How to Fix a Leaky Chimney: A Step-by-Step Diagnostic & Repair Guide
Before buying tools, identify the source. The 4 most common culprits are:
- Leak at Ceiling/Attic: Usually Flashing failure.
- Water inside Firebox: Usually a missing Cap or cracked Crown.
- Damp/Moldy Walls: usually porous Brickwork (Spalling).
- White Stains (Efflorescence): Moisture trapped inside masonry.
A leaky chimney is a homeowner’s nightmare. It starts as a drip, but quickly turns into rotted roof decking, mold in the attic, and structural instability. While you might be tempted to grab a bucket of tar, fixing a chimney requires specific materials compatible with masonry.
If you aren’t sure if the issue is structural or just maintenance, check our guide on chimney repair and homeowners insurance. Sometimes, storm damage is covered.
Fix #1: Repairing the Chimney Crown
The crown is the cement slab at the very top. It takes the hardest beating from the sun and rain. If it cracks, water pours directly into the flue walls.
The Solution: Flexible Sealant
Don’t use standard mortar to patch cracks; it will just crack again next winter. You need an elastomeric coating that expands with the heat.
ChimneyRx Brushable Crown Repair
A flexible, waterproof membrane that you paint over the cracks. It cures into a rubber-like seal.
Check Price
Sashco Mor-Flexx Repair
Textured caulk specifically designed to look like mortar but stretch like rubber.
Check PriceFix #2: Sealing the Flashing
The “flashing” is the metal strip connecting the chimney to the roof. If you see water stains on your ceiling near the fireplace, this is 90% likely the cause.
We have a detailed review on the best chimney flashing sealants, but the short version is: use a fabric-reinforced system.
- Step 1: Clean the metal flashing with a wire brush.
- Step 2: Apply a base coat of sealant (like FlashSeal).
- Step 3: Embed the reinforcement fabric while wet.
- Step 4: Apply a top coat. This lasts 7-10 years.
Fix #3: Waterproofing Porous Bricks
Bricks are like sponges. In heavy rain, they soak up water. When that water freezes, it pops the face of the brick off (spalling).
You must use a “breathable” water repellent. Never use silicone or Thompson’s Water Seal on a chimney; it traps moisture inside. Read our full guide on how to waterproof a chimney correctly.
ChimneySaver Water Repellent
Siloxane-based formula that penetrates deep and allows vapors to escape.
Buy on AmazonFix #4: The Missing Cap
If water is dripping directly onto your logs or gas insert, your chimney cap is missing or damaged. A good cap also stops animals.
Related: How to get a raccoon out of your chimney (often the result of no cap!).
Ensure you buy stainless steel. Check our list of the best chimney caps for rain to match your flue size.
Not Comfortable on the Roof?
Finding a leak can be dangerous. If you have a steep roof or complex masonry issues, let our certified partners handle it.
*Providing your zip code helps us match you with a local expert.
Frequently Asked Questions
Why does my chimney leak only when it rains hard?
This usually indicates “saturation.” Your bricks are soaking up water until they can’t hold anymore, and then it leaks through. This requires a waterproofing treatment rather than a patch.
Can I fix a chimney leak from the inside?
Rarely. Most chimney leaks originate on the exterior (roof, crown, or flashing). Sealing the inside only traps the water in the wall, leading to hidden mold.
How much does it cost to fix a chimney leak?
A simple flashing reseal might cost $200-$400 via a pro. However, if you need extensive tuckpointing (mortar repair), it can run $1,000+. Check our guide on best chimney services for pricing estimates.