Chimney Flashing & Leak Repair

Why chimneys leak more than the rest of the roof

A chimney is a rigid masonry or metal structure poking through a roof deck that expands and contracts with temperature — two different materials moving at different rates, joined only by flashing. That joint takes more stress through freeze-thaw cycling than almost any other point on the roof, which is why chimney flashing fails well before the surrounding shingles typically need attention.

Step flashing and counter-flashing, and what each one does

Step flashing is a series of individual L-shaped metal pieces woven in with each shingle course up the sides of the chimney, directing water down and away at every step. Counter-flashing is set into the mortar joints of the chimney itself and folds down over the step flashing, so water can never get behind it. A chimney with only one of the two — or with flashing sealed in place with caulk or roof cement instead of properly installed — is a leak waiting on the next hard freeze to open it up.

How this flashing actually fails

• Caulk and sealant substitutes — a common shortcut on older homes where flashing was patched with sealant instead of replaced. Sealant dries, cracks, and fails within a few years; proper flashing doesn't depend on it holding.
• Mortar joint deterioration — where counter-flashing is set into mortar, deteriorating mortar loosens the flashing's anchor point, even if the metal itself is fine.
• Rust-through on older galvanized flashing — decades-old flashing on older Rockford-area homes sometimes rusts through from the inside, well before it looks obviously bad from outside.

What repair or replacement involves

A flashing repair replaces the step and counter-flashing with new material — typically aluminum or galvanized steel — properly woven into the shingle courses and set into fresh mortar joints where needed, not resealed with caulk. On a full roof replacement, chimney flashing is replaced as a matter of course, not an optional add-on; reused old flashing is one of the most common causes of a callback leak.