BS476 Part 3 Roof Fire Test
In the UK the hazard that exists of fire spreading to the roof of a building from a nearby fire outside the building itself is assessed by testing representative specimens of the roof construction to BS 476: Part 3. This test is not concerned with the behaviour of roofs when subjected to the effects of fire from its underside, i.e. from within the building.
The Building Regulations for England and Wales, Scotland and Northern Ireland require roofs exposed to external fires to provide protection against fire penetration into the space beneath the roof and against the spread of flames across the surface of the roof. The ability of roofs to provide such protection is tested in BS 476-3.
The performance of the roof construction in this test is measured by the time for fire to penetrate to the underside of the test specimen and the length of flame spread along the surface. These are designated by two letters, the first concerning fire penetration and the second fire spread.
First Letter - Fire Penetration classificationA : Those specimens which have not been penetrated within 1 hour.B : Those specimens which are penetrated in not less than 30 minutesC : Those specimens which are penetrated in less than 30 minutesD : Those specimens which are penetrated in the preliminary flame test
Second Letter - Spread of Flame classificationA : Those specimens on which there is no spread of flameB : Those specimens on which there is not more than 533mm, (21 inches), spread of flameC : Those specimens on which there is more than 533mm, (21 inches), spread of flameD : Those specimens which continue to burn for 5 minutes after the removal of the test flame or with spread of flame more than 381mm, (15 inches), in the preliminary test.
Examples of typical roof designations are EXT. F.AA or EXT.S.ABX, where EXT = external., F = flat, S = Slope, AA and AB or the classifications achieved in the test for fire penetration and flame spread respectively. X indicates flaming droplets or some other observation recorded in the test report.
The fire performance classification achieved in BS 476 : Part 3 :1958 is used in the Building Regulations to limit the use, near a boundary, of roof coverings which will not give adequate protection against the spread of fire over or through them, as shown in the following table:
The Building Regulations therefore regulate the location of roof constructions by their resistance to fire penetration rather than by their resistance to flame spread, in accordance with the main concern of the Regulations to preserve life rather than property. Provided it achieves an 'A' classification for fire penetration, a roof construction can be used without restriction under the Building Regulations even if it achieves 'B' or 'C' rather than 'A' classification for flame spread. However an 'A' classification for flame spread is desirable and may be required by the owner or designer of the building.