Why do chimneys leak more than other parts of the roof?
A chimney is a solid mass that interrupts the water running down a slope and sits on a roof that moves around it. Metal at the edge is fastened to the roof on one side and the brick on the other, so seasonal movement works that seam open, and brick itself soaks up water.
Brick is porous. In winter, water that enters a mortar joint or a hairline crack can freeze, expand and widen the gap a little more each cycle. That is why a chimney problem sometimes begins as a masonry problem and ends as a roofing one.
A chimney also gives the roof a large vertical face above the shingles. Water meets that face and has to be carried around it. Where a roofer finds sealant instead of metal on that face, the leak is usually just a matter of time.
Is it the flashing, the crown or condensation?
Flashing leaks usually show up after rain or melting snow and appear beside the chimney. Crown or brick leaks tend to appear inside the chimney or on the masonry itself. Condensation shows as dampness near the flue in cold weather without rain. A roofer sorts them by inspection, not guesswork.
Stains near the chimney are not always from the roof. A cold flue can collect moisture inside an attic in winter, and a poorly vented attic can add to it. That is a ventilation and air-sealing issue, covered on the attic ventilation and roof insulation page, and no amount of flashing will fix it.
If a chimney is no longer used, ask whether the flue top has a cap. An open flue can let rain straight down the inside.
Who fixes what: roofer or mason?
A roofer handles flashing, saddles, shingles and sheathing around the chimney. A mason handles brick, mortar, crowns and flue liners. Many leaks need both, so the order matters: the masonry is sound first, then the metal goes in over it.
Counter-flashing is set into the mortar, so a roofer sometimes has to cut a groove into joints that are already crumbling. If the mortar will not hold the metal, the roofer should say so and recommend repointing before the flashing goes on.
A roofer is not a chimney sweep or an inspector of the flue and should not be asked to judge whether a fireplace is fit to use. That is a separate trade.
How does weather in Massachusetts test a chimney?
Freeze-thaw cycles, drifting snow and wind-driven rain all hit the one vertical mass on the roof. Boston averages about 43.8 inches of snow a year, with a record 108.6 inches in winter 2014-15, and meltwater that sits against a chimney finds any gap in the metal or the mortar.
In a heavy-snow winter, a roof slope can hold snow against the uphill side of a chimney for weeks. The melt that comes off the slope ends up exactly where flashing is weakest. That is also the point where an ice dam can send water back under shingles from the eaves, a separate problem covered in our ice dam prevention guide.
