Wood Foundations

Preservative-treated lumber is pressure-infused with chemicals to resist infestation and decay, and the process is so effective that manufacturers are now recommending treated plywood and lumber as an alternate material for crawl space, basement, and stem wall foundations. Using a wood foundation system eliminates the need to cast and cure concrete footings (although a slab floor may be included), and allows construction in any weather by the same crew that completes other wood framing. Wood foundations also resist cracking, and are easy to insulate and finish for additional interior living areas. Over 300,000 U.S. homes have been constructed with wood foundation systems.

Wood foundation systems were first developed in the 1960s, after the development of preservative-treated lumber and plywood allowed wood materials to be used in applications which previously would be subject to decay. Manufacturers that produce preservative-treated lumber, and related associations, have developed procedures and guidelines for constructing wood foundations, while other companies specialize in custom designed or panelized systems.

Because wood foundations are significantly lighter than concrete or block walls, they may be placed on gravel beds, eliminating the need to cast and cure a concrete footing. A concrete slab may form the floor within the foundation, but most manufacturers promote the use of a treated wood floor system. This eliminates concrete work from the schedule altogether, allowing construction to proceed in any weather, with work completed by the same crew that does the rest of the wood framing. Typically, walls are framed with 2x8 treated studs on 16" centers, or as specified by the designers. The exterior foundation surfaces are clad with ½" or 5/8" treated plywood, and polyethylene film is applied before backfilling.

For basements finished as living space, the cavities of wood foundation framing are easily insulated with conventional batts or blown cellulose. Plumbing, wiring, and drywall are installed as for any framed wall. Finishing concrete or block walls might require chase construction, special cutting equipment, furring strips or masonry adhesive. Manufacturers state that wood foundations are also more thermally efficient and elastic, therefore less subject to cracking and moisture penetration.

Permanent wood foundations must be preservative-treated in accordance with American Wood-Preservers' Association (AWPA) Standard C22, "Lumber and Plywood for Permanent Wood Foundations - Preservative Treatment by Pressure Processes."

	By eliminating the need for a concrete manufacturer and materials needed to work with concrete (for piping, electrical, etc.), costs are kept down.
	Wood offers better thermal properties than other foundation types. In addition, batts can be applied in a much simpler method because there is no need to attach special studs or adhesive to the concrete.

Initial Cost  

Manufacturers have estimated costs to be approximately $45 per foot of 8' wall, which includes foundation coating, 6 mil. poly vapor barrier, and hardware. Costs for materials with non-arsenate formulas may be 10 to 20% higher than for CCA treatment.

Operational Cost  

Not Applicable

Permanent wood foundations are permitted by the 2000 International Residential Code (IRC), which specifies fasteners, wood treatment, and gravel or crushed stone footings. Wood foundations are also accepted by most lenders and insurance companies. Codes often refer to American Wood Preservers Association (AWPA) standards. They require certain species of Pine or Fir, limited to 20% heartwood, treated to 0.6 preservative retention (compared to 0.25 to 0.4 for regular treated wood), kiln-dried, and stamped by the treater. The four treatment methods previously approved by AWPA are CCA, ACZA, ACQ Type B, and ACA. However, the Environmental Protection Agency will not permit CCA for residential use after 2003. Check with local code offices to determine the requirements in your area.

Not Applicable

Following excavation, a base layer of gravel, coarse sand, or crushed stone is placed and leveled. Panelized foundation sections can then be set in place, or the foundation can be site framed as for conventional wood walls. Fasteners must be stainless steel or other corrosion resistant material. Typical material dimensions are 2X8 studs on 16" centers, sheathed with ½" or 5/8" treated plywood. When wood floors are used, floor joists are placed on sleeper strips, directly on the aggregate bed. Wood joists also allow the incorporation of plumbing, wiring, and HVAC ductwork into the floor system, prior to the installation of treated floor sheathing.

Depending on the location and soil conditions, a sump pit, drain piping, and appropriate vapor barriers are recommended. Exterior plywood surfaces are covered with minimum 6 mil polyethylene prior to backfilling. The Southern Pine Council publishes a useful Permanent Wood Foundation Design Manual, which has detailed construction drawings and photos. Companies that specialize in wood foundations often provide design and specification services.

Companies that offer system design and/or panelization may provide transferable warranties for up to 75 years.

Wood foundations are easy to construct. The work can be done by framing crews under a wider variety of weather conditions, eliminating the need for a concrete contractor. Remodelers can easily frame foundations for additions, and modifications to existing wood frame foundations are simpler (and therefore less costly) than for block or concrete. Installation of wiring, plumbing, ductwork, insulation, and wall finishing are all simplified, and are accomplished with the same techniques as for other framed wall systems. The elasticity of wood is said to make the foundation structure less susceptible to cracking or shifting, and easier to insulate for improved thermal performance.