Why does my chimney leak only after heavy rain?
Short answer. A chimney that leaks only in heavy rain usually has a weakness that light rain does not reach: water piling up behind a chimney with no cricket, wind-driven rain soaking porous brick, or water overtopping flashing that barely laps. A cracked crown or missing cap can also let rain pour down inside during downpours.
The likely causes
- No cricket. Water and debris pile up on the uphill side of a chimney. The International Residential Code calls for a cricket, a small peaked diverter, where a chimney is more than 30 inches wide and does not sit at the ridge. Without one, a heavy rain dams water higher than the flashing.
- Porous brick and mortar. Brick absorbs water like a sponge. In a long, wind-driven Georgia storm it saturates and passes water inside.
- Short or failing flashing. Step and counterflashing that barely overlap work in light rain but flood when the volume rises.
- Cracked crown. The concrete cap on top of the brick cracks and lets water into the chimney.
- No chimney cap. Rain falls straight down the flue and shows up in the firebox.
- Chase cover. On framed chimneys with siding, a rusted or flat metal top pools water and leaks.
Reading the clues
Water in the firebox points to the cap or crown. A stain on the ceiling on the uphill side points to a missing cricket or flashing. Damp brick in the attic points to absorption or the crown. Note the wind direction during the storm, too; water on one side only usually means one face of the chimney.
The fix
Add a cricket and proper flashing where they are missing, rebuild or seal the crown, install a cap, and apply a breathable water repellent to porous brick. See chimney leaks and crickets and common causes of roof leaks.