How to Seal Leaks Around Chimney Flashing

Chimney flashing acts as the critical barrier between your roof's shingles and the brick or stone of the chimney stack. Because these two surfaces expand and contract at different rates during temperature changes, the seal often breaks over time, leading to water infiltration that ruins roof decking and ceiling drywall. A successful repair requires more than just slathering goop over the cracks. You must strip away the failing material to create a clean surface, then use a flexible, exterior-rated sealant that can withstand constant thermal movement. When done correctly, your repair should last several years and keep your attic dry through the heaviest downpours.

Clear Every Crevice First. Remove all debris, moss, and dirt from the flashing joints using a stiff wire brush. Ensure the area is completely dry before moving to the next step, as moisture will prevent the sealant from bonding.
Strip the Failing Seal. Scrape away cracked, hardened, or peeling old sealant using a putty knife or a flat-head screwdriver. Do not force the metal flashing out of the mortar joints; just remove the failing caulking.
Dissolve All Surface Oils. Wipe down the entire flashing perimeter and the adjacent brick with a rag soaked in mineral spirits. This removes microscopic oils and dust that would otherwise cause your new seal to fail prematurely.
Build a Stable Base. If the gap between the flashing and the brick is wider than 1/4 inch, press foam backer rod into the crevice. This saves sealant and provides a solid base for the bead.
Run a Perfect Bead. Load your polyurethane roof sealant into the caulk gun and apply a smooth, continuous bead along the flashing joint. Keep the tip at a 45-degree angle to force the material deep into the gap.
Seal Out Every Air Pocket. Immediately after applying, smooth the sealant bead with a gloved finger or a smoothing tool. This ensures the material has full contact with both the metal flashing and the masonry, forcing out any air pockets.