Footings provide a stabilising base on which a building's foundation can be built. The minimum width measurement of concrete footings is based on the load the soil can accept and the load it must support. A narrower footing can be used on soil with a high load-bearing value. The minimum dimension of the concrete footing increases with the weight of the building. Local building codes also can specify minimum dimensions.
Minimum Footing Width
Footing projections for foundation walls need to extend at least 2 inches past the edge of the wall. For a 6-inch wall, the minimum width of the footing will be 10 inches wide to meet the width of the wall and extend 2 inches past it on each side. Foundation walls, such as those in basements, require more support. "Masonry and Concrete" by Christine Beall recommends "the footing width should be a minimum of two times the thickness of the foundation wall it supports." If building codes and design recommendations give two possible footing widths, the greater measurement should be used in construction. Multistory buildings will have wider footings than single-story buildings. To support heavy structures like fireplaces, the footing will extend several inches to several feet beyond the foundation.
Other Footing Dimensions
Concrete footings must be at least 6 inches thick. Footing depth will reach compacted earth or bedrock or extend at least below the frost line. Footings must also be at least 12 inches below the top of the soil. They can, however, go deeper. According to "Architectural Graphic Standards" by Andy Pressman of the American Institute of Architects, "the bottom surface of the footing should not exceed a slope of 1 in 10." Footings can also be used to support stairs or house level changes. According to "Understanding Construction Drawings" by Mark W. Huth, "steps in footings are usually in increments of 8 inches to conform to standard concrete block sizes."
After the foundation is poured, the footing is attached to the foundation. This can be done using standee, loops of steel rebar that stick out of the footing, or with steel plates called footing sills. Steel bolts are then used to attach the foundation to the footing. "Architectural Drafting and Design" by Alan Jefferis and David A. Madsen states "all footing sills must have full bearing on the footing or slab and should be bolted to the foundation with 1/2 x 10" bolts embedded at least 7" into the concrete or reinforced masonry."
Pad footings are a rectangular concrete footing poured at the base of the foundation. Find the centerline length of the footing to use in the volume calculation. The volume of concrete in the footing is found by multiplying the footing length by the width by the depth. This is the volume of concrete within the footing in cubic feet. Divide the volume by 27 to convert it to cubic yards.
- "Architectural Drafting and Design"; Alan Jefferis, David A. Madsen; 2004
- "Understanding Construction Drawings"; Mark W. Huth; 2009
- "Estimating for Residential Construction"; David J. Pratt; 2011
- "Masonry and Concrete"; Christine Beall; 2000
- "How to Build Your Own Greenhouse"; Roger Marshall; 2006
- "Architectural Graphic Standards"; Andy Pressman, American Institute of Architects; 2007