Footings act as the base of a house’s foundation and transmit the superimposed load to the soil. The type and size of footings should be suitable to the soil condition, and in cold climates the footings should be far enough below the ground level to be protected from frost action, this depth being established by local building codes.
Poured concrete footings are more dependable than those of other materials and are recommended for use in house foundations. Where fill has been used, the foundations should extend below the fill to undisturbed earth.
In areas having adobe soil or where soil moisture may cause soil shrinkage, irregular settlement of the foundation and the building it supports may occur. Local practices that have proven successful should be followed in such cases.
The size of the wall footings should comply with local code requirements. One method of determining the size, often used with firm soil, is based on the proposed wall thickness. In this case, footings should project beyond each side of the all one-half the wall thickness. See Figure 1.
If soil is of low load-bearing capacity, wider reinforced footings may be required. A few rules applying to footing design and construction:
1. Footings must be at least 6 inches thick
2. If footing excavation is too deep, fill with concrete– never replace dirt
3. Use side forms for footings where soil conditions prevent sharply cut trenches
4. Place footings below the frostline
5. Reinforce footings with steel rods where they cross pipe trenches
6. In freezing weather, cover freshly poured concrete with straw
Pier Post and Column Footings
Footings for piers, posts or columns should be square and include a pedestal on which the member will bear. A protruding steel pin is ordinarily set in the pedestal to anchor a wood post.
Footings vary in size depending on the allowable soil pressure and the spacing of the piers, posts or columns. Common sizes are 24 by 24 by 12 inches and 30 by 30 by 12 inches. The pedestal is sometimes poured after the footing. Minimum height should be 3 inches above the finish floor and 8 inches above finish grade for basementless areas and similar situations. See Figure 2.
Stepped footings are often required where the lot slopes, and the vertical part of the step should be poured at the same time as the footing. The bottom of the footing is always placed on undisturbed soil and located below the frostline, and each run should be level.
The vertical connection between footings at the step should be constructed of concrete at least 4 inches thick and of the same width as the footings. On steep slopes more than one step may be required. The vertical distance between steps should not exceed 2 feet and the horizontal distance between steps should be not less than 2 feet.
Adapted from Wood Frame House Construction by L.O. Anderson and O.C. Heyer