Nobody can deny the need for a good house foundation. Improperly built foundation footings--unlike poorly installed gutters or gapped hardwood flooring--can eventually bring down the house.
Building code has a few things to say about foundation footings. Like any code, it is not a guide to building footings so much as it is a set of parameters you need to observe.
Code Basics: IBC Chapter 18, Soils and Foundations
Derived from the International Building Code (IRC) for 1- and 2-story residences, this nutshell guide is intended to give you a general sense of footings code.
Your situation will vary. For example, soil is different from place to place, and thus the load-bearing value of the soil will change. For a complete picture, purchase the most recent International Building Code or consult a free version of the IBC online. The relevant section is Chapter 18, Soils and Foundations.
Section 1808 covers foundation basics, both shallow and deep. Foundations for most residential are what are called "shallow foundations," covered in Section 1809.
Site Grading (1804.4)
Ground immediately adjacent to the foundation footings (on the exterior of the footings) should slope down at a 5% minimum slope. This must continue for at least 10 feet.
Soil: Load-Bearing Value (1806.2)
Code calls load-bearing values (LBVs) "presumptive." This means that a soil test is the only way to really know the load-bearing value (LBV) of the soil for your intended footers.
|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 and Width of Footings In Undisturbed Soil (1809)
- Depth: Footings should extend to a minimum of 12".
- Width: Footings should have a minimum width of 12".
Undisturbed soil is soil that has never been turned over, tilled, graded, hoed, or anything of that nature, by man or machine. Undisturbed soil is significantly stronger than soil that has been disturbed.
Levelness of Footings (1809)
- Top: Top of the footings must be level, no exceptions.
- Bottom: Preference for the bottom of the footings is that it is level. Exception: if building on a sloped grade, you can step the footings as much as 1 unit vertical per 10 units horizontal (or a ten-percent slope).
Spread footings help widen the load carried by the footings. The "spread" part is a base that looks like an upside-down "T" and transfers the weight across a larger area.
The spread footing should be no less than 6 inches thick. It should project, on both sides, no less than 2 inches.