The flammability of exterior claddings

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The flammability of exterior claddings

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REPRINTED FROM F I R E TECHNOLOGY

The Flammability of

Exterior Claddings

J.

H.

McGUIRE,

SFPE

Division of Building Research National Research Council (Canada)

The author reports on a series of tests conducted to study the vertical flame-spread characteristics of certain exterior cladding materials. The results take into account several types of construction.

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NE of the causes of moisture problems in buildings is rain penetration, and to combat it, external rain screens may shortly become more popular. I t might well prove convenient to use combustible materials, but for high rise buildings, this might constitute a fire hazard. To allow a n exterior cladding fire to envelop a tall building would be to invite multiple interior ignitions and the subsequent destruction of the whole interior. To assess the hazard that various exterior claddings might present, the ad hoc tests described in this report were carried out.

T E S T P R O C E D U R E

Assemblies, 55% in. wide by 8 f t high, were constructed with cedar shingles, cedar shakes, drop siding (painted and unpainted), vertical siding (painted), imitation brick siding, imitation stone siding, and sheathing grade plywood. Several types of back-up construction were used as indicated in Figures 1 through 6. Each assembly was clamped to an 8-in. thick, 55%-in. wide, concrete block wall by ties screwed to the side mem- bers of the specimen and anchored to the block wall. An overall view of the test assembly is shown in Figure 7.

The source of ignition was a propane-fired burner (see Figure 8), fed a t a pressure of 1.28 in. water gage. With the jets vertical, the burner pro- duced free burning flames 20 in. high. I n use, the burner was located about 5 in. symmetrically below, and in front of, the bottom edge of the specimen. The jets were directed toward the bottom edge of the specimen, so that the base of the assembly was bathed in flame.

NOTE: This paper is a contribution from the Division of Building Research, Na- tional Research Council, Canada, and is oublished with the approval of the Director of the Division.

60 BATTERYMARCH ST., BOSTON, MASS. 02110 ' Printed in U.S.A.

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138 Fire Technology

The time for which the fuel supply was left on was dependent upon the behavior of the specimen. As substantial as the burner flame was, once it had effectively ignited the exterior cladding material, it played little part in the further development of the fire, except in the unpainted drop siding test (Test No. 8).

In testing imitation stone siding (Specimen No. 7), which was similar to the imitation brick siding (Specimen No. 6), slightly different ignition conditions were adopted. The burner tube was raised a few inches so that the flames played only on the front surface of the specimen.

L

55%''

-i

Figure 1. Specimen construction for T e s t 1. 2 Figure and 3. 2. Specimen construction for Tests

C

551"

-

I 551"

-i

Figure 3. Specimen construction for Tests

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Exterior Claddings 139

Figure 5. Specimen construction for Tests 6 and 7. Air gap closed for Test 7.

Figure 6. Specimen construction for Tests 9 and 10. Air gaps at top aizd bottom were left open and insulation was not used for Test 10.

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140 Fire Technology D r i l l 0 . 0 6 7 D i a

19 H o l e s _{6 0 "}

"

-I

Figure 8. Jet locations for propane burner.

R E S U L T S

As had been expected, exterior claddings behaved in one of two ways -

either they decisively propagated fire upwards within a short time after ignition, or they made little contribution t o the propane flames. Specimen behavior is indicated by the maximum extent of flaming given in the fourth column of Table 1. Claddings falling into the first category are distin- guished as those whose flames projected above the top of the specimen. About the time this occurred, the propane supply was cut off. This made little difference to the performance of cedar shingles in Test No. 1, cedar shakes in Test No. 3, imitation brick in Test No. 6, imitation stone in Test No. 7, and plywood in Test No. 10. I t was inferred from this behavior that assemblies of these types would indefinitely propagate fire vertically. In Test No. 8 (unpainted drop siding), flames did project above the assembly; however, when the propane was cut off a t this time, behavior was anomalous, and flaming subsided rapidly. About 2 % min after the burner was shut off, it was re-ignited; and in the course of the next 6 min, fire again developed.

TABLE 1. Test Results

Time to

Test or M a x . extent max. height Rating Specimen Fig. of flaming or top E162-66T

No. Ref. Cladding material ( f t ) ( m in:sec) test Cedar shingles Cedar shingles Cedar shakes Drop siding (painted) Vertical siding Imitation brick Imitation stone Drop siding (unpainted) Plywood sheathing - . grade Plywood sheathing above t o p 4:30 124 4 2:05 124 above t o p 1 : l O - 4 1:35 126 3 1:00 109 above tor, 3:50 206 above t o p 3:05 251 above t o p 2:20 158 3.5 5:OO 101 grade above t o p 5:30 101

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Exterior Claddings 141

Flame-spread tests, according to the Tentative Method of Test for Surface Flammability of Materials Using a Radiant Heat Energy Source (ASTM E162-66T), were carried out on samples of all test specimens. Mean results are given in the sixth column of Table 1. The result of the test on imitation brick siding (Test No. 6), which involved a thermally thick assembly, suggests that a cladding with a rating exceeding 200 is likely to propagate fire vertically. As the behavior of the unpainted drop siding in Test No. 8 might also be considered undesirable, critical rating might well be taken as 150 rather than 200.

Since the assemblies in Tests No. 1 and 10 were not thermally thick in relation to the time scale of the test, burning could take place on both the front and rear surfaces of the specimen. This feature influenced performance. The results suggest that, in assemblies that are not thermally thick, a cladding with a rating exceeding 100 will probably propagate fire vertically. Vertical siding used in Test No. 5 had a rating of 109. The assembly included a cavity in which flaming could occur, but the specimen did not propagate fire indefinitely. The siding, roofing felt, and plywood sheathing made the assembly thermally thick.

ACKNOWLEDGMENT: The author wishes to acknowledge the contribution of Mr. R. Huot and Mr. K. Begley in carrying out the tests reported in this paper,