Why brick chimneys leak in a place this wet

Nothing shields a chimney once it clears the roofline. Rain lands on its top, beats on all four faces, and runs down the roof into its base. Mobile ranks among the rainiest cities in the country, so that happens often, and storms off the Gulf drive rain sideways into the brick. Then the humid air keeps the brick from drying out between showers.

A lot of chimneys here also see very little fire. Winters are mild, and many older houses have fireplaces that were closed off long ago. A chimney that is never used still takes all the rain, and nobody is looking up the flue to spot a problem. So a leak often shows up as a stain on a ceiling or a damp smell well after it started.

The crown is the slab of concrete or mortar that covers the top of the brick around the flue. It keeps rain from settling on the stack and steers it away from the brick. A good one slopes down away from the flue and sticks out past the brick a little, so water drips clear instead of running down the face.

Plenty of crowns were never built that way. A thin layer of mortar troweled over the top is common, and it cracks as it ages and as the flue tile and the brick move at different rates in the heat. Every crack lets water straight down into the middle of the chimney, where it soaks the brick from the inside. Crown cracks start a large share of chimney leaks, and they are hard to spot from the yard.

Small cracks can sometimes be sealed. A crown that is broken up or sloped the wrong way usually needs to be replaced.

The cap is the cover that sits over the flue opening, usually metal with a screen around the sides. It keeps rain from falling straight down the flue and keeps birds and squirrels out. Without one, every shower drops water onto the damper and into the firebox, and that water rusts the metal and soaks the brick inside.

A cap and a crown do different jobs. The cap covers the hole. The crown covers the brick around the hole. A chimney can have a perfectly good cap and still leak through a cracked crown.

Mortar joints on a chimney wear faster than the joints on the walls below because they get more weather. Once the joints sink back or crack, rain gets behind the face of the brick and into the core of the stack. The fix for that is repointing: cut out the worn mortar and pack in new.

The brick itself soaks up water too, especially the old soft brick on older Mobile houses. Brick that stays wet starts to lose its faces, and you will see broken faces, crumbling corners and white salts on the stack. Spalled brick near the top is common, since that part catches the most rain.

Look for green growth or moss on one side of the chimney. It marks the side that stays damp the longest, and that is often where the trouble starts.

Flashing is the metal that closes the gap around the base of the stack where shingles stop and brick starts. There are usually two layers. One piece is worked in with the shingles, and a second piece, the counter flashing, is set into a mortar joint in the brick and folds down over the first. Wide chimneys often have a small peaked section on the uphill side, called a cricket or saddle, that splits the water around the chimney.

Flashing fails often, and the stain it leaves looks just like a masonry leak. In general, the metal and the roofing are a roofer's job. The mason's part is the brick and the mortar joint the counter flashing is tucked into. If that joint has opened up or the brick around it is loose, water gets behind the metal even when the metal is fine. So it helps to have both looked at.

A thick bead of caulk along the top of the flashing usually means someone has chased this leak before. It can hold for a while, but it dries out and cracks in the sun.

A musty smell is easy to blame on Mobile's humidity in general. But a fireplace that smells worse after a rain is telling you water got into the chimney. These are the other signs to look for.

• Brown rings or blotches on the ceiling or plaster near the chimney breast
• Paint or plaster bubbling or flaking on the chimney breast
• A damp smell around the fireplace that gets stronger after rain
• Rust on the damper, the grate or the metal parts of the firebox
• Water, wet ash or bits of tile in the firebox after a storm
• White salts or dark damp patches on the brick inside the firebox or on the hearth

Leaks travel. Water that gets in at a cracked crown can run down inside the stack and show up at the ceiling below the flashing, so the stain is not always under the part that failed. The way to sort it out is to check each part in turn: the crown and cap first, then the joints and brick, then the flashing.

Sometimes the answer comes from running a hose on one part of the chimney at a time while someone watches inside. Leave that to whoever is doing the repair, starting low and working up, so the test does not soak the whole chimney at once.

And the inside of the flue and its liner are their own trade. A chimney sweep can look at those.

It helps to know who handles which part before you start making calls.

• Masonry: the crown, the brick and mortar of the stack, and the joint that holds the counter flashing
• Roofing, in general: the flashing metal, the shingles around the chimney and the cricket
• Chimney sweep or liner work: cleaning and checking the flue, and repairing or replacing the liner
• Caps: set by more than one trade, so ask whoever is working on the chimney

Brick Repair and Renovations does chimney repair on Mobile houses: loose and cracked brick, worn joints, cracked crowns and leaks around the chimney. The chimney repair page covers that work. Donald W Carpenter tells you what the brick needs, and what it does not.

Text him a picture of the chimney taken from the street, one zoomed in on the crown, and one of the stain indoors if there is one, along with your part of Mobile. Call or text (251) 305-5966.