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 so much as they are parameters that you need to observe and should be adjusted according to your local code requirements.
The ground immediately adjacent to the foundation footings—on the exterior of the footings—should slope down at a 5-percent minimum slope. This must continue for at least 10 feet.
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 human by or machine. Undisturbed soil is significantly stronger than soil that has been disturbed.
Code refers to load-bearing values (LBVs) as "presumptive." This means that a soil test is the only way to really know the load-bearing value (LBV) of the soil for the footings on a given site.
Disturbed soil can eventually be classified as undisturbed after enough time; a soil test can determine this.
|Soil Type||LBV Per Square Foot|
|Sandy Gravel or Gravel||3,000|
|Sand, Silty Sand, Clayey Sand, Silty Gravel, Clayey Gravel||2,000|
|Clay, Sandy Clay, Silty Clay, Clayey Silt||1,500|
Depth, Width, and Thickness of Footings
Depth of Footings
Footings should extend to a minimum depth of 12 inches below previously undisturbed soil. Footings also must extend at least 12 inches below the frost line (the depth to which the ground freezes in winter) or must 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.
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.
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 1 unit vertical per 10 units horizontal (or a 10-percent slope).
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 no less than 6 inches thick. It should project, on both sides, no less than 2 inches.
Strength of Footing Concrete
For foundations and slabs, except for garages, the minimum strength of the concrete should be 2,500 psi. Vertical walls exposed to the weather may be required to have 2,500 psi concrete minimum strength, on 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 1- and 2-story residences. This summarized guide is intended to give you a general sense of code requirements for foundation footings. The IBC encompasses the International Residential Code (IRC) but includes provisions for commercial buildings as well as for 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/or IRC as their 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 will need to observe.