Why do chimney leaks happen more in winter?
Snow and ice work at the chimney joint all winter, and the freeze and thaw cycle opens the gap between the metal and the masonry. Wind drives snow into the joint, and the meltwater then runs behind the flashing.
A chimney is a fixed block that sits in a roof that moves with the temperature. Each cold night and warm day changes the gap a little, and the sealant and the metal take the strain. Over a winter the joint can open enough to let meltwater through.
Snow that drifts against the uphill side of the chimney sits in one place for weeks. As it melts, the water pools against the flashing, and any opening there sends the water inside.
Can the chimney flashing be sealed with caulk?
Caulk can close a small gap for a time, but it is not a lasting fix for a lifted or corroded flashing. Caulk on a masonry joint also tends to fail when the joint moves, so the flashing and the counter-flashing should be re-set properly.
Sealant has a limited life in an outdoor joint. It stiffens in the cold and cracks in the sun, and it does not bridge a gap that has opened because the metal has been pushed out of place.
A repair that re-sets the metal and seals it into a proper groove lasts longer than caulk laid over the top. The technician uses sealant as part of the repair, not as the whole repair.
Should the chimney be inspected from inside too?
Yes. The firebox, the smoke chamber and the chimney walls can show water staining from a flashing leak. A fireplace that has been unused for a while should be checked for water before it is used again.
Water that enters at the top of the chimney runs down the flue and can stain the smoke chamber or the firebox. A stain or a damp patch inside the fireplace points to a leak at the top, even when the ceiling below looks dry.
A chimney professional is the right person for a flue inspection, and a roofing technician is the right person for the flashing. The two checks can be coordinated so the roof and the flue are looked at together.
Does a chimney cap help with a leak?
A cap keeps rain and snow out of the top of the flue and protects the masonry from water. It does not stop a leak at the flashing, but a missing or cracked cap adds water that the flashing has to handle.
A cap that is rusted or tilted lets water pour into the top of the chimney. The water soaks the brick and the mortar and runs down to the flashing line, where it adds to the problem.
The cap is checked during the flashing repair, and it is repaired or replaced where it is worn. Each step is recorded so you can see which parts were changed and why.
What does a chimney repair leave behind?
A repair leaves a re-set flashing, a sealed counter-flashing and a cap in good condition, along with written findings. The homeowner receives notes on what was changed and what to watch for after the next storm.
Ask the technician to show you the repaired joints from the ground, and to point out what was changed. A photograph of each side, taken on the day, makes the next inspection easier.
Watch the chimney after the next heavy snow and the next thaw. Any new stain, drip or rust line should be reported, because it may show a joint that has been affected by the new load.
