Why Is My Chimney Leaning Away From the House?

Signs the chimney is moving

On a house with an outside chimney, the first sign is often a gap between the chimney and the siding or brick wall that was not there before. Flashing tears at the roof joint, caulk at the side joint splits open, and from across the street the top of the chimney no longer lines up with the corner of the house. Inside, a crack can open between the hearth and the floor, a mantel can pull away from the wall, or new cracks can appear in the plaster beside the chimney breast.

• A gap between chimney and wall that widens over the seasons
• Torn flashing or split caulk at the roofline
• The chimney top out of line with the house corner or the roof edge
• A crack between hearth and floor, or a mantel pulling away from the wall
• Diagonal, stair-step or horizontal cracks in the brick

A footing that cannot carry the load

A full masonry chimney is extremely heavy, and an outside chimney usually sits on its own footing, separate from the house foundation. When that footing was built too small, too shallow, or on fill or soft soil, the chimney settles at its own pace. Because it was only loosely tied to the house wall, it tips outward as it sinks, opening a wedge-shaped gap that is wider at the top than at the bottom.

This is the common story on detached houses with exterior chimneys: Queens and Staten Island detached homes, New Jersey colonials and Cape Cods, and Delaware and Pennsylvania suburban houses built with the chimney on a gable end. The lean can develop slowly for years and then speed up after a wet season.

Water at the base

Water is the usual accelerator. A downspout that dumps beside the chimney, a low spot in the yard that holds water against it, a leaking hose bib or a cracked drain line can soften the soil under the footing. Frost heave in winter lifts and drops the soil around a shallow footing, and tree roots nearby can push from one side and dry out the soil on the other.

Some of this is within a homeowner's reach. Extending a downspout away from the chimney, regrading so water runs away from the base, and fixing a leaking outside faucet all reduce the pressure on a footing that is already struggling. They do not reverse a lean, but they can slow it while a repair is planned.

When only the top is leaning

Sometimes the chimney is straight up to the roof and only the section above it tilts or curves. That usually points to the masonry rather than the footing. Mortar on the wettest side washes out faster, and in an unlined or poorly lined chimney, exhaust from an oil or gas boiler condenses inside the masonry and reacts with the mortar. The affected joints swell more on one side than the other, and over many heating seasons the top bends.

A top leaning toward the house can press on the roof framing. A top leaning sideways or outward is at risk of dropping bricks onto the roof, the driveway or the sidewalk.

Planned offsets in the attic

Some chimneys were built with a deliberate offset inside the attic, so that they come out of the roof near the ridge or clear of a dormer. That slope is part of the design and is stable when it was built properly. What matters is change: fresh cracks, gaps opening at the roof deck, loose bricks or sand on the attic floor beside the chimney, or framing that has shifted. Those mean the chimney is moving, not just slanted by design.

Checking whether it is still moving

A chimney that leaned years ago and has stopped is a different decision from one that is still going. Two simple records from the ground or from inside the house help the technician tell the difference.

• Photograph the chimney from the same spot across the street or yard every few weeks
• At ground level, pencil a mark and a date at each end of the gap or crack you can reach safely
• Measure the width of the gap at the bottom, where you can reach without a ladder
• Note new cracks indoors around the hearth, mantel and chimney breast

Making it safe in the meantime

Stop using the fireplace, keep people and cars out of the area where the chimney would land if it fell, and stay off the roof. When the boiler or water heater uses that chimney, a shifted or cracked flue can leak exhaust into the house, and carbon monoxide is dangerous. Make sure a CO alarm works on each level; if one goes off, leave the house together and phone 911, or the gas company if you heat with gas.

Have the chimney inspected promptly, before the next storm or a hard freeze. If it vents your sole source of heat, ask the heating technician whether the boiler can run safely in the meantime, and do not run a generator or a fuel-burning heater indoors as a stopgap.

What the inspection looks at

A leaning chimney is examined from the ground up. At the base, the technician looks at the footing where it can be seen, the soil and grading around it, downspouts, and any cracks in the foundation wall beside it. Along the height, the gap at the wall is measured at several points to tell a tilt from a bulge, and the brick is checked for stair-step and horizontal cracks that show where the stack is bending.

On the roof, the crown, the top courses and the flashing show whether the top has moved on its own. Inside, the flue is examined for cracked or shifted tiles, because a liner that has separated at the joints can leak exhaust into the house even before any brick falls. The hearth, mantel and any framing near the chimney are checked for signs that the movement has reached the structure of the house. Those findings decide whether the job is a rebuild above the roof, a rebuild from the footing, or something smaller.

How a leaning chimney gets fixed

The repair follows the cause. When only the top is leaning, the usual approach is to take the chimney down to sound brick and rebuild it, often with a liner for the boiler flue so the exhaust stops attacking the mortar. When the footing has failed, the chimney may need to come down further and be rebuilt on a proper base, and an engineer is often involved to design the footing.

Steel ties and brackets that anchor a chimney to the house frame can help a sound chimney resist small movement, but they do not stop a chimney whose footing is still sinking. We look at it, explain which situation you have, and put the plan and the price in writing, at no charge, before any work.