Why floor thickness matters less than your subfloor

Ask a room full of flooring buyers what they are comparing and you will hear thickness, wear layer and price. Almost nobody says subfloor. That is backwards. The subfloor is the only part of the assembly you cannot fix after the floor goes down, and it is the condition nearly every manufacturer’s warranty is written against.

Our own catalogue makes the argument better than we can. Products carry an installation method, and every installation method is really a statement about what the subfloor has to be.

Every installation method is a demand on the subfloor

Two caveats. They describe products, not projects, and one product can carry more than one label — a click-lock plank is also a floating floor — so the categories overlap rather than partition. They count only listings where the method is published, a fraction of a catalogue of 118,651 items.

Now read down the right-hand column. Not one of those methods asks how thick the plank is. Every one asks something about the subfloor.

Flatness is a measurement, not a feeling

“The floor looks flat” is not a specification. Every manufacturer publishes a flatness tolerance — a maximum deviation over a fixed span — and it lives in the installation guide, not the marketing copy. They differ between products, so check the one you are buying.

Checking takes ten minutes and a straightedge:

1. Lay a long straightedge — a good level, or a clean length of aluminium — across the floor and rotate it around the room.
2. Slide a tape or a stack of shims into any gap. Write down the worst number you find, and where it is.
3. Compare it against the tolerance in your product’s installation guide.
4. Anything out of tolerance gets ground down or filled with levelling compound before a single box is opened.

Unglamorous work, and the difference between a floor that is silent in year five and one that ticks.

Thickness cannot fix deflection

Deflection is the floor system flexing under load — joists, sheathing and span, not the finish. It is easy to confuse with subfloor unevenness because both feel like a floor that moves, but they are unrelated problems and only one of them can be solved from above.

Going from a 4 mm plank to an 8 mm plank does not stiffen a floor structure. A finish measured in millimetres cannot brace joists measured in metres. If the floor bounces when you cross it, that is a framing conversation — blocking, a second layer of sheathing, sometimes a mid-span support — and it happens before flooring, not instead of it.

This matters most for tile. Tile cannot flex with what is beneath it, which is why thin-set installations — 564 products in our catalogue — come with a deflection limit expressed as a fraction of the span.

Moisture is the half nobody tests

Flatness is visible. Moisture is not, and moisture is the one that writes off whole floors.

• Concrete never entirely stops giving up moisture. A slab that looks bone dry can read high on an in-situ relative humidity probe, and below-grade slabs in Canadian basements are the worst offenders.
• Test rather than assume. In-situ RH probes and calcium chloride tests both produce a number you can hand to an installer. A meter waved at the surface does not.
• Seasonal swing is a Canadian problem specifically. A house that runs humid in August and bone dry at -20 °C in February puts a wood-based floor through a full expansion and contraction cycle every single year.
• Hold indoor humidity between 35–55%. That range is on wood flooring warranties for a reason, and a hygrometer costs less than one box of hardwood.

Where the money actually goes

Here is what the catalogue says about price. These count the flooring itself — trims, tools, adhesives and accessories are excluded — and they are median list prices per listed unit — per box, per piece, per roll — not per square foot. That distinction matters more than the numbers do.

One figure we deliberately left out of that table proves the point. Broadloom carpet’s median across 1,247 products is $46.30 — not because carpet is dramatically cheaper than vinyl, but because it is almost certainly listed by the square foot while a vinyl product is listed by the box. Never compare medians across categories sold in different units. Including ours.

What the table is genuinely good for is scale. Hardwood is the most expensive box in the building, and the reflex on a tight budget is to drop a grade. Consider the other lever: buy the floor you want and spend the difference on levelling compound, a moisture test and a proper underlay. A mid-priced floor on a prepared subfloor will outlast a premium floor on a bad one.

A subfloor checklist to run before you order

1. Identify the substrate: plywood, OSB, concrete slab, or an existing floor you plan to go over.
2. If it is a slab, test it. Record the reading and give it to your installer in writing.
3. Straightedge the whole room and note the worst deviation, not the average one.
4. Walk the floor listening for movement. A bounce is a framing problem no flooring product fixes.
5. Read the installation guide for the product you want before you buy it. The approved-method list narrows your options more than the price does.
6. Budget levelling compound, a moisture barrier and underlay as line items, not as contingency.
7. Acclimate the flooring in the room it will live in, for the period the manufacturer specifies.

The honest conclusion

Thickness is worth understanding. It decides the height at your door, how a plank feels underfoot, and how much unevenness it will tolerate. But it is a comfort spec, not an insurance policy — every installation method in our catalogue is defined by what it needs the subfloor to be.

So if you have $3,000 to spend and the choice is $2,900 on the floor with $100 for preparation, or $2,400 on the floor with $600 spent getting the subfloor right, take the second. The floor you can replace in fifteen years. The mistake underneath it, you cannot.

Shop what this guide covers

• self-levelling compound and primers — the $600 in that last paragraph.
• underlayment — the rest of it.
• setting materials — mortars and membranes, for the tile case with a deflection limit.
• resilient flooring adhesives — bought against the moisture reading, not against a rule of thumb.
• How thick should your floor actually be? — the spec this article spends its length arguing with.