Why Chimneys Leak & How the Repair Is Properly Done
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A chimney interrupts the roof's water path with a solid masonry obstacle, which is why it is the most common leak location on an otherwise healthy roof. Four separate pieces of metal work together — apron, step, cricket and counter flashing — over ice-and-water membrane. This guide explains how that assembly works, the five ways it fails, how to tell a flashing leak from a cracked crown, the full step-by-step repair and how to choose between copper, galvanised steel and aluminium.
Chimneys are the most common leak point on a roof, because they are a large obstruction in the middle of a water path with a joint between two entirely different materials all the way round.
The four flashings a chimney needs
Apron flashing across the down-slope face, lapping over the shingles below.
Step flashing up both sides, one piece per shingle course, woven in as the shingles go on.
A cricket or saddle on the up-slope side, required on any chimney wider than 30 inches across the slope.
Counter flashing let into a cut reglet in the masonry, lapping over everything below it and sealed at the top.
All four have to be present and lapped in the right order. Miss one and the others cannot compensate.
Diagnosing the leak
Stains on the ceiling below and around the chimney breast, worst after driving rain.
Rust streaks on the flashing itself, or flashing lifted away from the masonry.
A thick smear of roof cement around the base — that is somebody's previous repair, and it will have cracked.
Damaged or missing crown at the top of the chimney, which lets water into the masonry from above rather than at the roof line.
Deteriorated mortar joints, which soak up water that then appears lower down.
Repairing it properly
Remove the old flashing entirely. Bent, nail-holed flashing reused under new work is the classic cause of a repeat leak.
Cut a fresh reglet into a mortar joint for the counter flashing rather than surface-mounting and sealing it.
Replace step flashing one piece per course, woven with the shingles — never one long piece up the side.
Build a cricket if the chimney is over 30 inches wide and does not have one.
Seal the reglet with a proper masonry sealant, and check the crown while you are up there.
What not to do
Do not fix a chimney leak with roof cement. It is the most common attempted repair and the most reliable way to be back on the roof in two winters — cement cracks as it shrinks and as the roof moves, and a thick smear of it hides the missing detail underneath rather than replacing it.
Frequently asked questions
Why do chimneys leak?
Because a chimney is a large obstruction in the middle of a water path with a joint between two entirely different materials all the way round. It needs four separate flashings, all lapped in the right order.
What flashing does a chimney need?
Apron flashing on the down-slope face, step flashing up both sides one piece per course, a cricket on the up-slope side if the chimney is over 30 inches wide, and counter flashing let into the masonry over all of it.
Can I seal a chimney leak with roof cement?
It is the most common attempted repair and the least durable. Cement cracks as it shrinks and as the roof moves, and a thick smear of it hides the missing flashing detail rather than replacing it.
Should chimney flashing be replaced with a new roof?
Yes. Flashing that has been lifted off is bent and has nail holes in the wrong places. Reusing it under a new roof is the classic cause of a leak in an otherwise new installation.