Why does my chimney leak when it rains?
Short answer. A chimney that leaks only when it rains is letting water in at the top or at the roofline: an open flue without a cap, a cracked crown, porous brick and mortar, or failed flashing where the chimney meets the shingles. The timing narrows it down, because cap and flashing leaks show up during the rain while water soaked into the brick can keep seeping for a day or more afterward.
Use the timing
- Starts within minutes of any rain: an open flue with no cap, or a large flashing gap.
- Only in wind-driven storms from one direction: the flashing on that side, or the brick face taking the wind.
- Starts hours into a storm and continues after it stops: water absorbed into the brick and crown, slowly migrating inward.
- Only in long, heavy downpours: water piling up behind the chimney on the uphill side, often where there is no cricket.
Why brick lets water in
Brick and mortar are porous and soak up water like a sponge in driving rain. Mortar erodes over decades, and hairline cracks in the crown let water into the core of the chimney. A breathable, water-repellent treatment made for masonry can reduce absorption once cracks and joints are repaired. Paint and film-forming sealers can trap moisture and make brick spall.
Georgia storm patterns
Summer thunderstorms drop a lot of rain in a short time, and pine needles collect behind chimneys, holding water against the uphill flashing. That combination is why many chimney leaks here show up only in the biggest storms.
What to check
- Is there a cap, and is it intact?
- Is the crown cracked or crumbling?
- Are mortar joints eroded?
- Is the counterflashing set into the mortar, is the step flashing sound, and is there a cricket on a wide chimney?
See chimney leaks and crickets and pine needles and leaves on the roof.