Chimney guides

Why Your Chimney Only Leaks in Heavy Rain

When a chimney leaks matters as much as that it leaks. The timing tells you which of four things is wrong, and they need four different repairs.

Step flashing woven into roof courses where a chimney meets the roof

Short answer

A leak that appears only in wind-driven rain is almost always the flashing — the metal sealing the joint where the chimney passes through the roof. Rain arriving horizontally gets under laps and drives up flashing faces in a way vertical rain never does. A cracked crown, by contrast, leaks in any rain. Continuous staining with no drip is usually a corroded chase cover. Porous brick absorbing water is the fourth and least common.

The four candidates, told apart by timing

You can narrow this down from inside the house before anyone gets on the roof, which is worth doing because it stops you paying to repair the wrong thing.

Only in driving rain — flashing

Wind pushes water up and under. The classic Connecticut version is a nor'easter or an onshore blow along the Sound finding a gap that holds perfectly well in ordinary weather.

In any rain — the crown

The crown is the slab at the top that throws water clear of the flue. On most older houses it is a thin mortar wash that cracked years ago, and it admits water every single time it rains.

Continuous staining, no visible drip — the chase cover

On a framed and clad chase rather than a masonry stack, the sheet- metal lid ponds water and rusts through. The runoff stains, and water gets into the framing without ever dripping into a room.

Slowly, in prolonged wet — the brick itself

Old brick is porous. In a long wet spell it can take up enough water to show inside. This is the least common and the one most often blamed, because it is the one a sealant salesman can fix.

Why sealant over the flashing is the wrong answer

The repair offered most often for a flashing leak is a bead of sealant or a trowel of roof cement across the joint. It stops the leak for a season, then shrinks and cracks, and the water goes back to where it was — except now it is hidden under a black smear.

Correct flashing is two separate metal layers: step flashing woven in with the roof courses as they climb the chimney, and counter-flashing let into a groove cut in the mortar joint and folded down over it. The groove is the part that makes it last. Metal stuck to the surface of the masonry fails when the adhesive does.

On slate, it is skilled work

A good deal of older Connecticut housing has slate. Slates have to be lifted and relaid rather than nailed through, and a broken slate becomes the next leak. Ask whoever quotes what experience they have with your roof covering, not just with chimneys.

Do it while the roof is open

If a reroof is coming, raise the chimney detail before the roofing contract is signed. The worst outcome is new covering laid over old flashing to save a day.

How to confirm it before spending

A hose test settles it. Someone runs water on the roof in sections, working upward — the tiles or shingles below the chimney first, then the flashing, then the crown — while someone else watches inside. Whichever section produces the leak is the answer.

It takes half an hour and it is the difference between a repair and a guess. Any company that proposes work on a leak without either a hose test or a clear explanation of the timing is guessing.

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Frequently asked questions

There is a stain but no active drip. Does it still need fixing?

Yes. Staining means water has been arriving and the framing behind has been wet. A stain that has stopped growing usually means the water found a different route, not that it stopped.

Could it be condensation rather than a leak?

It can be, particularly on an unlined flue serving a modern gas appliance. That produces damp that appears in cold weather rather than with rain — which is another reason the timing matters.

How long should flashing last?

Copper will outlast the roof. Aluminum gives decades. Sealant gives two or three years, which is why the distinction is worth insisting on.