The use of vapour permeable underlays (VPUs) is quickly gaining acceptance in all types of pitched roof. Originally introduced for warm roofs – that is for buildings with a room-in-the-roof – the development of VPUs mean they now offer more strength than 1F bituminous felts which are traditionally used on cold pitched roofs, where the attic space is unoccupied.

Correctly installed, such underlays allow water vapour to pass through from the interior and disperse harmlessly into the atmosphere, so condensation does not become a problem in the roof space. However, as with a Goretex raincoat, the passage of water vapour is one way only and VPUs provides a secure waterproof barrier against the weather. Any rainwater that penetrates the tile junctions onto the VPU will run off harmlessly into the gutter, whilst the tape-sealed lap system ensures the roof remains wind tight.

The popularity of VPUs stems from the fact that they are much lighter and easier to handle than 1F felt, as well as aiding roof-space ventilation. However, installers should be aware of the correct installation procedure. Klober have produced this step by step guide on installing VPUs on a traditional cold roof.

Step 1 Neither VPUs nor bituminous felt are UV stable, so an Eaves Carrier should be stapled to the rafters at the eaves as a continuous support for the membrane and to protect the material from sunlight. The Eaves Carrier sheets should overlap one another by at least 100mm.

Step 2 Lay the first roll of VPU with the bottom edge over the eaves carrier to ensure the membrane will be covered by the roof finish. Double-sided tape can be used to seal the membrane to the eaves carrier.

Step 3 Drape VPU over the rafters and fix along rafter centres using a Staple hammer and 8 x 11mm nails at 450mm centres. Using a staple hammer is quicker and means the roofer can keep one hand free, which is safer. Furthermore, lightweight membranes do not require heavyweight felt tacks to fix them in place.