How to tell if a chimney is leaning, and why it moves

Is my chimney leaning? What to look for

A leaning chimney may not look tilted from across the street. What you notice first is the chimney pulling away from the house at the joint, and the signs inside. Outside, look for a gap between the brick and the siding that opens wider toward the top, caulk that has stretched or torn along that line, and flashing pulled loose where the chimney meets the roof.

To check the stack itself, stand back and sight its edge against a straight vertical line such as the corner of the house, or hang a string with a weight from a window and compare. Cracks that step diagonally through the joints near the base of the chimney are another sign of movement. Inside, watch for a gap opening between the fireplace surround and the wall, a hearth cracking or separating from the floor, and a damper that has started to bind.

Why a chimney needs its own footing

A masonry chimney is heavy, and all of that weight bears on a small area. Section R1003.2 of Florida's residential code sets the minimum footing under a masonry chimney: a slab of solid masonry or concrete no thinner than 12 inches, projecting 6 inches or more beyond the face of the foundation or support wall all the way round, and resting on natural undisturbed earth or engineered fill. Section R1001.2 sets the same minimum for a masonry fireplace and its chimney.

A footing built smaller or thinner than that, or poured on soft or disturbed soil, can settle unevenly under the load. Because a chimney is tall and narrow, a small drop on one side of the footing becomes a noticeable tilt at the top.

What makes a stack move

The causes fall into two groups: the ground under the chimney and the chimney itself. The first group tilts the whole stack from the base. The second can make only the upper part lean while the lower chimney stays true.

• A footing that is undersized, too thin, or missing, with the chimney resting on a slab edge.
• Water softening the soil under the footing, from downspouts that discharge at the chimney base, grading that sends water toward it, or a plumbing leak nearby.
• Soil that has washed out or settled beside the footing over time.
• Mortar that has eroded in the upper courses, letting the section above the roof shift on its joints.
• Water damage inside the masonry, from a failed crown or an uncapped flue, weakening the top of the stack.

Why the fireplace stays cold for now

When a chimney moves, its flue moves with it. Joints between clay flue tiles can open and tiles can crack or shift, so smoke, heat and gases may no longer stay inside the liner. The air space the code requires between a chimney and the framing beside it is small, two inches for an interior chimney under section R1003.18, and a moving chimney can close that gap or open paths for heat toward wood.

A leaning stack can also drop brick or crown pieces onto the roof or the ground below. Until someone has looked at it, close the damper, do not light the fireplace, and keep everyone away from the ground directly under the chimney.

When the chimney pulls away at the roof

A chimney separating from the house at the roof line brings damage of a different kind. As the stack tilts or shifts, the flashing that sealed it to the roof is stretched, torn or pulled out of its joint, and the roof deck around the chimney can be soaked by the water that follows. A leak near the chimney can be the first thing a homeowner notices, well before the tilt itself.

Any repair to a chimney that has moved includes new or reset flashing, because the old flashing was shaped for where the chimney used to be. Roofing and decking damage found around it is listed with who repairs it.

Who needs to look at it

A chimney evaluation finds how far the stack has moved, whether the movement is in the footing or only in the upper chimney, and what has happened to the flue and the flashing. When the footing itself has moved, the question of why and whether it is still moving belongs to a structural engineer or a foundation contractor, and that work is theirs, not a chimney crew's.

A sound estimate for a leaning chimney therefore says who does what. The footing is assessed and stabilized first if needed, and the masonry, flue and flashing work follows.

What an evaluation records

A useful evaluation gives numbers and locations, not only an opinion. It should leave you with a clear picture of how far the chimney has moved and where.

• How far the stack is out of plumb, measured, and in which direction.
• The width of the gap at the wall and roof, measured at marked points so later readings can be compared.
• A map of cracks in the chimney, the fireplace face and the hearth.
• The condition of the flue, since movement opens liner joints.
• The state of the flashing and the roof around the chimney.
• Whether the footing needs a structural engineer or a foundation contractor to look at it.

The repair options

What gets done depends on where the movement is and whether it has stopped. Our chimney rebuild page describes the masonry side of that work.

• Only the part above the roof has shifted, from mortar loss or water damage: that section is taken down to sound masonry and rebuilt, with a new crown and flashing.
• The footing settled once and has stopped: the chimney is repaired or partly rebuilt to plumb, the flue is checked and repaired, and the joint with the house is resealed.
• The footing is still moving: it is stabilized by a foundation contractor first, and masonry work waits until that is done.
• The chimney is badly out of plumb or cracked through: a full rebuild from a sound footing.

What to watch in the meantime

If the gap is small and you are waiting on an evaluation, measure it and mark it with a pencil line and the date at a couple of spots. Check again after heavy rain. A gap that keeps widening tells the evaluator the chimney is still moving. Keep downspouts discharging away from the chimney base, and resist the urge to keep filling a growing gap with caulk, which hides the movement without stopping it.