What's Under Your Floor: Subfloor Types in Colorado Homes
In most of Colorado the honest answer is plywood, and that is genuinely good news. The subfloor question here is less about what the boards are made of than about how dry the air above them gets.
Start with the odds, because in Colorado they are lopsided
Guides written for the Midwest or the Northeast open on the assumption that an older house hides
a plank subfloor. Colorado does not work that way. The housing here is young, and outside two
cities it is very young.
| City | Median year built | Built before 1940 |
|---|---|---|
| Thornton | 1997 | 0.3% |
| Aurora | 1985 | 1.0% |
| Arvada | 1979 | 1.3% |
| Lakewood | 1977 | 2.7% |
| Fort Collins | 1992 | 4.6% |
| Longmont | 1991 | 4.8% |
| Colorado Springs | 1986 | 5.3% |
| Boulder | 1977 | 7.5% |
| Denver | 1975 | 16.9% |
| Pueblo | 1965 | 21.2% |
Read the bottom two rows and then the top four. A plank subfloor is a live possibility in a Baker
or Congress Park bungalow and in older Pueblo; in Thornton it is close to a rounding error. If you
are in one of the newer suburbs, you almost certainly have sheet goods, and almost every material
is available to you without prep.
The three things you are actually likely to find
Plywood (roughly 1950s onward)
Sheet plywood, usually ½ to ¾ inch, over joists. This is what the installation instructions on
the box assume. Flat, continuous, and it accepts any floor covering directly. For the majority of
Colorado houses this is the end of the subfloor conversation.
OSB (roughly 1980s onward)
Oriented strand board, and given the state’s build dates this is extremely common here — a house
from 1997 is far more likely to sit on OSB than on plywood. It performs like plywood while dry. Its
one real weakness is standing water: wet OSB swells at the panel edges and does not shrink back.
That matters in a basement, under a dishwasher, and nowhere else much.
A concrete slab
Slab-on-grade at the main level, or the floor of a finished basement. Not a subfloor problem so
much as a different problem: concrete carries moisture and will hand it to anything you glue or lay
on top. Nailed solid hardwood is off the table without a subfloor built over it first.
And the exception: plank
1×6 boards across the joists, straight or diagonal. Structurally sound, but the boards move
independently, so the surface is neither flat nor continuous. In Colorado this belongs to
pre-war Denver and older Pueblo, not to the state as a whole.
How to find out which you have, in about five minutes
- Pull a floor register. The duct cut goes through the whole assembly, so the
exposed edge shows you the finished floor and the subfloor under it in one look. - Go stand in the basement. If joists are exposed, the underside of the subfloor
is right above your head — individual boards or a sheet product, obvious immediately. - Lift a corner of carpet in a closet. Tack strip comes up easily and nobody
sees the closet. - Check the build year first. It is not proof, but on the numbers above it is a
strong prior. A 1997 Thornton house has sheet goods.
What each floor needs
| Floor | Plywood / OSB | Concrete slab | Plank |
|---|---|---|---|
| Solid hardwood, nailed | Yes | No — needs a built subfloor first | Yes, run across the boards |
| Engineered, floating or glued | Yes | Yes, with a moisture check | Overlay first |
| Luxury vinyl plank | Yes | Yes | Overlay first |
| Laminate | Yes | Yes, with a vapor barrier | Overlay first |
| Tile / stone | Backer board plus a deflection check | Ideal | Overlay plus uncoupling |
| Carpet | Yes | Yes, over pad | Yes |
The Colorado question the subfloor does not answer
Elsewhere, identifying the subfloor tells you most of what you need. Here it tells you less than
the air does. Colorado averages around 30% relative humidity, and indoor air in a heated house in
January routinely drops below 20%. Wood delivered to a national specification arrives too wet for
this state, and it gaps over the first heating season no matter how good the plywood under it is.
So having established you have a sound sheet subfloor, the next number to ask about is moisture
content, not thickness. What altitude and dry air do to a wood floor
is the page that matters more for most Colorado houses than this one.
What prep costs, when you need it
If you are in the minority with a plank subfloor, flattening it runs from roughly $1 a square
foot for an underlayment to $2–$4 for a plywood overlay, on top of the flooring. Over a slab,
building a subfloor is the larger number. Most Colorado houses pay neither.
An estimate, not a quote. These are planning ranges for Colorado, not a quote. What you actually pay depends on your rooms, your subfloor and the material you choose.
Frequently asked questions
My house is from the 1990s. Do I need to check at all?
A quick look through a register is still worth five minutes, mainly to confirm OSB rather than
plywood and to see whether anything has been wet. But you are very unlikely to find a surprise that
changes your material options.
Can I nail solid hardwood over a basement slab?
Not directly. You would build a subfloor over the concrete first — sleepers or a floating panel
system — which costs height and money. Engineered or vinyl plank over the slab is the usual answer
in a Colorado lower level.
What is the difference between the subfloor and the underlayment?
The subfloor is structural and spans the joists. The underlayment is a thin layer on top whose
only job is to give the finished floor something smooth and sound to sit on.
Is OSB worse than plywood?
Not for flooring purposes, dry. The distinction only becomes real where water is a possibility,
which is why it comes up in basements and bathrooms and almost nowhere else.