Chimney Flashing Repair Where the Roof Meets the Brick

Where a chimney passes through a roof there is a gap that has to move a little with the seasons. Flashing bridges it in two layers: step flashing woven into the shingles, and counter flashing set into the mortar joints of the brick and lapped down over it. When either layer pulls loose, rusts through or was sealed with caulk alone, water runs down the brick and into the attic or the ceiling.

In our climate flashing gets more water than most, and moss makes it worse. A leak at the flashing can soak framing for months before a stain shows indoors, which is why we check it at every free inspection. Needles and leaves pile up behind the chimney, moss takes hold in that wet debris, and the flashing on the uphill side ends up sitting in standing water after every storm. We look there first, then work around the rest of the chimney.

Signs you need this

• A water stain on the ceiling next to the chimney
• Damp or dark wood in the attic around the chimney
• Caulk or tar smeared over the flashing, cracked or peeling
• Metal pulling away from the brick or lifted at the corners
• Moss growing thick along the chimney's uphill side

How we do it

1. Find the leakWe check the flashing from the roof and look at the framing from the attic when we can reach it.
2. Decide reseal or replaceSound metal with failed sealant is resealed. Rusted, cut or tar-patched flashing is replaced.
3. Install new flashingNew step flashing goes in with the shingles, and counter flashing is set into cut mortar joints and sealed.
4. Check the uphill sideOn a wide chimney, a small peaked diverter called a cricket may be added so water does not pool behind it.

How does moss lead to flashing leaks?

Oregon State University Extension describes moss holding moisture and debris against roofs, and lifting shingles on asphalt roofs so water reaches the layers underneath. The uphill side of a chimney is a natural dam for needles and leaves, so moss often grows thickest right where the flashing has to work hardest.

Clear moss gently by hand or with a soft brush rather than a pressure washer. OSU also notes that zinc sulfate, a common moss treatment, corrodes copper, so it should be kept off copper flashing.

What does flashing repair cost?

Resealing flashing that is still sound is typically $300 to $600. Installing new step and counter flashing is typically $800 to $1,800, depending on the size of the chimney, the roof type and whether a cricket is needed.

If a roofer is replacing the roof, it is a good time to replace the chimney flashing as well. We can work alongside your roofer so the shingles and flashing lap correctly.

Why does caulk alone keep failing?

Caulk and roofing tar harden in sun and crack as the chimney and roof move slightly through the seasons. A bead of sealant holds for a while, then opens up, and the leak returns.

Proper counter flashing is set into a cut joint in the brick, so the metal does the work and the sealant only protects the edge. That kind of flashing lasts far longer than a smear of tar.