This informal CPD article Flat Roofs & Fire Risk was provided by CARLISLE Construction Materials, waterproofing solutions for flat roofs, facades, buildings and landscapes.
While all roof constructions are susceptible to localised fire spread from within a building or adjacent structures, flat roofs are at a greater risk of being the original fire source. As an accessible surface area, flat roofs are an easy target for arson attacks. Given arson is the most common cause of fire in the UK, annually accounting for 50% of fire service call-outs, this poses a notable concern for any building with a flat roof.
Additionally, the maintenance and refurbishment of flat roofs also present a fire hazard. Grinding, welding and torch-applied roofing are all hot works which can cause a fire to ignite.
Finally, plant and machinery are commonly installed on flat rooftops. The mechanical failure of these can lead to a roof fire. With solar panels specifically, there are known incidences of faults causing fires by ‘arcing’.
Approved Document B – Fire Safety Update
The design and construction of flat roofs needs to take into consideration resistance against fire outbreak on both the inside and the outside of the building. On the 30th August 2019 the new edition of 'Fire Safety-Approved Document B' (ADB) came into force and with it there are a number of changes that affect flat roofing.
The National Classification system set out in BS 476-3:2004 as the principle determinant of external fire performance for roofs (with exception of a listing that sets out the old classification system for historic projects), is now no longer referred to.
The European Classification system set out in BS EN 13501-5 is now the main reference for external fire performance of roofs in the UK. This classification still covers external fire penetration and spread of flame of roof systems in their response to fire from outside the building, not to be confused with individual comprising components tested for reaction to fire. The test is not concerned with the behaviour of roofs when subjected to the effects of fire from the underside, i.e. from within the building.
To advance and clarify the fire standards across Europe for External Fire Performance the roofing industry has been going through a changeover from the test and classification of BS 476-3:2004 to the tests of TS 1187 (planned to become BS EN 1187 in the next few years) and classification using BS EN 13501-5, the culmination of which has now come in the new ADB 2019 edition.
The standardisation into one European test proved indefinable as many countries within the EU had differing regulations, therefore, four test standards were required to cover the legislation in place within the various member states at the time. As such the TS 1187 has four tests for roof covering systems: t1 for Germany, t2 for Scandinavia, t3 for France and t4 for the UK (and used in the Republic of Ireland).
For TS 1187 Test 4 it was recognised that Approved Document B focused upon saving human lives rather than protecting the building itself and therefore set the highest requirement of A for penetration - not penetrated within 1 hour and C for spread of flame - more than 533mm. This meant that Test 4 needed to replicate the penetration test but did not require the spread of flame full test because the preliminary test allowed the tester to determine if classification D or better was achieved.
If the preliminary test specimen did not burn for more than 5 minutes after removal of the test flame and spread of flame was not more than 381mm then class C is achieved. This is why Test 4 only requires the preliminary test and penetration test.
It should also be recognised that the classification is for all types of roof system, but virtually all commonly used flat roof build-ups achieve Broof (t4) when tested. However, to meet the Building Regulations for fire it would need to be proved that the 'as installed' system has a valid test certificate. For this reason, late product substitution has a high risk of not complying with the Building Regulations.
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