Warm or cold deck flat roof?

Flat roofing insulation panels

There a two distinct styles of flat roof construction in home flat roofs. These are cold deck and hot deck flat roofs and as the names suggest the difference is about how the roof handles insulation.

Older roofs are typically built with a timber under deck providing the support for both the waterproof covering above and the thermal insulation below. With a traditional roof insulation is likely to consist of mineral wool sheets suspended below the deck. As with all roofs moisture control is essential. With an older cold deck flat roof vapour control is achieved by ensuring a gap of 25mm or more between the ceiling and the insulation layer and between the insulation layer and the deck above. These gaps help warm moist air escape before condensing on the cold deck above. The cold deck method works well when constructed correctly and in correct conditions but condemnation problems.
Cold deck flat roofing diagram

The warm deck roof approach is different. The insulation layer is found on top of the deck with the waterproof layer above it creating the ‘warm deck’ underneath. This completely removes the risk of condensation building up on the deck and timbers. Because the insulation is built on top of the roof it can be a dense foam (PIR) layer with excellent insulating properties more than twice as effective as a typical cold flat roof. A range of modern roof protection coverings can be applied to a warm deck roof including single ply membranes and GRP fibreglass both of which come with extreme life expectancies.

hot or warm deck flat roofing diagram

For most domestic purposes a warm deck flat roof is superior to a cold deck. Converting a cold flat roof into a modern warm deck roof can be surprisingly economical. Provided the existing roof is in a good state of repair and has a suitable structure a warm deck overlay can be applied. After cleaning & preparing the existing roof we install an insulation layer on top creating a thermal envelope with the old roof as the vapour barrier. Finally we install a roof covering system with necessary drainage and guttering. The result is an attractive, highly durable and energy efficient flat roof. This can make a huge difference to home insulation saving money from heating bills for years to come. If you’d like us to take a look at your cold deck flat roof just contact us and we’ll be happy to help.