What a cracked flue tile puts at risk

Why a masonry chimney has a liner at all

Unlined brick and mortar make a poor flue. They absorb moisture and acids from smoke, and they pass heat to whatever surrounds the chimney. Section R1003.11 of Florida's residential code therefore requires a lining in every masonry chimney, matched to the appliance that uses it. Clay flue tile made to ASTM C315 is the traditional choice, stacked in short sections with mortar joints between them.

The liner is also what makes the chimney's small clearances safe. Under section R1003.18, an interior masonry chimney may sit two inches from combustible framing, and one standing wholly outside the walls may sit one inch away. Those gaps assume a sound liner keeping the fire's heat inside the flue.

How clay tiles end up cracked

Fired clay handles steady heat well. What it handles badly is sudden, uneven heat, and a chimney fire is the extreme case: the tile's inner face heats up and grows much faster than its outer face, and the tile splits, sometimes lengthwise. Some cracked flues have had a fire that nobody noticed at the time.

• Chimney fires and very hot fires in a cold flue, which crack tiles through thermal shock.
• Settling or movement of the chimney, which cracks tiles across and opens joints.
• Water from an uncapped flue, which soaks the joints and the masonry behind the tiles.
• Acidic condensate from gas or oil appliances that vent into the flue, eating away the joint mortar.
• Construction faults such as dry or offset joints and missing mortar, present from the day the flue was built.
• Aggressive cleaning tools run through a flue that was already weak.

What gets through a crack

In a cracked chimney flue, heat is the first concern. A cracked or open-jointed liner lets hot gases reach the brick of the chimney wall, and the wall passes that heat on toward the framing it sits beside. Wood that is heated again and again slowly chars, and charred wood catches fire more easily than new lumber, and for that reason liner condition and the clearances around the chimney are part of the annual NFPA 211 inspection.

Smoke and flue gases come next. Gases that escape the liner can move through the masonry and out through gaps into an attic, a wall cavity or a room. Where a furnace or water heater uses the flue, those gases include carbon monoxide. Creosote can also collect in cracks and joints, where a brush does not reach it and where it can burn. Water is the slow problem, soaking the chimney wall from the inside and feeding the staining and spalling you see on the outside.

Finding flue tile cracks

Most of a flue cannot be seen from the fireplace. The flue is swept so soot does not hide hairline cracks, then examined from the top down and from the smoke chamber up along its full length, and each defect is located by height. Where it is hard to tell whether a joint leaks, a smoke test can show gases escaping into the chimney wall.

The useful result is a written map: which tiles are cracked, which joints are open, whether any sections have shifted or fallen, and whether the flue is the right size for what it serves. That map decides the repair, and you should be able to see it before agreeing to one.

Reading the findings

Flue reports use a handful of terms, and each points to a different fix.

• Hairline crack: a fine line through a tile that is otherwise in place. A candidate for resurfacing.
• Open crack: a crack wide enough to see through to the masonry behind the tile.
• Open or missing joint: mortar gone from between two tiles, leaving a gap around the flue.
• Offset joint: tiles that have shifted so their edges no longer line up, which also traps deposits.
• Spalled tile: flakes or chunks lost from the inside face of the tile, from moisture or condensate.
• Fallen or missing tile: a section gone from the stack, which points to relining.

Is a cracked flue tile dangerous to use?

A flue with known cracks stays out of service pending a full check. NFPA 211 expects chimneys to be kept sound and repaired where necessary, and a liner with cracks is the part of the chimney that protects the house from the fire. Close the damper, and if a furnace or water heater uses that flue, get it checked promptly and keep a working carbon monoxide alarm near the sleeping areas.

That advice holds even when the fireplace seems to draw normally. A cracked flue can move smoke up the chimney perfectly well while still letting heat and gases into the walls around it.

Three ways a damaged flue is fixed

The map from the inspection points to one of three paths, and the choice depends on where the damage is and how much of the flue it covers. Each is described in more depth on our chimney flue repair page.

• Replacing the top tiles. When the damage stops near the crown, the affected tiles are cut out from the roof and new ones set in, and the crown is patched with a fresh bond break around the tile.
• Resurfacing. If the flue is straight and still well supported, a coating system carrying a UL 1777 chimney liner listing can be applied over the inside walls to seal cracks and worn joints, following its maker's procedure.
• Relining. Shifted or fallen tiles, damage along the whole height, or a flue sized wrong for its fireplace or appliance all point to a new liner, such as a metal or cast-in-place system listed to UL 1777, sized for what it serves.

What to ask for in writing

Whatever path is proposed, the estimate should say where each defect is, which repair method was chosen and why the others were not, and the product and its listing when a resurfacing system or liner is used. It should also confirm the finished flue will still be large enough, since sections R1003.14 and R1003.15 of the code tie flue area to the appliance or fireplace the flue serves, and a resurfacing coat reduces that area slightly.