There are two main types of roof construction, warm roofs and cold roofs, the differences are where the installation is positioned. With a cold roof the installation is placed in between the rafters, whilst with a warm roof the installation is placed on top of the rafters – the benefit of a warm roof is that there are no ‘cold spots’ – areas where there is no installation such as where the rafters are.
Cold roof construction
Installation is packed between the rafters leaving airspace of approximately 50 mm between the insulation and the top of the rafters. Roofing felt used today is a high-performance fabric rather than the bitumen-based felt of years ago, the felt is fixed to the outside of the rafters. Over the felt treated tile battens are fixed and the tiles hung. If a low profile roofing such as asphalt shingles is being used a deck of plywood is fixed over the rafters but under the felt, and this replaces the tile battens. It is important with a cold roof that the roof soffits have vents in them to keep the free flow of air above the installation.
Warm roof construction
With a warm roof, rigid insulation is fixed to the outside of the rafters, on top of this goes the roofing felt followed by the treated tile batten and finally the tiles, if asphalt shingles are being used a plywood deck is created over the insulation and under the felt. Because the insulation is on the outside there is no need for an airspace or ventilation in soffit boards. It is often easier to achieve target u-values (the rate at which heat passes through the roof) with a warm roof than with a cold roof.
SIP’s are becoming popular in garden room construction and can be used in the roof replacing the rafters and insulation. The SIP’s are covered with felt and finished with tile battens and tiles as described above.