Insulating flat roofs in accordance with new British Standard and fire performance revisions

The recent review of the flat roof design British Standard and Approved Document on Fire feature new design considerations for insulating flat roofs. Ensure your project requirements are being met by following Bauder’s guidance on achieving minimum U-values, thermal performance of inverted roofs and fire safety.

BS 6229:2018
The 2018 update of British Standard 6229 – flat roofs with continuously supported flexible waterproof coverings, brings in significant changes that affect the design of flat roofs and the installation of a waterproofing system and associated insulation. The document comprises updated practices that directly impact the design of roofs on new buildings and those to be refurbished.

Minimal U-Values and Control of Condensation
Under the revised British Standard, the minimal U-value levels permitted at any point on a roof, must be 0.35W/m²K (clause 4.7.2). ‘At any point’ includes the minimum thickness of tapered roof areas and gutters to maintain the thermal performance of a heated building. This is a new design consideration under the standard’s revision. In such roofs, the risk of surface condensation is removed if continuity of insulation, including at upstands and roof penetrations exists.

This uplift in the standard looks to eradicate the practice of thinly insulated gutter soles and excessively low points in tapered schemes.

For reference: 0.35W/m²K is achieved using approximately 60mm BauderPIR FA-TE or 100mm of BauderROCK.

Thermal Performance of an Inverted Roof
When designing an inverted roof construction, the principal thermal insulation layer is located above the roof structure and waterproofing, resulting in the waterproofing, structural deck and support structure being at a temperature close to that of the interior of the building. In order to prevent water from passing around the insulation, through the joints, and reaching the waterproofing layer, where it would have a cooling effect on the building, the correct installation of a Water Flow Reducing Layer (WFRL) is required. However, the construction tolerances and installation of the WFRL can result in a less than desirable reduction of water flow and as such the corrective action is much reduced. The advice Note in clause – Inverted Roofs to counter this reduction suggests increasing the design thickness of the thermal insulation of an inverted roof where a Water Flow Reducing Layer (WFRL) is being relied upon by “not less than 10%”. Bauder provides in project specifications both the design thickness for the target U-value plus the advisory minimum 10% increase in thickness for the specifier to choose if they wish to follow the advice.

To ensure comparable tendering Bauder recommends that you should clearly state if you are following the advice in BS 6229:2018 Clause

Update to Fire Safety Approved Document B
On the 30th August 2019 the new edition of ‘Fire Safety-Approved Document B’ (ADB) came into force and with-it considerations that affect insulation within flat roofing design. For a full understanding of the testing procedures, classifications and further guidance, please visit

Specifying roof insulation based on fire performance
Individual construction products are covered within BS EN 13501-1 for which insulation as a separate component will be encompassed and allocated a Class according to their reaction to fire test results with letter classifications from ‘A1’ through to ‘F’. On the whole, materials manufactured from plastics will achieve an ‘E’ rating, which will include the insulants Expanded Polystyrene (EPS), Extruded Polystyrene (XPS) and Polyisocyanurate (PIR) and are excluded from ‘Specified Attachments’ – fixed balconies, but they are perfectly acceptable in a flat roof build-up where the whole build up achieves BROOF(t4).

There are currently a number of ‘grey areas’ in the legislation that further complicate this. Whilst there are several non-combustible insulants that can be used on balconies or vertical flashings, the waterproofing membrane used will typically be Class ‘E’. The MHCLG need to be more specific on their requirement for flat roofs.

Non-combustible insulants, such as cellular glass along with mineral wool, are clearly desirable materials to include in a flat roof specification because of fire performance, and it is important to consider and balance the factors for inclusion within a roof system. In general, non-combustible insulants are not as thermally efficient as PIR insulation and therefore extra thicknesses, increased weight and reduced compressive strength can be a limitation in some applications. PIR has the advantage of being highly efficient, which reduces the height and weight of a roof covering build-up whilst also offering good compressive strength meaning greater versatility on a project.

Within a Bauder warm roof waterproofing system (excluding ‘Specified Attachments’) the insulation, be it mineral wool, cellular glass or PIR, is not directly exposed and is therefore protected through the performance of the cap sheet and its system classification of BROOF(t4); thus these insulants in-situ all conform to Building Regulations for external fire on roofs in the same way – not one achieving a higher rating than the other.