What a Cracked Flue Tile Means for Your Chimney

What the flue tile does

Most masonry chimneys are lined with square or rectangular clay tiles stacked from the smoke chamber to the top. The tiles keep heat and corrosive gases off the brick, give smoke a smooth path and keep the chimney's outer walls cooler. The Florida Building Code expects masonry chimneys to have a liner like this.

A cracked chimney flue no longer seals, and the brick behind the tile starts doing a job it was not built for.

Why tiles crack

Clay expands when it heats. A very hot fire, and especially a chimney fire, heats the tiles faster than they can expand evenly, and they split. Water from a missing cap or cracked crown soaks the tiles and the mortar between them. Settling of the chimney and age do the rest.

Flue tile cracks often have no outside sign at all, which is why they are usually found during an inspection rather than noticed by the homeowner.

How cracks are found

A crew looks up the flue with a strong light from the firebox and down it from the top. Pieces of clay on the smoke shelf, mortar gaps between tiles and smoke staining on the outside of the chimney are clues. A chimney fire, or a loud roar and heavy smoke from the chimney in the past, is reason enough to check the whole flue.

The three repair paths

Which repair fits depends on where the damage is and how widespread it is.

• A cracked top tile above the crown can be replaced, often when the crown is recast
• Small cracks and joint gaps lower down can be sealed with a listed resurfacing system
• Widespread cracking, or a flue that does not suit the appliance, is fixed with a stainless steel liner

Should you use the fireplace meanwhile?

Is a cracked flue tile dangerous? It can be, and a flue with known cracks should not be used until it is repaired. Heat escaping through a crack can reach nearby framing without anyone noticing, and smoke can find its way into the house. Our flue repair and liner pages explain each repair in more detail.