Our Victorian coach house has three gables, with a slate roof above and sandstone cappings, and there are persistent patches of damp not at the apex of the triangle which forms the roofline but around its base. These patches are obvious from the inside, and also from the outside where they are bursting through the paint/render where previous owners have applied it - one gable is just bare brick.
Builders have tried lots of things: plan A was to raise the capping stones and put sections of lead underneath, but of course the stones were too fragile and would have been ruined if they'd been moved. So instead these builders put lead sections above each slate, and sand-and-cemented them into position. They also repointed the gaps between the capping stones to make sure that they weren't letting water in that way.
The next builder said that those lead sections should have been under the slates - I've seen both methods in this area.
The next one suggested covering the sandstone cappings with lead to prevent water penetration, but when it came to it they used a waterproof stick-on stuff instead (not quite as unsightly as it sounds, but still noticeable in a conservation area.
Nothing works - it's getting worse. And yes, I can seal it from the inside, which works for a while, but in the meantime it won't be doing the brick and lime mortar any good, will it?
Builders have told me that it was coming through the porous sandstone capping stones; now they say that it can't possibly be, and must be coming through the wall itself - but all walls have been repointed and (in the case of the worst wall) sealed with a waterproofing paint.
So where is it coming from, and how can we stop it?
all ideas gratefully received
Carol

You say sandstone capping, Does your gable end wall extend above the roof and this wall is capped? If this is so, there should be lead flashing going between the wall and the slates. As the water ingress is at the bottom of the roof, it would seem its the volume/speed oof the water that is causing the water ingress.
The lead flashing is in two parts, one , the "soaker" is a strip of lead bent into a long L shape and put UNDER the slates tucked right up tight against the wall, the theory beinng any water getting on this will run down it and drip out into the gutter/down wall. this will leave the little gap between the actual wall and the vertical part of the soaker. This is protected by pieces of lead let into the brick work and run down the wall to over lap the vertical edge of the soaker. Any water running down the wall will run down this piece of lead onto the soaker and will drip out. If the soaker has got grot on it , water finding its way on to it will drip of at the grot point into the roof void instead of running down it.
Try going up into the loft, get someone to turn a hose on the roof, perhaps you can see whats happening.
Frank