How to Prepare a Subfloor for Hardwood
A hardwood floor is only as good as what sits under it. Most of the problems we get called about — squeaks, gaps, cupping, dips you can feel through the boards — trace back to a subfloor that was never properly prepared.
The step nobody sees
Subfloor prep is invisible once the floor is down, which is exactly why it gets skipped. But every flaw underneath eventually shows up on top — a process the trade calls telegraphing. A dip of a few millimetres becomes a spot that flexes underfoot. A subfloor that moves against its joist becomes a squeak. Moisture in a slab becomes cupped boards a year later.
Flat
Within tolerance across the whole floor, not just where it looks obviously level.
Dry
Measured, not guessed. Moisture is the single most common cause of floors failing early.
Solid
Firmly fastened to the joists, with no soft spots, movement or failing material.
Identify what you actually have
Four subfloor types turn up in Lower Mainland homes, and they behave very differently.
| Subfloor | Typically found in | What to know |
|---|---|---|
| Plywood | Most homes built or renovated since the 1970s | The benchmark. Holds fasteners well and is straightforward to level and repair. |
| OSB | Newer subdivisions | Fine for nail-down, but swells permanently if it gets wet. Damaged sections are replaced, not dried out. |
| Concrete slab | Basements, slab-on-grade, condos | No nailing. Engineered, floating or glue-down only — and moisture testing is not optional. |
| Particleboard | Many 1970s–80s homes | Not an acceptable base for nail-down hardwood. It does not hold fasteners and disintegrates when wet. It has to come out. |
| Old plank | Pre-1940 character homes | Often laid diagonally. Usually needs a plywood overlay to give a consistent, flat nailing surface. |
Check the floor is flat
Flat is not the same as level. A floor can slope gently across a room and still be perfectly fine for hardwood; what matters is that it has no humps or dips over a short distance.
| Installation type | Flatness tolerance |
|---|---|
| General NWFA guideline | 3/16" over 10 ft — or 1/8" over 6 ft |
| Nail-down, fasteners 1½" or longer | 3/16" over 6 ft — or 1/4" over 10 ft |
| Glue-down, or shorter fasteners | 1/8" over 6 ft — or 3/16" over 10 ft |
You check it with a long straightedge laid across the floor in several directions, marking anywhere light shows underneath. High spots get sanded or ground down; low spots get filled with a levelling compound and allowed to cure properly before anything goes on top.
Measure the moisture
This is the step most often skipped and the one that causes the most expensive failures. Wood moves with the moisture around it, and a floor installed into the wrong conditions will cup, crown or gap months after everyone has gone home.
- Wood subfloors — read with a moisture meter, and compare against the flooring itself. The two need to be close before installation; a large gap between them is a warning, not a detail.
- Concrete slabs — the industry method is ASTM F2170, using in-situ relative humidity probes drilled into the slab and left undisturbed for 72 hours before reading. A surface meter gives a quick indication but tells you nothing about moisture deeper in the slab, which is what actually matters over time.
- Crawlspaces — an unventilated crawlspace feeds humidity straight into the floor above it. Worth looking under before choosing a product, not after.
Deal with squeaks and soft spots now
A squeak is almost never the flooring. It is the subfloor moving against the joist underneath it. Once hardwood is down, fixing that means taking the floor back up — so it is worth being thorough while the surface is open.
- Walk the whole floor deliberately and mark every squeak and soft area before anything else happens.
- Re-fasten the subfloor into the joists with screws, not more nails. Nails are what worked loose in the first place.
- Cut out and replace any section that is water-damaged, delaminating or spongy underfoot.
- Remove old staples, nail heads and ridges of dried adhesive. Anything proud of the surface will telegraph through.
Clean, then let the wood settle
Vacuum thoroughly — grit left under a floor grinds and eventually creaks. Then give the flooring time to acclimatize in the room where it will live, under the conditions it will actually experience: heating on if the heating will be on, windows shut, normal humidity.
This is not a formality. Wood delivered from a cold warehouse into a heated house will lose moisture and shrink. Installed before that happens, it shrinks in place — and the gaps that open up between boards are permanent.
When to call someone in
- You find particleboard, or the subfloor is spongy anywhere
- The floor is out of flatness tolerance over a large area
- It is a concrete slab and you have no way to test moisture properly
- The house is pre-1940 and the joists have settled
- The flooring warranty requires documented subfloor conditions
None of these are unusual, and none of them are disasters. They just need doing properly before a floor goes down. We assess subfloor condition as part of every free estimate — and if the answer is that it needs work, we tell you before you buy the flooring rather than on install day.
Frequently asked questions
How flat does a subfloor need to be for hardwood?
The National Wood Flooring Association tolerance is 3/16" over a 10-foot span, or 1/8" over 6 feet. Nail-down installations using fasteners of 1½" or longer are allowed slightly more: 3/16" over 6 feet, or 1/4" over 10 feet. Always check the flooring manufacturer's own specification, since it overrides the general guideline for warranty purposes.
Can I install hardwood over particleboard?
Not for nail-down hardwood. Particleboard does not hold fasteners reliably and breaks down when it gets wet. It needs to be removed and replaced with plywood — which is why identifying it before install day matters.
How do you test a concrete slab for moisture?
The industry method is ASTM F2170, using in-situ relative humidity probes drilled into the slab and left undisturbed for 72 hours before reading. A surface meter gives a quick indication but does not measure moisture deeper in the slab, which is what actually affects the floor over time.
Why is my floor squeaking?
Usually because the subfloor moves against the joist below it, not because of the flooring itself. The fix is re-fastening the subfloor to the joists with screws before the new floor goes down. Once hardwood is installed, correcting it is far more disruptive.
Do I need to acclimatize hardwood before installing?
Yes. Wood moves with the moisture in the air around it. Flooring should sit in the room where it will be installed, under normal living conditions, so it reaches equilibrium before it is fastened down. Skipping this is a common cause of gapping and cupping later.
Can you prepare the subfloor if I supply the flooring?
Yes. Subfloor preparation, levelling and repair is work we do on its own as well as part of a full installation. Call 778 965 0206 and we will take a look.
Keep reading
Not sure what is under your floor?
We check subfloor type, flatness and moisture as part of every free on-site estimate — and tell you straight if something needs doing first.
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