What Do Cracks in a Chimney Mean?

Four things to note about any crack

Before deciding whether a crack matters, note four things about it. Where it is: the crown on top, the brick outside, the flue liner, the plaster on the chimney breast, or the firebox. Which way it runs: straight up, along the mortar in steps, or level across. How wide it is: a hairline you can barely catch with a fingernail, or a gap you could slide a coin into. And whether it is changing: an old crack that has looked the same for years is a different matter from one that opened this winter.

Put those four together and most cracks sort themselves into routine maintenance or something that needs attention soon.

Crown cracks: small, common and wet

The crown is the concrete or mortar slab that caps the top of the chimney around the flue tiles. Many crowns on older houses were troweled on thin, in ordinary mortar, with no overhang and no gap around the flue tile for expansion. Those crowns crack early, first in fine lines running out from the flue tile and later into loose pieces.

A hairline crown crack does not threaten the chimney on its own. What it does is let water into the top courses of brick, where freeze-thaw cycles widen every opening. Sealing a sound crown with a crown repair coating, or rebuilding a broken one with a proper overhang and drip edge, is small work compared with rebuilding the brick it protects.

Vertical cracks through brick

A vertical crack that runs straight through the bricks, rather than winding around them along the joints, means the masonry was pushed apart by a strong force. The usual causes are intense heat from a chimney fire, uneven settling of the chimney, and steel inside the masonry rusting and swelling. A rusted steel lintel over a fireplace opening, for example, can expand enough to crack the brick above it.

A vertical crack near the top of the stack can also come from an unlined or poorly lined flue running very hot, or from water freezing in a cracked crown and splitting the brick below. Either way, it is a crack to have inspected rather than filled.

Stair-step and horizontal cracks

A crack that zigzags along mortar joints like a staircase usually means one part of the chimney is moving relative to another. On an outside chimney, that often traces back to the footing settling, especially where a downspout dumps water at the base. On a chimney attached to the house, it can come from the house and the chimney settling at different rates.

A horizontal crack is more worrying, particularly when the masonry above it sits slightly offset from the masonry below. That pattern can mean the stack is leaning or being pushed sideways by wind load or swollen mortar. Keep people away from the area below and have the chimney checked promptly.

Plaster and firebox cracks indoors

Fine cracks in the plaster over a chimney breast are common in older brownstones, rowhouses and colonials. Plaster on masonry moves as the house moves through the seasons and as the fireplace heats and cools. A crack that keeps reopening, runs along the joint where the breast meets the wall, or shows soot or brown staining is different. That can mean the chimney is shifting, water is getting behind the plaster, or smoke is finding its way out of the flue.

Inside the firebox, firebrick and its mortar take direct heat, and small cracks and eroded joints appear over time. Wide gaps or loose firebrick let heat reach the masonry and framing behind the firebox, so the firebox is repaired before the fireplace goes back into regular use.

The cracks you cannot see

The flue liner sits inside the chimney where nobody looks. Clay tiles crack from sudden heat, and a chimney fire can crack several at once, sometimes without anyone in the house noticing more than a roar and a hot smell. NFPA 211 sets a yearly inspection and another one whenever a chimney fire or similar event could have done harm, because a cracked liner lets heat and gases reach the brick and the wood framing around it.

On boiler flues in older houses, cracked or missing tiles also let wet exhaust soak into the masonry, which feeds the outside cracks and stains described above.

Tracking a crack at home

For a crack you can reach safely from inside the house or from the ground, a pencil line across each end with the date written beside it shows whether it is growing. Check it again after a few weeks and after a cold snap. A photo taken from the same spot each time helps with cracks you can only see from a distance.

Skip the caulk. Filling a crack hides the evidence a technician needs to read, and it does nothing about the force that opened it. Roof-level cracks are for a crew with proper roof gear to examine up close.

• Note the date, length and width each time you check
• Look for fresh mortar dust or brick chips below the crack
• Watch for new staining or a smoke smell near indoor cracks
• Report any crack that has grown since the last check

Matching the repair to the crack

Each kind of crack has its own fix, and the fix works only when the cause is dealt with first. A thin, cracked crown is rebuilt with an overhang that throws water clear of the brick. Cracked joints in sound brick are cut out and repointed with a mortar suited to the brick. Cracked bricks are cut out and replaced with matching units. A lintel that has rusted and lifted the brick above it is cleaned, treated or replaced before the masonry is rebuilt over it.

Cracked flue tiles are handled by replacing the liner, most often with a stainless liner matched to the appliance's flue size or with a poured liner on some masonry chimneys. Cracks caused by a moving footing or a leaning stack come last in this list for a reason: the movement has to be stopped first, and that can mean rebuilding part or all of the chimney.