If a house leaks at a chimney — and in Gresham, with eight months of rain a year, a lot of them do — the odds are heavily against the masonry being the cause. It is the joint between two trades: the roofer’s flashing and the mason’s wall.
Why this joint is hard
A chimney passes through a plane that is supposed to shed water, and the two materials on either side of that intersection move differently. The roof deck flexes with load and temperature. The masonry stack does not. Anything rigid spanning between them fails.
The traditional solution is a two-part flashing, and the two parts are what people get wrong.
Base flashing — step flashing up the sides, an apron at the low side, a cricket or saddle at the high side — is attached to the roof and laced into the shingle courses. It moves with the roof.
Counter-flashing is let into a mortar joint in the masonry, bent down, and laps over the base flashing without being attached to it. It moves with the chimney.
The lap between them is what makes the joint watertight while allowing differential movement. Sealant across that lap, or one continuous piece of metal fastened to both, converts a working detail into a rigid one that tears in a season or two.
The failures, in order of frequency
- Surface-mounted counter-flashing sealed with caulk. Fast, cheap, and it is the single most common defect. Sealant on a masonry face is a two to five year detail; a reglet cut into a joint is a thirty year one.
- No cricket on a wide chimney. Anything wider than 30 inches across the slope needs a saddle behind it. Without one, water and debris pond against the back and find their way in.
- Step flashing replaced with a continuous bent strip. Water gets behind it at every course and there is no lap to stop it.
- Reglet cut into brick instead of the mortar joint. It weakens the unit and it never seals well.
- Everything done right, but the crown is cracked. Water enters the top of the stack and appears at the ceiling looking exactly like a flashing leak.
Diagnosing it before you pay for the wrong repair
Water tests in stages, from the bottom up, with someone inside watching. Hose the roof surface below the chimney first, then the base flashing, then the counter-flashing line, then the stack itself, then the crown — ten minutes each, and stop at the first sign of water inside. Doing all of it at once tells you only that it leaks somewhere.
Common outcomes and rough costs: reflash a chimney properly with reglet-set counter-flashing and a cricket, $900 to $2,800; rebuild a cracked crown, $500 to $1,500; repoint the exposed stack, $600 to $2,000; and the one nobody expects, replace a rusted-out flashing that was buried under two roof layers, which usually means opening the roof around the chimney.
Because the fix straddles two trades, the useful question to ask is who owns the detail. On a reroof, the flashing is roofing work and the reglet cut is masonry work — a Gresham roofing contractor who cuts and sets counter-flashing as part of the scope will produce a better joint than one who caulks to the brick and leaves the masonry to somebody else.
Frequently asked questions
How long does chimney flashing last? Properly detailed copper or stainless, 40 years or more — often longer than the roof around it. Galvanized steel, 15 to 25 years. Aluminum in contact with wet masonry corrodes and should not be used against fresh mortar at all. Caulk-only details, 2 to 5 years.
Can flashing be replaced without redoing the roof? Around a chimney, yes. Shingles are lifted and the step flashing woven back in, which is normal repair work on an asphalt roof in decent condition. On a brittle old roof, expect some collateral shingle replacement.
Is a cricket really necessary on my chimney? If the chimney is more than 30 inches wide across the slope, codes generally require one, and it is good practice below that on a low-slope roof or where debris collects. It costs a few hundred dollars during a reroof and it removes the most common ponding point on the whole roof.