition zone, this plot tracks the maximum char height at least two board widths downwind
from the ignition zone. In this plot, time is along the horizontal axis, and the locations of thestringers are marked with horizontal lines.
On the right is a plot showing the contours of the burned area of the fence over time.
Contours have been obtained at five points in time spaced evenly between the initial and final
times. The order of the sequence is displayed using color, with blue indicating the earliest
contour and red the contour at the end of the experiment.

Fig. E.4. Data Box C – Flame spread as a function of time for fence experiments.

E.3.2. Mulch Only
For experiments with mulch only, presented in Appendix F, data box C contains a single plot
showing flame spread, as shown in Fig. E.5. In this plot, the distance of the char front fromthe wall of the structure is plotted as a function of time, with time along the vertical axis. The
regions occupied by the ignited mulch bed and the target mulch bed next to the shed areshaded. The gap between the ignited mulch bed and the target mulch bed (in Series 2
experiments) or the wall (in Series 1 experiments) is shown in white. The example to theright shows the gap between mulch beds for a separation distance of 0.91 m (3 ft) between
the end of the ignited mulch bed and the shed wall.

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Fig. E.5. Data Box C – Flame spread as a function of time for mulch experiments.

E.4.

Data Box D: Table of Timing Values and Environmental Factors

At the lower left of the case description is a table containing average values and standard
deviation for timing and environmental variables, as shown in Fig. E.6.
The test duration was obtained from subtraction of the time at which the fan was turned onfrom the time at which the fan was turned off, as determined from videos recorded fromcameras mounted to the right and left of the test object. Other times listed in this table
include the time for the flame front to reach the end of the fence or mulch bed, the time forthe first firebrand ignition to occur in the target mulch bed, the time for ignition of the spot
fire that puts flames on the wall of the shed, and the time for those flames to reach the wall.
Expanded uncertainties (at the 95 % confidence level) for these values are estimated from theType B uncertainties of determining the time from the videos, as explained in Appendix A.
Environmental variables include the average values and expanded uncertainties of applied
wind speed, ambient wind speed and direction, and ambient temperature. The value used tocharacterize the applied wind speed is an average of the mean values for the lower fourbidirectional probes along the centerline of the probe array, as discussed in Appendix C.
These probes measure the wind field that strikes the edge of the fence facing the fan.
Ambient data are collected from probes mounted on a 3.7 m (12 ft) pole about 7.9 m (26 ft)
south-southeast of the wind machine propellers and 17.7 m (58 ft) south-southwest of thetarget shed, as described in Section 2.7.2.

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Fig. E.6. Data Box D – Table of timing values and environmental factors.

E.5.

Data Box E: Applied Wind

Wind speeds measured by the 13 bidirectional probes and averaged over the duration of theexperiment (the time during which the fan is on) are displayed in the three plots indata box E, as shown in Fig. E.7.
The pseudocolor plot on the left displays the mean wind speed value for each probe as acolored cell in a rectangular array. The array represents the probe locations in Fig. 17 or Fig.
C.3. The scale along the right of the plot relates color to wind speed. If any of the probes are
not operational during the experiment, they appear as white.
The plot at the top right of data box E shows the mean wind speeds for probes #9, #8, #7, #6,
and #5, which extend horizontally out from the centerline at a height of 1.22 m (4 ft) above
the ground. The probe distance is plotted along the horizontal axis, and the wind speed isalong the vertical axis. The plot typically indicates a minimum at the centerline, which is aflow field artifact from the hub region of the wind machine propellers, as described in
Section 2.2.3. The wind speed decreases rapidly away from the central flow field.
The plot at the bottom right shows the mean wind speeds for probes #3, #4, #9, #10, and #13,
which are arrayed along the centerline of the wind field. For this plot, the height of eachprobe is plotted along the vertical axis, and the wind speed is along the horizontal axis. The
wind speeds are typically highest in the central region and lower at the position of the top
probe. The tilt of the wind straightener, described in Section 2.2.2, helped to extend higher
wind speeds toward the ground.

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Fig. E.7. Data Box E – Applied wind.

E.6.

Data Box F: Ambient Wind

Data box F, located at the lower right of the case description, displays a wind rose and timeplots of temperature and wind speed based on data taken over the duration of the experiment,
as shown in Fig. E.8. The instruments used to measure ambient temperature and winds havebeen described in Section 2.7.2.
In the wind rose plot on the left, the wind speed and direction of the ambient winds arebinned. The length of each spoke indicates the percentage of time that the wind blew fromthat direction during the experiment. The concentric rings are labeled to show the frequency.
The colored bins within each spoke indicate wind speed ranges, which are listed in the scale
to the right of this diagram.
On the upper right of data box F is a plot of ambient temperature as function of time. The
time begins when the fan is turned on and ends when it is turned off, which defines thetiming for the experiment.
On the lower right is a plot of ambient wind speed as a function of time. The red line applies
a moving average filter with a 300 s span to the data, indicating the value and trend of theambient wind speed over the experiment.

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Fig. E.8. Data Box F – Ambient wind.

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Appendix F. Case Details: Mulch Only
This appendix provides the data from all experimental cases on mulch without a fence
present. The data includes a description of the experiment, photographs from before andduring, flame spread plots, critical times, and ambient and applied winds. The data for eachexperiment can be read as described in Appendix E.

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Appendix G. Case Details: Fence Only
This appendix provides the data from all experimental cases in which the fence was mounted
in an empty mulch pan that did not contain mulch. The data includes a description of theexperiment, photographs from before and during, flame spread plots, critical times, andambient and applied winds. The data for each experiment can be read as described inAppendix E.

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Appendix H. Case Details: Fence and Mulch
This appendix provides the data from all experimental cases in which the fence was mounted
in a mulch pan containing mulch. The data includes a description of the experiment,
photographs from before and during, flame spread plots, critical times, and ambient andapplied winds. The data for each experiment can be read as described in Appendix E.

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Appendix I. Case Details: Parallel Fences
This appendix provides the data from all experimental cases in which fence panels were
mounted parallel to each other, with or without mulch beneath. The data includes a
description of the experiment, photographs from before and during, flame spread plots,
critical times, and ambient and applied winds. The data for each experiment can be read as
described in Appendix E.

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