Chimney and Wall Flashing Leaks: What Proper Flashing Looks Like, Why Caulk Is Not Flashing, and the Cricket Nobody Installed

The four pieces of a chimney flashing

1. The apron. A bent piece of metal on the downhill face of the chimney, over the shingles below it and up the brick 4 to 6 in.
2. Step flashing. On each side, a stack of L-shaped pieces, one per shingle course, each lapped over the one below and woven into the shingles as they go up, with the vertical leg against the brick. Water running down the side of the chimney steps from one to the next and out onto the roof.
3. Counter flashing. A second metal piece on all four faces, its top edge let into a groove cut into a mortar joint (a reglet) 1 to 2 in. deep, bent down over the step flashing and the apron so water on the brick runs over the outside of them. This is the piece that makes the joint a joint and not a bead of sealant.
4. The cricket (saddle). On the uphill side of any chimney wider than about 30 in., a small peaked roof that splits the water coming down the slope and sends it round the chimney. Without one, snow and water sit against the uphill face, and that is where the rot is on every chimney that leaks. The Ontario Building Code asks for one on wide chimneys and a lot of 1980s roofers did not read that part.

Under all of it, on a roof done right, a square of ice and water membrane wrapped up the base of the chimney, so even the flashing’s own nail holes are sealed.

Why caulk is not flashing

The shortcut, and the thing most “chimney repairs” are: a bead of roofing cement or polyurethane sealant along the top of the step flashing where it meets the brick, instead of counter flashing cut into the mortar. It stops the leak that day. Then the brick heats and cools, the roof moves, the sealant goes hard in the sun and splits in the cold, and in one to three winters the leak is back, now with the sealant hiding what is under it. A roofer who quotes a chimney “repair” at $200 is quoting caulk. A proper re-flash is $800 to $2,000 and it lasts as long as the roof. If the mortar joints are soft and crumbling, the counter flashing has nothing to sit in and a mason has to repoint first ($500 to $1,500 for the above-roof part), which is why old chimneys are expensive and why a lot of unused ones get taken down below the roof line and roofed over instead ($1,500 to $3,000).

What good looks like from the ground

• Two layers of metal visible at the chimney: the lower step flashing stepping up with the shingles, and the counter flashing over it with a straight top edge disappearing into a mortar joint, not a bead along the brick.
• The counter flashing’s top edge in a horizontal mortar line, cut in, not surface-mounted with a line of sealant.
• A peak behind a wide chimney (the cricket), shingled or metal.
• No roofing cement smeared anywhere. Black goop on a chimney is a repair that did not work.
• Metal in one condition: all new, or all old. New shingles with the 1990s rusted flashing left in place is the roofer saving $600 and the owner buying a leak.

Roof-to-wall flashing

Where a lower roof runs into a wall (a one-storey addition against a two-storey, a dormer’s side walls, a garage against the house), the same step flashing is woven into the shingles, with its vertical leg behind the wall’s cladding. Behind the siding, behind the brick’s weep course, behind the stucco. Water runs down the wall, onto the flashing, and out onto the roof. Done wrong, the flashing is on the surface of the siding with a bead of caulk along its top, which is the same one-winter fix as the chimney. On brick, the counter flashing is let into the mortar like a chimney. A kick-out flashing, a small angled piece at the bottom of the run, throws the water out into the eavestrough instead of behind the siding at the corner; its absence is the cause of the rotten wall corner on about half the Ontario houses with a roof-to-wall joint. Re-doing a wall flashing means the bottom courses of siding come off: $500 to $1,500 per run.

Finding the leak

Chimney and wall leaks show as stains on the ceiling or wall near the chimney, often on the uphill side, often first in a thaw or a wind-driven rain from one direction. In the attic, water trails on the sheathing beside the chimney, rot at the uphill corner, and sometimes the chimney’s own brick damp on the attic side (which can also be a cracked crown or a missing cap at the top, a mason’s job, not a roofer’s). The hose test on the leak diagnosis page settles it: wet each face of the chimney separately, downhill first.

What it costs

Sources and further reading

• Ontario’s Building Code: the official page, with the current regulation (O. Reg. 163/24). Roofing is Section 9.26, attic ventilation 9.19.
• Canadian Roofing Contractors Association: the national association; its roofing specifications are what good quotes are written to.