
Originally Posted by
RGSP
The reduced ventilation is no worse than what would happen if the roof was re-felted and re-slated, and can easily be coped with.
The real problem is that the roofing battens are encapsulated in a wet space between the slates and the foam, as are the slating nails. It's worse with old battens which are not preservative treated, but even new battens will not stand that for long, neither will the nails. Within a few years the whole roof covering is just sitting there more or less under its own weight, with no positive fixing. The foam holds it together well to start with, but it does deteriorate with time, and once the adhesion starts to go, and slates (or tiles) start coming off, there's not much alternative but to replace the whole roof, and the tiles or slates often aren't re-usable after foam either.
There may be examples where foam is working extremely well, but I do know of quite a number of cases where the above has happened.