03/02/2025
Spotted in the wild yesterday—a wood foundation! Otherwise know as a permanent wood foundation (PWF). I have only ever read about these—never have I actually seen one.
They are built from pressure treated 2x8 framed walls sheathed with pressure treaded, heavy-duty below-grade contact rated plywood. A poly sheeting barrier is applied to the exterior of the ply—to give more water resistance to the wall. There will also be a good gravel (capillary layer) backfilled against the walls to create a path of least resistance for water. This water is captured at the base of the wall via drain tile—and is gravity fed into a sump pit. These are common details on a concrete foundation as well.
The upside of PWF is the speed at which you can build. There’s no waiting for concrete to properly cure. You can begin setting first floor joists immediately. The floor joists are also part of the foundation assembly—as the floor system is engineered to help the foundation walls resist soil pressure. PWF’s also eliminate a lot of concrete—which we know has a ton of carbon inputs.
This house was built in 1992 and after 33 years—I saw zero signs anywhere of structural failure, water intrusion or foundation failure. Nor was there a musty/mildewy smell. It was pretty cool coming across one of these—as there may only be around a half million in existence in the US.
