7 Signs Your Chimney Needs Repair Before Winter
Check seven things before cold weather: a cracked or flat chimney crown, spalling brick faces above the roofline, mortar joints you can rake with a key, white efflorescence staining, rusted or lifted flashing, a stack leaning off plumb, and damp or stained plaster on the ceiling below. Any one of them means water is already inside the stack.
A chimney is the hardest-working masonry on a St. Louis house. Every other wall has a roof over it. The stack above the roofline takes rain and sun on four sides, has nothing sheltering the top, and holds the most exposed mortar joints in the building. It is also the part nobody looks at, because you need to be on a roof or holding binoculars to see it properly.
Late September is the right time to check. Mortar work needs temperatures above about 40 degrees to cure, so a problem found in October can usually still be fixed this year. The same problem found in January waits until spring, and spends the winter getting worse.
1. Is the crown cracked, or is it just a mortar wash?
The crown is the concrete slab that caps the masonry at the top of the stack. It has a job: shed water off the brickwork and out past the face of the chimney. A proper crown is cast with a slope, projects an inch or two beyond the brick, and carries a drip edge underneath so water falls clear instead of running back down the wall.
What many St. Louis chimneys actually have is a mortar wash. Somebody troweled mortar across the top, flush with the brick, and called it done. Mortar is not a casting material. It cracks in a few winters, the cracks funnel water straight down into the core of the stack, and the freeze damage works from the inside out.
Look for cracks wider than a hairline, missing chunks at the corners, and a top surface that sits flush with the brick rather than overhanging it.
2. Is the crown being confused with the cap?
These are two different things and the words get used interchangeably, which causes real confusion on estimates.
| Part | What it is | What it does |
|---|---|---|
| Crown | Cast concrete slab across the whole top of the masonry | Sheds rain off the brickwork, projects past the face with a drip edge |
| Cap | Metal hood over the flue opening, usually with mesh | Keeps rain, birds and animals out of the flue itself |
You need both. A cap with no crown lets water into the masonry. A crown with no cap lets rain straight down the flue, where it mixes with soot and produces an acidic mess that attacks the liner and the damper. If a quote says “chimney cap” and the price is four figures, ask which one they mean.
3. Are the brick faces flaking off?
Spalling is the face of the brick shaling away in flakes or popping off in a thin plate, leaving a rough, lighter-colored patch behind. On a chimney it usually starts on the north face and near the top, because those areas stay wet longest.
Spalling means water got into the brick and froze there. Once the face is gone the brick is opened up, absorbs faster, and goes downhill quickly. Spalled brick cannot be patched. It has to be cut out and replaced.
4. Can you rake the mortar joints with a key?
Do the key test from a ladder at gutter height if you can do it safely, or have someone do it from the roof. Drag a door key along a mortar joint. If it scratches a line, the joint is sound. If it digs in and brings mortar out as sand, the joint has failed.
Joints recessed more than about a quarter inch from the brick face are past due. On a stack, failed joints are worse than on a wall, because there is no roof overhead to reduce how much water arrives.
5. Is there white staining on the brick?
Efflorescence is the white powdery deposit that shows up on a brick face. It is salt. Water moved through the masonry, dissolved soluble salts inside it, carried them to the surface, and evaporated, leaving the salt behind.
The deposit itself is mostly cosmetic and often washes off. What it tells you is not cosmetic at all: water is moving through that masonry in volume. On a chimney, heavy efflorescence usually points back to the crown or to open joints at the top.
Keep the two failure signs separate in your head. Efflorescence is evidence of water movement. Spalling is evidence of water movement plus freezing. Efflorescence is the early warning and spalling is the damage.
6. Is the flashing doing anything?
Where the stack passes through the roof there should be two layers. Step flashing is woven into the shingle courses. Counter flashing is set into a cut mortar joint on the chimney and laps over the step flashing.
Common failures we find on The Hill and across the older city stock: counter flashing that was surface-mounted and sealed with caulk instead of being let into a joint, galvanized flashing rusted through at the back side, and tar smeared over the whole assembly by a previous roofer. Tar buys a year or two. It then cracks, and the water goes somewhere you cannot see.
7. Is the stack plumb, and is the ceiling below stained?
Sight the chimney against a vertical line, a downspout, a corner of the house or a door frame in the distance. A visible lean means the mortar has failed enough that the stack is moving, or that the structure under it has settled. That is a rebuild conversation rather than a pointing conversation.
Inside, look at the ceiling and the wall where the chimney passes through. Brown rings on plaster, blistered paint, a musty smell in a closet backing onto the stack, and rust on the damper all say water is coming down inside the masonry. Damp at the ceiling near the chimney is the sign most often blamed on the roof when the chimney is the source.
What to do with what you found
Work through the list in order and write down what you see.
- Crown cracked or flat: recast the crown with an overhang and a drip edge
- Brick faces spalling: cut out and replace the affected brick
- Joints raking to sand: repoint the stack above the roofline
- Heavy efflorescence: find the water path before sealing anything
- Flashing rusted, caulked or tarred: strip and reflash with counter flashing set in a joint
- Stack leaning or bricks loose in the hand: rebuild above the roofline
- Ceiling stains below: treat as urgent and get eyes on the stack
One more thing about timing. Standard cold weather practice is to stop placing mortar once temperatures are below 40 degrees unless materials are heated and the work is covered and protected for a day or two after. St. Louis gives usable windows well into December, but they are windows rather than a season.
Getting the stack looked at
If more than one of these seven turned up, get someone on the roof before the first hard freeze. We photograph every stack we assess so you can see what we are describing rather than take our word for it. Our chimney repair page covers crowns, rebuilds and flashing and what each one runs.