Why wood foundations are durable
If you're considering a wood basement, don't worry about durability and strength. If in doubt, it's best to get an
opinion from someone who's built a few dozen and had
to guarantee their performance—me! I'm positively
gleeful over the 20 trouble-free years we've had with
ours at home and have yet to hear a single complaint
from any of the dozens of customers I've built them for.
The clients I still hear from love the advantages wood
foundations have to offer. You can build them yourself,
they're dry and warm, and they're easy to finish (inside
and out). And because they're simple wood frame construction,
they're easy to alter even after the basement's
in. In fact, I added six windows and doors in my walkout
basement three years after the house was finished.
You won't find too many unhappy wood foundation
owners as long as the installation is done properly. Successful
wood foundations depend on using the proper
wood, good drainage and waterproofing. Most people
considering a wood foundation are concerned about
rot and strength. “Foundation-grade” wood used in
foundations is treated by steam-impregnating it with a
chemical called CCA (chromated copper arsenate) at a
concentration of at least .6 lbs. of chemical per cubic
foot of wood so that the chemical penetrates deep into
the core of the wood. The copper part of the compound
is toxic to fungus, mold and bacteria, while the arsenate
is toxic to pests like carpenter ants and termites.
Long-term durability isn't an issue with a sound
wood foundation. That's been well documented by U.S.
Dept. of Agriculture's Forest Service testing. Proper
construction techniques are everything. When wood
foundations fail, it's always a case of poor construction
techniques. The key elements are properly sized
sheathing and framing, secure floor tie-ins and proper
Strength is a matter of engineering
Strength is simply a matter of following the guidelines
laid out in the wood foundation manual put out by the Southern Pine Council. Framing member widths and
spacing as well as sheathing thickness depend mostly
on backfill height. The deeper into the ground, the
stronger the wall needs to be. In our example, 2x8s are
spaced every 16 in. with 5/8-in. sheathing, with the wall
resting on 2x10s, which in turn rest on an 8-in. thick
gravel footing. The concrete floor resists ground pressure
at the bottom, while the top is anchored to the
floor system above with joist hangers and clips. This
design is acceptable for most normal-height backfill
scenarios. Any and all water will filter through the
gravel backfill, footings and under-slab fill where it's
collected by a perforated sump basket for pumping
away from the house, or drained to daylight if your
home's on a hill.
Hire a wood foundation built and the cost is roughly
equivalent to concrete block and a little cheaper than
poured walls. If you build it yourself, you'll get it for
half the price of a conventionally built masonry basement.
But keep in mind that finishing is much easier
and cheaper because you're finishing a stud wall.
If you plan to install a wood foundation, get help with the design and
construction first by ordering a booklet from The Southern Pine Council
called “Permanent Wood Foundations.” Search “Southern Pine Council” for more information.
Wood Foundation Details
Wood foundations require foundation grade pressure treated wood, studs sized for soil pressure, the proper fasteners, solid gravel footings, damp-proofing and good drainage.