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.
Details
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 or 5/8 inch 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.
For many years, CCA (chromated copper arsenate) was the predominant
chemical used for pressure treatment. However in early 2002,
manufacturers announced that preservatives containing arsenic and
chromium would be phased out of the residential market. For residential
uses including foundations, CCA products will be replaced by chemical
formulas such as Ammoniacal Copper Quat (ACQ-A, B, C), Amine Copper
Quat (ACQ-D) and Copper Azole-Type A (CBA-A). For foundation systems,
wood is typically treated with a higher level of preservative content
(0.6 lb. per cubic foot instead of 0.4 lb. per cubic foot for wood in
soil contact), so it is critical to use only materials rated for
foundation applications.
Installation
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 or 5/8 inch 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 mill 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.
Benefits/Costs
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.
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. Companies that offer system
design and/or panelization may provide transferable warranties for up
to 75 years.
Limitations
Some people may be skeptical about the long-term durability or strength
of wood foundations. However, accelerated aging tests, and use for over
40 years attest to the durability of this system. Permanent wood
foundations for residences have been constructed for decades in the U.
S.
Preservative: treated lumber manufactured prior to 2002 and shortly
thereafter, is likely to contain chromated copper arsenates (CCA). Some
groups have expressed concern about the toxicity of chromium and
arsenates, although there is no scientific consensus on potential
risks.
Code/Regulatory
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.
Courtesy of the U.S. Department of Housing and Urban
Development (HUD/FHA), There are no warranties, either expressed or implied,
regarding the accuracy or completeness of this information.
