NIST Technical Note 1794

The Performance of Concrete Tile 

and Terracotta Tile Roofing 

Assemblies Exposed to 

Wind-Driven Firebrand Showers

Samuel L. Manzello

http://dx.doi.org/10.6028/NIST.TN.1794
NIST Technical Note 1794

The Performance of Concrete Tile 

and Terracotta Tile Roofing 

Assemblies Exposed to 

Wind-Driven Firebrand Showers

Samuel L. Manzello 

Fire Research Division

Engineering Laboratory

http://dx.doi.org/10.6028/NIST.TN.1794

March, 2013

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 1794

Natl. Inst. Stand. Technol. Tech. Note 1794, 12 pages (March, 2013) 

http://dx.doi.org/10.6028/NIST.TN.1794

CODEN: NTNOEF
iii

Table of Contents

page

Abstract iv

1.0 Introduction 1

2.0 Experimental Description 1

2.1 NIST Firebrand Generator (NIST Dragon) 1

2.2 Fire Research Wind Tunnel Facility, Building Research Institute (BRI), Japan 3

2.3 Concrete Tile and Terracotta Tile Roofing Assemblies 5

3.0 Results 8

4.0 Discussion 10

5.0 Summary 10

6.0 Acknowledgments 11

7.0 References 11
iv

Abstract

Concrete tile roofing assemblies (flat and profiled tile) as well as terracotta tile roofing 

assemblies (flat and profiled tile) commonly used in the USA, Australia, and elsewhere were 

exposed to wind-driven firebrand showers with an average mass flux of 10 g/m2s at a wind speed 

of 9 m/s. The purpose of these scoping experiments was to determine if firebrands were able to 

penetrate the tile assemblies and melt the sarking material. Wind-driven firebrand showers were 

generated using the NIST Firebrand Generator (NIST Dragon) installed inside the Building 

Research Institute’s (BRI) Fire Research Wind Tunnel Facility (FRWTF) in Tsukuba, Japan. No 

decking was included in the roof support structure as these experiments were intended to 

replicate Australian construction details. The results, however, are relevant to USA construction 

since the same concrete (flat and profiled) and terracotta (flat and profiled) tiles are used in both 

countries. Underlayment or sarking, in the form of a layer of aluminium foil laminate bonded 

with a fire retardant adhesive to a polymer fabric, was placed under the tile battens. The results 

showed that firebrands penetrated the tile gaps and subsequently melted the sarking material for 

both types of concrete tile roofing assemblies (flat and profiled tile) and the profiled tile 

terracotta roofing assembly when exposed to wind-driven firebrand showers. The flat tile 

terracotta roofing assembly performed best probably due to its interlocking design. For these 

tiles, the firebrands were observed to become trapped within the interlocking sections of the tiles 

and as a result, the firebrands were not transported past the tiles towards the sarking material. 
1

1.0 Introduction

Wildland-Urban Interface (WUI) fires have become a problem of great concern 

worldwide. There have been significant WUI fires in Australia, Greece, Portugal, Spain, and 

USA over the past five years [1]. The fires in Australia in 2009 claimed the lives of more than 

150 people [1].

 

The WUI problem can be viewed as a structural ignition problem [1]. For years, post-fire 

studies have identified firebrands as a significant source of ignition in these fires (see Ref. [2] for 

a review of many of these studies). The coupling of the NIST Firebrand Generator (NIST 

Dragon) to the Building Research Institute’s (BRI) Full Scale Fire Research Wind Tunnel 

Facility (FRWTF) in Tsukuba, Japan has made it possible to expose actual building elements to 

wind-driven firebrand showers. A recent paper has been published that summarizes all of the 

findings obtained using the NIST Firebrand Generator installed in BRI’s FRWTF [3].

Post-fire studies have long identified a building ignition mechanism in which very small 

(on the order of 1mm) firebrands penetrate under a non-combustible tile roof covering to ignite a 

building [2]. Although current standards exist (e.g. ASTM E108 [4]) to test ignition of roofing 

decks to firebrands by placing a burning wood crib on top of a section of a roof assembly under 

an air flow, the dynamic process of multiple wind-driven firebrands landing and then being 

transported under non-combustible tiles/gaps as a function of time is not taken into account. A 

recent experimental campaign demonstrated the vulnerabilities of curved ceramic tile roofing 

(Spanish tile roofing) assemblies to ignition under a controlled wind-driven firebrand attack 

using the NIST Firebrand Generator. Further details regarding those experiments are provided

elsewhere [5].

Recent major bushfires in Australia have led to the establishment of a Royal Commission 

to examine the suitability of building codes for the construction of houses in bushfire prone areas. 

With many wildland fires propagated by the spread of firebrands, the resistance of building 

elements to wind-driven firebrands has become a topic of great interest worldwide as evidenced 

by papers submitted on this topic from all over the world to the first ever Special Issue devoted 

to WUI fires in the fire safety science archival literature [6].

The objective of this investigation was to observe the performance of tile roof assemblies 

using the test method developed by Manzello et al. [5] for several types of tile roofing 

assemblies. Specifically, these scoping experiments were conducted to determine if firebrands 

were able to penetrate the tile assemblies and melt the sarking material. The outcome of these 

tests is expected to be of value to both Australian and USA tile manufacturers as well as fire

authorities/code officials in these countries.

2.0 Experimental Description

2.1 NIST Firebrand Generator (NIST Dragon)

Figure 1 shows side and front views of the NIST Firebrand Generator (NIST Dragon). A 

brief description of the device is provided here for completeness and follows prior descriptions 
2

very closely [5]. With the exception of the flexible hose, all components of the apparatus were 

constructed from stainless steel (0.8 mm in thickness). Two propane burners were used to ignite 

the fuel (tree mulch) to generate firebrands. Recently, an improved Continuous Feed Firebrand 

Generator has been developed that is capable of generating firebrand showers of unlimited 

duration [7]. This device was not used for the present experiments because: (1) industry was 

interested in a direct comparison to the experiment protocols used for the Spanish tile roofing 

assemblies to those described here, (2) the batch-feed version of the Firebrand Generator, shown 

in Figure 1, was far less expensive to operate and provided a simple cost-effective tool to 

determine if further work is required (i.e. if firebrand penetration and melting of sarking 

occurred). 

The bottom panel in Figure 1 displays the procedure for loading the Norway Spruce 

(picea abies Karst) tree mulch into the apparatus. Norway Spruce (picea abies Karst) was 

chosen because it belongs to the Pinaceae family, which includes such species as Ponderosa Pine 

(Pinus Ponderosa) and Douglas-Fir (Pseudotsuga menziesii); a common conifer species found in 

the USA, Japan, Australia, and elsewhere. The mulch for the Firebrand Generator was produced 

from 6.0 m tall Norway Spruce trees.

The mulch pieces were deposited into the firebrand generator by removing the top 

portion of the apparatus The mulch pieces were supported using a stainless steel mesh screen 

(0.35 cm spacing). Two different screens were used to filter the mulch pieces prior to loading 

into the firebrand generator. The first screen blocked all mulch pieces larger than 25 mm in 

diameter. A second screen was then used to remove all needles from the mulch pieces. The 

reason for this filtering procedure is described below. The mulch loading was fixed at 2.8 kg. 

After the Norway Spruce tree mulch was loaded into the Firebrand Generator, the top 

section of the Firebrand Generator was coupled to the main body of the apparatus (see Figure 1). 

The blower was then switched to provide a low flow for ignition (1.0 m/s flow inside the duct 

measured upstream of the wood pieces). The two propane burners were then ignited individually 

and simultaneously inserted into the side of the generator. Each burner was connected to a 0.635 

cm diameter copper tube with the propane regulator pressure set to 344 kPa at the burner inlet; 

this configuration allowed for a 1.3 cm flame length from each burner [5]. The Norway Spruce 

mulch was ignited for a total time of 45 s. After this period, the fan speed of the blower was 

increased to 2.0 m/s for the flow inside the duct, measured upstream of the wood pieces. The 

principle behind the operation of the apparatus was rather simple; after ignition, the mulch would 

begin to burn and the density decreased at which point the low air flow passing through the 

support mesh was able to loft the burning mulch and force it to exit the device as firebrands at 

low velocity. The timing and fan blower speed timing was developed to produce glowing rather 

than flaming firebrands. [5].These procedures were used to generate a continuous flow of 

glowing firebrands for approximately six min duration. Manzello et al. [5] have shown that the 

firebrands produced using these filtering and ignition procedures generate firebrands with mass 

up to 0.2 g and surface area up to 1000 mm2. This is within the range of firebrands generated 

from burning conifer species (Douglas-Fir and Korean Pine Trees) [8-9].
3

2.2 Fire Research Wind Tunnel Facility (FRWTF), Building Research Institute (BRI), 

Japan

The NIST Firebrand Generator was installed inside the test section of the FRWTF at BRI 

(see Figure 2) in Japan. The facility was equipped with a 4.0 m diameter fan used to produce the 

wind field and was capable of producing up to a 10 m/s wind flow. The uniformity of the wind 

flow velocity distribution was verified using a 21 point hot wire anemometer array. The array 

was translated across the entire 15 m test section to map out the flow distribution. The flow was 

uniform (within 10 %) across the entire length of 15 m test section [3]. 

Figure 1. Schematic drawings of the NIST Firebrand Generator (NIST Dragon). The Top Panel 

Shows the Side View of the Assembled Firebrand Generator and the Bottom Panel Shows Front 

Views of the Firebrand Generator Disassembled to Load Firebrands (left side of panel) and the 

Assembled Firebrand Generator (right side of panel).

Firebrand exit

51 cm 79 cm 38 cm

30.5 cm 

Diameter

Supporting Brace

Firebrand Mesh

Propane Burner

Flexible Hose 

15 cm Diameter

1.5 kW Blower

Electrical Generator

(Gasoline, 240 V 1Φ )

Firebrand Generator Assembled

Side View

51 cm 79 cm 38 cm

30.5 cm 

Diameter

30.5 cm 

Diameter

Supporting Brace

Propane Burner

Line to Propane Gas Cylinder

Firebrand Mesh

Firebrand exit

51 cm 79 cm

Firebrand Generator Assembled

Front View

Firebrand Generator Disassembled 

To Load Firebrands

Front View

Norway Spruce Mulch

Poured into Generator

2.1 kg Initial Mass Mulch 

 

Mulch Poured into

Generator
4

9.85 m

Calcium Silicate Board

h = 5.0 m

15.0 m

5.0 m

Firebrand Generator

2.0 m

Flow Direction

Test Section

h = 13.8 m, w = 5.0 m

h 0 0 

Roofing Section Location

To determine the average total firebrand mass flux generated by the NIST Firebrand Generator, 

experiments were conducted to collect the generated firebrands from the device using an array of 

water filled pans. Water was necessary to quench combustion of the generated firebrands [5]. 

The firebrands were subsequently removed from the water pans, dried, and the total mass 

generated over 6 min duration from the device was determined; more details regarding the drying 

procedure are available elsewhere [5]. Based on these measurements, an average total firebrand 

mass flux of 10 g/m2s was directed by the NIST Dragon at the roofing assemblies at a wind 

speed of 9 m/s. It is important realize the all the generated firebrands that depart the mouth of 

the NIST Firebrand Generator do not land on the surface of the roofing assemblies. The average 

total firebrand mass flux arriving at the roofing assembly was estimated from video records to be 

0.3 g/m2s. Approximately 60 % of the firebrand mass generated from the Firebrand Generator 

arrived at the roofing assembly surface

Figure 2. Schematic of BRI’s Fire Research Wind Tunnel Facility (FRWTF).
5

2.3 Concrete Tile and Terracotta Tile Roofing Assemblies

A custom assembly was constructed to mount the tile roofing assemblies. In all 

experiments, the tile roofing assemblies were mounted 2.0 m downstream of the NIST Firebrand 

Generator (see Figure 2); the same location as used by Manzello et al. [5]. In these experiments, 

no decking was included in the supporting structure as these experiments were intended to 

replicate Australian construction details. Underlayment (sarking) in the form of a layer of 

aluminium foil laminate bonded with a fire retardant adhesive to a polymer fabric, was placed 

under the tile battens. The batten spacing was adjusted depending on the type of ceramic or 

terracotta tile roofing assembly being tested. A photograph of the custom assembly for the 

experiments is shown in Figure 3. The sarking (green material) as well as the battens used to 

support the tiles are clearly visible in the figure. Figures 4-8 display the four types of tile roofing 

assemblies tested, including two concrete tile roofing assemblies (flat and profiled tiles) as well 

as two terracotta tile roofing assemblies (flat and profiled tiles). The differences between the flat 

and profiled tiles are obvious in the images. A 25 degree pitch was used for all roofing 

assemblies. The overall dimensions of each roof assembly were 1.2 m by 1. 2 m. The flat 

terracotta tiles had an interlocking design. The tile roofing assemblies were constructed in an 

attempt to have uniform gaps across the tiles.

Figure 3. Custom Assembly used to Mount the Various Tile Roofing Assemblies. The Sarking 

(green material) as well as the Battens used to Support the Tiles are Clearly Visible. In this 

Picture the Batten Spacing was Adjusted for Terracotta Tiles (flat tiles).
6

Figure 4. Concrete Tile Roofing Assembly (profiled tile) Exposed to Wind-Driven Firebrand 

Showers. The dimensions of each tile were 420 mm long by 320 mm wide. The Height, from the 

Wind Tunnel Floor to the Base of the Gutter of Roof Deck was 1250 mm.

Figure 5. Concrete Tile Roofing Assembly (flat tile) Exposed to Wind-Driven Firebrand 

Showers. The Dimensions of Each Tile were 420 mm long by 329 mm wide. The Height, from 

the Wind Tunnel Floor to the Base of the Gutter of Roof Deck was 1250 mm.
7

Figure 6. Terracotta Tiles Roofing Assembly (flat tile) Exposed to Wind-Driven Firebrand 

Showers. The Dimensions of Each Tile were 425 mm long by 265 mm wide. The Height, from 

the Wind Tunnel Floor to the Base of the Gutter of Roof Deck was 1250 mm.

Figure 7. Terracotta Tile Roofing Assembly (profiled tile) Exposed to Wind-Driven Firebrand 

Showers. The Dimensions of Each Tile were 423 mm long by 265 mm wide. The Height, from 

the Wind Tunnel Floor to the Base of the Gutter of Roof Deck was 1250 mm.
8

3.0 Results

A total of four experiments (all under the same wind tunnel speed of 9 m/s) were 

conducted. WUI fires in the USA and bushfires in Australia have been observed to occur under 

wind speeds in excess of 20 m/s [10]. A wind tunnel speed of 9 m/s was selected for these 

scoping experiments, because this is near the upper limit of the FRWTF. A typical experiment is 

shown in Figure 8 for terracotta tile roofing assembly (flat tile). The results shows that, for both 

types of concrete tiles (flat and profiled), firebrands were observed to accumulate in the gaps of 

the tiles. The firebrands continued to burn until they were able to penetrate the tile gaps and the 

underlayment (sarking) was observed to melt. Figure 9 displays burn patterns present in the 

sarking for experiments conducted for profiled concrete tiles. The tile gaps are on the order of 3 

mm. Penetration of the tile gaps was not a surprise as prior work by Manzello et al. [11] 

observed firebrands penetrating a 1 mm wire mesh. 

Figure 8. Typical Experiments Exposing Terracotta Tile Roofing Assembly (flat tiles in this 

image). The Wind Tunnel Speed was 9 m/s and the Roofing Assembly was Located 2.0 m from 

the NIST Dragon. The Average Total Firebrand Mass Flux was 10 g/m2s.
9

The terracotta tile roofing assembly constructed of profiled tiles (no interlocking) 

performed similarly to both types of concrete tiles used; namely firebrands were observed to 

penetrate the tiles and melt the sarking. For the terracotta tile roofing assembly (flat tile), these 

tiles are interlocked, and it was observed that, after the firebrands penetrated the tile gaps, the 

firebrands were trapped within the interlocking between the tiles so essentially no penetration 

was observed onto the underlayment (sarking – see Figure 10). 

Figure 9. Images of Sarking Placed Under Concrete Tiles (profiled tile) Taken Immediately after 

the Experiment was Completed. The Tiles have been Removed.

Figure 10. Photograph of sarking placed under terracotta tiles (flat tiles) taken immediately after 

the experiment was complete. The tiles have been removed. No firebrand penetration was 

observed.
10

4.0 Discussion

A series of scoping experiments were conducted to expose concrete and terracotta tile 

roofing assemblies to wind-driven firebrand showers with an average mass flux of 10 g/m2

s at a 

wind speed of 9 m/s. The firebrands were generated using a batch-feed version of the NIST 

Firebrand Generator, which provided about 6 min of firebrand showers. In all experiments, the 

tile roofing assemblies were placed 2.0 meters downstream of the firebrand generator. The 

results showed that firebrands can penetrate a profiled terracotta tile roofing assembly and a (flat 

and profiled tile) concrete tile roofing assembly and melt the underlying sarking material. For a 

flat terracotta tile roofing assembly, these tiles are interlocked, and it was observed that, after the 

firebrands penetrated the tile gaps, the firebrands were trapped within the interlocking between 

the tiles, so essentially no firebrand penetration to the underlayment was observed.

These experiments were conducted with the roofing assemblies installed by the 

manufacturer. This may be a best-case scenario because in realistic applications the gaps 

between tiles may change with time due to earthquakes, settling of the structure, aging of the 

roof, or other possible reasons. Manzello et al. [5] simulated this effect and observed greatly 

reduced performance of roofing assemblies as compared to well-aligned Spanish tile roofing 

assemblies. In these experiments, the sarking material was observed to melt, yet full penetration 

of the sarking material by glowing firebrands was not observed. This is important because 

penetration by burning firebrands could ignite materials below the sarking, such as fine fuels 

found in attic spaces, as shown by Manzello et al. [11]. In addition, it was also not possible to 

conduct experiments for wind speeds larger than 10 m/s. Wind speeds in excess of 20 m/s, 

however, have been observed in real WUI fires and bushfires [10]. Manzello et al. [12] 

quantitatively showed that the surface temperature of glowing firebrands increased as the applied 

airflow was increased. In that work, building materials were observed to ignite under higher 

applied airflows, as compared to no ignition observed for lower applied airflows. Specifically, 

firebrand temperatures were observed to increase from 550 °C at an applied wind speed of 1.3 

m/s to 675 °C when the wind speed was increased to 2.4 m/s [12]. The duration of an actual 

WUI fire firebrand attack may be longer than the one simulated in this study, which would result 

in the tile roofing assembly being exposed to a greater number of firebrands, increasing the 

probability that a greater number of firebrands would deposit under the tiles. If the wind speed 

was higher, the temperature of the firebrands could be higher as compared to the present 

experiments, providing even more favorable conditions to melt and ultimately penetrate the 

sarking by melting the thin aluminum foil backing of this material (as Al has a melting 

temperature of 660 °C and glowing firebrand temperatures have been shown to exceed that [12]).

 

5.0 Summary

Concrete tile roofing assemblies (flat and profiled tile) as well as terracotta tile roofing 

assemblies (flat and profiled tile) commonly used in the USA, Australia, and elsewhere were 

exposed to wind-driven firebrand showers with an average mass flux of 10 g/m2

s at a wind speed 

of 9 m/s. The purpose of these scoping experiments was to determine if firebrands were able to 

penetrate the tile assemblies and melt the sarking material. The results showed that firebrands 

penetrated the tile gaps and subsequently melted the sarking material for both types of concrete 
11

tile roofing assemblies (flat and profiled tile) and the profiled tile terracotta roofing assembly 

when exposed to wind-driven firebrand showers. The flat tile terracotta roofing assembly 

performed best probably due to its interlocking design. For these tiles, the firebrands were 

observed to become trapped within the interlocking sections of the tiles and as a result, the 

firebrands were not transported past the tiles towards the sarking material. 

Based on the findings of these experiments, a potential cost-effective mitigation strategy 

would be to use a continuous underlayment of firebrand-resistant sarking. Clearly, full-scale 

experiments are required to observe vulnerabilities of structural components to wind-driven 

firebrand showers and, yet bench scale test methods afford the capability to evaluate firebrand 

resistant technologies such as firebrand resistant sarking. The bench scale Dragon’s LAIR 

(Lofting and Ignition Research) facility, described in detail elsewhere [13], is a unique 

experimental platform to evaluate sarking material performance to wind-driven firebrand 

showers. Once promising technologies are identified, full scale tests, similar to those described 

in this paper, could be conducted to test the performance to provide firebrand resistant tile 

roofing assemblies.

6.0 Acknowledgements

This work was funded by the US Tile Roofing Institute (TRI). TRI partnered with the 

Roofing Tile Association of Australia (RTAA) and sent two RTAA members to Japan to 

construct all the roofing assemblies used in these experiments. As these experiments were not 

covered under the BRI/NIST agreement, BRI’s FRWTF facility was rented for this work. The 

assistance of Dr. Yoshihiko Hayashi in arranging the rental is appreciated. The technicians of 

the NIST’s National Fire Research Laboratory (NFRL) are acknowledged for preparing the 

shipment of the NIST Dragon to Japan.

7.0 References

[1] Mell, W.E., Manzello, S.L., Maranghides, A., Butry, D., and Rehm, R.G., The wildlandurban interface fire problem – current approaches and research needs. International Journal of 

Wildland Fire, 19 (2010) 238-251.

[2] Manzello, S.L., Foote, E.I.D., Characterizing Firebrand Exposure During Wildland-Urban 

Interface Fires: Results of the 2007 Angora Fire, Fire Technology, published on-line (2012). 

DOI: 10.1007/s10694-012-0295-4.

[3] Manzello, S.L., Suzuki, S., and Hayashi, Y., Enabling the Study of Structure Vulnerabilities 

to Ignition from Wind Driven Firebrand Showers: A Summary of Experimental Results, Fire 

Safety Journal, 54:181-196, 2012.

[4] ASTM E108, “Fire Standards and Flammability Standards” ASTM International , West 

Conshohocken, PA, (2003) DOI: 10.1520/E0108-10A.
12

[5] Manzello, S.L., Hayashi, Y., Yoneki, Y., Yamamoto, Y., Quantifying the Vulnerabilities of 

Ceramic Tile Roofing Assemblies to Ignition During a Firebrand Attack, Fire Safety Journal 45 

(2010) pp. 35-43.

[6] Manzello, S.L. (Invited Guest Editor), Special Issue on WUI Fires, Fire Technology, 

published-on line (2013). DOI: 10.1007/s10694-012-0319-0.

[7] Manzello, S.L., and Suzuki, S., Experimentally Simulating Wind Driven Firebrand Showers 

in Wildland-Urban Interface (WUI) Fires: Overview of the NIST Firebrand Generator (NIST 

Dragon) Technology, 9th Asia-Oceania Symposium on Fire Science and Technology, Hefei, 

China, October, 2012.

[8] Manzello S.L., Maranghides A., Mell W.E. (2007) Firebrand Generation from Burning 

Vegetation. International Journal of Wildland Fire 16, 458-462.

[9] Manzello S.L., Maranghides A., Shields J.R., Mell W.E., Hayashi Y., Nii D (2009) Mass and 

size distribution of firebrands generated from burning Korean pine (Pinus koraiensis) trees. Fire 

and Materials 33, 21-31.

[10] Mitchell J.W. (2009) ‘Power Lines and Catastrophic Wildland Fire in Southern California,’ 

In Proceedings of the 11th International Conference on Fire and Materials pp. 225-238, San 

Francisco, CA.

[11] Manzello, S.L., Park, S.H., Shields, J.R., Suzuki, S., Hayashi., Y., Determining Structure 

Vulnerabilities to Firebrand Showers in Wildland-Urban Interface (WUI) Fires, Fire Safety 

Journal 46 (2011) 568-578.

[12] Manzello S.L., Park S.H., Cleary, T.G., Investigation on the Ability of Glowing Firebrands 

Deposited Within Crevice to Ignite Common Building Materials. Fire Safety Journal 44, (2009) 

894-900.

[13] Manzello, S.L., and Suzuki, S., The New and Improved Dragon’s LAIR (Lofting and 

Ignition Research) Facility, Fire and Materials, 36: 623-635, 2012.