STRUCTURAL ELEMENTS
Minimising fire risk on the ‘fifth facade’
Will Wigfield from Rockwool shares best practice advice on how to limit fire risk in flat roof constructions, and explains the limitations of current testing protocols for flat roof insulation
W
hile flat roofs have long been used to house plant equipment and machinery, their scope and role in contemporary building design is expanding. Now, this ‘fifth facade’ is frequently used to create green and blue roofs, harvest energy through PV arrays, or provide garden terraces and social spaces. With this expanding role however, there is also expanding risk. In addition to the hazard presented by hot works – both in the construction phase and during subsequent maintenance – electrical malfunctions in roof-mounted equipment, such as solar panels, have been known to cause roof fires.
If a flat roof is relatively accessible – whether at low level, featuring a terrace, or reachable via scaffolding – it can become an easy target for arsonists. Yet with the right material choices, not only can these fire risks be mitigated, the roof can even perform as a means of escape or place of refuge.
As is the case throughout the building envelope, the use of non-combustible insulation in flat roof design should be considered best practice. However, when selecting materials, changes to Approved Document B (ADB) in 2019 have caused some confusion in the market regarding testing, standards and legislation for fire performance.
A different class
The national classification system (BS 476- 3) was previously the principal determinant of external fire performance in roofs. While the European classification system historically ran alongside it, changes to ADB in 2019 saw the national classification become obsolete, with EN 13501-5 ‘Fire classification of construction products and building elements – Part 5: Classification using data from external fire exposure to
roofs tests’, becoming the main reference point for assessing fire penetration and spread of fire in roof applications. Under the national classification, performances of AA, AB or AC allowed the unrestricted use of a flat roof system and could be applied anywhere on the roof. BS EN 13501-5 is now the recognised standard for indicating the performance of a flat roof system when exposed to fire from an external source. Applicable to ADB, Test 4 of DD CEN/TS 1187:2012 provides the method for evaluating the performance of a roof under the conditions of ‘thermal attack,’ which includes external fire spread and penetration by fire. The highest achievable rating is termed BROOF
(t4). ADF JANUARY 2022 The tests required to
achieve a BROOF(t4) rating do not subject the roof system to a fully developed fire
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