Concrete Wall Dimensions
By code, foundation walls must always extend above the finished grade at least 6 in. However, 18 in. is recommended to prevent splashback from deteriorating wood siding (see Board Siding). The height above grade may be lowered to 4 in. if a masonry cladding, such as brick veneer, is used.
Maximum heights of foundation walls should be based on the required wall thickness and reinforcement required, as specified below.
Concrete Wall Thickness
The design thickness of foundation walls depends on soil conditions, the amount of reinforcement in the wall, the height of the wall, and the height of unbalanced fill. Unbalanced fill refers to the difference in ground-level height between the inside and outside of the foundation wall.
While code will allow plain, or unreinforced, concrete and masonry block foundation walls (refer to Figure R404.1.1 (1), p. 67 of the 2000 International Residential Code), this is not recommended. Code sets minimums for safety, not necessarily for quality. To avoid the callback nightmare of a cracked foundation, all foundations should include reinforcing steel. An exception can be made for a plain concrete or masonry foundation wall less than 5 ft. tall with less than 4 ft. of unbalanced fill. In this case, use minimum 4,000-psi concrete (see Specifying Ready-Mix).
Typically, a foundation wall should be thicker than the wall it supports. An 8-in.-thick wall is standard for supporting wood-framed walls, but only if the unbalanced fill height is less than 6 ft., the foundation is located on a well-drained site, and the foundation is properly reinforced.
Where poor soils exist, in seismic zones, and on hillside sites, a thicker 10- or 12-in. poured concrete wall may be required. In each of these cases, an engineer’s review is recommended and may be required by local code.
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