RESEARCH FOREST PRODUCTS LABORATORY FIRE PERFORMANCE PAPER 

OF STRUCTURAL 

FLAKEBOARD 

FOREST SERVICE
 FPL315 U.S. DEPARTMENT OF AGRICULTURE 
 

MADISON, WISCONSIN 53705 
 

FROM 1979 

FOREST RESIDUE 
 
Abstract 
 

Fire performance properties were determined for the Forest Service (FS) structural 

flakeboard made from forest residues and for 

three commercial structural flakebard products. Tests include fire endurance of waII systems, fire penetration, room corner-wall performance, 8- and 25-foot tunnel furnace, FPL 

rate of heat release, and smoke development 

by NBS smoke density chamber. Walls with the 

FS board met HUD Minimum Property Standards for a 20-minute exterior dwelling wall. The 

board also met Class B flamespread criteria, 

and in general, performed equal to or better 

than the commercial reference boards. 
FIRE PERFORMANCE OF 

STRUCTURAL FLAKEBOARD 

FROM FOREST RESIDUE 

By 
 

CARLTON A. HOLMES,1 Forest Products Technologist, 
 

HERBERT W. EICKNER, Chemical Engineer, 

JOHN J. BRENDEN, Chemical Engineer, 

and 

ROBERT H. WHITE, Forest Products Technologist 

Forest Products Laboratory,2 Forest Service 

U.S. Department of Agriculture 

Introduction 

The U.S. Forest Service has initiated a major research and development program on the 

feasibility of producing a structural flakeboard 

panel from the logging residues left in the forests (23).3 This residue is estimated to be 3 to 4 

billion ft3 of wood per year. Structural panel 

board might be used for exterior wall and roof 

sheathing, and for subflooring and other floor 

systems, as future needs in housing expand. 

This would reduce projected shortages of lumber and plywood. 

As a part of this program, the Forest Products Laboratory (FPL) has developed an experimental Forest Service structural flakeboard 

panel utilizing forest logging residues from the 

Pacific Northwest (18, 23). More than 200 4- by 

8-foot panels were made for a comprehensive 

physical and mechanical testing program. The 

research reported in this paper is intended to 

provide information on the fire performance of 

this board in comparison with three commercial 

structural flakeboards fabricated from wood 

mill residues or round wood. This information is 

intended to be useful in gaining acceptance by 

architects, builders, and code officials for this 

new product. 

Material 

Forest Service Structural Flakeboard 

The Forest Service structural flakeboard in 

this evaluation was developed as a result of 

FPL research. It is a 1⁄2-inch-thick, three-layer, 

random-oriented flakeboard with faces of 0.02­

by 1-inch-wide by 2-inch-long disk-cut flakes 

and core of 0.05-by random width by 2-inchlong ring-cut flakes. The face-core-face weight 

ratio was 15:70:15. 

The residue mix used in preparing the 

flakes was primarily Douglas-fir, 75 percent 

>6-inch-diameter sound wood, 121⁄2 percent 

sound wood containing some decay. 61⁄4 percent <6-inch-diameter sound wood and 61⁄4 

percent bark. 

1Authors’ responsibilities lor specific areas of the study 

were: C.A. Holmes, tunnel furnace and room cornerwall performance; H.W. Eickner (former project leader, 

now retired), planning and conduct of the study; J.J. 

Brenden, smoke yield and rate of heat release; and R.H. 

While, fire penetration and fire endurance. Recognition 

is given to C.A. Jackson, Engineer, who prepared the 

study plan while at the Forest Products Laboratory. He 

is now at Weyerhaeuser Company, Tacoma, Wash. 

2Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 

3Underlined numbers in parentheses refer to Literature 

Cited at end of this report. 
The board was fabricated from flakes with 

5 percent added phenolformaldehyde resin 

and 1 percent wax emulsion, based on ovendry 

flake weight. The prepared three-ply mat was 

pressed for 10 minutes at 350°F, with pressure 

to reach final board thickness in 1 minute. The 

final density of the board was 42 to 45 pcf 

(ovendry weight and volume at 65 pct RH). 

Commercial Structural Flakeboards 

Commercial Board A 

Board A was a 1⁄2-inch-thick structural 

building particleboard or flakeboard. Larger 

flakes were in the core layer with smaller particles in the outer layers bonded with urea-formaldehyde resin at about 7 percent of the dry 

board weight. Board was of medium density 

range, 40 to 46 pcf. 

Commercial Board B 

Board B was the same as Commercial 

Board A except that it was phenolic bonded. It 

was marketed as exterior structural grade. 

Commercial Board C 

Board C was a 1⁄2-inch-thick exterior grade 

structural flakeboard building panel of medium 

density, averaging 40 pcf. It was made of aspen 

flakes bonded with phenolic resins at 3 to 4 percent of the dry board weight. 

Methods 

Tunnel Furnace Tests 

Flame spread properties of the FS structural flakeboard and the commercial boards 

were determined by the 25-foot tunnel furnace 

test of ASTM E 84-70 (2) and the 8-foot tunnel 

furnace test of ASTM E 286-69 (4). These test 

methods also give comparative values on heat 

contribution and smoke development. Selectgrade red oak flooring is the reference standard in both test methods. The test results from 

burning red oak are assigned a value of 100. 

The zero value is assigned to results obtained 

with asbestos cementboard (25-ft furnace) and 

asbestos millboard (8-ft furnace). The 25-foot 

furnace tests were conducted in the laboratory 

of the Hardwood Plywood Manufacturers Association, Arlington, Va. 

FPL Room Corner-Wall Jests 

The room corner-wall test, developed at 

the FPL in 1949, was used to simulate a "real 

fire" situation in the study of the performance 

of different materials in the 8-foot furnace 

(21, 25). Correlation was good between the values obtained on these materials by the two test 

methods. The study of material fire performance by a realistic corner test has gained considerable interest in recent years to augment 

the results obtained by the smaller laboratoryscale tests including the 25-foot tunnel furnace 

(13,15,27). 

In the FPL method, the test is conducted in 

the corner of an enclosed room, 8 feet wide by 

8 feet high by 12 feet long. The test specimen 

was mounted in this corner (fig. 1). The two wall 

pieces are 2 feet wide by 8 feet high; a 5- by 5­

foot area on the ceiling above the corner was 

covered by the specimen material. Ventilation 

is provided by an opening in the floor, and 

smoke and combustion gases are vented from 

the room through two flues at the top of the 

walls 5 feet either way from the test corner. The 

ignition source in the corner was a 5-pound 

wood crib of 20 hard maple sticks. The crib was 

ignited by burning 50 ml of alcohol. Temperature measurements were obtained with ironconstantan thermocouples at various locations 

on the specimen and in the room during the 

test. Continuous smoke measurements were 

obtained by light attenuation sensed by a 1P39, 

gas-filled phototube. Measurements were 

taken in the stack and across the center of the 

room at the 5-foot level. Radiant and convective heat energy from the fire corner was measured by a thin-foil calorimeter located in an adjacent corner. 

Observations were made, after ignition, on 

the progress of the flames up the walls and 

across the ceiling to the flues. Flame spread indexes were calculated for the specimen materials by the following: 

Flame = 100 x 6 min (1) 

spread (time in min for flames on test 

index specimen to reach flues) 

The 6 minutes is the time required for flames on 

a red oak standard specimen to reach the flues. 

The flame spread index for red oak is thus 100 

2 
Figure 1 .–Forest Service structural flakeboard specimen in place ready for test by FPL room cornerwall procedure. 

(M 145 915) 
 

3 
 
and for asbestos millboard 0. If flames do not 

reach the flues during the test of the specimen: 

100 x (maximum distance in exFlame cess of 5.5 feet reached by flames 

spread = on test specimen in 6 minutes) (2) 

index 7.5 feet 

The 5.5 feet in equation 2 is the maximum distance reached by the flames from the wood crib 

on asbestos millboard. The 7.5 feet is the distance from that 5.5-foot point to the flues (8 ­

5.5 + 5). Thus, 7.5 feet is the maximum distance that flame can travel over a specimen, 

beyond the flames from the crib, to reach the 

flues. 

Smoke Density Chamber Test 

In the evaluation of the fire performance of 

any material or group of materials, smoke yield 

is an important consideration. Hazards from 

smoke are the result of two basically different 

phenomena: First, smoke obscures vision so 

that means of fire escape are more difficult to 

find and fires are harder to fight; and second, 

smoke contains toxic substances which are 

life-threatening. Thus the potential smoke development from materials is an important factor 

in the understanding of how the materials will 

react to an imposed fire exposure. 

Equipment 

The structural flakeboards that comprise 

the group of materials being examined here 

were subjected to smoke yield determination in 

a closed, instrumented chamber that had been 

used for several preceding smoke studies 

(5, 6, 9). The chamber is an early version of an 

apparatus for smoke measurement developed 

by the National Bureau of Standards (NBS) 

(14). with a few of the modifications made in recent commercial models (17). Using the apparatus, smoke yield can be measured for both 

flaming and smoldering exposures. Specifically, the data will help answer the question of 

what the potential smoke yield is from each of 

a group of structural flakeboards. 

4 
 

Procedure and Calculations 

Specimens for this study were 3 by 3 by 1⁄2 

inch. Prior to smoke yield determinations, the 

specimens were conditioned to moisture equilibrium at 80°F and 30 percent relative humidity. The test procedure was similar to that used 

previously (6, 9). 

The calculation portion of the test procedure involved determination of a parameter 

known as the “specific optical density.” Ds. The 

mathematical derivation of Ds has been discussed (5, 6). Briefly, Ds is a characteristic 

smoke density parameter based on the length 

of the light path, the specimen area, the chamber volume, and the percentage of light transmitted through the smoke. For the specimen 

and apparatus used in this research, 

100 Ds = 132 log ( ) 

percent light (3) 

transmitted 

The calculations result in “smoke accumulation” or Ds-time curves (e.g., fig. 2). These 

curves describe the rate at which smoke accumulates in the chamber from fire-exposed test 

specimens. 

Parameters and calculations from the Ds ­

time data are: 

Dm is the maximum value of Ds, t0.9Dm is the time (min) when 0.9Dm 

occurs. 

t16 	 is the time at which Ds = 16 (room, 

12 x 20 x 8 ft; burning area, 10 ft2; 

light path length, 10 ft; 16 pct light 

transmitted), 

SON5 is the sum of Ds at 1, 2, 3, 4, and 5 

min, and 

SOI is the “smoke obscuration index.”: 

SOl = DmRav 

100 t16 

(4) 

where: Rav is the average rate of increase of Ds 

between the start of the run and t0.9Dm. When the specific optical density, Ds, 

reaches a maximum, that value is denoted by 

Dm . Because Dm relates to the maximum smoke 

yield (or maximum obscuration of vision) during 

relatively long fire exposures, it is important 

that, if the smoke yield of wood is to be reduced, the Dm parameter must be lowered. 
Figure 2.--Relationshipof specific optical density to time (Ds-time curve) (typical of smoldering exposure). 

(M 143326) 

The parameter t0.9Dm is related to the time 

necessary for maximum smoke obscuration to 

occur. Thus, larger values of t0.9Dm show 

longer times to reach Dm, and hence longer 

time for escape and for firefighting. When this 

test method was being developed, it was common practice to report tm, the time (in minutes) 

from the start of a test at which the maximum 

value of Ds (Dm) was reached (5). On accumulating experience with the smoke chamber, it 

became apparent that the Ds-time curves for 

many materials had plateaus in the region of 

Dm. Consequently, what is reported is t 0.9Dm 

, 

the time at which Ds reaches a value of 0.9Dm. 

In the region of 0.9Dm, Ds is usually changing 

rapidly enough with time to permit a more accurate estimateof t0.9Dm than tm. 

The time from the start of a run until Ds_= 

16 is the parameter t16 (in minutes). It is sometimes referred to as the “critical time factor” in 

smoke chamber work. As such, it enters into 

the calculation of the smoke obscuration index 

(SOI). The parameter t16 provides a convenient 

reference point at which to compare the flakeboards because it can be related to a model 

room. However. a criticism of the t16 value is the 

arbitrary nature of the hypothetical room on 

which it is based. In practice, for many materials, Ds = 16 is so low compared to Dm that t16 

is very short and as a result, determining t16 

from the Ds-time curves is sometimes a problem. Because t16 represents a critical time factor, low values are associated with relatively 

high smoke hazards and vice versa. 

The smoke obscuration index was developed to represent the salient features of the Ds ­

time curves by a single number or index. Thus, 

materials could be compared by this test 

method using a single criterion. In formulating 

the SOI, the following factors were considered 

important in representing the smoke buildup in 

rooms: (1) The maximum smoke accumulation 

(Dm); (2) the average rate of smoke buildup, Rav; 

and (3) a critical time factor (taken as t16). Low 

SOI values are desirable and high values are 

not. 

The parameter “smoke obscuration number for the first 5 minutes” –SON5 – another 

attempt to represent several features of the Ds ­

time curves by a single number. Here, the Ds, 

values are taken at 1-minute intervals, and are 

added for the first 5 minutes of the run. This 

technique emphasizes the first short period of 

fire exposure. 

Rate of Heat Release 

In considering the fire hazards associated 

with any material, several characteristics might 

be listed: flammability, ignitability, potential for 

heat release, smoke yield, and others. One of 

the most important (although one of the least 

studied) is the rate at which the material releases heat during fire exposure. The rate of 

heat release is important because in a fire, 

there is an energy balance: If the rate of heat 

release is greater than the rate of heat losses to 

the surroundings, the fire will intensify; conversely, if the loss to the surroundings is 

greater, the fire will diminish. 

In addition, code officials generally recognize that many materials will not pass the current definition of “noncombustible” in the 

model codes (1, 10, 16, 20, 24), but the materials 

will releaseonly limited amounts of heat during 

the initial and critical periods of fire exposure. 

5 
There is also some criticism of using limited 

flammability to partially define “noncombustibility.” Thus, at one time consideration was 

given to defining combustibility in terms of heat 

release based on a ”potential heat” method 

(19), with the low levels used to define low combustibility or noncombustibility. This method 

measures the total heat release. It is generally 

agreed that a better or a supplementary measure of “combustibility” would be a determination of the rate of heat release. This would assess efficiently the relative heat contribution of 

materials–thick, thin, untreated, treated-under fire exposure. 

Equipment 

The apparatus used to determine the rate 

of heat release of the structural flakeboards in 

this research consisted of: a furnace in which 

one surface of the test material was exposed to 

fire (fig. 3a); a fuel-air mixture supply system 

(fig. 3b) for an exposure burner within the furnace; and instrumentation to control furnace 

operation and to record various responses (fig. 

3c). (See also references 7, 8). 

Procedure 

The flakeboards were cut into 18- by 18­

inch test specimens from larger sheets of available stock. Specimens were conditioned at 

80°F and 30 percent RH for at least 2 months 

prior to determining the rate of heat release. 

The method used here can be looked on as 

constituting the “substitution method” of measuring rate of heat release. This method follows 

the general procedure used by the Factory Mutual Fire Insurance Company with their materials calorimeter (12). 

Duplicate determinations were made on 

each of the four types of flakeboards. Data 

were calculated from the heating value of additional propane gas controlled in a second, or 

“substitution,” run with a noncombustible 

specimen to produce the same furnace fluegas thermal response as with the test material 

(7, 8). A correction was made for any variation 

in heat flow through the walls of the test 

chamber. 

Figure 3.–Apparatus for measuring rate of heat release: Furnace, a; gas-air-water supply system, b; 

and electronic control cabinet, c. 

6 
 
Fire Penetration 

The four flakeboards were tested for fire 

penetration in the FPL 20- by 20-inch vertical 

panel furnace using ASTM E 119 (3) time-temperature fire exposure conditions. The boards 

were tested as a solid material. Results of the 

tests were duration of fire resistance and char 

rate. 

Equipment and Specimens 

The vertical furnace had a 201⁄4-inch 

square opening on its side into which the specimen was inserted. Fiberglass insulation or high 

density mineral wool was used to fill the gap between the specimen and the edge of the furnace opening. The furnace was equipped with 

pipe outlets for discharging natural gas into the 

furnace. All air for combustion was admitted by 

natural draft through vents at the bottom of the 

furnace with baffling to get proper distribution. 

Five 30-gage iron-constantan thermocouples were placed beneath asbestos pads on the 

unexposed side of the specimens. Locations 

were at the center of the panel and at the center 

of each quadrant. Inside the furnace, a single 

iron-capped and four semiprotected thermocouples were located on a plane 2 inches from 

the exposed surface of the panel. 

The 20-inch-square test panels were either 

1⁄2 inch or 2 inches thick. The 2-inch-thick specimens consisted of four plies of the 1⁄2-inchthick flakeboard panels glued together with 

phenol-resorcinol glue. A 30-gage copper-constantan thermocouple was inserted between 

each ply at the center of the panel. 

Procedure 

The panels were subjected to fire exposure 

on one face. The gas supply of the furnace was 

regulated so that the temperature of the ironcapped thermocouple, opposite the center of 

the panel, followed the ASTM E 119 time-temperature curve. The gas was turned off when 

there was a burnthrough or flaming along the 

edges of the unexposed face of the panel. The 

panel was then removed and water applied to 

the exposed surface to extinguish any remaining flames or glowing. Measurements were 

made of the thickness of the uncharred wood 

remaining. During the tests, recorders monitored the temperatures indicated by the various 

thermocouples. 

Fire-resistance times and char rate were 

determined in the one-ply tests. Fire resistance 

is a property characterized by the duration of 

exposure to time of failure. Failure is reached 

when the fire has burned through some part of 

the test specimen or when the thermocouples 

in contact with the outside surface of the panel 

indicate an average temperature of 250°F 

above the initial temperature or an individual 

temperature of 325°F above the initial temperature. Char rates were computed from the 

times of burnthrough or when a thermocouple 

indicated 550°F. Past work has indicated that 

the base of the char layer in solid wood can be 

characterized by a temperature of 550°F (22). 

The results of the four-ply tests gave initial 

char rate, steady-state char rate, and average 

char rate. The times for thermocouples within 

the panel to reach 550°F were used to compute 

the initial and steady-state char rates. The initial char rate is the actual thickness of the 1⁄2­

inch ply divided by the time that the thermocouple–about 1⁄2 inch from the fire-exposed surface-reached 550°F. The steady-state char 

rate is the slope of the line obtained by a leastsquare analysis of the time-distance data of the 

three thermocouples between subsequent 1⁄2­

inch plies. The average char rate is either the 

thickness of panel charred divided by the duration of the fire exposure or the total thickness 

of the panel divided by the time of burnthrough. 

Fire Endurance of Walls 

The four flakeboards were used in loadbearing wood-frame walls tested for fire endurance in the Forest Products Laboratory large 

vertical furnace. The tests were conducted according to ASTM Standard E 119-73 (3) except 

for some modifications described in “Procedure.” 

Wall Construction 

For each of the four flakeboards. two walls 

were tested with different interior facing. For 

one wall, the interior facing was %-inch gypsum 

wallboard panels vertically applied to the frame 

with 1-inch-long. fourpenny, gypsum wallboard 

nails, spaced 8 inches on center. All joints were 

taped and covered with joint compound, and 

nailheads were indented and covered with joint 

7 
compound. One coat of latex paint was applied. 

For the second wall, the interior facing was the 

1⁄2-inch-thick flakeboard panel vertically attached to the framing with sixpenny nails 

spaced 8 inches on center at edges and 12 

inches at intermediate locations. 

The rest of the construction was the same 

for both types of walls. The wall framing consisted of nominal 2- by 4-inch studs (marked 

“Eastern woods stud grade”) spaced 16 inches 

on center and symmetrically across the frame 

with 1-inch-thick end studs spaced approximately 12 inches from adjacent studs to accommodate the 10-foot width. A double plate 

was used at the top and a single plate at the 

bottom. The plates were ponderosa pine-sugar 

pine, Standard or better. The plates were attached to the studs by end nailing with two sixteenpenny common nails. 

Glass-fiber roll insulation, 31⁄2 inches thick, 

with asphalt-impregnated paper vapor barrier 

was attached within the stud spacings. The 

tabs of the insulation were stapled to the 2-inch 

stud edgesfacing fire exposure. 

For exterior sheathing, the 1⁄2-inch flakeboard panels were vertically nailed to the framing with eightpenny common nails, spaced 6 

inches on center at edges and 12 inches at intermediate locations. Cedar bevel siding, 1⁄2 by 

8 inches, was attached with sevenpenny siding 

nails to give 6 inches of horizontal exposure. 

Procedure 

The wall assemblies, 8 feet high and 10 feet 

wide, were constructed in the test frame of the 

wall furnace (11) at the Forest Products Laboratory. Panel height was limited to 8 feet and 

the applied load was 1,250 pounds per linear 

foot (the computed maximum loading for the 

first floor wall of a two-story house of average 

size). The load is greater than the maximum design load of 984 pounds per linear foot which is 

based on the compression of the plates. The 

allowable load for compression failure of the 

studs is 1,280 pounds per linear foot. The total 

load of 12,500 pounds was applied to the wall 

using the hydraulic jack system of the furnace 

frame. Five minutes after the application of the 

load, the furnace was ignited and the fire exposure conditions as prescribed in ASTM 

Standard E 119-73 were applied to the interior 

face. Observations were made of the furnace 

temperatures and pressures, temperatures on 

the unexposed face, panel deflection, time for 

burnthrough, time for structural failure, and 

condition of wall during test. Walls were tested 

until failure occurred. 

The finish resistance, also called finish rating, of an interior fire-exposed facing material 

is often interpreted as a measure of the degree 

of protection provided by the facing material to 

the substrate materials and load-bearing members. The finish resistance of the interior flakeboard facing was determined by six thermocouples between the flakeboard on the fire-exposed side and the wood studs. The finish 

resistance was considered to be the time for 

the average of the six thermocouples to reach 

250°F above ambient or a single thermocouple 

to reach 325°F above ambient. 

Results and Discussion 

Tunnel Furnace Tests 

The flame spread values obtained in the 

two tests with the FS structural flakeboard in 

the 25-foot rating furnace were 70 and 72 with 

an average of 71 (table 1). This meets the acceptance flame spread criteria under building 

codes for class B material–75 or under flame 

spread. This classification will permit its use in 

many building occupancies, and applications 

where flame spread of building materials is limited; for example, in exitways and corridors. 

The heat contribution of 51 is low compared to 

red oak and the smokedeveloped of 165 is also 

acceptably low. The commercial boards obtained FSC values of 147, 127, and 189, and 

thus met the acceptance flame spread criteria 

for class C material-200or under. Materials of 

this class are not permitted by most codes in 

many building types. 

Densification of the surface is postulated 

as the reason for the lower flame spread obtained by the FS structural flakeboard compared to the considerably higher values obtained by the commercial boards. In an evaluation of the density gradients in a sample of 32 of 

the 200 plus FS boards, the density of the top 

surface increment ranged from 53 to 59 pounds 

per cubic foot (fig. 4). Density of the bottom surface increment ranged from 45 to 54 pounds 

per cubic foot. Average density of the 32 

boards was 45 pounds per cubic foot. 

8 
The somewhat higher flame spread value 

of 97 average for the FS structural flakeboard 

in the 8-foot furnace compared to the 71 average in the 25-foot furnace cannot be fully explained (table 1). There are fundamental differences in the construction and operation of the 

two furnaces and test results on some materials 

do not always compare. However, with wood 

materials, the two furnaces usually rank the 

materials in the same order. This was true for 

the boards tested in this study. The commercial 

boards had flame spread values in the lower 

range of class C in the 8-foot furnace whereas 

in the 25-foot furnace the values were in the 

midrange. Commercial Board C had the highest 

flame spread by both test methods. 

Smoke developed values were somewhat 

higher in the 8-foot furnace than in the 25-foot 

furnace. This is usually the case since the specimens in the 8-foot furnace are exposed to a radiant plate and a small pilot ignition flame which 

tends to promote nonflaming combustion and 

consequently high smoke development. In the 

25-foot furnace the specimens are exposed to 

a large 41⁄2-foot long impinging flame promoting 

greater flaming combustion and consequently 

less smoke. 

FPL Room Corner-Wall Tests 

The performance of the FS structural flakeboard and Commercial Boards A and B in the 

corner-wall tests were quite similar (table 2). 

Commercial Board Chad a slightly higher flame 

spread, earlier ignition time, higher maximum 

heat flux, higher temperature development in 

the room, but less smoke development than the 

other boards. The somewhat higher flame 

spread of this Board C over the other boards 

was also obtained in the 25- and 8-foot furnace 

tests. 

Although there were a limited number of 

specimens, the flame spread data can be used 

to obtain an estimate of the correlation between the different test methods. The best correlation of the flame spread data was obtained 

between the 8-foot furnace and the corner-wall 

test: 

9 
 
Estimated 

correlation 

coefficient 

25-foot furnace and 8-foot 0.73 

furnace 
 

25-foot furnace and corner- .33 
 

wall test 
 8-foot furnace and corner- .88 
 

wall test 
 

Smoke Density Chamber Test 

Smoke yields from the four structural flakeboards and 3/8-inch-thick Douglas-fir plywood 

are given in table 3. In assembling the table, 

arithmetic means of three smoke-yield determinations were used at each combination of 

material and exposure (flaming or nonflaming). 

Determinations were in serial order. 

Because of the importance of Dm values, 

table 3 gives the range of Dm as well as the 

arithmetic mean of Dm for the three replicate 

smoke determinations made on each combination of flakeboard and fire exposure. 

Table 3 shows some rather wide variations 

in Dm values. Commercial Board B and Douglas-firplywood tended to have lower maximum 

Dm while Commercial Board C had by far the 

highest Dm observed. With Dm in the range of 

640 to 730, the corresponding light transmission for Commercial Board C is near the lower 

limit of measurement with this apparatus. Thus, 

using the criterion of maximum values for Ds. 

under nonflaming fire exposure conditions, 

Commercial Board C is undoubtedly the smokiest material tested as a part of this study. 

Table 3 shows that, in general, Dm results 

Figure 4.–Density gradients through panel thickness in FS structural flakeboards. Density is based on 

weight and volume at standard conditions (73°F, 65% RH). 

(M 146 236) 

10 
-- -- -- 

Table 2. - - Results of room corner-wall tests on 1⁄2-inch structural flakeboards and two reference materials 

Maximum light 

obscuration by 

smoke 

Maximum temperatures obtained 

during test 

Board Corner-wall 

flamespread Time wall Time ceiling Time flames Maximum Time 

Percent of 

total On ceiling Onceiling Center of 

index ignited ignited reached flues heat flux obscuration in corner1 at flues2 room3 

Min Min Min W/cm2 Min Pct °F °F °F 

Red oak4 100 4.5 5.7 6.0 0.09 12.6 26 1,470 1,478 260 

Asbestos millboard4 0 .03 7.6 44 458 272 132 

FS structural flakeboard4 106 4.2 5.2 5.8, 5.5 .16 8.2 45 1,475 1,191 478 

Commercial Board A5 98 4.6 5.7 6.1 .25 5.4 72 1,510 1,230 600 

Commercial Board B5 102 4.6 5.7 5.9 .18 5.7 56 1,515 1,255 580 

Commercial Board C5 113 4.1 5.1 5.3 .29 7.8 40 1,625 1,240 720 

1On specimen surface, ceiling, in corner 6 in. from either wall. 

2On specimen surface. ceiling, 1 in. from flue, 

3In air, 3 ft down from center of room ceiling. 

4Values are averages from two tests. 

5Values are from one lest. 
Table 3.- -Smoke yield from four 1⁄2-inch Structural flakeboards and a reference material1 

Board Dm (Range) t 0.9Dm t16 SOl SON5, 

Min Min Min-2 

NONFLAMING COMBUSTION 

FS structural flakeboard 506 (487-521) 13.1 1.9 173 489 

Commercial Board A 530 (510-549) 12.2 2.0 148 369 

Commercial Board B 442 (418-480) 7.1 2.4 198 509 

Commercial Board C 692 (641-729) 8.2 1.8 458 501 

Douglas-fir plywood, 

exterior grade, A/C, 3/8-inch 440(376-494) 7.9 2.1 163 383 

FLAMING COMBUSTION 

FS structural flakeboard 104 (84-134) 22.9 11.0 2 5 

Commercial Board A 71 (36-90) 21.2 12.6 1 2 

Commercial Board B 244 (134-333) 18.5 11.1 11 5 

Commercial Board C 110 (94-138) 22.3 16.4 1 0 

Douglas-fir plywood, 

exterior grade, A/C, 3/8-inch 106(60-159) 13.5 8.1 3 15 

1Each value is the mean of three test runs in Serial order 

are lower for flaming fire exposure than for nonflaming exposure. This observation is typical of 

wood and wood-base products. Also shown is 

that Commercial Board B gave a somewhat 

higher Dm measurement under flaming conditions than the other four materials tested. 

Table 3 shows that the FS structural flakeboard and Commercial Board A have the longest times to Ds = 0.90m under nonflaming exposure. Commercial Board C, Commercial 

Board B, and Douglas-fir plywood have a much 

shorter t 0.9Dm , indicating that they tend to 

reach Dm quickly. During flaming exposure, Douglas-fir plywood had the shortest 

t0.9Dm while the other four materials tended to 

have relatively long times to reach Ds = 0.9Dm. 

All of the materials tested tended to have a 

shorter t0.9Dm under nonflaming than under 

flaming exposures. 

Inspection of table 3 shows that Commercial Board B has the highest t16 value under 

nonflaming exposure conditions. Under these 

same conditions the other four materials had t16 

times that were nearly equal but shorter. The 

variation among the four is probably not significant, Under flaming exposure conditions Commercial Board C has the longest t16 time while 

the other materials reached Ds = 16 more 

quickly. It is typical for most woods and woodbase materials that they have shorter t16 times 

resulting from nonflaming exposures that result 

from flaming exposures. 

Under nonflaming exposures, Commercial 

Board C had a much higher smoke obscuration 

index, SOI, than the other test materials (table 

3). Typically, all of the flakeboards had a much 

lower SOI under flaming exposure, with even 

the highest value of 11 for Commercial Board B 

12 
being low compared to the corresponding nonflaming exposure. 

Commercial Board A and Douglas-fir plywood had the lowest SON5 under nonflaming 

exposure (table 3). Under these conditions the 

other three test materials tended to release 

their smoke more quickly. Under flaming exposure the SON5 for all the materials was very low, 

this being typical for wood and wood-base materials in general. 

Rate of Heat Release 

In these rate-of-heat-release studies, there 

was relatively little difference among the four 

types of flakeboards (figs. 5-9; table 4). In all of 

the figures, there is a period of time from zero 

(when the specimen was inserted in the specimen opening) to the time when measurable 

heat release rates begin to occur. This is the 

"induction" period during which the specimens absorb enough heat to raise their temperatures to the point at which active burning 

with exothermic chemical reactions can proceed. During this period it is probable that negative rates of heat release can be observed; the 

physical significance of the negative rates is 

that, in this case, heat is being absorbed by the 

specimen rather than being liberated. 

Under the exposure conditions generated 

in the FPL rate-of-heat-release apparatus, the 

heat release curves tend to have two distinct 

peaks with a valley between them. The first 

Figure 5.–Rate of heat release versus time for FS structural flakeboard (two replicates) 

(M 146 066) 

13 
Figure 6.–Rate of heat release versus time for Commercial Board A (two replicates). 

(M 146 067) 

Figure 7.–Rate of heat release versus time for Commercial Board B (two replicates). 

(M 146 068) 

14 
Figure 8.–Rate of heat release versus time for Commercial Board C (two replicates). 

(M 146 069) 

Figure 9.–Rate of heat release versus time for %-inch-thick Douglas-fir plywood, exterior grade 

(untreated). 

(M 141 212) 
 

15 
 
Table 4.--Duplicate rate-of-heat-release. determinations on four 1⁄2-inch structural flakeboards and a reference material 

Percent of total 

Board Maximum heat Time of Average rate area under curve 

release rate maximum rate (first 10 min) in first 10 minutes 

Btu/min/ft2 min Btu/min/ft2 Pct 

FS structural flakeboard 966 14 422 43.0 

1,115 15 505 36.0 

Commercial Board A 	 945 4 440 40.0 

901 4 403 37.6 

Commercial Board E 	 994 14 371 28.9 

899 14 453 33.9 

Commercial Board C 	 989 4 647 53.5 

958 4 573 56.4 

Douglas-fir plywood, exterior grade, 3⁄4-inch 611 3 308 27.8 

peak, occurring at 4 to 5 minutes into the test, 

represents the heat released from flaming combustion of the specimen surface. As time goes 

on, a char layer forms at the surface and the 

rate of heat release is reduced. During all of this 

time, however, the sample is accumulating sensible heat and its average temperature is increasing. At about 12 to 16 minutes, the specimen is heated to a temperature at which it begins to crumble, exposing uncharred material 

of a relatively large surface area. Thus a second 

peak of rate-of-heat-release is generated. After 

the second peak, the heat-release rate falls off 

rapidly to zero. Values at both peaks are in the 

range of 900 to 1,100 Btu/min/ft2 (17 to 21 W / 

cm2). Values near the bottom of the valley between the peaks are in the range of 400 to 600 

Btu/min/ft2 (7.5 to 11 W/cm2). 

Replicate heat release determinations on 

each of the four flakeboards show good agreement between replicates. Except for Commercial Board B, all of the flakeboard types show 

consistency between replicates with respect to 

which of the two peak heat release rates is 

highest. However, on the basis of the data 

given here, it is not possible to state which of 

the types of flakeboard might have the best fire 

Performance from the standpoint of rate-ofheat release characteristics. Considering previously published data (8), the flakeboards tend 

to have relatively high initial (flaming) rates of 

heat release and tend to be consumed more 

quickly than solid wood–for example, 1-inchthick red oak or 3⁄4-inch-thick exterior grade 

Douglas-fir plywood (fig. 9). The flakeboards 

are not consumed as quickly nor with heat release rates as high as tempered hardboard, but 

the induction period for these materials tends 

to be slightly longer than for plywood. In understanding the performance of all of these materials in rate-of-heat release tests, however, it is 

important that variations in specimen thickness 

and density be kept in mind. 

Fire Penetration 

The fire penetration performance of the %­

inch-thick FS structural flakeboard was similar 

to that of the commercial boards (table 5). For 

all boards, the steady-state char rates from 0.99 

to 1.26 inch per hour were about what would be 

expected for a wood product of their density. 

Douglas-fir with a dry specific gravity of 0.6 and 

6 percent moisture content would have a char 

rate of 1.27 inch per hour (22). 

As expected, the char rate is initially 

higher. The production of a char layer reduces 

16 
the char rate until a steady rate is obtained. The 

thermocouples between the plies in the four-ply 

panels revealed high correlation between char 

depth (at 550°F) and time in all the four-ply 

panels. The high linear correlation in the fourply tests indicates the constant char rate that 

occurs after the first 1⁄2 inch of charring. Data 

based on total duration of fire exposure and 

thickness of wood charred generally agreed 

with the line obtained by the least-squares analysis of the time-distance data of the three thermocouples between the plies. Some data 

based on time of burnthrough indicated a 

higher char rate at burnthrough. The initial char 

rate in the four-ply specimens is lower than the 

char rate for the one-ply specimens (table 6). 

This could be due to heat being conducted to 

the rest of the four-ply specimen instead of accumulating and increasing the temperature. A 

few of the char rates for the one-ply panels 

would be reduced slightly if only one center 

thermocouple is used as is the case with the 

four-ply panels. 

Fire Endurance of Walls 

In the fire endurance tests of the walls, the 

flakeboard interior facing was observed to begin to break up between 11 and 12 minutes and 

to be almost completely gone in 14 to 16 minutes. The poorer performanceof the flakeboard 

in the large furnace compared with the performance in the small vertical furnace is possibly due to the difference in the type of burners 

in the two furnaces or differences in the boundary conditions of the unexposed side of the 

panel. 

Based on area under the time-temperature 

curves, the actual time-temperature exposure 

deviated from the standard curve by + 0.8 to 

+ 4.6 percent. The average internal furnace 

pressures were -0.002 to -0.041 inch of water 

near the bottom and + 0.024 to + 0.042 inch of 

water near the top. Temperatures as measured 

by thermocouples beneath asbestos pads on 

the unexposed surface were less than 250°F 

above initial temperatures until after burnthrough. 

The performance of the FS structural flakeboard was better than or equal to the performance of the commercial flakeboards in the wall 

tests (table 7). The walls with %-inch gypsum interior facing performed slightly better than the 

walls with the flakeboard as the interior facing. 

A previous test of 3/8-inch Douglas-fir plywood 

exterior sheathing and 3/8-inch gypsum interior 

facing resulted in a burnthrough at 29 minutes 

30 seconds and structural failure at 33 minutes 

30 seconds (11). These results for 3/8-inch plywood and 3/8-inch gypsum are within the range 

of results for 1⁄2-inch flakeboard and 3/8-inch 

gypsum. In general, the fireendurance of these 

walls with flakeboard facings were as expected 

for this type of wall construction. 

The fire endurance of all eight walls was in 

excess of the 20 minutes required for exterior 

walls in the Minimum Property Standards for 

Single-and Double-Family Dwellings of the US. 

Department of Housing and Urban Development (26). 

The value determined for the finish resistance provided by the FS structural flakeboard 

in the wall configuration tested was 10.1 minutes, slightly higher than the other boards (table 8). 

17 
-- 

-- 

-- 

-- 

Table 5. --Fire penetration tests of 1⁄2-inch, one-ply specimens of structural flakeboards 

Time an individual 

Time average thermocouple reached 

temperature 

Material Test reached 250° F 

above ambient1 

325° F above 

ambient1 

550° F1 Time of 

burnthrough 

Char 

rate2 

Min Min 

FS structural flakeboard 1 

2 

3 

Average 

11.0 

11.2 

12.0 

11.4 

11.3 

11.9 

12.9 

12.0 

Commercial Board A 1 

2 

Average 

11.9 

13.0 

12.4 

12.9 

14.5 

13.7 

Commercial Board E 1 

2 

Average 

10.4 

11.6 

11.0 

11.2 

13.0 

12.1 

Commercial Board C 1 

2 

Average 

11.6 

10.6 

11.1 

10.2 

11.7 

11.0 

Min 

13.4 

14.3 

15.3 

14.3 

15.2 

15.6 

15.4 

13.4 

14.9 

14.2 

11.7 

Min In./hr 

15.7 2.26 

14.7 2.04 

15.9 1.87 

15.4 2.05 

2.01 

16.0 1.95 

1.98 

2.24 

15.0 2.05 

2.14 

12.3 2.51 

11.5 2.53 

11.9 2.52 

'Five thermocouples were under asbestos pads on the unexposed surface. A thermocouple was located in the center of the panel and the center of each quadrant. 

'Computed from the time a thermocouple reached 550°F or time of burnthrough and the thickness of the panel. 
-- 

Table 6. - - Fire penetration tests of 2-inch, tour-ply specimens of 1⁄2-inch structural flakeboard 

Char rate 

Material Test Steady -
 

Initial1 state2 Average3 
 

FS structural 

flakeboard 	 

Commercial 

Board A 	 

Commercial 

Board B 

Commercial 

Board C 

In./hr 

1 1.76 1.24 1.34 

2 1.72 1.23 1.34 

3 1.59 1.38 

Average4 1.69 1.24 1.35 

1 1.62 1.13 1.39 

2 1.57 .99 

3 1.57 1.16 1.32 

Average4 1.59 1.07 1.36 

1 1.68 1.26 1.38 

2 1.73 1.12 1.24 

Average4 1.70 1.17 1.31 

1 1.96 1.14 1.31 

2 1.89 1.19 1.33 

Average4 1.92 1.16 1.32 

1Calculated from time for thermocouple 1⁄2 in. from original exposed surface to reach 550°F. 
 

2The slope of linear line obtained by least-squares analysis of times tor thermocouples 1⁄2, 1, and 11⁄2 in. from original exposed 
 

surface to reach 550°F. 
 3Calculated from total duration of fire exposure and thickness of wood charred at end of test or thickness of panel and time of 
 

burnthrough. 
 

4Average is the mean of the test results except for the steady-state char rate in which it is the slope obtained from me least-
 

squares analysis of the combined data. 
 

Table 7. - - Summary of fire endurance of load-bearing wood-frame walls by ASTM E 119 tests 

1⁄2-Inch 

flakeboard 

sheathing 

Interior facing 

Time of 

structural 

failure 

Time of 

burnthrough 

Min:sec Mi n:sec 

FS structural 3/8-inch gypsum board 35:37 33:20 

flakeboard 1⁄2-inch flake board 30:44 31:35 

Commercial 3/8-inch gypsum board 35:22 33:54 

Board A 1⁄2-inch flakeboard 27:40 27:35 

Commercial 3/8-inch gypsum board 29:20 28:25 

Board B 1⁄2-inch flakeboard 21:13 

Commercial 3/8-inch gypsum board 28:40 27:45 

Board C 1⁄2-inch flakeboard 25:50 26:20 

19 
Table 8 - - Finish resistance of interior fireexposed flakeboardon wood-frame walls in ASTM E 119 furnace tests 

1⁄2-Inch Time average temperature Time single thermocouple 

interior exceeded 250° F temperature exceeded 325° F 

facing above ambient1 above ambient1 

FS structural 

flakeboard 

Commercial 

Board A 

Commercial 

Board B 

Commercial 

Board C 

Min 

10.4 

9.1 

8.0 

10.2 

Min 

10.1 

8.9 

7.7 

9.8 

1Six thermocouples were placed between the interior facing and the studs. Three were 2 ft from the top of the wall and three were 

2 ft from the bottom of the wall. 

Conclusions 

1. The Forest Service structural flakeboard 

obtained a flame spread of 71, or class B, in the 

25-foot rating furnace of ASTM E 84, while the 

three commercial boards tested were class C. 

By 8-foot furnace and room corner-wall tests, 

the fire performance of all boards was generally 

equivalent. Commercial Board Chad the highest flame spread value in all test procedures. 

2. The FS structural flakeboard was intermediate in most smoke yield characteristics obtained in the smoke density chamber test. Commercial Board C had the highest smoke yield 

under nonflaming exposure conditions. 

3. Under exposure conditions of the FPL 

rate-of-heat-release test method, little difference was indicated between the four types of 

structural flakeboards. Results were relatively 

high compared to solid wood and exterior 

grade Douglas-fir plywood. 

4. Fire penetration performance of the FS 

structural flakeboard was similar to that of the 

commercial boards. Its steady-state char rate 

of 1.24 inch per hour was what would be expected for a wood product of that density. 

5. Fire endurance performance of loadbearing wood-frame walls constructed with FS 

structural flakeboard was better than or equivalent to that of the commercial flakeboards. All 

walls tested met the 20-minute requirement of 

the Minimum Property Standards for exterior 

walls of one- and two-family dwellings. 

6. Temperature measurements between 

the interior facing and the wood studs indicated that 1⁄2-inch-thick FS structural flakeboard has a finish resistance of 10 minutes in a 

wood-stud wall with glass-fiber insulation. 

20 
 
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