How to Inspect and Seal Chimney Flashing

Flashing is the critical metal bridge between your roof and your chimney. When installed correctly, it diverts water away from the junction; when it fails, it acts as a funnel, sending moisture directly into your attic or living room walls. Most leaks here don't stem from total flashing failure, but rather from the breakdown of the sealants or the mortar that holds the metal to the masonry. Performing this maintenance is a matter of precision and patience. You aren't just slapping goop over a crack; you are restoring a drainage system. Done well, this repair will prevent wood rot, interior water stains, and expensive structural damage to the chimney stack itself. Keep your footing secure, take your time, and ensure every bead of sealant is fully bonded to both the metal and the brick.

  1. Strip the Joint Clean. Remove any loose, peeling, or failing old caulk with a stiff wire brush or a utility knife. Sweep away all dust, debris, and moss from the area where the metal flashing meets the brick, as sealant will not adhere to a dirty surface.
  2. Spot Hidden Damage. Examine the metal flashing for pinholes, rust, or loose fasteners. If the metal is punctured or corroded through, it must be replaced or patched with a sheet metal flashing kit rather than just sealed.
  3. Secure Loose Fasteners. If the metal has pulled away from the masonry, check the fasteners. If they are loose, replace them with masonry screws or masonry anchors, ensuring they are driven into the mortar joints rather than the brick itself.
  4. Fill Gaps With Support. If the gap between the masonry and the flashing is wider than a quarter-inch, press a foam backer rod into the joint. This creates a solid base for the sealant so it doesn't just sink into the void.
  5. Seal With Precision. Load a tube of high-performance polyurethane roofing sealant into your caulking gun. Apply a consistent bead along the top edge of the flashing, bridging the gap between the metal and the brick.
  6. Shape Water Drainage. Use a caulk finishing tool or a gloved finger to smooth the bead of sealant. Ensure it creates a clean slope that sheds water away from the chimney rather than trapping it against the joint.