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
While 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).
While the tests required to achieve a BROOF(t4) rating consider surface spread of flame, penetration of the roof membrane, and the presence of droplets or charring – they do not subject the roof system to a fully developed fire, nor do they consider fire penetration from the underside.
BROOF(t4) is achievable by virtually all commonly-available roof build-ups on the market, even those that incorporate combustible insulation with a reaction-to-fire rating as low as F.
As such, it’s important to note that a BROOF(t4) rating does not define the combustibility of the component parts of a roof system – including the insulation. Therefore it cannot be used alone to satisfy requirements that are defined by Euroclass reaction-to-fire ratings, such as those in ADB concerning the crossing of compartment walls (A2-s3, d0), or attachments to buildings covered by Regulation 7 (A2-s1, d0).
A simple and straightforward way to determine the combustibility of a building product is by checking its Euroclass reaction-to-fire rating, which can be found on the product’s declaration of performance.
In a bid to reduce fire risk and minimise room for installation error, industry is starting to adopt a simplified approach to insulation design, using a non-combustible material across the whole flat roof area.
This proactive approach brings tremendous benefits. Designers can futureproof buildings and specifications against a changing regulatory landscape, and there’s the potential for efficiency gains onsite by removing the need for close coordination when using a combination of different insulation materials. But above all, architects and their clients can be assured they are also providing a safer environment for future building occupants.
Will Wigfield is flat roof product manager at Rockwool