Pressure Treated Wood Foundations - 6/23/2003 - Home Foundation Structure Framing

Pressure Treated Wood Foundations

Question:
I am considering buying a house that was built five years ago with a pressure-treated plywood foundation. I am interested in a short rundown on pressure treated wood foundations. What are the plusses and minuses, particular items to look at during a home inspection, etc.?

Answer:

Description: A pressure-treated plywood foundation, typically referred to as "Permanent Wood Foundation" or simply a wood foundation, is a foundation wall type constructed of pressure-treated 2x6 or 2x8 wood studs and pressure-treated plywood. The wood preservative is similar to that used for decks, but has a higher level of preservative content (0.6 lb. per cubic foot instead of 0.4 lb. per cubic foot). The use of this preservative, in combination with proper exterior waterproofing and drainage, prevents fungal decay. There is not enough experience with these foundations yet to know if they will eventually suffer from decay, but at least one manufacturer is offering a 75 year warranty. Wood foundations may be used for full basements (finished or unfinished), crawl space walls, stem walls below slab edges, and floors below grade.

wood foundations can provide several advantages including:

• reduce construction time and construction in any weather;
• ability for carpentry crews to build the foundation, then move directly to framing the rest of the home;
• energy efficiency -- stud cavities can be easily insulated;
• dry basements;
• absence of cracking; and
• ease of finishing-off the interior -- drywall can simply be applied to the studs.

These types of foundations vary in cost--they may be more or less expensive than conventional foundation types.

Inspecting: When looking at a house with a wood foundation become familiar with the design practices for wood foundations or hire an engineer or home inspector who is. Be sure all requirements were followed faithfully by the builder. Understand that many details will be concealed.

Key items to check include:

• sizing, grade, and spacing of wood framing members, and grade and thickness of sheathing materials (typical basement walls use 2x6 or 2x8 studs at 16" or 12" on center, and 5/8" thick plywood.);
• proper preservative treatment of wood members, as indicated by the treater’s stamp;
• use of proper corrosion-resistant fasteners;
• exterior polyethylene water barrier (protected above grade), proper backfill, and proper drainage;
• floor slab poured against bottom plate of wall;
• Proper fastening of walls to floor system.
• signs of excessive bowing or cracking of studs, or of movement of top of wall; and
• signs of water entry.

Permanent Wood Foundations: Design and Construction Guide is available from the Southern Forest Products Association (SFPA) website or by calling 504-443-4464.