Natural Spaces now offers pre-assembled foundation wall sections utilizing high quality ACQ or
CCA pressure treated “FDN” (foundation) grade stamped stud and plate lumber. We design well above the minimum design specs stated in the American Plywood Assoc. PWF Design Manual. We offer an option for ACQ-treated stud and plate lumber, which is considered to be environmentally safe. ACQ is an entirely different preservative process from CCA. Ask for our brochure on ACQ.
All of the lumber below ground is a special “foundation grade” treatment that is a much heavier treatment than the regular treated lumber you find for use on decks and railings above grade.
Our standard structural wall unit uses a 2X8 treated stud spaced 12″ or 16″ o.c. The structural sheathing is 1/2″, 5/8″ or 3/4″ treated plywood. Stainless steel nails are used throughout. Backfill boards, concrete slab screed boards and 2 X10 footing plates are included in our PWF system. Stuff the competition forgets to tell you. Steel brackets, braces, bolts, screws, etc., must be either triple zinc plated, painted with Rustoleum, or must be stainless steel.
Because of the custom nature of each dome and each site, we can only give an approximate cost. Specific quotations will be given when specific plans are produced and a specific site is selected.
Wood Foundation Walls: 8′ – 4 1/2″ high wall. 2×8 studs @ 16″ o.c Basic wall system without extensions, 3′ – 6″ in ground.
The details behind the permanent treated wood foundation systems:
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 400,000 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 forms the floor within the foundation. Typically, walls are framed with 2×8 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 a heavy membrane waterproofing 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.
Some people may be skeptical about the long-term durability or strength of wood foundations. However, accelerated aging tests, and use for over 50 years attest to the durability of this system. Permanent wood foundations for residences have been constructed for decades in the U. S.
Natural Spaces Domes first started building treated wood foundation systems in 1974. These early domes have been remodeled over the years, allowing inspection of the treated wood foundation. No deterioration has been found.
On any foundation, it is the waterproofing material that will keep it dry. We do not suggest the use of black tar – the real cheap stuff – as a waterproofing. It will dry out in 5-8 years, cracking, pealing, letting water seep into your foundation wall. Tar is only considered a damp proofing – it is not a water proofing material. 6 mil PVC will also dry out, become brittle with age and disintegrate. ANY portion of the 6 mil PVC exposed to sunlight will breakup within one year.
The premium long-term waterproofing recommended is the membrane system. The Grace Bituthene Waterproofing Membrane System is a self-adhesive, cold-applied sheet membrane consisting of a top surface of high strength, 4 mil. cross laminated, high density polyethylene film adhered to 56 mils of rubberized asphalt. Rubberized asphalt promotes full adhesion to the substrate while simultaneously providing crack-bridging and crack-cycling properties. This 60 mil thick membrane is supplied in 36″ wide rolls 66.7′ long inter-wound with a disposable coated paper release sheet.
All corner wall joints in the permanent wood foundation walls receive an extra layer of the membrane. Strips 12″ wide are applied vertically over each corner before the final layer of Grace Bituthene is applied. Corner joints should be filled with mastic first. If backfilling material includes rocks with sharp edges, a protection fabric over the membrane should be used before backfilling or install 3/4” or more rigid insulation board.
U.S. Code Acceptance
Permanent wood foundations are permitted by the 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.
Following excavation, a base layer of crushed stone is placed and leveled. Panelized foundation sections can then be on a 2×10 or 2×12 treated wood footing plate. 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″ or 12″ centers, sheathed with ½” or 5/8″ treated plywood.
Depending on the location and soil conditions, a sump pit, drain piping, and appropriate vapor barriers are recommended. Exterior plywood surfaces are covered with 45 to 60 mil membrane waterproofing prior to backfilling. The Southern Pine Council publishes a useful Permanent Wood Foundation Design Manual, which has detailed construction drawings and photos. Natural Spaces Domes specialize in wood foundations and provides 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.
Foundation Waterproofing System
Grace Bituthene waterproofing membrane
We use Bituthene waterproofing on all our Permanent Wood Foundation Systems. We also use it on some low slope roof areas not subject to heat buildup by the sun. Bituthene is a self-adhesive, cold-applied sheet membrane consisting of a top surface of high strength, 4 mil. cross laminated, high density polyethylene film adhered to 56 mils of rubberized asphalt. Rubberized asphalt promotes full adhesion to the substrate while simultaneously providing crack-bridging and crack-cycling properties. This 60 mil thick membrane is supplied in 36″ wide rolls 66.7′ long inter-wound with a disposable coated paper release sheet. Thermal stability is up to 135°F (57°C)
Grace Bituthene – 36″ x 66.7′ long (1 roll/box): $180 per roll