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How to Seal Chimney Flashing and Prevent a Leaky Chimney
Sealing this small part of your roof makes a big difference in keeping you dry
Photo: Richard Nantais/ Adobe Stock
Photo: Richard Nantais/ Adobe Stock
SKILL LEVEL
Simple
COMPLETION TIME
1 hour
COST
$25–$50
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What you'll need:
TOOLS
- Ladder
- Putty knife or scraper
- Stiff-bristle brush
- Caulk gun
SUPPLIES
- Weather-resistant silicone caulk
Your home's exterior is a network of materials that keep you dry and safe from water damage. Flashing refers to metal sheets installed over gaps and transitions in a roof, especially around chimney brickwork. While damaged flashing warrants full replacement, smaller issues can still lead to moisture problems. Learn how to seal chimney flashing to close gaps in your defenses.
Buying Caulk to Seal Chimney Flashing
Many types of caulk degrade quickly when exposed to heat and weather, and you can’t use just any product to seal your chimney flashing. The best caulk for chimney flashing is a high-quality, weather-resistant silicone caulk designed specifically for roofing applications. Expect to spend an average of $25 per tube, though some products cost more.
How to Seal Chimney Flashing
Photo: P A Thompson/ Getty Images
If you inspect your chimney and discover that the caulk around your flashing is wearing away, you need to reseal it. To seal the chimney flashing, remove the old caulk and clean the surface of the flashing. Then, carefully apply new weather-resistant silicone caulk along flashing edges and in mortar joints.
Inspect the Chimney Flashing
Start with a close inspection of the caulk on your chimney flashing. Telltale signs that it needs to be sealed include dry or flaking areas, visible gaps between the caulk and the brickwork, or sections where the caulk is missing completely.
If your flashing is seriously damaged, or if you discover spongy areas underneath, then simply sealing it won't suffice. Instead, you'll need to replace the chimney flashing, which is a much more complex and risky project to DIY.
Remove Old Sealant Material
Use a putty knife or scraper to remove old, dried-out caulk material. Be careful not to damage the surrounding brickwork or the flashing itself in the process, and avoid pulling the flashing away from the chimney.
Clean Off Debris
Use a stiff brush to scrub the area around the flashing to remove moss, grime, dust, and other debris. Leaving debris on any surface will prevent your caulk from holding properly.
Apply New Silicone Caulk
Use your caulk gun to carefully apply caulk to the following areas:
Along the top edges where the flashing meets the chimney
In mortar joints where flashing is inserted
Over nail heads
Use your putty knife or scraper to smooth the caulk into an even layer. Avoid air gaps to ensure an effective seal. Allow the caulk to dry per the manufacturer's instructions, then inspect your work and repeat as needed to correct any gaps or flaws in the seal.
DIY vs. Hiring a Pro
Sealing chimney flashing as a DIY project is inexpensive and takes very little time. The cost to seal a chimney can be much higher if you hire a pro, since labor alone will cost $130 or more.
However, any project that involves working on a roof is inherently risky for homeowners. You should always have a spotter when working on a ladder or roof. Additionally, removing material from a chimney poses a risk to the masonry, which can be costly to repair.
To avoid the risk of personal injury or damage to your roof or chimney, consider hiring a local roof repair company to handle sealing your chimney flashing.
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