Why is my chimney leak causing damp walls and stains?
Short answer. Damp patches, peeling paint and musty smells on the walls or ceiling around a chimney usually come from one of two sources: rain getting in through the flashing, crown, cap or brick, or condensation inside the flue from gas appliances venting into an oversized or unlined chimney. The timing of the damp tells you which one you are dealing with.
Use timing to diagnose
- Appears after rain, fades in dry weather. Rain entry. Look at the flashing, crown, cap and brick.
- Appears in the heating season, regardless of rain. Condensation from a furnace or water heater flue. Water vapor in exhaust gases condenses on cool masonry and soaks through, sometimes with brown or tar-like staining.
- Constant, worse in humid summer months. Humid outdoor air drawn down an unused, uncapped flue condensing in the cool fireplace and walls. A well-fitted damper or top-sealing damper and a cap help.
Where the damp shows
- Ceiling next to the chimney. Usually flashing, especially on the uphill side or where a cricket is missing.
- Wall around the fireplace. Water running down inside the masonry, or condensation.
- Inside the firebox. Rain down an uncapped flue, or a cracked crown.
Fixes by cause
- Flashing: replace step flashing and cut-in counterflashing, and add a cricket if the chimney is wider than 30 inches.
- Crown and brick: repair the crown, repoint mortar, and use a breathable masonry water repellent.
- Cap: install a cap that covers every flue.
- Condensation: have an HVAC professional evaluate the venting. Lining the flue or rerouting the appliance vent is often the fix.
Do not just repaint
Stain-blocking primer covers the mark but not the cause. Damp masonry and drywall can grow mold, so fix the source, let the area dry, then repair the finish.