- Sitework
- Concrete
- Footings
- Anchors
- Structural Slabs
- Poured Walls
- Concrete Block
- Pier Foundations
- Insulated Foundation Systems
- Waterproofing and Dampproofing
- Retaining Walls
- Drainage
- Radon Abatement
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.
Reinforcing Concrete Walls
Follow general guidelines for reinforcing foundations (see Rebar).
For appropriate sizing and spacing of rebar in concrete foundations, follow the minimum code requirements shown in Figures in Sizing and Spacing in Walls.
Rebar should always be placed near the tension side of the concrete (see Placing Foundation Rebar). On a full-height foundation wall, which is held in place by floor framing at the top and by the footing at the bottom, the tension side is toward the inside (Figure: Rebar for Poured Foundation Walls in Placing Foundation Rebar)
Horizontal rebar is most effective along the top and bottom of the foundation elevation.
Stepped Foundation Walls
Foundation walls on sloped sites are often built as a half-wall with a wood-frame cripple wall (Figure A). In a half-height foundation wall, the hinge joint between the concrete and the cripple, or pony, wall creates a structural weakness. The half-height portion of the wall must be treated as a retaining wall, and reinforced, buttressed, or otherwise supported to resist the lateral loading of unbalanced soil (see Retaining Walls)
Figure A: Full-Height vs. Half-Height Foundation Walls
Stepped Foundation Corners
When any foundation wall turns a corner, maintain the height for a distance equal to half the height of the higher wall before stepping it down (Figure B).
Figure B: Stepped Foundation Corners
Structural Bracing
Always frame the floor deck before placing backfill. If this is not possible, install temporary bracing before placing backfill (see Bracing Before Backfill).
Where soils impose an excessive lateral load, additional intermediate structural support for foundation walls can be provided by the cast foundation elements (Figure C).
Figure C: Cast-In Bracing for Foundation Walls
Control Joints for Concrete Walls
Some surface cracking is inevitable in the face of concrete walls. Rebar and wire mesh are placed to keep cracks from widening but will not prevent them from occurring. If minor, random cracking is unacceptable, use control joints to confine cracks to intended locations.
Place control joints no more than 20 ft. apart, and within 10 ft. from corners. Locate control joints at natural points of weakness, such as door and window openings, corners, and changes in elevation or section (Figure D). In a stepped foundation, control joints should also be placed along a vertical in line with the point where the foundation changes elevations.
Figure D: Control Joints for Concrete Walls and Slabs
Bracing Before Backfill
Backfill should be placed only after the floor framing has been completed to brace the top of the foundation wall against the backfill load. If this is not possible, provide temporary foundation bracing, as shown in Figure E.
Figure E: Temporary Foundation Bracing
Placing Backfill
Careful backfilling helps prevent foundation damage and soil settlement. Backfill only with well-draining granular fill (see Drainage).
Minimum Backfill Amount
Place the granular backfill in the zone from footing to near grade, widening to a minimum 30-degree angle from the footing up (Figure F).
Figure F: Backfilling Foundation Walls
Foundation Backfill Checklist
- Use coarse granular material. Sandy or gravelly soils make the best backfill.
- Compact the backfill. Place and compact the backfill in 6-in. lifts. Use hand compactors near the foundation, not machinery (see Compacting Soil). Uniform-graded gravel is largely self-compacting, but sandy, silty, or clay soils will settle if not compacted.
- Slope final grade away from foundation. A minimum pitch of 6 in. in 10 ft. will direct surface water away from the foundation. For added insurance against settling, build final grade up deeper and at a steeper slope so it will still slope away from foundation after settlement. For additional drainage details, see Drainage.