How to Repair Leaking Chimney Flashing

CHIMNEY flashing acts as the critical barrier between the rigid masonry of your chimney and the flexible, weather-exposed surface of your roof. When this metal transition fails, water inevitably follows the path of least resistance, trickling down the masonry into your attic or living spaces. Ignoring a leak here often results in rotting roof decking and structural damage that costs far more than a simple afternoon of sealing. Done well, a flashing repair creates a watertight seal that allows for natural thermal expansion without cracking. You are not just patching a hole; you are restoring the integrity of your home's primary moisture defense. We focus on identifying the specific failure points—usually rusted metal or dried-out, cracked sealant—and replacing them with materials designed to withstand extreme UV exposure and temperature swings.

  1. Spot the Failure Points. Climb to the roof and locate the flashing. Use a wire brush to scrub away all loose, dried-out caulk, moss, and dirt around the base of the chimney where the metal meets the brick.
  2. Strip Away the Broken Seal. Use a utility knife or a specialized scraping tool to pry out old, hardened caulk from the joints. If the metal flashing is rusted through, carefully pry it away from the brick or roof surface.
  3. Prime for Better Bond. Once the surface is clean and dry, apply a thin coat of metal primer to any exposed, rusted flashing sections. This prevents further oxidation and ensures the new sealant creates a strong chemical bond.
  4. Lock In the New Seal. Apply a generous bead of high-performance polyurethane roofing sealant into the gap between the masonry and the metal flashing. Use a caulking gun to maintain even pressure.
  5. Shape the Protective Bead. Press the sealant firmly into the joint to eliminate air pockets. Ensure the bead overlaps onto the masonry by at least half an inch and extends well onto the shingles.
  6. Verify Full Waterproofing. Check for any pinholes or missed spots in your bead. Ensure that the flashing remains tight against the chimney walls and does not lift when pressed.