By Joan Ferrer, Ravago Building Solutions.
There has been an increased focus on fire safety since the tragic fire at Grenfell Tower, with the current consensus suggesting combustible cladding materials helped spread the fire, with a chimney-effect sucking flames up the building because of air gaps and the lack of efficient fire breaks.
After the Independent Review of Building Regulations and Fire Safety led by Dame Judith Hackitt, the Building (Amendment) Regulations, SI 2018/1230 barred the use of combustible materials in external walls and specified attachments of relevant buildings greater than 18 metres in height, and there are now calls for this to be extended to structures taller than 11 metres.
However, when contemplating your choice of insulation for an inverted flat roof it would be wrong to assume that non-combustible materials are necessary, and it is troubling to see unsubstantiated fears about fire safety being used to promote stonewool or cellular glass insulation. Tried and trusted solutions already comply with BROOF(t4), the highest performing fire classification for a roof system, and while new Building Regulations may usher in additional changes to fire safety regulations for cladding, there is no evidence to justify a shift from safe established approaches in roofing.
The (t4) of BROOF(t4) indicates the use of test 4 from European Specification ENV 1187, the most intensive test, and the only one which meets the UK’s tough fire safety requirements. This test uses burning brands, wind and supplementary radiant heat to assess the spread of fire across the external surface of a roof, fire penetration and the production of flaming droplets or other debris. To achieve the UK fire classification as outlined in BS EN 13501-5:2016 there can be no penetration of a roof system within 60 minutes and a maximum flame spread of 0.38m during the preliminary test stage.
The exemplary track record of roof systems with non-combustible coverings is recognised by European Commission Decision 2000/553/EC of 6 September 2000, that implements European Council Directive 89/106/EEC, exempting them from the testing regime. It explicitly states “The external fire performance of many roof covering products/materials is well established and sufficiently well known to fire regulators in the Member States that they do not require testing for this particular performance characteristic.”
It is easy to understand why the fire properties of insulation used in an inverted roof have been determined to have no relevance to fire safety, as the non-combustible concrete deck beneath and gravel ballast or paving slabs above enclose the material.
However, to assuage concerns about fire safety, some systems using XPS products from Ravago Building Solutions have now been tested, easily achieving BROOF(t4) classification, which considering decades of safe usage was not a surprise to anybody.
What about green roofs? They aren’t referenced in the European Decision, however, Fire Performance of Green Roofs and Walls, published by the UK government in August 2013, extends the exemption from testing to green roofs provided that the growing layer is at least 80mm with a maximum of 50% organic content.
XPS is of course not just a safe material for use in inverted roofs, it also boasts exceptional long-term thermal performance, and its high compressive strength means roofs can be used for everything from terraced gardens to hosting heavy plant equipment.
ETAG 031 Guideline for European Technical Approval of Inverted Roof Insulation Kits recommends the use of XPS, not some of the non-combustible alternatives which have never been subjected to rigorous immersion, diffusion and freeze-thaw tests to assess the impact of water absorption on performance over an extended period. Recent revisions to BBA Bulletin 4 Inverted Roofs – Drainage and U value corrections determines that an accurate U-value calculation requires the use of a moisture correction factors – you don’t have these values for previously unused products.
Ultimately, there is no reason to complicate your decision. We know that XPS is safe for use in inverted flat roofs and it offers exceptional thermal performance – specify it with confidence.