NIST Technical Note 1778

Exposing Wood Decking 

Assemblies to 

Continuous Wind-Driven 

Firebrand Showers

Samuel L. Manzello

Sayaka Suzuki

http://dx.doi.org/10.6028/NIST.TN.1778
NIST Technical Note 1778

Exposing Wood Decking 

Assemblies to 

Continuous Wind-Driven 

Firebrand Showers

Samuel L. Manzello

Sayaka Suzuki

Fire Research Division

Engineering Laboratory

http://dx.doi.org/10.6028/NIST.TN.1778

December, 2012

U.S. Department of Commerce 

Rebecca Blank, Acting Secretary

National Institute of Standards and Technology 

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director 
ii

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. 

National Institute of Standards and Technology Technical Note 1778

Natl. Inst. Stand. Technol. Tech. Note 1778, 20 pages (December, 2012) 

http://dx.doi.org/10.6028/NIST.TN.1778

CODEN: NTNOEF
iii

Table of Contents

page

Abstract iv

1.0 Introduction 1

2.0 Experimental Description 3

3.0 Results and Discussion 9

4.0 Summary 17

5.0 Acknowledgments 17

6.0 References 17
iv

Abstract

A series of experiments were conducted to examine potential vulnerabilities of wood decks to 

continuous, wind-driven firebrand showers. Sections of wood decks (1.2 m by 1.2 m) were 

constructed and attached to a re-entrant corner assembly. The deck/reentrant corner assembly 

was then exposed to continuous, wind-driven firebrand bombardment generated by the newly 

developed NIST full-scale Continuous Feed Firebrand Generator (NIST full-scale Continuous 

Feed Dragon) installed in the Building Research Institute’s (BRI) Fire Research Wind Tunnel 

Facility (FRWTF). The type of wood decks used, as well as the placement within a reentrant 

corner assembly, was based on input received from a decking ignition vulnerability workshop 

that NIST held in California in 2011. Three different wood decks types were exposed to winddriven firebrand showers at wind speeds of 6 m/s; Cedar, Douglas-Fir, and Redwood. For each 

wood deck tested (exposed to a total firebrand mass flux of 17.1 g/m2

s), firebrands were 

observed to accumulate on the deck surface, and due to continual glowing firebrand 

bombardment, each wood deck type was observed to ignite by flaming ignition. The average 

time to flaming ignition was 437 s for Cedar, 934 s for Douglas-Fir, and 756 s for Redwood. 

Therefore, wood decks were observed to be vulnerable to ignition from continuous, wind-driven 

firebrand showers. Results of these experiments are discussed in detail. 
1

1.0 Introduction

Wildland-Urban Interface (WUI) fires have caused significant destruction to communities in 

Australia, Greece, Portugal, Spain, and the USA [1]. There have been three significant WUI 

fires in California over the past six years. The 2003 Cedar Fire in California resulted in $2B in 

insured losses and destroyed more than three thousand homes [1]. The 2007 Southern California 

Fire displaced more than 300,000 people, destroyed over one thousand structures, and resulted in 

$1B paid by insurers in 2007 alone [2]. WUI fires continue to burn in the USA; most recently in 

Texas in 2011 and Colorado and California in 2012. In 2009, fires in Victoria, Australia caused 

the death of more than 150 people, destroying more than two thousand structures.

From a pragmatic point of view, the WUI fire problem can be seen as a structure ignition 

problem [1]. Ignition resistant structures under WUI fire exposure was listed as one of the major 

recommendations in the USA GAO 2005 report, Technology Assessment: Protecting Structures 

and Improving Communications During Wildland Fires [3], and was the subject of a Homeland 

Security Presidential Directive [4]. In spite of these facts, little effort has been spent on 

understanding the processes of structure ignition in these fires. 

As both vegetation and structures burn in WUI fires, pieces of burning material, known as 

firebrands, are generated, become lofted, and are transported by the wind. This results in 

showers of wind-driven firebrands. Post-fire studies indicate that firebrand showers have a 

significant role in the spread of disastrous WUI fires [4-9]. Interestingly, post-fire damage 

studies have suggested for some time that firebrands are a significant cause of structure ignition 

in WUI fires, yet for over 40 years, firebrand studies have focused on understanding how far 

firebrands fly, known as spotting distance [10-21]. Few studies have examined firebrand 

generation [22–24] and the ultimate ignition of materials by firebrands [25–28]. For these 

reasons, the prior firebrand studies are of limited help in designing firebrand resistant structures.

Scientifically based building codes and standards are needed to guide construction of new 

structures in areas known to be prone to these fires in order to reduce the risk of structural 

ignition in the event of a firebrand attack. Proven, scientifically based retrofitting strategies are 

required for homes located in areas prone to such fires. To meet these objectives requires 

knowledge regarding the types of materials that can be ignited by firebrands as well as 

vulnerable points on a structure where firebrands may easily enter. The reason that prior 

firebrand investigations have not been able to quantify the vulnerabilities of structures to ignition 

from firebrand showers is that it is difficult to develop a measurement method to replicate winddriven firebrand bombardment on structures that occur in actual WUI. Entirely new experimental 

approaches are required to address this problem. To address this problem, a new firebrand 

research area targeted on quantifying structure vulnerabilities to wind-driven firebrand showers 

has been developed by NIST. This type of firebrand research is now possible due to the 

development of the Firebrand Generator.

A unique experimental apparatus, known as the NIST Firebrand Generator (NIST Dragon), 

has been constructed to generate controlled, repeatable firebrand showers commensurate with

those measured from burning conifers and a real WUI fire. The purpose of the NIST Dragon is

to simulate wind-driven firebrand showers observed in long-range spotting; therefore, glowing 
2

firebrands were the initial emphasis. Yet, due to careful design of the NIST Dragon, it is also 

possible to generate flaming firebrand showers. Another very important characteristic of the 

NIST Dragon is that the firebrand size and mass produced using the device can be tailored to 

those measured from full-scale tree burns and actual WUI fires, which are in stark contrast with 

the size of firebrands referenced in existing test standards and wildfire protection building 

construction recommendations [29]. In collaboration with the California Department of Forestry 

and Fire Protection (CALFIRE), NIST has quantified firebrand distributions from a real WUI 

fire (Angora Fire) for the first time [29]; more information such as this is needed.

Since wind plays a critical role in the spread of WUI fires, NIST has established 

collaboration with the Building Research Institute (BRI) in Japan. BRI maintains one of the only 

full-scale wind tunnel facilities in the world designed specifically for fire experimentation, the 

Fire Research Wind Tunnel Facility (FRWTF). BRI’s FRWTF, constructed in the late 1990s, is 

one of the first wind tunnel facilities in the world designed specifically with fire experiments in 

mind. Parametric studies have been conducted using the NIST Dragon installed in BRI’s 

FRWTF to expose roofing assemblies, building vents, siding treatments, walls fitted with eaves, 

and glazing assemblies to wind-driven firebrand showers [30-35]. In addition, the dangers of 

firebrand accumulation in front of structures have been quantified for the first time. It is worth 

noting that NIST Dragon has now been reproduced by other research laboratories. For example, 

the Insurance Institute for Business and Home Safety (IBHS) has used the NIST Dragon concept 

to generate firebrand showers in their full-scale wind tunnel facility [36]. 

In WUI fires, decking assemblies have been observed to be an ignition vulnerability based on 

post-fire damage surveys. The Office of the State Fire Marshall (OFSM) in California adopted 

the test method known as State Fire Marshall (SFM) STANDARD 12-7A-4 [37]. The SFM test 

method is intended to determine the response of decks to firebrand exposure and the firebrand 

exposure is very similar to the ASTM E108 [38] roofing test. ASTM E108 has been used for

more than 100 years. Namely, a firebrand is simulated by placing a burning wood crib (either 

Class A, Class B, or Class C firebrand) on top of a section of a deck assembly under an air flow. 

This test standard does not adequately simulate the processes and conditions observed in real 

Wildland-Urban Interface (WUI) fires. The dynamic process of multiple firebrands bombarding

decking materials as a function of time is not taken into account in this standard. Firebrand 

showers have been observed in actual WUI fires over and over again [1]. Based on firebrand 

attack from real WUI fires, it is expected that multiple firebrands would accumulate within 

gaps/crevices of decking materials. In addition to not simulating a dynamic firebrand attack, no 

attempt is made to relate the size and mass of the firebrand used in this standard to actual 

firebrands produced from burning vegetation and structures. There is no evidence to suggest that 

this test is a ‘worst-case’ firebrand exposure for decking assemblies.

 To this end, a series of experiments were conducted to examine possible vulnerabilities of 

wood decks to continuous, wind-driven firebrand showers. Sections of wood decks (1.2 m by 

1.2 m) were constructed and attached to a reentrant corner assembly. The deck/reentrant corner 

assembly was then exposed to continuous, wind-driven firebrand bombardment generated by the

recently developed NIST full-scale Continuous Feed Firebrand Generator (aka the NIST fullscale Continuous Feed Dragon) installed in the FRWTF. While previously the full-scale NIST 

Dragon (Batch Feed) coupled to BRI’s FRWTF has been used to expose building elements to 
3

firebrand showers, the duration of exposure using the existing apparatus is limited. To develop 

test methods needed to evaluate different building materials resistance to firebrand showers 

requires the capability to generate firebrand showers of varying duration. Therefore, a new fullscale Continuous Feed Firebrand Generator was developed in support of these decking assembly 

experiments. 

The type of wood decks used, as well as the placement within a reentrant corner assembly, 

was based on input received from a decking workshop NIST held in California in 2011 [39]. 

The Los Angeles (LA) Basin Chapter of the International Code Council (ICC; 

http://www.iccsafe.org) hosted a workshop on June 15, 2011 in Norwalk, CA. The workshop 

was moderated by Mr. Ruben Grijalva, former CALFIRE director and Mr. Stuart Tom, Certified 

Building Official (C.B.O.) from the city of Glendale, CA, served as the official note taker. Mr. 

Neville Pereira, LA Basin ICC Chapter President, arranged the venue and location. At that 

workshop, input was obtained for this experimental series from interested parties in California 

(e.g. building officials, Office of the State Fire Marshall (OFSM), code consultants, construction 

industry, and product manufacturers) since large WUI fires have occurred in this state recently. 

Specifically, guidance was desired in order to conduct experiments that will have the potential to 

provide the scientific basis for improvements of existing codes and development of new codes. 

The results of the decking workshop form the basis for this study. 

2.0 Experimental Description

Figure 1 is a photograph of the new full-scale Continuous Feed Firebrand Generator. This 

version of the device is modified from the NIST Dragon [30-35] and consists of two parts: the 

main body and continuous feeding component. The feeding system was connected to the main 

body and was equipped with two gates to prevent fire spread (described in more detail below). 

Each gate was alternatively opened and closed. A blower was connected to the main body and 

the purpose of the blower is also described below. All components of the full-scale Continuous

Feed Firebrand Generator were constructed of stainless steel. A major challenge when 

constructing this device was designing a completely contained feeding system shielded from the 

wind tunnel flow. 

The feeding system consisted of a pneumatic cylinder coupled to a cylindrical container 

where wood pieces were stored (see Figure 2). The pneumatic cylinder was contained inside a 

metal sleeve. Inside the metal sleeve, the sliding rod of the pneumatic cylinder was connected to 

a plate that allowed the volume of wood contained within the sleeve to be varied. This volume 

was set precisely to allow a specific mass of firebrands to fall into this volume. When the air 

pressure was applied, the sliding rod of the pneumatic cylinder moved forward, forcing the wood 

pieces within the volume of the metal sleeve to the first gate, where they are then dropped into

the second gate that leads to the Dragon where they are ignited (see Figure 3). Care was taken 

to select the pneumatic cylinder (15 cm bore with 15 cm stroke; maximum pressure of 1.7 MPa 

and maximum load of 31 kN); smaller sized pneumatic cylinders were observed to be unable to 

force the wood pieces to the first gate and would jam. The gate system was required to contain 

the fire from spreading from the Dragon to the feed system and each gate was driven by 

pneumatic cylinders as well. For all tests, Douglas-fir wood pieces machined to dimensions of 

7.9 mm (H) by 7.9 mm (W) by 12.7 mm (L) were used to produce firebrands (see Figure 4). The
4

same-size wood pieces were used to feed the bench-scale continuous Firebrand Generator in past 

studies and have been shown to be commensurate with sizes measured from full-scale burning 

trees, as well size distributions obtained from actual WUI fires [23, 29]. 

An operational parameter that was varied was the blower speed. When the blower was set to 

provide an average velocity below 3.0 m/s measured at the exit of the Dragon when no wood 

pieces were loaded, insufficient air was supplied for combustion and this resulted in a great deal 

of smoke being generated in addition to firebrands. At blower velocities above 3.0 m/s, smoke 

production was mitigated, but many firebrands produced were in a state of flaming combustion 

as opposed to glowing combustion. It has been suggested that firebrands fall at or near their 

terminal settling velocity. As such, when firebrands contact ignitable fuel beds, they are most 

likely in a state of glowing combustion, not open flaming [13]. It is possible for firebrands to 

remain in a flaming state under an air flow and, it is reasonable to assume that some firebrands 

may still be in a state of flaming combustion upon impact. The purpose of this device is to 

simulate firebrand showers observed in long-range spotting and therefore glowing firebrands 

were desired. 

Figure 1 Full-scale continuous Feed Firebrand Generator installed in BRI’s FRWTF. The 

device was not producing firebrands at the time of this photograph.

As in prior experiments using the NIST Dragon, the new experimental device was installed 

inside BRI’s FRWTF (see Figure 5). The facility was equipped with a 4.0 m diameter fan to 

produce the wind field. The cross section of the FRWTF is 5.0 m wide by 4.0 m high. The 

experimental device was installed inside the test section of the FRWTF. A parametric study was 

conducted to determine the influence of various conditions for the firebrands to arrive at the deck 

location. This included varying: the distance of the deck/wall assembly from the full-scale

Continuous Feed Firebrand Generator, the orientation of the deck/wall assembly, the orientation 

of deck board alignment for a given distance and wall/deck assembly orientation, and the wind 
5

tunnel speed for the above mentioned conditions. Having completed the parametric studies, 

experiments were then conducted to expose the decking assemblies to worst case situations, 

namely, those conditions where the greatest number of firebrands arrived and accumulated on the 

surface of the decking assembly to see if it was possible to ignite the decking assemblies due to 

continual application of wind-driven firebrand showers.

 As a result of these scoping investigations, the reentrant corner was placed at a distance of 

3.25 m downstream of the full-scale Continuous Feed Firebrand Generator (see Figure 5 and 6). 

The dimensions of the reentrant corner assembly were 1.2 m wide by 2.1 m high on each side. 

The walls of the reentrant corner were lined with gypsum board since these experiments were 

focused on deck assembly ignition, not the ignition of the wall assembly itself. Each of the 

decking assemblies was installed inside the reentrant corner assembly at the base (ground level). 

In all experiments, the decking assemblies were 1.2 m by 1.2 m. Three different types of wood 

decking boards were used: Cedar, Douglas-Fir, and Redwood. The dimensions of all decks 

boards were: 25.4 mm thick by 137 mm wide (Cedar), 25.4 mm thick by 139 mm wide 

(Douglas-Fir); 25.4 mm thick by 137 mm wide (Redwood). The spacing of the decks boards was 

5 mm. The boards were installed using a wood supporting frame (boards were 38 mm thick by 

140 mm heigh) with members spaced 16” (406 mm) on center.

Figure 2 Schematic of the feeding system for the full-scale Continuous Feed Firebrand 

Generator.
6

A typical experiment was conducted in the following manner. The deck assembly was 

installed inside the reentrant corner. The wind tunnel speed was set to the desired level (e.g. 6 

m/s). Wood pieces were first loaded into the cylinder storage container and the air compressor 

needed to provide compressed air for the pneumatic cylinder and gate system was switched on 

(air compressor pressure was set to 0.7 MPa). The blower was set at 3.0 m/s and two propane 

burners were ignited and inserted into the side of the device. The propane burners were operated

continuously during the experiment. The pneumatic piston was then activated and the sliding rod 

was positioned to allow wood pieces to enter the volume in the metal sleeve. The sliding rod 

was moved to push the wood pieces (200g) to the first gate. The gate was opened, closed, and 

the second gate was then opened, and the wood fell into the Dragon. The feeding was varied to 

determine the optimal conditions for continuous firebrand showers. It was observed that 200g, 

fed into the Dragon every 15 sec provided an adequate firebrand generation rate to ignite 

building materials. For completeness, 200 g corresponded to approximately 400 wood pieces 

deposited every 15 sec.

Figure 3 Side view of the main body (or Dragon) of the full-scale Continuous Feed Firebrand 

Generator. The location where the feeding system provided wood pieces into the device is 

shown.

The number flux (number of firebrand generated/m2s), at the exit of the device, was

measured at a feeding rate of 200g every 15 sec (800g/min). To determine the number flux, the 

number of firebrands was counted at every frame of the video recording, summed every second, 

and then summed again at every ten seconds. Based on the analysis, the number flux reached a 

steady value of 342.0/m2 300 sec after feeding began. The first firebrands began to be generated

~ 100 sec after feeding was commenced.

Mass flux data (mass of firebrands generated/m2

s) were calculated by multiplying the 

number flux and the average mass of each firebrand at a feed rate of 200g every 15 sec. To 

measure the firebrand mass, another set of experiments was conducted using a series of water 
7

pans were placed downstream of the NIST full-scale Continuous Feed Firebrand Generator. 

Water pans were required in order to quench combustion of the firebrands. If the water pans 

were not used, the firebrands would continue to burn and by the time collection was completed; 

only ash remained. After the experiment was finished, the pans were collected and the 

firebrands were filtered from the water, using a series of fine-mesh filters. Firebrands were dried 

in an oven, at 104 °C, for 16 hours. As in previous work, the mass versus drying time was 

monitored to determine the duration need to completely dry the firebrands. The mass and

dimension of each firebrand were measured using precision calipers (1/100 mm resolution) and a 

precision balance (0.001 g resolution). The mean mass and standard deviation of each firebrand 

were obtained and observed to be 0.05 ± 0.02 g. Therefore, the mass flux of generated 

firebrands was calculated to be 17.1 g/m2s.

Figure 4 Picture of wood pieces (bulk – image on top shows pieces inside 215 L drum; 571 mm 

diameter opening) needed to feed the full-scale Continuous Feed Firebrand Generator. Detailed 

image (bottom) of a single wood piece along with a metric ruler in cm.

A great deal of effort has been made to link the firebrand sizes using this device with those 

from actual burning vegetation, and actual WUI fires. Specifically, firebrand sizes produced 

using this device is commensurate with the characteristics of firebrand exposure at a single 
8

location during a severe WUI fire in California [29]. This is incredibly important since empirical 

characterization of firebrand exposure is extremely limited, especially with respect to firebrand 

size distributions during actual WUI fire conditions. Consistently small sizes of windblown 

firebrands, similar to those generated using this device, were observed by data collection 

adjacent to a home that survived severe interface fire exposure. This is in stark contrast with the 

size of firebrands referenced in existing test standards (e.g. ASTM E108 [38]) and wildfire 

protection building construction recommendations. The interested reader is referred to the recent 

paper of Manzello and Foote [29]. 

Figure 5 Schematic of the Fire Research Wind Tunnel Facility (FRWTF). The location of the reentrant corner/deck assembly with respect to the full-scale Continuous Feed Firebrand Generator

shown

9.85 m

Calcium Silicate Board

h = 5.0 m

15.0 m

5.0 m

Continuous Dragon

 Location

3.25 m

Flow Direction

Test Section

h = 13.8 m, w = 5.0 m

h = 4.0 m, w = 5.0 m

Re-entrant corner/deck 

assembly
9

Figure 6 Orientation of deck boards with respect to the direction of the firebrand-laden flow. 

Note that this drawing is for illustration purposes and is not intended to reflect the total number 

of deck boards used per experiment or the actual dimensions of the decking assembly itself.

3.0 Results and Discussion

Figure 7 displays a typical experiment exposing a cedar decking assembly exposed to 

continuous wind-driven firebrand showers. The wind tunnel speed was fixed at 6 m/s and two 

repeat experiments were conducted for each deck board type. A wind tunnel speed of 6 m/s was 

used since it is similar to that used in the State Fire Marshall (SFM) STANDARD 12-7A-4 [37]; 

this conformance would allow a more meaningful comparison to this standard. The moisture 

content (dry basis) of the deck boards, determined by oven drying at 104 °C, was determined for 

all of the experiments (see Table 1).

Figure 7 Picture of a typical experiment exposing a decking assembly to continuous wind-driven 

firebrand showers. In this particular experiment, cedar was used as the decking material.

Firebrand Laden Flow

Wall

Wall
10

For all three wood deck types considered, smoldering was observed to transition to flaming. 

Figure 8-10 displays flaming ignition for Cedar, Douglas-Fir, and Redwood, respectively. 

These experiments clearly demonstrated that it is the accumulation of glowing firebrands that 

pose an ignition danger. Specifically, glowing firebrands accumulate within the gaps of the deck 

boards and, when stacked together these accumulated firebrands provide intense heat flux to the 

surface of the deck board, producing self-sustaining smoldering ignition of the boards that 

eventually transitions to flaming ignition.

Figure 8 Flaming ignition of Cedar decking assembly.

Figure 9 Flaming ignition of Douglas-Fir decking assembly.
11

Figure 10 Flaming ignition of Redwood decking assembly.

It was not possible to determine the time for the exact onset of smoldering ignition. This was 

due the simple fact that the board gaps eventually were covered with firebrands. However, 

intense smoke generation, an indication of smoldering ignition, was observed. Therefore, the 

time to reach flaming ignition was determined for each wood type (see Table 1). The time was 

defined as the time sustained flaming was observed on the deck surface less the time the first 

firebrand landed on the deck surface. The time to reach flaming ignition was the longest, on 

average, for the Douglas-Fir decking assemblies. Cedar decking assemblies reached flaming 

ignition in the shortest time. The time to reach flaming ignition for Redwood decking assemblies 

occurred in between the other two wood types. Firebrand generation was continued until 

sustained flaming ignition was observed. 

After the full-scale Continuous Feed Firebrand Generator was turned off, the wind tunnel 

was switched off, and the accumulated firebrands were swept off the surface. The deck boards 

continued to smolder after the wind tunnel was turned off. Figure 11-13 are pictures taken of the 

same decks board that are shown in Figure 8-10 but with the firebrands removed to show the 

areas of ignition. As can be observed, even after the wind tunnel was switched off, smoldering

ignition was still occurring within the deck boards. Interestingly, for the Redwood decking 

assembly, flaming ignition was still occurring in combination with smoldering.

As mentioned earlier, the total number/mass flux of firebrands generated from the full-scale

Continuous Feed Firebrand Generator was determined. However, all of the generated firebrands 

that depart the mouth of the Dragon did not land on the surface of the deck. The total number of 

firebrands arriving on the surface of the deck was determined to estimate the number and thus 

mass required for ignition. Based on this analysis, approximately 20% of the generated 
12

firebrands were able to accumulate within the center of the decking assembly surface. Therefore, 

the firebrand mass necessary to produce ignition for Cedar was 120 g (T1) and 89 g (T2), 

Douglas-Fir 283 g (T1) and 165 g (T2), Redwood 85 g (T1) and 278 g (T2).

Figure 11 Cedar decking assembly after accumulated firebrands were swept off and wind tunnel 

was off. Smoldering ignition is still occurring.

Figure 12 Douglas-Fir decking assembly after accumulated firebrands were swept off and wind 

tunnel was off. Smoldering ignition is still occurring.
13

Figure 13 Redwood decking assembly after accumulated firebrands were swept off and wind 

tunnel was off. Smoldering ignition and local flaming ignition is still occurring.

To explain the differences in ignition times between the different wood types, it is important 

to consider the heat and mass transfer processes that take place at the gap between the deck 

boards in contact with glowing firebrands. The deposited firebrands heat up the surface,

resulting in the production of pyrolysates. As a result, flammable air/fuel mixtures are formed 

above the gaps between the deck boards. Continued heat supplied from the firebrands 

contributes to exothermic gas-phase reaction, leading to ignition. The net heat flux, q"net to the 

fuel bed from the impinged firebrands is given as (see Figure 14):

( ) " " " " " qnet qFB qconv qrad m Lv = − + +  (1)

where q"FB is the heat flux from the firebrands, q"conv is the convective heat flux, q"rad is the 

radiative heat flux, " m is the mass loss rate per unit area, and Lv is the heat of gasification of 

cedar. The ignition time for thermally thick materials is known to be proportional to the roomtemperature density, ρ, of the material and inversely proportional to the square of the net heat 

flux to the fuel bed, q"net [40-41].
14

ig "2

net

t

q

ρ ∝ (2)

Therefore, for wood with similar moisture content, and the same net heat flux, the wood 

with the highest density is expected to take the longest time to ignite. The densities were 

measured for the three wood types and the Douglas-Fir boards (534 kg/m3

) were found to have 

the largest density, followed by Redwood boards (437 kg/m3), and then Cedar boards (361 

kg/m3). Since the configuration of the decking assembly, the wind speed, and the number flux 

(and mass flux) of firebrands generated was the same in all experiments, it is reasonable to 

assume that when the firebrands were deposited on a given decking assembly, Douglas-Fir

would take the longest to reach flaming ignition under conditions of the same q"net (on average).

q"FB

q"conv q"rad .

m"Lv

Fuel Bed

Glowing Firebrand

Figure 14 Heat and mass transfer processes that take place at the fuel bed in contact with a 

glowing firebrand. 

Continual firebrand bombardment provided the heat source in these experiments to transition 

smoldering ignition to flaming ignition. Ohlemiller [42] reported that for wood to transition 

from smoldering ignition to flaming ignition, the wood must be arranged in a configuration to 

minimize heat losses, such as a crevice. The firebrands accumulated between the gap (5mm) 

between the deck boards, creating such a configuration to minimize heat losses. Similar to the 

results reported in this study, Ohlemiller [42] found wood transitioned consistently from 

smoldering ignition to flaming ignition in wood crevices but that the time of this transition

varied; suggesting the reason for large variations in the measured times to flaming ignition.

Wood Type Time to Flaming Ignition Moisture Content (%)

Cedar 501 sec [T1]; 372 sec [T2] 11.6 [T1]; 11.3 [T2]

Douglas-Fir 1180 sec [T1]; 688 sec [T2] 8.8 [T1]; 10.8 [T2]

Redwood 356 sec [T1]; 1159 sec [T2] 11.2 [T1]; 9.8 [T2]

Table 1 Summary of time to flaming ignition and moisture content. Note that T1 and T2 refer to 

Test 1 and Test 2, respectively. The moisture content was similar for all wood decking 

assemblies tested.

Although few studies exist with respect to firebrand ignition of building materials, it is 

nonetheless useful to compare these detailed results to prior studies. Manzello et al. [33-34]

developed the NIST Dragon’s LAIR (Lofting and Ignition Research) facility to simulate wind 

driven firebrand showers at bench-scale. This facility consists of a reduced-scale Firebrand 
15

Generator (known as the NIST Baby Dragon) coupled to a bench-scale wind tunnel. The 

Dragon’s LAIR facility has now been upgraded to include the NIST continuous feed Baby 

Dragon. The interested reader is referred elsewhere for a very detailed description of the new and 

improved Dragon’s LAIR facility [43]; capability is presented briefly below. The success of the

new bench-scale continuous Feed Baby Dragon provided the impetus to construct the full-scale 

Continuous Feed Firebrand Generator presented in this report; the full-scale Continuous Feed 

Firebrand Generator was scaled up from this bench scale device. 

The efficacy of the Dragon’s LAIR facility to determine ignition regime maps of building 

materials exposed to continuous wind-driven firebrand showers has been determined [41]. To do 

this, cedar crevices were constructed for ignition testing. Two pieces of cedar were aligned at an 

angle of 60 degrees. This angle was selected since consistent ignition behavior has been 

observed for other building materials ignited by firebrands using this configuration [see 41]. The 

dimensions of each cedar piece used were 114 mm wide by 448 mm long. These dimensions 

were selected since the length covered nearly the entire wind tunnel length and the low width 

allowed for less flow obstruction. Since the cedar used in these experiments is used for siding, 

each piece was tapered. Specifically, the largest edge thickness was 11 mm and tapered down to 

3 mm. Due to the arrangement of the crevice, the thick edge of each cedar piece was offset and 

this allowed for a nominal thickness of 7 mm at the center of the crevice, where ignition 

(described below) was observed. The cedar pieces were held in place using a custom mounting 

bracket. The location of the cedar crevice was placed about 760 mm from the exit of the mouth 

of the NIST continuous feed Baby Dragon. This location was selected simply due to the fact that 

the firebrands were observed to land within the crevice using the wind speed selected in these 

experiments (6 m/s). The moisture content of the cedar pieces was varied using an oven. 

Specifically, experiments were conducted using cedar held at 11 % moisture content (dry basis) 

as well as oven dried). Cedar, in this form, was selected since it is a common material used for 

both siding and roofing assemblies. For cedar crevices held at 11 % moisture content (dry basis), 

it was observed that flaming ignition was observed to occur within 185 sec ± 67 sec after the 

deposition of continuous wind-driven firebrand showers when firebrands were introduced at a 

rate of 0.03 g/s. Under higher firebrand generation rates (0.1 g/s), the time to flaming ignition 

was measured to be 103 sec ± 25 sec. At the highest firebrand generation rate tested (0.14 g/s), 

the time to flaming ignition 74 sec ± 4 sec. In these bench scale tests, all the generated 

firebrands are collected within the crevice. Thus, the mass required to reach flaming ignition 

varied from 6 g ± 2 g (0.03 g/s) to 10 g ± 3 g (0.14 g/s). These masses were similar within the 

range of uncertainty in the experiments. 

Dowling [25] performed experiments to investigate ignition of wood bridge members due to 

firebrand deposition. In these laboratory experiments, Dowling [25] burned wood cribs and the 

resultant firebrands were collected and deposited into a 10 mm gap between the wood bridge 

members (deck plank and gravel beam). The mass of the firebrands (7 g to 35 g) generated was 

varied by altering the initial mass of the wood crib. The greater the wood crib mass, the greater 

the mass of the resultant firebrands. It was observed that 7 g of firebrands were able to produce 

smoldering ignition of the wood members within the 10 mm gap. The state of the firebrands 

upon deposition into 10 mm gaps, i.e. glowing or flaming, was not specified. 
16

When comparing the mass required for cedar crevice ignition of Manzello and Suzuki [41], 

to the deck results reported here, it was observed that the decking assemblies required 

significantly more mass of firebrands to produce flaming ignition. This was expected since the 

firebrands were not allowed to accumulate in such a tight, V-shaped crevice as the bench-scale 

experiments, resulting in larger heat losses (see Equation 1). Nevertheless, flaming ignition of 

decking assemblies was observed with total firebrand masses from 85 g to 280 g. This is not a

significant amount of mass and could easily be generated during WUI fires.

Figure 15 Picture of the underside of the decking assembly (cedar); overall view.

Figure 16 Detail of the underside of the decking assembly (cedar). The ignition process was 

observed to spread to the wood supporting members. 
17

Finally, it is important to show that ignition of deck boards also began to produce smoldering 

ignition in the supporting members under the decking assembly. Figure 15-16 shows the 

underside of the cedar decking assembly (overall view and detail of one of the sections). Even 

long after the experiments are completed, sustained smoldering ignition was occurring in the 

supporting members.

4.0 Summary

An experimental campaign was conducted to expose wood decking assemblies to winddriven firebrand showers. These experiments have represented the dangers of the dynamic 

process of continual, wind-driven firebrand showers landing onto decking assemblies for the first 

time. For each wood type decking assembly tested (Cedar, Douglas-Fir, and Redwood), the 

accumulation of glowing firebrands resulted in flaming ignition of the deck boards. These 

experiments have confirmed what has been observed in post-fire studies: wood decks may be 

ignited by firebrand showers. Future work will consider composite decking assemblies to 

determine its performance when exposed to continuous wind-driven firebrand showers and 

conduct experiments using protocols outlined in SFM STANDARD 12-7A-4. 

5.0 Acknowledgments

The authors would like to acknowledge the many contributions from Mr. Edward 

Hnetkovsky, Mr. John Shields, Mr. Laurean DeLauter, Mr. Tony Chakalis, and Mrs. Doris 

Rinehart, all of the Engineering Laboratory (EL) at NIST. Dr. Matthew Bundy, NIST National 

Fire Research Laboratory (NFRL) Associate Director, is acknowledged for making NFRL 

technicians available to support this work by preparing all materials for shipping to Japan. Dr. 

Yoshihiko Hayashi of BRI (Japan) is appreciated for allowing us to use the FRWTF to conduct 

the experiments outlined in this paper. The Science and Technology Directorate of the U.S.

Department of Homeland Security sponsored the production of this material under Interagency

Agreement IAA HSHQDC-11-X-00565 with the National Institute of Standards and Technology 

(NIST).

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