Chimney flashing leaks: what to look for and who fixes it

What flashing is

The roof moves and the chimney doesn't. Flashing covers the gap between them so water runs past the joint instead of into it.

On a brick chimney it's two layers. Step flashing is a row of L-shaped pieces woven in with the shingles, one piece per course, with one leg on the roof and one against the brick. Counterflashing is metal set into the mortar joints of the chimney and folded down over the step flashing.

The residential building code calls for flashing along a wall like this to be at least 4 inches high and 4 inches wide, and to send water away from the wall onto the roof. The counterflashing is what keeps water from getting behind the step flashing at the top.

The cricket on a wide chimney

A cricket, also called a saddle, is a small peaked roof built on the uphill side of the chimney. It splits the water coming down the roof and sends it around both sides, so it doesn't pond against the back of the chimney.

The International Residential Code calls for a cricket on the ridge side of any chimney more than 30 inches wide, measured across the slope, covered in sheet metal or the same material as the roof. On a wide chimney without one, the back of the chimney is the first place we check for a leak.

Why tar and caulk patches fail

The quick fix is a coat of roofing tar or caulk smeared over the joint. Maine Preservation calls these three- to five-year fixes at best.

A patch sits on top of the joint and does nothing about the flashing underneath. Once it cracks, water gets in behind it, and the tar hides where the water is coming from. Layers of tar around the base of a chimney usually mean the flashing under them was already failing when the first coat went on.

Signs of a flashing leak

Some you'll see inside, and some you can see from the ground with a pair of binoculars.

• A water stain on the ceiling or wall beside the chimney, or on the chimney breast itself
• Water marks or dark wood on the rafters or roof deck around the chimney in the attic
• A leak that only shows up in wind-driven rain from one direction, or when snow is melting
• Flashing that's rusted, bent, lifted away from the brick, or covered in tar
• Counterflashing that has pulled out of the mortar joint, often because the mortar around it has crumbled

A stain alone doesn't prove it's the flashing. Water can also get in through the crown, a missing cap or the brick, and run down to the same spot. Leak around the chimney: roofer or chimney company? goes through each source.

Michigan snow and ice

Snow piles up on the uphill side of a chimney all winter. As it melts, the water sits against the back of the chimney, right where the flashing and the cricket have to keep it out. The Great Lakes region averages about 42 freeze-thaw cycles a year, according to the University of Michigan's GLISA center. Water that gets into a gap and freezes opens it a little wider.

Michigan State University Extension explains that ice dams form when attic heat melts snow on the roof, and the water refreezes at the colder eaves. Water then backs up under the shingles and can leak into the attic. An ice dam leak shows up near the outside walls of the house. A leak that's right next to the chimney, during snowmelt, points more toward the flashing or the cricket.

Who fixes it

Flashing is shared ground between the roof and the chimney. Plenty of roofers do it well, and we do it too.

The counterflashing is set into the brick, so when the mortar joints holding it have failed, the joints need repointing before new counterflashing will hold. That part is masonry work. If the shingles around the chimney are worn out too, or you're about to reroof, the flashing is best done with the new roof so it's woven in with the new shingles.

A standard chimney inspection includes the flashing, along with the crown, the cap and the brick. We'll tell you where the water is getting in, and if the answer is the roof and not the chimney, we'll say so. Flashing, crown and masonry repairs are all on the chimney repair page.