What's Under Your Floor: Subfloor Types in Older Minnesota Homes
Minnesota does not have one answer to this question. It has two, and which one applies to you is decided almost entirely by which side of the metro you live on.
Two Minnesotas, and the split is unusually clean
In most states housing age is a gradient. Here it is closer to a switch. The core cities and the
Iron Range built out before the war; the ring suburbs are a different century of construction.
| City | Median year built | Built before 1940 |
|---|---|---|
| Minneapolis | 1951 | 42.8% |
| Duluth | 1951 | 42.8% |
| St. Paul | 1952 | 41.6% |
| St. Cloud | 1981 | 11.6% |
| Rochester | 1989 | 7.7% |
| Bloomington | 1972 | 3.3% |
| Brooklyn Park | 1980 | 3.1% |
| Plymouth | 1988 | 1.8% |
| Woodbury | 1997 | 1.3% |
| Eagan | 1988 | 1.0% |
Better than two in five houses in Minneapolis, St. Paul and Duluth predate 1940. Barely one in
fifty does in Woodbury or Eagan. If you are in a Longfellow or West Seventh bungalow, plan on
plank. If you are in Plymouth, plan on sheet goods and stop worrying.
What you will find, by era
Diagonal plank (roughly pre-1950)
1×6 boards run at 45 degrees across the joists. The diagonal is deliberate — it is what braces
the frame against racking — so this is structurally excellent. What it is not is flat. Individual
boards cup and shrink, and a century of heating seasons leaves gaps between them.
Straight board (roughly pre-1960)
The same construction laid perpendicular to the joists instead of diagonally. Same flatness
problem, plus a constraint: you cannot nail new hardwood parallel to the boards beneath it, so
running across them becomes the only direction available.
Plywood (roughly 1950s onward)
Sheet plywood over joists, typically ½ to ¾ inch. Flat, continuous, takes anything directly.
This is the postwar ring and most of Rochester and St. Cloud.
OSB (roughly 1980s onward)
Oriented strand board — Woodbury, Eagan, Plymouth, newer Brooklyn Park. Behaves like plywood
while it is dry, swells permanently at the edges if it is ever soaked.
How to tell, without pulling up the floor
- Look up from the basement. Minnesota houses have basements, which makes this
the easiest state in the country to answer the question in. Exposed joists put the underside of
the subfloor directly in view. - Pull a floor register. The duct opening cuts the full assembly and the edge
tells you the finished floor thickness and what is under it. - Lift a closet corner of carpet. Tack strip lifts easily; a closet is where a
mistake costs nothing. - Use the build year as a prior. On the numbers above it is a good one at both
ends of the range.
What each floor needs
| Floor | Plank subfloor | Plywood / OSB |
|---|---|---|
| Solid hardwood, nailed | Yes — run across the planks | Yes |
| Engineered, floating | Underlayment or overlay first | Yes |
| Luxury vinyl plank | Underlayment or overlay first | Yes |
| Laminate | Underlayment or overlay first | Yes |
| Tile / stone | Overlay plus uncoupling; check deflection | Backer board plus deflection check |
| Carpet | Yes | Yes |
The part that is specific to a cold state
An old plank subfloor in Minnesota has spent eighty-odd winters in a house heated to seventy
while the outside air sat below zero. That is an enormous annual swing in indoor humidity, and the
gaps you find between the boards are its record. They are not damage; they are the floor at its
January width.
Which matters when you lay something new on top. Board width and moisture content have to be
chosen for the dry end of that swing, not the average — what winter humidity does to a wood floor
covers the numbers. And if the job is a lower level rather than a main floor, the subfloor question
changes shape again: see basement flooring in Minnesota.
What the prep costs
Bringing an old plank subfloor to a flat continuous surface generally runs from about $1 a
square foot for an underlayment to $2–$4 for a plywood overlay, on top of the flooring itself. In
Minneapolis and St. Paul this is the single most common reason a real quote lands above an online
estimate. In Woodbury it usually does not arise at all.
An estimate, not a quote. These are planning ranges for Minnesota, not a quote. What you actually pay depends on your rooms, your subfloor and the material you choose.
Frequently asked questions
Can I install straight over a plank subfloor?
Solid hardwood and carpet, yes. Everything else wants a flat continuous surface first. The
boards move independently, and a floating floor laid over that telegraphs every gap and eventually
fails at the joints.
Do I have to take the old hardwood out before adding plywood?
Not necessarily — sound old hardwood makes a perfectly good base for an overlay. The real
constraint is height, because every layer raises the floor against your doors, your appliances and
the top step of your stairs.
The gaps between my floorboards are wide. Is the subfloor failing?
Almost certainly not. Wide gaps in February on an old Minnesota floor are seasonal, and they
close again by July. Failure looks like movement underfoot or a soft spot, not a gap.
What is the difference between subfloor and underlayment?
The subfloor is structural and spans the joists. The underlayment sits on top of it and exists
only to give the finished floor a smooth, sound surface.