Footings serve to spread the load onto the soil for adequate bearing, and to provide a smooth, level surface for wall forms or for block and mortar work. Footings also resist lateral soil pressure at the base of foundation walls.
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Footing Layout
Footings can be larger than required dimensions, but should not be smaller. Footings should be straight and level to within 1/4 in. over 20 ft., and should be no more than 1/2-in. out of square in 20 ft.
Footing Width
The bearing width of footings varies according to soil strengths and loading conditions (below).
Figure: Minimum Width of Concrete or Masonry Footings (in.)
The width of a foundation footing is based on the loadbearing value of the soil. Load-values assume undisturbed, native soils of known type or tested values of compacted soils.
Footing Tie-In
Footings should be struck off level, but should never be troweled smooth. In addition, footings must be tied firmly to the wall above using either a keyway poured into the footing or reinforcing bar projecting from the footing into the concrete (below).
Figure: Footing Tie-In
To tie a footing to the wall above, a keyway should be about 1 in. deep by 2 in. wide. When rebar is used, the typical rebar layout is one #4 bar every 3 ft. o.c.
Footing Thickness
Footing proportions for plain concrete (unreinforced) footings are set by code (Footing Dimensions, left and center). Generally, footings should be at least as deep as the thickness of the wall they support. The wall should be centered on the footing so that the projection of the footing on each side equals half the wall or footing thickness.
If a wider footing is required, the footing must be reinforced (Footing Dimensions, right). Typically, 1/2-in. or 5/8-in. rebar will be required on 1-ft. centers, set about 3 in. up from the bottom of the footing.
Figure: Footing Dimensions
If a particular soil bearing capacity requires a wider footing, code allows the footing projection to increase, but the footing thickness must also be increased so that the footing depth equals the distance it projects from the wall (center). An alternative is to reinforce the wider footing (right), if local code allows.
Frost Depth
The footing should be placed at least 1 ft. below the frost line so it will not heave when the soil freezes. Frost depth varies depending on local climate; check with local codes for the local frost depth. On a hillside foundation (such as a basement walkout), plan on building a frostwall on the downslope side (below).
Figure: Frostwall for Basement Walkout
On a hillside foundation, a frostwall should be added on the downslope side to extend the footing below the frostline.
As an alternative to building a frostwall, a walk-out basement may be insulated to protect against frost damage (see Insulation for Walkout Basements)
