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The following requirements listed below are from the International Building Code (IBC) for 1 – 2 story residences.

Site Grading

The slope of the ground immediately adjacent to the foundation footings must be a minimum of 5% for 10 ft [2].

Load-Bearing Value of Soil

Foundation must rest on undisturbed soil because it is stronger than disturbed 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 human or machine. The code refers to load-bearing values (LBVs) as “presumptive”. This means that a test must take place to determine if the soil is disturbed or not. Disturbed soil can be reclassified as undisturbed soil after a certain time [2].

A chart displaying how much square footage is needed for testing depending on the type of soil [2]

Depth of Footings

In general, footings should extend to a minimum depth of 12 inches below undisturbed soil as well as 12 inches below the frost line or must be frost protected. Certain structures such as sheds or decks have different regulations and requirements [2].

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 1,500 LBV soil. For 2,000 LBV soil or greater, two-story buildings may have 12-inch minimum wide footings.” [2].

Levelness

Top of the Footings

The top of the footings must be level [2].

Bottom of the Footings

Preference is that the footings are level but if you’re building on a sloped grade, you can have a 10% slope (one unit vertically per 10 units horizontal) [2].

Strength of Footing Concrete

The minimum strength of the concrete for foundations and slabs (except garages) should be 2500 psi. Vertical walls that are exposed to the weather may require a stronger minimum strength of 2500 psi – 3000 psi depending on weather conditions [2].