Building Code Foundation Requirements

A good foundation is essential for every house. Improperly built foundation footings can bring down a house. So, building codes naturally have a few things to say about foundation footings.

Like any code recommendations, these are not how-to guides to building footings. Review these parameters and adjust them according to your local code requirements.

Site Grading

The ground immediately adjacent to the foundation footings—on the exterior of the foundation—should slope down at a 10% minimum slope. This must continue for at least ten feet.

If obstructions do not permit the preferred 10% grading, 5% grading may be approved, but you need an alternative method of diverting water from the foundation. When there is a ramp or a door landing, an impervious surface may be installed and sloped at a minimum of 2%.

Load-Bearing Value of Soil

Foundations must rest on firm, undisturbed soil. ​Undisturbed soil is soil that has never been turned over, tilled, graded, hoed, dumped, dug, scraped, compacted, amended, or anything of that nature by humans or machines. Undisturbed soil is significantly stronger than soil that has been disturbed. After enough time, disturbed soil can eventually be classified as undisturbed, and a soil test can determine this.

Load-bearing values (LBVs) are "presumptive," meaning that a soil test is the only way to know the soil's load-bearing value (LBV) for the footings on a given site.

Depth, Width, and Thickness of Footings

Depth of Footings

Footings should extend to a minimum depth of 12 inches below previously undisturbed soil. Footings must also extend at least 12 inches below the frost line (the depth to which the ground freezes in winter) or be frost-protected.

These rules may not apply to accessory buildings (such as sheds) if they are below a certain square footage specified by your local building code. Also, deck footings may have a different depth requirement. Some decks, such as those not attached to the house, may not have the exact depth requirements.

Width of Footings

Footing width can vary according to the structure, site, and conditions. Under code, generally, one-story buildings with footings on undisturbed soil with LBVs between 1,500 and 4,000 should have a minimum width of 12 inches.

Two-story buildings require a minimum of 15-inch wide footings for soil with a 1,500 LBV. For soil with 2,000 LBV or greater, two-story buildings may have 12-inch wide footings.

Levelness

Requirements for levelness are different for the top and bottom of the footing:

Top of the Footings

The top of the footings must be level, with no exceptions.

Bottom of the Footings

The preference for the bottom of the footing is that it is level. Exception: If building on a sloped grade, you can step the footing by as much as one unit vertical per 10 units horizontal (or a 10% slope).

Spread Footings

Spread footings help distribute the load carried by the footings over a wider area. The spread part is a base that looks like an upside-down "T" and transfers the weight across its area.

The spread footing should be at least six inches thick and project at least two inches on both sides.

Strength of Footing Concrete

The minimum strength of concrete for foundations and slabs, except for garages, should be 2,500 psi. Vertical walls exposed to the weather may require 2,500 psi concrete minimum strength, up to 3,000 psi in some severe weathering conditions.

Building Code Basics

These code specifications are derived from the International Building Code (IBC) for one- and two-story residences. Use this summary only as a general sense of code requirements for foundation footings.

The IBC encompasses the International Residential Code (IRC) but includes provisions for commercial and residential buildings. The IRC is equally suitable for residential buildings. In the IRC, the chapter relating to foundations and footings is chapter 4.

Keep in mind that each construction project is unique. For example, the soil is different from place to place, and thus, the load-bearing value of the soil will change. Also, code rules are set and enforced at the local level, typically through each city's building department.

Most local code authorities use the IBC and IRC as model codes but may omit, modify, or expand any code specification to suit local conditions and legal requirements. While the IBC and IRC are merely suggested guidelines, the local code is the law of the land that you must observe.