Pressure-treated wood foundations (also known as permanent wood foundations, or PWFs) have been around at least since the 1940s, and my own experience with them dates back 20 years or so. During my first year on my own as a framing contractor, I was framing homes for a large builder who used wood foundations on many of his projects. Although I was confident of my ability to frame the job from the foundation up, my experience with PWFs was limited. Fortunately, the builder was good enough to assign one of his own experienced carpenters to help me out on my first foundation.
That job went well, and today I still use variations of that same design. I've tweaked some elements, but the overall process has changed very little.
Benefits of Wood
For me as a builder, a key benefit of the wood foundation is that it keeps my guys working in lean times. I employ carpenters, and with a crew trained in foundation installation I can keep that part of construction in-house, which helps with scheduling and quality control. It also means one less sub on the job, which saves money. The trades are ready to go as soon as the walls are built, and insulation and drywall are ready with no additional framing required.
The idea of wood foundations might initially make some builders uneasy, but I'm very comfortable with them. Like any foundation, a PWF can be done wrong, but if that happens it's probably the fault of the builder, not the foundation system. I've stayed in touch with many former customers, and I know that the wood foundations I built 15 years ago are as good as ever. For me, that's convincing evidence that the approach works. The builder I learned from years ago is still using them, too.
Choose your foundation. That said, I don't use wood foundations everywhere. I work in more than 40 Michigan counties, and I use conventional concrete foundations more often than not. Although the cost of a PWF is comparable to that of a poured foundation, it's a better value if the basement will be used as living space, thanks to the ease with which a wood foundation can be insulated and finished. PWFs are especially good on sloping lots where there's a walkout area toward the front of the structure.
You can't use just any green-tinted lumber for a PWF. Lumber and plywood meant for use in foundations are engineered specifically for that application, and each piece should be clearly marked as such. The grade stamp should also specify a preservative retention of .60, indicating that each cubic foot of material contains .60 pound of preservative. Southern yellow pine is the most commonly used species in PWFs because it accepts preservatives readily. Pressure-treated dimension lumber follows the same grading system as ordinary framing lumber. We ordinarily use a No. 2 grade for foundation framing.
Conveniently for us, wood foundations are well-known in our work area. The salespeople at the yards we deal with understand our requirements and have never failed to send us the right material.
Health and the environment. Foundation-grade PT lumber is treated with chromated copper arsenate, or CCA. Although CCA-treated lumber was widely used for decades, the EPA sharply restricted its use in 2004 after concerns were raised about its possible toxicity. (CCA is still widely and legally used in marine, agricultural, and industrial construction, however.) One of its few remaining uses in residential construction is for permanent wood foundations.
I take common-sense precautions when working with CCA-treated lumber and have never experienced any ill effects from exposure to it. Since the preservative is said to resist leaching into the soil, I don't think there's anything to worry about once the treated lumber is completely enclosed in a wall.
Most of my customers seem to agree, because I've never had anyone express any concern about this issue - including the owner of the home shown on these pages, who is an environmentally conscious organic farmer. From an environmental standpoint, in fact, a wood foundation is arguably a much "greener" choice than concrete, which takes enormous amounts of energy to manufacture.
The right fasteners. For long life, all fastenings used in a PWF must be stainless steel - hot-dipped fasteners won't cut it here. Stainless fasteners are available to fit both coil and stick nailers and cost about 50 percent to 60 percent more than conventional fasteners.
The code does permit us to use regular galvanized framing connectors, though, and we've never had any problems with them. There are probably two reasons they're allowed: First, the connectors are on the dry side of the wall, so there's no moisture present to cause rust or corrosion. Second, CCA-treated lumber seems to have a fairly low potential for corroding metal - most problems with damaged hangers and fasteners are associated with the newer lumber treatments that have come along since CCA was phased out for general use.
The bible for wood foundation design is the "PWF Design and Construction Guide," available as a free download from the Southern Pine Council (SPC) at southernpine.com. In our area, code officials stick pretty much to the guide, because the state building code doesn't cover the subject in any detail. Its tables and construction details let you choose the right fastening schedule, lumber dimensions, and stud spacing for most common situations.
The design guide typically calls for deeper and more closely spaced framing members than you'd find in an above-grade wall. For example, a 9-foot basement I built recently - 2 feet of which was above grade - was framed with 2x8s spaced 16 inches on-center and sheathed with 3„4-inch foundation-grade plywood. Foundations that will be backfilled to a depth of 86 inches or less can be designed with the prescriptive specs in the guide, but deeper walls will require an engineer's stamp.
Load paths. A key difference between a wood and a concrete foundation has to do with how each accepts point loads: In a concrete foundation, a point load that falls within a wall - from a structural ridge, for example - can be transmitted to the mudsill wherever the load-bearing vertical member happens to end up. But because a treated wood foundation is essentially just another studwall, the load path in a PWF has to remain continuous all the way to the footing at its base. These loads should be accounted for and called out on the plan to make sure they are properly transferred to the footing. Depending on the load, it may be necessary to beef up the section of footing beneath the point load.
Shear walls. Most buildings that have a front-to-back grade differential of more than 4 feet will need at least one internal shear wall to resist the lateral thrust from the uneven soil pressure. In structures from 24 to 48 feet wide, the shear wall must be located in the middle third of the basement. Buildings more than 48 feet wide will require two shear walls, which must be no farther than 24 feet from one another or from an end wall.
An internal shear wall can often be omitted where the grade differential occurs between the shorter sidewalls, since the long exterior walls that connect them often provide adequate shear resistance in themselves. The SPC design guide provides detailed shear-wall nailing requirements for a range of situations.
Whenever shear walls will be required, it's important to work out their locations during the design stage, because they have a big effect on the quality of the living space. The goal is to come up with a floor plan that allows the shear walls to serve as useful partitions rather than obstructions.