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Permanent Wood Foundations

Permanent Wood

Foundations

The Permanent Wood Foundation (PWF) is an innovative building system that saves builders time and creates living areas that enhance a home’s value. Builders and homeowners choose the PWF system for many reasons, including design flexibility, faster construction, and larger, more comfortable living spaces.

But what exactly is a permanent wood foundation? It is a load-bearing foundation wall framed with pressure-treated lumber and sheathed with pressure-treated plywood designed to resist and distribute earth, wind, and seismic loads and stresses that may crack other types of foundations. The walls are designed to withstand backfill and vertical loading, and are suitable for crawlspace, split-entry, or full-basement designs.

They are supported laterally at the top by the floor system, at the bottom by a cast-in-place slab or pressure-treated wood basement floor, and at the base by backfill and foundation footings of crushed stone, gravel, coarse sand, or poured concrete. The footings distribute the vertical load from the structure to the soil.

Permanent wood foundations vs. Wood-frame Exterior Walls

Like conventional wood-frame walls, the wood foundation is adaptable to virtually any design. It fits a variety of floor plans and can be used for both level and sloping sites. There are, however, some differences:

  • Because PWF walls are used in below-grade applications, all lumber and plywood must be pressure-treated with preservatives for decay and termite resistance.
  • It is necessary to use stainless steel nails.
  • You must incorporate an offset footing plate.
  • Framing anchors to connect foundation studs and floor joists to the top plates of foundation walls in high backfill conditions are required.

Engineering Design and Performance

The engineering design analysis for PWFs was developed jointly by the U.S. Department of Agriculture’s Forest Service, the American Wood Council (AWC), and the American Wood Protection Association (AWPA), with field evaluations by the National Association of Home Builders Research Foundation, Inc. The APA-the Engineered Wood Association conducted additional laboratory and in-ground structural testing.

The system’s durability is demonstrated by long-term, in-ground tests conducted by the Forest Service. In these tests, pressure-treated wood has withstood severe decay and termite conditions over decades of exposure.

The PWF is accepted by the major model building codes, by federal agencies, and by lending, home warranty, and fire insurance institutions. And it has been proven by years of success in more than 300,000 homes and other structures throughout the U.S.

Advantages of Permanent Wood Foundations

Builders and homebuyers are increasingly choosing a permanent wood foundation for several reasons:

  • Design Flexibility. PWFs can be used in a variety of building types and sizes, including single and multi-story houses, condominiums, and apartments built onsite or manufactured elsewhere. They can also satisfy special site constraints and be adapted for a variety of soil conditions, including low-bearing capacity soils, expansive soils, or high water tables
  • Simplified Scheduling. A builder’s or subcontractor’s carpentry crews install the PWF, reducing the need to schedule other subcontractors. The PWF can be installed under nearly any weather condition, even below freezing, so the building season is extended. On remote sites, high delivery costs and delays related to concrete setting and form stripping are eliminated.
  • Faster Construction. The PWF is easily installed by a small crew, often in less than a day, or even in just a few hours. As soon as the foundation is framed and sheathed, floor and wall construction can proceed. Shorter construction times mean less need for interim construction financing — and greater productivity.
  • Comfortable Living Areas. PWFs incorporate drainage features that prevent the moisture problems typical of ordinary foundations that can result in warm, dry, below-grade living spaces. They also make it easy to install thick, economical batt-type insulation, which means less heat loss through the foundation wall and greater long-term energy savings.
  • Energy Efficiency. Research has found building an 8-inch basement wall with an R-19 insulation value costs about 30% more with concrete than a PWF. Cement blocks can cost up to 62% more.
  • Ease of Finishing. Since nailable studs are already in place, plumbing, wiring, and interior wall installation are simplified.
  • More Living Space. Wood foundation walls don’t need to be as thick as concrete or masonry walls. Less space is required for insulation, too, because it fits into the cavities of the wood-framed wall instead of furring strips or wall studs. Finally, PWFs are easy to remodel or modify.
  • Radon Gas Resistance. The gravel layer beneath the basement floor serves as a collection system for soil gas, which is easily vented to the outside.

Learn all about the Permanent Wood Foundation system in our PWF Design & Construction Guide. This guide describes the PWF’s features and advantages, in addition to construction tips for a variety of building styles, including both crawl space and full basement foundations.