How to Replace Chimney Flashing

Chimney leaks are notorious for causing silent, long-term damage to the structure of your home. The flashing serves as the critical transition point between the rigid masonry of your chimney and the flexible or shingled surface of your roof. When it fails, water doesn't just drip; it travels down the chimney chase, rotting roof decking and destroying attic insulation before you ever see a water spot on your ceiling. Successfully managing this project requires patience and a focus on the 'weather-lap' principle—ensuring every piece of metal sits over the one below it. When done well, the system acts like a series of shingles, shedding water away from the chimney rather than trapping it. You are aiming for a seamless metal perimeter that allows the house and chimney to shift independently without breaking the waterproof seal.

  1. Strip the Old Flashing. Carefully pry up the existing shingles and remove the old metal flashing pieces. Use a flat bar to clear away old roofing cement and loose debris, taking care not to puncture the underlayment underneath.
  2. Weave Base Flashing In. Install L-shaped pieces of flashing against the chimney side, weaving them between each course of shingles. Each piece should overlap the one below it by at least two inches.
  3. Cut the Mortar Groove. Use an angle grinder with a diamond blade to cut a clean, horizontal groove about one inch deep into the mortar joints of the chimney bricks. This groove will house the top edge of your counter-flashing.
  4. Anchor Counter-Flashing. Insert the top edge of the metal counter-flashing into the cut mortar joint. Secure it in place using masonry wedges to hold the metal tight against the brick.
  5. Seal Every Gap. Fill the groove where the counter-flashing enters the mortar with high-quality polyurethane sealant. Smooth the bead with a pointing trowel to ensure a water-tight slope.
  6. Verify the Water Shed. Double-check that all shingles are laid back down properly over the base of the flashing. Re-nail any loose shingles using roofing nails and dab the nail heads with a bit of sealant.