We have just bought an 1870's house which has an old slate roof without any felt, just mortar stuck to inside of slates (torching?). It leaks a bit, one chimney has no flashing. some slates have slipped and there is a bit of a sag visible in the structure.
It clearly needs repair, we can't decide between the options:
1. patching up the roof and then a DIY spray foam kit, or
2. patching up the roof and then professionally applied spray foam or
3. a complete re-roof, salvaging some of the old slates and fitting felt and new battens etc.
We have a limited budget but want the best for our house.
Roofers recommend new roofs, spray foam companies recommend foam, it's hard to find unbiased advice on this topic, please help.