How to Repair Leaking Roof Flashing Around a Chimney

Chimneys are notorious leak points because they create a permanent interruption in the roof's weatherproofing. When the metal flashing—the L-shaped barrier designed to divert water away from the masonry—pulls away from the brick or rusts through, rain tracks directly into your attic. Addressing this quickly prevents rot in your roof deck and rafters. Done well, a chimney flashing repair looks clean, sheds water effectively, and remains flexible enough to handle the expansion and contraction of your home during temperature shifts. You are essentially creating a new, watertight seal that forces water to run over the shingles rather than behind them.

  1. Strip to Solid Surface. Remove all loose debris, moss, and crumbling mortar from around the base of the chimney. Use a stiff wire brush to scrub the metal flashing and the adjacent masonry until you reach a clean, solid surface.
  2. Eliminate Every Loose Bit. Use a utility knife or flat-head screwdriver to scrape away old, cracked caulk or bitumen from the joints where the flashing meets the brick. Be aggressive, as any remaining loose material will cause the new seal to fail.
  3. Test Metal Integrity. Check the metal flashing for pinholes, rust, or loose nails. If the flashing is severely corroded, it must be removed and replaced with new aluminum or galvanized steel flashing pieces.
  4. Bridge the Gap Completely. Apply a generous bead of high-quality exterior polyurethane roof sealant or install self-adhering flashing tape along the joint. Ensure the material bridges the gap between the brick and the metal flashing entirely.
  5. Layer Flashing for Overlap. If your local code requires it or if the gap is wide, install metal counter-flashing to overlap the base flashing. Anchor this into the masonry using lead plugs or masonry anchors.
  6. Verify Every Seal Point. Double-check that all edges are sealed and that no gaps remain where water could pool. Wait 24 hours for the sealant to cure before testing with a light spray from a garden hose.