What is a concrete footing?

Depth

below the local frost lineIRC R403.1.4, depth set by the building department

Bearing surface

undisturbed soilnever on fill or topsoil

Frost depth

no national numberpublished by each jurisdiction

Width and thickness

from load and soil bearingcode table or engineer

Reinforcement

engineer-specifiedACI 318 governs

Short version

• A footing turns a concentrated load into a pressure the soil underneath can actually carry.
• Footings sit on undisturbed or properly compacted soil — never on topsoil, fill, or a frozen surface.
• Exterior footings go below the frost line, and the frost line is a local number, not a national one.
• Width and thickness come from the load and the soil’s bearing capacity, through a code table or an engineer.
• A frost-protected shallow foundation is the engineered alternative where digging deep is impractical.

What does a concrete footing actually do?

A footing converts a concentrated load into a distributed pressure. A foundation wall or a column delivers a large load over a small contact area, and most soils cannot carry that pressure without settling. Widening the base spreads the same load over more square inches, dropping the pressure to something the soil can support.

The second job is stability. A footing gives the foundation a level, uniform bearing surface, resists differential settlement between one part of a structure and another, and — when it extends below the frost line — keeps the structure out of the zone where freezing soil moves.

Both jobs depend on what the footing sits on. Soil bearing capacity is the input the whole design runs on. Building codes provide presumptive bearing values for common soil types, and the building department or a geotechnical report determines what applies to your site. USDA’s soil survey resources are useful for identifying what soils are mapped in an area, but they do not substitute for the authority having jurisdiction.

General information, not engineering advice. Structural work should be designed by a licensed engineer.

What types of concrete footings are there?

Footings fall into a few standard forms chosen by what they support. A continuous or strip footing runs under a foundation wall and spreads a line load. An isolated spread footing, sometimes called a pad footing, is a rectangular block under a single column or post. A monolithic thickened-edge slab, common in warm climates, casts the slab and its perimeter footing in one placement.

Sloped sites use stepped footings, which drop in stages so each segment stays level while following the grade. Where competent soil is deep, pier or pile caps transfer the load through a footing into deep elements rather than into the soil directly beneath.

ACI 332 is the residential code requirements document for structural concrete in houses, and ACI 318 covers structural concrete generally, including footing design and reinforcement. Reinforcement layouts that look standard on residential jobs — continuous bars in strip footings, a mat in larger pads — are typical practice, but bar size and spacing are engineer-specified under ACI 318 rather than fixed by convention.

How deep does a footing have to be?

Footing depth is governed by frost, by soil, and by the code — and the controlling number is local. IRC section R403.1.4 requires exterior footings and foundations to extend below the frost line as established for that jurisdiction, and the IRC also sets a minimum depth below undisturbed ground regardless of frost. Whichever requirement is deeper governs.

The frost line itself is not printed as a national value. The IRC references a locally determined frost depth, and each building department publishes the figure that applies within its boundaries. That is why you cannot look up a correct footing depth for your project in a national table — you call the building department, or you read their published foundation requirements.

Depth is not only about frost. The footing has to reach soil capable of carrying the design pressure, which may be deeper than the frost requirement on a site with topsoil, organic soil, or fill. Expansive clays introduce their own depth and detailing requirements. And a footing must never be placed on frozen ground, since the soil settles when it thaws.

Frost-protected shallow foundations are the recognized engineered alternative. Insulation placed to retain ground heat around the foundation keeps frost from reaching the bearing level, allowing a shallower footing — a system that has to be designed, not improvised.

Regional variation

Published frost depths vary enormously across the United States, from effectively zero along the Gulf Coast and in much of the Southwest to five feet or more in northern New England and the northern Plains. Two towns in the same state can carry different published values, and a county may differ from the city inside it.

Treat any number you find online — including a range like this one — as background only. The frost depth that governs your permit is the one your local building department publishes, and it is the only one an inspector will accept. Call before you dig footings, not after.

Soil varies just as much and matters just as often. The same house design lands on dense glacial till in one region, expansive clay in another, and loose sand in a third, and the footing that works on one will not work on the others. Where soils are difficult, a geotechnical investigation replaces the presumptive values entirely.

How wide and thick should a footing be?

Footing width comes from dividing the load by the allowable soil bearing pressure, and thickness comes from the structural behavior of the footing itself. Wider is not automatically better — beyond a certain projection, an unreinforced footing cracks in bending rather than distributing more load, which is why width and thickness are designed together.

Residential codes carry prescriptive tables that give minimum widths and thicknesses for common combinations of wall type, number of stories, and soil bearing value, and those tables are what most permit sets rely on. Anything outside their assumptions — unusual loads, poor soils, tall walls, large point loads — moves the design to an engineer.

Placement details matter as much as dimensions. Excavate to undisturbed soil rather than backfilling to level, keep the bearing surface clean and unfrozen, and place concrete the same day the excavation is opened where you can, since rain slaking the bottom of a trench changes the soil you designed for.

Common mistakes

• Using a frost depth from the internet — the governing number is published by your building department, and only that one clears an inspection.
• Bearing on fill or topsoil — the footing settles as the loose material consolidates, and the cracking shows up in the structure above.
• Placing concrete on frozen ground — the soil settles when it thaws, leaving the footing unsupported.
• Over-widening an unreinforced footing — past a certain projection the footing cracks in bending instead of spreading more load.
• Treating a deck or porch pier as exempt — exterior piers are footings, and they follow the same frost depth and bearing rules as the house.