How to Tell If Chimney Flashing Is Leaking

How chimney flashing is built

Flashing is the sheet metal that seals the gap between a masonry chimney and the roof covering around it. On a pitched roof it is done in two layers. Step flashing, short L-shaped pieces, is woven in with each course of shingles up the sides of the chimney, with an apron across the front and a saddle or cricket behind on wider chimneys. Counter flashing is set into a slot cut into a mortar joint and bent down over the step flashing, so water running down the brick lands on the roof instead of behind the metal. On a flat roof, the membrane is turned up the chimney and covered by counter flashing in the same way.

The two-layer design exists because brick and roof framing move independently. Wood swells and shrinks with the seasons, the house settles, and the chimney sits on its own foundation. The counter flashing lets the two slide past each other without opening a gap.

Why flashing fails on older homes

On older houses around the region, the causes tend to show up in a predictable order. Most common is flashing that was never set into the brick at all: the metal is just pressed against the chimney and sealed with tar or caulk, which dries, cracks and lets water behind it within a few seasons. Next is counter flashing that was set into a joint but has pulled loose as the mortar around it eroded. Then come rusted steel flashing, pieces lost or bent during a reroof, and flat roofs where the membrane was turned up the chimney without any counter flashing over it.

Wind-driven rain from nor'easters tests every one of these weak points, which is why many flashing leaks only show up in storms from one direction.

Signs inside the house

Go into the attic with a flashlight during or right after a heavy rain. Look at the roof boards and rafters beside the chimney. Dark streaks, damp wood, rusty nail tips, and a drip line running down the side of the brick all mean water is getting in at the roof line. Old leaks leave rings and soft, darkened wood even when the attic is dry.

On the floors below, flashing leaks tend to show in a few recognizable ways.

• A ceiling stain that starts at the chimney breast and spreads outward
• Peeling paint or bubbling plaster down one side of the chimney breast
• Leaks that appear only in wind-driven rain from one direction
• In a top-floor apartment under a flat roof, a stain right where the chimney meets the ceiling
• A dry firebox while the wall next to it is wet

Signs you can see from outside

Stand back in the yard, across the street or at an upper window that faces the chimney, and look at its base. Counter flashing that has pulled out of the mortar joint, gaps between the metal and the brick, rust streaks, bent or missing pieces and loose caulk are all warning signs. Thick black tar or roofing cement smeared around the chimney is a clue too: it usually means someone patched a leak instead of fixing it. Tar shrinks and cracks with sun and cold and starts letting water through again, often behind the patch where nobody can see.

Stay off the roof. Chimney bases are often on the steepest, highest part of the house, and wet shingles around them are slippery.

Telling flashing apart from other leaks

A chimney can let water in through the flashing, the crown, an uncapped flue or the brick itself, and from inside they can look alike. Where the water appears is the strongest clue.

How the source gets confirmed

These are clues, not proof. Water can run along a rafter or down the outside of the flue before it appears, so the source is confirmed on the roof. A technician looks at every side of the flashing, the crown, the cap and the brick above the roof. Where it is still unclear, a controlled water test runs water over one area at a time, starting low and working up, while someone watches inside. That isolates the flashing from the masonry above it.

It is slow, careful work that needs proper roof access and fall protection, which is why it is not a homeowner job with a ladder and a hose.

How leaking flashing is fixed

A small gap in otherwise sound flashing can sometimes be resealed as a short-term measure. Flashing that has rusted, lifted, been buried in tar or was never set into the brick is replaced. New step or base flashing is worked into the roofing, new counter flashing is cut into a mortar joint and sealed, and on wide chimneys a cricket behind the chimney may be added to split the water around it. On flat roofs the membrane turn-up is repaired and properly covered.

If the brick or crown above the roof is also letting water in, it makes sense to fix that in the same visit, since new flashing under wet masonry still leaves a damp chimney. Every repair comes with a free written estimate before work starts.

Why it is worth acting soon

A flashing leak rarely stays small. Each storm wets the roof sheathing and rafters beside the chimney, and wood that stays damp grows mold and eventually rots. Inside, plaster softens and paint fails. In winter, water in the joint freezes and pushes the flashing further away from the brick. Fixing flashing while the framing is sound is a roofing and masonry job; waiting can turn it into a carpentry job as well.

Insurance is another reason not to wait. Many policies treat sudden damage differently from damage that builds up over time, and a long-running leak can be hard to present as sudden. Your agent can explain how your policy reads.