Chimney Flashing Leak: How to Seal It Yourself
Water on the ceiling or in the attic right beside the chimney usually means the flashing has failed, not the chimney. Flashing is the metal that seals the joint where the chimney meets the roof. If it's still in place, you can often seal it yourself in an afternoon with a brush-on sealant and fabric. Everything below ships with No Sales Tax in any state.
Quick answer: what to use on leaking chimney flashing
- The sealant: FlashSeal chimney flashing sealant, $134.99 per gallon in black, brown or white. One gallon covers about 28 linear feet.
- The fabric: FlashSeal Reinforcement Fabric, $53.99 for a 6 in x 150 ft roll. You embed it in the sealant.
- Leak coming through the top instead? CrownCoat seals a cracked crown, from $256.99.
All ship with No Sales Tax.
Is it the flashing?
A chimney shouldn't leak when it rains, so it's worth finding the real source before you buy anything. Water gets into a chimney in four main ways.
↔ Swipe the table to see more →
| What you see | Likely source | Fix |
|---|---|---|
| Wet ceiling or attic right where the chimney meets the roof | Flashing | FlashSeal and fabric |
| Cracks across the concrete top, water stains high on the brick | Crown | CrownCoat, see the chimney crown repair guide |
| Damp brick, white chalky deposits, flaking brick faces | Brick and mortar | ChimneySaver water repellent, see the chimney sealer guide |
| Water in the firebox or on the damper | Down the flue | A chimney cap, see how to measure for a chimney cap |
Seal it or replace it?
FlashSeal is made to repair flashing that has been tarred, retarred or cracked, and to stop leaks around the base of chimneys, stacks and vents. It sticks to asphalt shingles, aluminum flashing and brick. If the flashing is still in place, sealing is a fair first step. If the metal has come away from the chimney, is badly rusted or is missing, sealant is the wrong fix, and that's a job for a roofer.
Stay safe. Only go on the roof in dry weather, with a ladder that extends at least 3 feet past the roof edge and someone at the bottom. If a steep or high roof sounds like a bad idea, call a roofer or chimney sweep.
How to seal chimney flashing with FlashSeal
- Check the weather. Apply at 45°F and rising, and allow at least 12 hours of dry, clear weather. Don't use it in cold, damp or rainy weather, because it can wash away.
- Clean the joint. Scrub the flashing, shingles and brick with a stiff brush to get rid of dust, dirt and debris.
- Cut the fabric. Measure and cut enough reinforcement fabric to completely cover the repair.
- Start low. Work the lowest side of the chimney first, and overlap the fabric the way shingles overlap, so water runs over the joints and not into them.
- Brush on the first coat. Stir well, then apply about 1/8" of FlashSeal. Fill all the cracks and holes, and run it about an inch past where the fabric will end.
- Embed the fabric. Press the fabric into the wet coat with the brush. The sealant should fill the holes in the fabric.
- Top coat. Brush on a liberal top coat that completely covers the fabric and extends about an inch past the problem area.
On a hot day it dries quickly, so work in 2 to 3 foot sections. Duct tape gives a clean edge. FlashSeal is dry to the touch in 30 to 60 minutes and fully cured in 6 to 12 hours. Clean tools with soap and water while it's still wet.
How much do you need?
One gallon of FlashSeal covers about 28 linear feet. Add up all four sides of the chimney. A chimney measuring 2 ft x 3 ft has 10 linear feet of flashing, so one gallon is plenty for most single chimneys. A 150 ft roll of fabric covers far more than one chimney, so one roll is all you need.
Which color?
FlashSeal comes in black, brown and white. Pick whichever comes closest to your roof. White can be tinted with standard mortar dye: move what you need to another container and add dye a drop at a time.
Frequently asked questions
How do I fix roof flashing around a chimney?
If the flashing is still in place, clean it with a stiff brush, then embed reinforcement fabric in FlashSeal, working from the lowest side up. If the metal is loose or missing, have a roofer replace it.
Is it normal for a chimney to leak when it rains?
No. Find out whether the water is coming through the flashing, the crown, the brick or down the flue, using the table above.
Can I use FlashSeal over old tar?
Yes. Saver Systems says FlashSeal repairs flashings that have been tarred, retarred or cracked. Brush off dust, dirt and debris first.
Can I do this in winter?
Only on a day that's 45°F and rising, with at least 12 dry hours ahead. Keep the pail from freezing, too.
Does FlashSeal work around skylights and vents?
Yes. It's made for chimneys, stacks and vents, and Saver Systems says it's also great for sealing around skylights.
Related guides
- Chimney crown repair: seal the cracks yourself or rebuild?
- Chimney sealer: how to waterproof a brick chimney
All our sealants and coatings are on the chimney repair and waterproofing page. Not sure where your leak is coming from? Email a photo to info@stovesource.com or text 1-888-866-8206. Every chimney repair product ships with No Sales Tax in any state.