How to Construct a Cold Flat Roof
In a cold roof construction, the Sarnafil membrane is installed directly to the structural decking (usually timber with firrings to fall). There must be an adequate void (minimum 50mm) between the deck and the insulation (traditionally mineral wool) to allow free movement of air, and to prevent moisture vapour being transmitted from the building outwards where it could condensate in the roof construction, causing harmful interstitial and/or surface condensation (interstitial condensation).
A suitable air-permeable breather membrane is required over the thermal insualtion and a continuously sealed air and vapour control layer should be installed on the warm side of the insulation, at ceiling level. Minimum 25mm deep timber battens support the ceiling, will provide a suitable service void necessary to enable cables and pipes to be installed without penetration of the AVCL.
Cold roofs are generally considered ineffective for energy use reduction and building performance and providing adequate ventilation is extremely problematic. Sarnafil membranes can be used in cold roofs but we strongly advise these should be avoided in the first place.
This advice is also supported by the latest SPRA Design Guide and current version of ‘BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings – Code of practice’, which also states that “this type of roof is not now recommended. This is because of the difficulty of forming and maintaining an effective AVCL below the insulation and of providing sufficient cross ventilation above the insulation”
- PVC membrane Sarnafil® G 410 EL Felt fully adhered to the insulation with Sarnacol® 2142 V
- SarnaTherm G PIR, thermal insulation bonded to the air and vapour control layer with appropriate Sika® insulation adhesive, or mechanically fastened to the substrate
- Self adhesive air and vapour control layer Sarnavap® 5000 E SA or S-Vap HD
- Primer 600 or Primer 610 Spray, where required
- Concrete (or timber/steel) deck
- FPO membrane Sarnafil® TS 77 mechanically fixed to the substrate with Sarnafast® SBF 6.3 mm and Sarnafast® SFT Thermally Broken Tube Washer
- New SarnaTherm S PIR thermal insulation
- Existing or new build-up on concrete deck