Our last house was an 1850's stone terrace with stone roof that had been
back pointed or torched. We had a firm trun it over and felt it as it was
letting water in left right and centre. All seemed well and we sold the
house. We then built an extension on the newer 1930's house and found a
retired 73 yr old roofer to do the slates, he really knew his stuff, even
telling where the slates came from which I checked with the supplier. In
talking I mentioned our old house and he said he lived down the road from it
and remembers seeing the job ! he pointed out that turning them wasn't the
thing to do as the newly exposed surface will now start to break up in
strata and the stresses in the stones may cause them now to crack. They
should have been cleaned off and replaced the same way around. He also said
that the felt will reduce the ventilation to the roof timbers compared to
the original installation and that properly laid stone roofs didn't need
felting. Anyway, 7 yrs later the roof still appears to be ok when we drive
past but I did get the impression he knew his roofs. His other gripe was
that there weren't aprentices these days and his knowledge will die with
him.

--
Pete Cross