While it's true that poor footings can stand up for years before incremental changes start to happen--peas slide off the dinner table, windows don't close, doors stick--why not build them right on the first go-around?
Building code has a few things to say about foundation footings. Like any code, it's not a prescription for how to build footings so much as a set of parameters you'll want to observe to "keep your house in order."
Here are foundation footings in a nutshell, derived from the International Building Code (IRC) 2006 for 1- and 2-story residences. This "nutshell guide" is intended to give you a general sense of footings code, and your situation will most likely vary. For example, soil varies from location to location and thus the load-bearing value of the soil will change. For a complete picture, purchase the most recent International Building Code (Compare Prices - IBC) on seek a free copy of the IBC online.
Soil: Load-Bearing ValueA soil test is the only way to really know the load-bearing value (LBV) of the soil for your intended footers. But below is a guide to basic soil types and their LBVs:
- Bedrock: 12,000 (Pounds Per Sq. Foot)
- Sedimentary rock: 6,000
- Sandy gravel or gravel: 5,000
- Sand, silty sand, clayey sand, silty gravel, and clayey gravel: 3,000
- Clay, sandy clay, silty clay, and clayey silt: 2,000
Depth of FootingsFootings should extend to a minimum of 12" below undisturbed soil. Undisturbed soil is soil that has never been turned over, tilled, graded, hoed, or anything of that nature, by man or machine.
Minimum Width of Foundation FootingsThe soil's LBV and type of house (1- or 2-story or other) help determine the minimum sizing of footings. Here we deal with 12 inch wide footings, the most common.
These foundation footings work in soil LBV 1,500 and upward and for one or two story typical light-framed (balloon) residences. The only exception is that for a two-story house on 1,500 LBV, minimum footing width is 15 inches.
12 inch minimum footings will also carry a one-story brick-veneered or hollow masonry house on LBV 1,500 or above.
Spread FootingsSpread 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.