Sprayed-on foam roof undercoatings might seem, to the uninitiated, like a good idea. However, coating the underside of a slated or tiled roof will compromise two vital attributes: the roof's ability to move and to breathe.
Traditional pitched roofs are constructed from a timber frame, which will expand and contract in response to changes in moisture and temperature. Slates and tiles are individually nailed to battens, and free to move relative to each other, in order to accommodate this movement. If they become bound together by the rigidity of the foam, this could set up stresses which might damage the verges, the ridges, or the individual slates or tiles.
The chief disadvantage, however, is the reduction in breathability, which might lead to wood rot in the roof timbers. The insulating properties of the foam will mean that in cold weather the battens and rafters - deprived of the slight warming effect of heat escaping from below - will be nearer dewpoint temperature, and hence more likely to become damp from condensation.
And once moisture has condensed out on the timbers, the enclosing effect of the foam means that it is likely to be trapped there for extended periods. For this reason, foam undercoatings contravene the requirements of the Building Regulations, which insist on a clear ventilated air gap between roof insulation and the slate or tile covering.
Readers have also reported that although foam spraying is often promoted as a cheap solution to roof problems, it can sometimes cost more than re-roofing using traditional methods. This is compounded by the fact that when the time comes to strip the roof off and do the job properly, it will be found that the foam sticks so tightly to the slates or tiles that they will not be re-usable, and will have to be dumped.
The National Federation of Roofing Contractors does not endorse foam undercoating, nor does it recognise it as a discipline of the industry. A spokesman has told me that the only acceptable use for this technique would be to temporarily extend the life of a dilapidated industrial or farm building by a few years, until it can be replaced or re- roofed properly.