Horizontal 'flat' roofs are commonly found on extensions, as well as on whole homes, and have usually been constructed with little or no insulation. To avoid heat loss through a flat roof, a layer of insulation board can be placed on top of the existing roof deck, and a new roof deck then fitted on top of the insulation board. Where possible, the cheapest and most efficient way to insulate a flat roof is to program the job with other scheduled maintenance or repair works.
It is recommended that flat roofs are insulated to become ‘warm deck’ roofs, meaning that a rigid insulation board is placed above the structural deck. A new waterproof finishing membrane is then fixed above the insulation. Insulating a flat roof from underneath ('cold deck') is not recommended, as this method can lead to issues with moisture build up.
The impact of flat roof insulation on external wall insulation should be considered. If the home is also due to have external wall insulation installed, the roof should be extended at the edges to cover the top of the wall insulation. Careful detailing between wall insulation and roof insulation helps to avoid fire and thermal bypass risks. Timber firring strips should also be fixed to ensure that the finished roof will drain correctly away from the home.