Q. My house was built in 1937 and has a stone chimney. When I had new asphalt shingles put on, a leak developed around the chimney flashing where the chimney comes up through the roof. I can see the leaks in the attic. A contractor returned and tarred under the flashing. Still leaks. Then a new metal cap was installed, and the stone repointed. Still leaks. Another man suggested sealing the mortar in the stone. Now what?
A. Stone chimneys are very difficult to install flashing against. Here's why: With brick chimneys, flashing (lead, copper, or aluminum) is folded into an "L" shape; one leg of this "L" is set against the brick, while the other half of the "L" sits on top of the shingles. Everything is tarred, then counter-flashing is applied over the part of the flashing that is against the bricks, in a series of steps, so it's also called step flashing. This counter-flashing is folded to form a small lip, perhaps 1-inch wide, and this lip is mortared into the brick, making a waterproof connection.
This works well with brick, which has horizontal mortar joints, but not with stone, because the mortar joints of stone wiggle all over the place, making it impossible to stick counter-flashing into. Sometimes a mason can form step flashing to fit into a short, curved mortar joint (between the uneven stones, for example).
A drastic move would be to remove the chimney to below the roofline and build a brick chimney of equal size, so a roofer can insert the flashing into a straight mortar line. You can also try applying stucco to the stone, making the stucco thick enough to cover the flashing.
The Globe Handyman on Call also appears in the Sunday Real Estate Section. Peter Hotton is available 1-6 p.m. Tuesdays to answer questions on house repair; call 617-929-2930. Hotton chats on line about house matters 2-3 p.m. Thursdays. Hooton can be reached at firstname.lastname@example.org