The short answer
Carefully, and with as little stripped as possible. Stone slate roofs fail at the pegs, battens and bedding long before the stone itself wears out, so the usual repair is to refix slipped slates and renew fixings, then lay the same stone back in its courses.
Brinkworth's parish church has stone rubble walls and a stone slate roof, and that tells you a good deal about how the village was built. Stone slate is a slow, heavy, hand-laid covering, and it is repaired in a different way from slate or tile.
Q1.1What goes wrong first on a stone slate roof?
The slates hang from battens by a peg or nail through a hole near the top. Pegs rot and nails rust, so slates slip while the stone is still perfectly good. Moss packs into the joints and holds water against the timber. Where the ridge and the gable ends sit, the mortar that once sealed the edges cracks and drops out, and wind then gets underneath.
Q1.2Can the slope be repaired rather than replaced?
Where only a few slates have slipped, we refix them with new fixings and leave the remainder alone. Where a whole slope is sagging, we lift it in sections, set the stones out on the ground by size, fit new battens, and put the stones back starting with the largest pieces along the eaves and finishing with the smallest under the ridge. Damaged stones are made up from matching stock rather than from whatever is easiest to find.
Q1.3Why photograph the slope before stripping it?
Before any slate comes off, we photograph the slope so the pattern can be copied exactly. A stone roof has a gentle wave from decades of settling, and we work with that instead of flattening it. Our page on Cotswold stone roofing describes the method in more depth, and roof repairs covers smaller faults.
For a roof you are unsure about, the survey photographs show what has slipped, what has cracked and what is still sound, so the written quote is for the repair the roof needs and nothing beyond it. The workmanship on any Brinkworth job carries our ten-year written guarantee.