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External Fire Performance of Roofs by David George
BS 476: Part 3 - 'External Fire Exposure Roof Test' was first introduced in 1958. It was revised in 1975 when the test procedures were modified and the results expressed in a new way and a new version of the test, BS 476: Part 3: 1975, was issued. The 1958 version of the test was withdrawn at the same time. However, the 1975 version proved unsuitable and, despite having been withdrawn, the 1958 version continued to be used and to be referred to in the Building Regulations for England and Wales, Scotland and Northern Ireland.
BS 476: Part 3: 1958 has recently been rewritten to refer to metric rather than imperial units and to conform to the current format used for British Standards. The instrument used to measure the heat radiation to which the test specimens are exposed has also been changed because the original one contained an asbestos disc. These changes have not significantly altered the test conditions but merely updated the test. This updated test has recently been issued as BS 476-3: 2004 - Fire test on building materials and structures - Part 3: Classification and methods of test for external fire exposure to roofs. It supersedes BS 476: Part 3: 1975 which has been withdrawn.
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 is tested in BS 476-3. The current Building Regulations refer to BS 476: Part 3: 1958, but in time will be amended to refer to BS 476-3: 2004. These two versions of the Standard are essentially the same.
BS 476-3: 2004 Fire tests on building materials and structures - Part 3 Classification and method of test for external fire exposure to roofs. ( BS 476: Part 3: 1958 - External Fire Exposure Roof Test.)
The tests described in this standard are designed to give information concerning the ability of a roof to resist fire from a nearby fire outside the building itself from penetrating the roof and spreading across its surface. A preliminary test is made in which a specimen section of the roof is subjected to the effects of flame in the absence of radiation. Unless the specimen is badly affected by this test further fire penetration and spread of flame tests are carried out with the surface of the test specimens exposed to radiant heat.
The tests are normally conducted with test specimens mounted either at an angle of 45o or horizontally, to represent sloping and flat roofs respectively.
The test flame used in the tests is a simulated town gas flame (230 +/- 10)mm long from a (9.5 +/- 0.1)mm orifice.
The effect of a wind speed of 6.7 m/s is simulated by applying suction to the underside of the roof specimen to reduce the pressure on the underside by (1.5+/- 0.1)mm of water below that on the upper side, for the duration of the test.
The heat radiation is provided by a radiant panel (915 +/- 5)mm square comprised of four (300 +/- 5)mm square combustion panels with their centres arranged at the corners of a square of (480 +/-5) mm side.
Test specimens are (840 +/- 10)mm square and must be representative of the complete "end use" roof construction including at least one specimen of any joints used in each of the materials to be tested.
Preliminary ignition test
One specimen is tested. The test flame is applied for 60s at the centre of the upper surface of the specimen and directed up the slope with the nozzle on the centre of the specimen and inclined at an angle of nominally 5o to it. In the case of specimens tested horizontally the direction of the flame shall correspond to that indicated in the test method. No external radiant heat is applied to the test specimen nor is the pressure on the underside of the specimen reduced.
The duration of flaming after the removal of the test flame, the extent of any spread of flame along the roof surface and any penetration of fire to the underside of the specimen are noted.
Fire penetration test.
Three specimens are tested. Within five seconds of being mounted in the test equipment the upper surface of the test specimen of the roof construction is exposed to radiant heat to achieve a radiation intensity of (12 +/- 1.5) kW/m2 uniformly over the surface of the specimen. This simulates exposure to an adjacent burning building. Suction is applied to the underside of the specimens to simulate the effect of wind and to facilitate the observance of fire penetrating the specimen.
Five minutes after the start of the test the test flame is applied to the surface for 60s in the direction indicated in the method, moving once up and once down the centre of the test specimen at a rate of 0.3m/10 sec., to simulate the effect of a burning brand falling onto the roof.
The time at which glowing or flaming appears on the underside of the specimen is recorded. Any droplets, mechanical failure or holes are also noted. The test is continued for 60 minutes, unless fire penetration occurs earlier.
Three specimens are tested. The upper surface of the test specimen of the roof construction is exposed to radiant heat within 5 sec of being mounted in the test equipment, to simulate exposure to an adjacent burning building. The radiated heat is adjusted so that the intensity diminishes at a specified rate in the plane of the specimen down its centre line. The test flame is immediately applied to the upper edge of the specimen for three minutes with its orifice on the upper edge of the specimen (115 +/- 5)mm from the centre line with the flame directed along the edge, the angle between the nozzle and the specimen being nominally 5o.
Attention is drawn to any flaming droplets from the edge of the specimen, mechanical failure or any development of holes by adding a suffix 'X' to the designation. However, this suffix does not lead to any restrictions in the Building Regulations.
BS 476 : Part 3 was revised in 1975 and the 1958 version was withdrawn. The test procedures were slightly modified and the results expressed in a new way. However, the 1975 version proved to be unsuitable and the 1958 version continued to be used and referred to in the Building Regulations despite being having been withdrawn and not available for purchase. This situation still continues.
BS 476: Part 3: 1958 has recently been rewritten to refer to metric rather than imperial units and to conform to the current format used for British Standards. The instrument used to measure the incident radiation has also been changed because the original one contained an asbestos disc. This updated version has been circulated for public comment and will be amended as necessary to accommodate any valid comments early in 2004. It will then be submitted to BSI for issue as BS 476: Part 3: 2004. The 1975 version will be withdrawn at the same time.
Approved Document B of the Building Regulations, England and Wales and Northern Ireland, and Schedule 9 of the Scottish Building Standard Regulations give notional classifications for certain roof coverings to BS 476; Part 3: 1958 on flat or pitched roofs. For example, a flat roof comprising of bitumen felt should (irrespective of the Felt specification) be deemed to be of designation AA if the felt is laid on a deck constructed from 6mm plywood, 12.5mm wood chipboard, 16mm (finished) plain edge timber boarding, compressed straw slab, screeded wood wool slab, profiled fibre reinforced cement or steel deck or aluminium deck with or without fibre insulating board overlay, or concrete or clay pot slab, and has a surface finish of:
a. bitumen bedded stone chippings covering the whole area of a depth of at least 12.5mm;
b. bitumen bedded tiles of a non-combustible material;
c. sand or cement screed; or
The Future - A European Standard.
Most countries have test methods for the resistance of roofs to external fire. These involve measuring the spread of flame along the surface of the roof specimen, and, in some tests, the time for the fire to penetrate to the underside of the specimen. Although all these methods have a common purpose they have significant differences: they use different sources of ignition; some involve blowing air over the test specimen to simulate wind; others use radiation to simulate the effect of an adjacent burning building; and some do both. Not surprisingly the results obtained in the various tests cannot readily be compared and even similar roofing constructions perform differently in the tests.
After many years work European Standards Committee TC127 developed a new European Standard for the external fire resistance of roofs to replace the existing National Tests, including BS 476; Part3. The result of their work was a draft European Standard, (prEN1187), consisting of three parts based, respectively, on the current German, Scandinavian and French external roof fire tests. It was the subject of considerable controversy and was rejected as a full European Standard on two occasions.
The delay in the availability of a full EN for the external fire resistance of roofs impeded the effort to co-ordinate the European approach for the fire safety of roofs. It also delayed the development of harmonised European Product Standards for roof waterproofing products, which must refer to it. To avoid further delay the CEN Technical Board published the draft standard as a CEN Report, (CR 1187), which made it available for reference in the Product Standards for roofing sheets. CEN TC127 also decided to ask CEN to issue the draft standard as a European Pre-Standard, which was done as ENV 1187:2001. This also made it available for reference but also enables further development of the Standard and ensures that it must be reviewed within two years of being issued. Because CR 1187 and ENV 1187 are not full European Standards their publication did not require the withdrawal of conflicting National Standards, such as BS 476: Part 3, which can still be used and referred to in National Building Regulations.
A classification system for use with ENV 1187 has been developed, ( prEN 13501-5 : Fire classification of construction products as building elements - Part 5: Classification using data from external fire exposure to roof test ). At present it is a draft standard and has not yet been adopted as a full European Standard. The classifications included in this Standard are given in the following table:
The classes referred to in the table correspond to those currently used in Germany, Scandinavia, and France with their National tests for the resistance of roofs to external fire, on which ENV 1187 methods 1,2 and 3 are based.
An extensive collaborative work programme (RADAR 3) involving the fire testing of 25 different roofing systems has been undertaken in the UK. The programme was managed, and the tests carried out, by Warrington Fire Research Centre and Fire Research Station of BRE. Organisations from all sectors of the UK roofing industry were also involved and provided the roofing products and constructed the test specimens selected for testing. As far as possible the same roofing systems were tested in accordance with BS 476: Part 3: 1958 and all parts of ENV1187. The results from the European tests were classified using prEN 13501-5:2002 and the Commission Decision of 21st August 2001.
The RADAR 3 work programme showed that there is poor correlation between the results obtained by testing roofing systems to BS 476: Part3: 1958 and to the three methods of ENV 1187:2001. In particular the ENV 1187:1002 tests do not provide the same differentiation between classes as achieved with the BS 476: Part 3: 1958 test, especially in relation to the penetration of fire through the roof. This is very significant for the UK where fire penetration through the roof system is the key parameter used to regulate the location of roofing systems. Adoption of ENV 1187:2002 in place of BS 476: Part 3:1958 in the UK Regulations will effectively necessitate acceptance of roofing systems in places where they are currently not permitted and may result in a reduction in the current safety level.
As this situation is unacceptable, the chief Fire Regulator for England and Wales has suggested three alternatives for consideration by the European Fire Regulators Group. The preferred one involves including the BS476: Part 3:1958 test method within the current European system for testing and classifying the external fire resistance of roofs. This can be done by adding BS 476: Part 3: 1958 to ENV 1187 as a fourth test method and amending prEN13501-5 to include new classes referring to ENV 1187, method 4. The European Fire Regulators Group supported this change and it has now been accepted by the European Commission and by CEN, the European Standard Organisation. CEN TC127 has now issued a modified version of ENV 1187 with an additional method 4 based on BS 476-3:2004 for consideration. Although the new method 4 is based on BS476-3: 2004 the test for surface spread of flame has been omitted because the UK Building Regulations do not require better than a 'C' classification for spread of flame and this can be achieved using the Preliminary test. Pr EN13501-5 is also being amended to include classifications based on the results achieved using the proposed method 4, and which will be suitable for use in UK Building Regulations. These standards will be adopted later in 2004 provided they are accepted by a majority of the EU Member States.
Because ENV 1187 is not an approved full European Standard there is no obligation for any country in the EU to use it in place of their National Standards for the resistance of roofs to external fire, nor to remove their own standards for the external fire resistance of roofs.. However it is likely that most countries will do so and amend their Building Regulations accordingly. In reality however little will change. For example, in the UK it is likely that BS 476-3: 2004 will continue to be used, possibly in conjunction with ENV 1187 method 4, the latter being used as part of the procedure to CE mark roofing products and the former to obtain more comprehensive information about the resistance of the roofing system to fire spread.
CEN TC 127 has been commissioned to develop a new single test for the resistance of roofs to external fire which can be adopted as a European Standard to replace all the conflicting national tests and ENV 1187. However, work has not yet begun and it will be many years before a new test method is available.
Existing National Standards for the resistance of roofs to external fire will remain in place and continue to be used for some years. In the UK BS 476: Part 3: 1958 will continue to be used and referred to in UK Building Regulations in its updated form as BS 476-3:2004. When an amended version of ENV 1187 with a method 4 based on BS476-3:2004, and of prEN 13501-5 is available, the UK Fire Regulatory Authority will have to decide how use them in the UK Building Regulations. This is unlikely to happen in the near future. In the longer term BS 476; Part 3 and ENV1187 will be replaced by a new harmonised European Standard, but this is many years off.