What are footings and foundations

Footings are below ground concrete structures that support and transmit the weight of an aboveground structure or a foundation to the soil. They must be below the frost line.

The building’s weight and the soil’s load-carrying capacity determine the footing’s size. For one-story structures and outbuildings, the rough formula is: a footing’s width is twice the thickness of the foundation wall; a footing’s thickness is the same as that of the foundation wall. How to build footings and foundations is specified by local building code; check with your municipality’s building department.

Forms and uses

Common types of footings are flat top, for a foundation wall or a slab; keyed, for anchoring concrete or masonry walls; and, for house additions, pier footings, on top of which are piers, posts, or blocks connected by rods. If the structure to rest on the footing will be of concrete blocks or poured concrete, footings must have connectors (steel rods or a keyed channel).
Excavate a trench to the depth required by the local building code and 16 to 20 inches wide-wider if it’s so deep you must get into it to work. If you intend to pour directly into the excavation, level the bottom so that the footing’s weight will be distributed evenly on undisturbed solid soil.

All footings require rebars (steel reinforcing rods). They are generally available in 20-foot lengths; you may
have to splice several with wire for the length you need. Before pouring, set the rebars on 22/3-inch thick bricks.
Build flat-top formwork using the same preparation, tools, and materials as for a basic slab. For a keyed footing, start at the center of the long forms and mark every 32 to 48 inches for crosswise 1 x 3 or 2 x 4 lateral braces. Then center and nail an oiled 2 x 4 perpendicular to these braces; nail the braces to the forms.

Pier forms themselves can be used as footings in areas where there is no frost. Buy premade piers at a lumberyard. For a job that will have footings connected to piers, posts, or columns, pour concrete in two stages: first, the footing, into which you place centered steel rods; then, 24 hours later, the pier or a sono-tube column.