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 drainage.
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.
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.
