Cracked Flue Tiles and What to Do About Them

What Flue Tiles Are

Most masonry chimneys built with a liner use square or rectangular clay tiles stacked inside the brick, joined with mortar. Together they form the flue: the smooth passage that carries smoke up and keeps heat away from the masonry. A gap of air or mortar sits between the tiles and the brick, so the tiles can expand on their own as they heat up.

How Flue Tile Cracks Happen

Water is often part of the story. A missing cap or a cracked crown lets heavy rain reach the flue, and clay and mortar that stay wet break down faster than dry ones.

• A very hot fire heats the inside of a tile faster than the outside, and it splits.
• A chimney fire, even a short one, can crack many tiles at once.
• Settling of the house or the chimney opens joints and cracks tiles.
• Rain coming down an uncapped flue wears the joints and adds stress as tiles wet and dry.

Is a Cracked Flue Tile Dangerous?

It can be. A cracked chimney flue lets heat reach the brick and, in time, the wood framing around the chimney, which is how house fires start from chimneys that look fine from the room. It can also let smoke and gases leak into the space between the liner and the brick and from there into the attic or rooms beside the chimney. Not every hairline crack is an emergency, but every crack deserves a look before the fireplace is used again.

The Three Repair Paths

Which path fits depends on how much of the flue is damaged and what the chimney serves. Tiles cannot easily be pulled out and replaced from inside a finished chimney, which is why relining is common when damage is widespread.

• Repair: a few cracks or open joints filled with heat-rated mortar.
• Resurface: many fine cracks sealed with a heat-resistant coating along the flue.
• Reline: broken, offset or missing tiles bypassed with a new stainless steel liner.

How Cracks Are Found

Flue tile cracks are invisible from the living room. They show up during a sweep or an inspection, when the flue is clean and lit from below or above. Pieces of clay on the smoke shelf, a smoky smell in rooms next to the chimney and a past chimney fire are all good reasons to have the flue checked.