What Is Under Your Oregon Floor: Fir, Plywood or Slab
Ten minutes with a pry bar in a closet will tell you more about your flooring options than an afternoon in a showroom, and in a prewar Oregon house it may find you a floor you already own.
Why this is the first question here
Oregon’s housing splits neatly by era, and the eras split by what is under the finish floor. Statewide, 10.7% of homes predate 1940 and 25.8% were built in 2000 or later — but those shares are not spread evenly. Portland is 26.8% pre-1940. Bend is 4.2%, with 52.0% built this century. Those two cities are not the same flooring problem and the difference starts below the surface.
Era one: board subfloor, and often a fir floor on top of it
In a house built here before roughly 1950, the subfloor is usually solid boards laid diagonally across the joists, and the finish floor above it is very often 3-1/4 inch vertical-grain Douglas fir. Oregon supplied the softwood that built the West and used it at home; oak was the imported option.
Douglas fir rates about 660 on the Janka hardness scale against red oak’s 1,290, which makes it sound fragile. The fir in a 1912 Portland house is not the fir sold today: it came from trees that had grown for centuries, the rings are packed tight, and tight-ringed old-growth fir is considerably denser than the fast-grown product with the same name. A hundred-year-old fir floor usually deserves better than the hardness table suggests.
What it means: refinishing is on the table, at roughly a third of the cost of replacing, and it wants an installer who has actually sanded fir before. A board subfloor also has gaps and squeaks that plywood does not, so a new floor laid over it may want a plywood layer added first.
Era two: plywood, and few surprises
From the fifties onward the board subfloor gives way to plywood sheet, and later to OSB. Joist spacing standardizes, the surface is flat and continuous, and there is nothing underneath worth rescuing. This covers most of Salem, Eugene, Springfield, Gresham and Beaverton, and it is the easiest starting point on this site.
What it means: straightforward. Check flatness with a straightedge, check for soft spots, and choose the material on how the room is used rather than on what the building will tolerate.
Era three: slab, and a moisture test
Slab-on-grade is common in Oregon’s newer housing, especially east of the Cascades and in the newer valley subdivisions, and it comes with a rule the wood-framed eras do not have: concrete gives up moisture for months after it looks and feels dry, and a floor bonded to a slab that is still releasing it will fail.
What it means: a slab needs testing, not guessing, before anything is glued to it. The relevant reading is a relative humidity probe placed in the slab rather than a surface test, and the flooring manufacturer will specify the limit. On a slab of unknown history, a floating floor over a vapor retarder sidesteps the question entirely, which is a large part of why floating floors dominate new construction.
How to find out, in ten minutes
- Pick a closet or an inconspicuous corner behind a door.
- Lift the carpet edge off the tack strip with pliers and fold it back. Vinyl usually peels at a seam.
- Look at what is under it: wide diagonal boards, plywood sheet, or concrete.
- If it is wood and it looks like a finished floor rather than a subfloor — tongue and groove, roughly 3 inches wide, a visible grain — you may have found an original fir floor. Take a photograph and show it to whoever quotes.
The daylight basement, which is its own case
Common across the west side of the state: a lower floor that is partly below grade, on a slab, with a heated room above it. It combines the slab moisture question with a temperature difference across the assembly. The answer is usually a floating floor over a vapor retarder rather than anything bonded, and carpet only over a slab that has been tested.