Roof replacement vs foam spray

foaming your roof stops it breathing and creates rot in your timbers it is about the same price wise as putting in roof felt which will last a lot longer, while it may be a 25yr insurance backed guarantee, most of these companies have now stop trading. basically would not touch a house where the roof has been foamed with a barge pole, if a tile slips after foaming it is impossible to re-seal without removing the whole roof covering. good luck Alex

Avoid foam, replace the roof if your budget allows.

The roof foam idea can actually work and it doesn't actually cause rot. Interstitial condensation is not always going to happen as heat is kept within the loft, due to insulation values. However the nails which keep the nails fixed to the battens, and the battens to the rafters will still erode over time. The roof will eventually need to be redone. The slates on your roof though will be worthless, and unusable again. Good quality Welsh slates would be reused with a small make up required to finish the job. However saying that around mid war time and onwards there was a lot of poor quality slates imported from Europe which are not of a good enough quality for reuse.