Footings and Frost Depth

Why footings are wide

A wall concentrates a house’s weight into a narrow line; soil can only carry so much per square foot. The footing spreads the load out. Wisconsin’s code says it plainly: the dwelling and attached structures (decks and garages included) must sit on a structural system that transmits and safely distributes the loads to the soil, and the footing’s bearing area has to be large enough that the soil’s bearing capacity isn’t exceeded ¹.

A quick check: suppose a bearing wall carries 2,400 pounds per running foot (roof snow, walls, floors, and the wall’s own weight), on soil good for 2,000 pounds per square foot. The footing needs 2,400 ÷ 2,000 = 1.2 feet of width, about 15 inches. Put the same wall on stiff clay (3,000 psf) and 0.8 feet would do.

The code’s presumed bearing values ¹:

No footing may go on soil bearing less than 2,000 psf unless it’s engineered, and if a weaker layer lies under the soil the footing sits on, the weaker value governs ¹.

Prescriptive sizes

Unless designed by structural analysis, plain concrete footings follow these minimums ¹:

• Continuous (wall) footings: at least 4 inches wider than the wall on each side and 8 inches deep (nominal). Footings in unstable soil, soil that can’t stand vertically for the footing’s depth, must be formed.
• Column or pier footings: at least 2 feet by 2 feet and 12 inches deep.
• Trench footings, poured with the wall where the soil allows: at least 8 inches wide.
• Chimney and fireplace footings: 4 inches beyond the masonry on each side, 12 inches deep.
• Deck footings for attached decks or decks that serve as an exit: sized for the load and the soil, like the house’s.

So an 8-inch poured wall gets a footing at least 16 by 8 inches.

What a footing sits on

Footings go on undisturbed or compacted soil free of organic material, or are reinforced to bridge poor spots ¹. Never on topsoil, unprepared fill, organic soil, mud, or frozen ground ¹. Don’t dig below a footing without shoring it.

Frost depth

The bottom of every footing, stoops and landings included, goes below the frost penetration level or at least 48 inches below adjacent grade, whichever is deeper ¹. The exceptions are frost-protected shallow foundations designed to ASCE 32, footings on clean bedrock, and the part of a foundation under a window areaway ¹.

One trap: a structure on a floating slab (like an attached garage slab or a porch on a slab) may not be rigidly attached to a structure on frost footings unless there’s an isolation joint the full height between them ¹. Otherwise frost heaves the slab, and it lifts or cracks whatever it’s tied to.

Holding the house down

The top of the foundation ties to the framing ¹:

• Anchor bolts at least 1/2 inch in diameter, embedded at least 7 inches in concrete or grouted masonry, no more than 72 inches apart and within 18 inches of each corner, each with a nut and washer tightened on the sill; or listed mechanical anchors, or an engineered system.
• Where floor joists run parallel to the foundation wall, solid blocking in at least the first joist space every 32 inches, so the floor braces the top of the wall against soil pressure.

What to take from this

Footings spread the load so the soil isn’t overloaded: width = load per foot ÷ soil bearing, with 2,000 psf as the minimum soil without engineering. Prescriptively, wall footings are 8 inches deep and 4 inches wider than the wall on each side, pier footings 2 by 2 feet and 12 inches deep, all on undisturbed soil, below frost or 48 inches, never on frozen ground. Bolt the sill every 72 inches and within 18 of corners, and isolate floating slabs from frost-footed walls.