Treated Wood Foundations

Topic Summary

Treated wood foundations are built on most types of soils (Group I - III), with the exception of unsatisfactory soils (Group IV) as defined by the U.S. Department of Agriculture. Similar to conventional foundation systems, sites should be cleared of organic material and top soil prior to the onset of work. The basement or crawl space is then excavated, and a footing is placed on undisturbed soil below the frost line. The footing under a treated wood foundation wall can be made of poured concrete or a wood footing plate on top of granular fill. The size of the footing is engineered based on the loads of the foundation walls. Adequate, unobstructed drainage around a treated wood foundation is achieved by having continuous granular fill on the sides of the building and underneath the structure. Granular material can be up to 1/2 inch crushed stone, 3/4 inch gravel or 1/16 inch sand depending on the where it is intended to be used. All granular material should be clean and free of silt, clay and organic material, and be covered with a 6-mil polyethylene sheeting. Sump pumps, perimeter drains, and dry wells may be used under the basement or crawl space floor and around the exterior to promote drainage. Storm water control, including gutters, downspouts, splash blocks and drainpipes, in addition to a finish grade surface that slopes 1/2 inch, is also important to direct water away from the structure.

The materials used to construct treated wood foundations include: plywood, preservative treated lumber, fasteners, termite protection, a moisture barrier, sealing, and insulation. The recommended plywood is APA-rated, all-veneer sheathing. Plywood must be attached to framing lumber on the exterior walls in 1/2 inch or 5/8 inch thicknesses, then wrapped with 6-mil polyethylene sheeting. Framing lumber can be fir or pine species and can be used as wall studs, footing plates, bottom plates and top plates. Lumber sizes are determined by the house size, number of stories, structural loads and spacing of members. The footing plate rests on top of granular material at the base of the foundation wall and can be 2x6, 2x8 or 2x10. Stud and plate size can be 2x4, 2x6 or 2x8. Studs are installed vertically at 16 or 24 inches on center. The bottom plate is installed horizontally along the top of the footing plate or concrete footing. The top plate is also installed horizontally along the top of the studs. Similar to the floors and walls above, the foundation walls are erected, fastened, sheathed and braced. As with balloon framing, a wood floor is constructed after the walls are erected. Wood floors can be a substitute for a concrete slab floor, but both require a 6-mil polyethylene vapor retarder above the granular fill and below the floor structure. Wood floors are constructed with sleepers along granular fill, and joists run in the opposite direction along sleepers, with wood blocking at the edges fastened to wall framing. A plywood sub-floor is fastened to the top of joists.

All framing lumber below grade in a treated wood foundation is required to be pressure treated with preservatives in accordance with the American Wood Preservers' Association (AWPA) Standard C22, "Lumber and Plywood for Permanent Wood Foundations - Preservative Treatment by Pressure Processes." Chromated Copper Arsenate (CCA) is a wood preservative that has been used since the 1940s; however, it contains arsenic and is restricted by the United States Environmental Protection Agency (EPA) for use in all residential applications today. Alternative wood preservatives include Acid Copper Chromate (ACC), Alkaline Copper Quat (ACQ), Ammoniacal Copper Citrate (CC), and Copper Azole (CA). An important consideration when building a treated wood foundation is the metal used in the fasteners. Due to the high levels of copper in wood preservatives, fasteners are required to be a corrosion-resistant. Type 304 or 316 stainless steel nails are recommended.

An essential part of the treated wood foundation system is moisture and termite protection. In addition to the 6-mil polyethylene sheet wrapped around the exterior plywood sheathing, all sheathing joints must be caulked with a high-performance acrylic latex or polyurethane caulk. Termite protection includes placing a vapor retarder under the basement floor, which will reduce the moisture that attracts termites. The use of preservative treated wood eliminates termites from the basement or crawl space framing, because termites can not penetrate the wood; however, additional steps should be taken to protect untreated floors above.

During construction the job site should be properly cleaned to remove all wood debris. Clearance between grade and wood on the interior of the crawl space and the exterior of a building is essential to allow inspection for termites. If clearances, as defined in "Termite Protection for Wood-Framed Construction," by the APA, are not able to be achieved, then the first floor structure and sub-floor should be pressure preservative-treated lumber as well. In addition, termiticide soil treatment can be used where termite hazards exist.

Several advantages make treated wood foundations an attractive option for a light frame structure foundation. The construction of a treated wood foundation is simple for a framing crew to erect, in comparison to concrete foundations. Having the framing crew construct the treated wood foundation work allows for cost savings as a result of a simplified and expedited schedule, since the schedule doesn't require other trades to be involved.

The treated wood foundation also offers advantages over concrete foundations such as the ability to install insulation between studs, which offers a higher R-Value in comparison to uninsulated concrete foundation walls, leading to greater energy efficiency. Furthermore, finishing the basement space does not require additional construction on the interior surface, since drywall can be fastened to the foundation wall studs, and plumbing, wiring, and heating/cooling can be installed within the wall cavity.

American National Standards Institute (ANSI) and American Forest & Paper Association (AFPA) standard, PWF 2007, "Permanent Wood Foundation Design Specification" has been adopted for reference in the 2009 International Residential Code.

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