Foundation Footing Sizes

Footings

• The load-bearing capacity of the soil is an important factor when considering foundation design. The structure is supported by the soil. With strong soil, a slab-on-grade foundation is sufficient for bearing the weight of a building. When the soil is weak, footings are added to increase the soil's load-bearing capacity. The weight of the structure is distributed through the footings and transferred to the soil below. In effect, the weight is spread so that the soil can carry the load.

Footing and Foundation Wall Design

• Foundation walls are placed on footings, which provide a level surface for the walls, as well as bearing the weight. The footings are below grade to prevent the wall from sliding and overturning. In cold climates, the footings are built below the frost line. The size of the footings is determined from the load-bearing capacity of the soil, and the working load of the building, estimated according to its size and type of construction. Poor soil requires larger footings.

Calculating Footing Size

• The footing size is based on how much soil needs to be under the footing to meet the load-bearing requirements of the building. The weight of the house is calculated by adding up the weight of the materials used for the construction of the building -- dead loads -- and the live loads from furniture and fixtures and other unattached objects. The loads are cross-referenced with the soil load-bearing capacity, measured in pressure per square foot, to calculate the footing size.

Footing Size

• Footing size for standard house construction is determined by referring to tables prepared by the Council of American Building Officials, but related construction matters are also considered. A narrow footing is often adequate for bearing the load but won't leave room for the typical 8-inch foundation wall. The standard footing size is 16 inches square by 8 inches in depth. This allows 4 inches of footing on each side of the wall, leaving room for the formwork for the walls and some leeway for adjustments.