Why does my chimney leak? Follow the water back to where it got in

Six entry points, from the top of the stack down

Think of a chimney as a tall box standing in the rain with a hole in its top. Water has a short list of ways in, and they sit in a fixed order from top to bottom. Knowing that order helps you read what you are seeing and describe it clearly when you call someone. The chimney leak causes below are listed in that order.

• The flue opening, if there is no cap over it
• The crown, the concrete or mortar cap poured across the top of the brick
• The brick faces and the mortar joints between them
• The flashing where the roof meets the chimney
• On factory-built units, the metal chase cover and its seams
• Inside the flue, condensation from a gas appliance venting into a flue too big for it

An open flue is a rain gauge

A flue with no cap catches every drop that falls on its opening, and a wind-driven storm pushes more in. That water runs down the liner walls, soaks the smoke chamber, rusts the damper and pools in the firebox. It can show up as a wet firebox or a rust-streaked damper before any ceiling stain appears, although it can also soak into the masonry and travel sideways.

A cap with a solid top and a screened side is the fix for this one entry point. It does nothing for water coming through the crown, the brick or the flashing, so a new cap on a leaking chimney sometimes changes nothing at all.

The crown on top of the brick

The slab around the flue is supposed to be sloped, to overhang the brick with a drip edge underneath, and to be separated from the flue tile by a caulked bond break. That is what the Florida Building Code, Residential asks for in R1003.9.1. On some chimneys the top is instead a thin mortar wash, flat or feathered at the edges, and the Brick Industry Association's Technical Note 19B warns against feathering a cap to its edge because the thin margin deteriorates sooner.

Crowns crack as concrete shrinks and as the flue tile heats and grows against them. Once a crack opens, water drops into the core of the chimney and runs down inside the masonry, sometimes for a full story or more before it shows. Crown leaks are easy to miss from the ground, and binoculars help: look for dark lines across the top, missing corners, or pieces lying on the roof.

Brick and mortar that soak up water

Brick is porous, and so is mortar. A sound chimney absorbs some rain and dries out again. Trouble starts when joints open up, when brick faces flake off and expose the softer core, or when someone has coated the brick with a film sealer that traps the water already inside.

Two clues point to the faces. The first is a white, crusty deposit on the brick, outside or in the attic, which is salt that water dissolved inside the masonry and left on the face as it dried. The second is a leak that follows rain from one direction, because the face turned toward that weather takes the soaking. A chimney leaking when it rains from one direction, and dry in other storms, fits this pattern.

Flashing where the roof meets the stack

The roof-to-chimney joint is made of two layers of metal: base flashing tied into the roofing and counter flashing tied into the brick, lapped over it. The Brick Industry Association's flashing guidance calls for the laps to shed water downhill and for every joint to be sealed. When the counter flashing has pulled out of its joint, when the step pieces have rusted through, or when the whole joint has been buried under roof cement, water runs straight in.

Flashing leaks tend to show close to the chimney: on the rafters beside it in the attic, or on the ceiling right next to the chimney breast. A leak on the uphill side is a classic flashing or debris-dam problem, because that face catches everything that runs down the roof.

Chase covers on factory-built chimneys

A factory-built fireplace vents through a metal pipe inside a framed box called a chase, and the box is capped with a sheet-metal chase cover. A flat cover holds water, rusts, and eventually lets water drip through its seams or around the collar where the pipe passes through. Rust streaks running down the siding of the chase are the tell.

Water from a failed chase cover can run down inside the chase and show up on a wall or ceiling some distance from the fireplace, and it can reach the fireplace unit itself. The siding and trim on the chase can also leak by themselves, like any wall.

Water that is not rain

Some chimneys drip on dry days, and that points away from the roof altogether. A gas appliance venting into a big masonry flue meant for wood fires sends up exhaust that cools as it climbs, until its vapor turns to water on the flue walls. It trickles down the flue walls, marks the brick, and can collect at the damper or seep through the chimney breast.

Section R1003.11 of the residential code expects every masonry chimney to be lined for the appliance it serves. Condensation of this kind is fixed with a correctly sized liner, not with sealant or flashing.

Chimney leak or roof leak? Clues you can check

A stain near a chimney is not proof that the chimney is to blame. Roofs leak at valleys, vents and nail holes too, and water from either source can end up in the same spot. A few observations help sort it out before anyone climbs a ladder.

• Timing: a leak during every rain points to the roof or flashing; a leak only in rain from one direction points to a brick face; drips on dry days point to condensation
• Where it shows in the attic: wet decking uphill of the chimney suggests roof or flashing, while a wet ring on the chimney brick itself suggests water coming down inside the masonry
• The firebox: water on the damper or the firebox floor points to the flue top or the crown
• Marks on the brick: white crust or dark damp patches on the chimney in the attic mean water is traveling through the masonry

Who to call for a chimney leak

If the clues point at the chimney, call a chimney contractor who will trace the leak before pricing a repair. If they point at the roof covering away from the chimney, call a roofer. Flashing sits between the two trades, and the useful thing is for whoever comes to test the flashing and write down whether it is the source.

Whoever you call, ask for a written finding that names the entry point and says how it was found. A quote to reseal the whole chimney or replace a cap, with no test behind it, is a guess.