Warm deck or cold deck flat roof that is the question?

Traditional cold deck flat roof

Traditionally, flat roofs simply consisted of a structural timber deck with a waterproof covering directly applied to the top of the deck. This design dates back many years and flat roof insulation was somewhat of an afterthought with this type of roof. Where a cold flat roof does incorporate insulation this is in the form of mineral wool stuffed in between the timber rafters/joists, with a minimum 50mm (2 inch) air gap left clear in between the insulation and the roof deck. This gap is necessary to allow air-flow, so that warm, moist air, is allowed to escape rather than coming in to contact with the ‘cold deck’ and condensing. Condensation can lead to; rotting decking and/or timbers, damp ceilings and damp insulation which is rendered ineffective. Good external ventilation is also essential to minimize the risk of condensation in a cold roof. This should ideally be in the form of ‘soffit vents’ to the underside of the fascia boards, but as many flat roofs were built without soffits, it is often in the form of unsightly ‘mushroom vents’ on top of the roof, or soffit vents incorrectly fitted in to the fascia boards. Good ventilation (otherwise known as a draught) is essential to allow condensation to escape, particularly where moisture levels are high (such as over bathrooms and kitchens). Unfortunately this means a great deal of heat is also lost in the process.


Warm deck flat roof

By moving the insulation to the outside of the structure, the roof deck is effectively brought inside, which results in a ‘warm deck’ (hence the name), this removes the the possibility of condensation forming on the underside of the deck and eliminates damp. Because this design works by conserving heat, no ventilation is required. Modern polyisocyanurate (PIR) foam rigid flat roof insulation with reflective foil to both sides provides more than double thermal efficiency compared to mineral wool, and unlike the cold roof method the insulation thickness is not limited by the depth of the timber rafters/joists. A vapour barrier or vapour control layer is an essential component of any warm roof.

Ashford Building Services can quickly and economically convert any cold deck flat roof to a warm-deck insulated flat roof, particularly where problems have been tackled sooner rather than later. Where the existing roof deck is in satisfactory condition and of a suitable material e.g. plywood, the insulation is fixed directly on top, often with the existing felt acting as the vapour barrier. The whole build-up will then be waterproofed with a high performance roof covering, such as GRP Fibreglass, which is certified by the British Board of Agreement to last at least 25 years. This ensures that your investment in energy efficiency is protected and will continue to pay dividends long in to the future, repaying itself many times over.