Chimney Foundations and Footings

Following the weight down

Picture a brick chimney as a tall column of solid masonry that has to deliver all of its weight to the soil without sinking unevenly. Every brick above the roof presses on the bricks below it, through the attic and the living spaces, past the firebox, and finally onto a pad at the bottom. That pad is the footing.

The Brick Industry Association's fireplace design note puts the goal plainly: a fireplace needs a foundation that can carry its weight without excessive or differential settlement. Differential settlement is the problem to avoid. If the chimney sinks a little while the house around it stays put, or one side of the footing sinks more than the other, the chimney tips, cracks or pulls away from the wall.

The house framing is not meant to share this job. The 2021 International Residential Code (IRC) says chimneys shall not support loads other than their own weight unless they are designed and built to carry the extra load. The reverse is also true in practice: the chimney is designed to stand on its own footing, not on joists or beams.

What the current code describes

For one- and two-family homes, New Jersey enforces the 2021 IRC, with state modifications, as its dwelling subcode (N.J.A.C. 5:23-3.21). Chapter 10 of that code sets out the footing for masonry fireplaces and chimneys in two matching sections (R1001.2 for fireplaces with their chimneys, R1003.2 for chimneys):

• Material: concrete or solid masonry.
• Thickness: at least 12 inches.
• Size: extending at least 6 inches beyond the face of the chimney or its supporting wall on all sides.
• Bearing: founded on natural, undisturbed earth or engineered fill, below frost depth. Where frost is not a factor, the code calls for at least 12 inches below finished grade.

The frost depth itself is a local design value, so the construction office that issues the permit is the place to confirm it. The code adds steel reinforcing and anchorage only in higher seismic design categories (D0, D1 and D2).

These figures describe how a new chimney is built. A chimney from 1925 was built to the practice of its day, and finding that it differs from current numbers is a fact to weigh. It does not by itself make the chimney defective. Whether it is performing is answered by looking for movement.

The footing in one picture

Where the base actually is: four common layouts

• Interior chimney with a basement. The chimney rises through the middle of the house and its base is visible in the basement, often with a cleanout door. The footing sits under the basement floor.
• Exterior chimney on a side wall. The stack runs up the outside of the house. Its footing sits beside the house foundation, and it is exposed to rain, roof runoff and frost on three sides.
• Chimney added or enclosed by an addition. A chimney that was once outside may now pass through a family room or garage. The footing did not move, but the drainage and soil around it may have changed during construction.
• Corbeled or offset chimneys. Some chimneys step out from a wall or shift sideways as they rise. The IRC limits corbeling: a chimney may not project from a wall or foundation by more than half the chimney's wall thickness, and no single course may project more than half the unit height or one-third of the unit bed depth, whichever is less. Offsets are limited so that the centerline of the flue above an offset stays within the center of the chimney wall below it. These rules exist because a projection or offset moves weight off the footing's center.

Factory-built metal chimneys are a different system, supported by their own listed parts. See masonry vs. factory-built chimneys.

What you can check without tools

You can learn a lot from the ground and the basement, as long as you stay off the roof and away from any masonry that looks loose. Look for:

• A gap between chimney and house that widens toward the top, or caulk that keeps tearing in the same joint.
• Stair-step cracks in the mortar near the base, or a crack that runs through both the chimney and the footing below it.
• A crack or drop in the basement floor next to the chimney base.
• Water at the base: a downspout emptying beside the chimney, soil sloping toward it, or puddles that sit there after rain.
• Trim, flashing or siding along the chimney that has been bent or torn, which can mean the chimney and wall are moving differently.

These are observations. A diagnosis takes an inspection. A gap can come from a footing that settled or from a stack that is deteriorating higher up, and the two lead to very different repairs. The leaning or separating chimney guide explains how they differ and when to stop using the fireplace.

Events that change what a footing sits on

A footing that has been stable for decades can start to move when the ground around it changes. Keep these events in mind, and mention them when you have the chimney inspected:

1. Drainage changesA new downspout location, a regraded yard or a new patio can send water toward the chimney base. Saturated soil carries load differently.
2. Digging nearbyAdditions, basement waterproofing, utility trenches and new footings next to the chimney can disturb the soil the footing bears on.
3. FrostThe code sends footings below frost depth so that freezing soil cannot lift them. An exterior chimney on a shallow footing has no heated basement around it to keep the ground from freezing.
4. New loadsFastening a deck ledger, a porch beam or a heavy fixture to a chimney adds weight the chimney was not designed for, which the IRC does not allow unless the chimney is designed to carry it.

Who evaluates a footing problem

A chimney inspection documents the condition of the masonry, the flue and the connection to the house, with photos, and can show whether the movement pattern points to the base or to the upper stack. A footing itself is mostly buried, so judging its size, depth and condition, and designing any fix, is work for a structural engineer, sometimes after a test pit is dug beside it.

Repairs fall into broad groups:

• Stabilizing the existing footing, for example with engineered underpinning, then repairing the masonry above.
• Taking the chimney down and rebuilding it on a new footing designed for it.
• Removing the chimney when it no longer serves an appliance. See should you remove an unused chimney?
• Fixing water at the source, such as downspouts and grading, as part of any of the above.

Masonry and rebuilding work on the chimney is covered on our chimney rebuilding page. The repair or rebuild guide covers how the upper stack factors into that decision.

Planning a project next to a chimney

If you are planning an addition, a patio, a deck or foundation waterproofing beside a chimney, bring the chimney into the plan early:

• Ask the designer or contractor to locate the chimney footing before excavation starts, and to keep new footings and trenches from undermining it.
• Do not attach ledgers, beams or railings to the chimney unless an engineer has designed the chimney to take that load.
• Direct downspouts and new hardscape drainage away from the chimney base.
• Photograph the chimney and the joint where it meets the house before work begins, so any later movement can be compared.

Questions homeowners ask

Sources

1. 2021 International Residential Code, Chapter 10: Chimneys and Fireplaces (model code text as published in the 2021 Seattle Residential Code, local amendments marked)
2. N.J.A.C. 5:23-3.21, New Jersey one- and two-family dwelling subcode (adopts the 2021 IRC with New Jersey modifications), via Justia Regulations
3. Brick Industry Association, Technical Note 19: Residential Fireplace Design (PDF)

This guide is general information. It can't diagnose a specific chimney; an inspection can.