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Subfloor Levelling Cost in Ontario: Flat vs Level, Self-Levelling Compound, Grinding, Plywood Overlays, the 3/16 in. Rule and Who Should Pay for It
Flooring & Tile
Flat Is Not Level, Level Is Not Flat, and the Floor You Are About to Buy Only Cares About One of Them
Almost every flooring complaint I get asked to look at, the gap, the click, the rocking tile, the plank that peaks in the middle of the room, started under the floor and was visible before the first box was opened. Subfloor prep is the line that gets left off the low quote, because it is the line the installer cannot see until the old floor is gone and the one the homeowner does not want to hear about. It runs from $0.75 a square foot to screw down and sand a plywood floor to $3.00 for self-levelling compound, and it is the cheapest money in the whole job.
Flat versus level
Level means the floor is horizontal: a marble put down stays put. Flat means there are no humps or dishes in it: a straightedge laid anywhere touches along its whole length. A floor can be level and lumpy, or flat and sloped, and old houses are often both at once. Flooring does not care about level. A 2 in. slope across a 20 ft room is invisible under hardwood and only matters at the fridge. Flooring cares a great deal about flat, because a plank spanning a dish is a plank with nothing under it, and a tile across a hump is a tile standing on one corner.
The numbers come from the flooring manufacturers and the trade standards, and they are tighter than most subfloors in Ontario. Hardwood and vinyl plank: 3/16 in. in 10 ft, which is a nickel’s thickness under a 10 ft straightedge. Large-format tile, anything with a side 15 in. or more: 1/8 in. in 10 ft, which is nothing. A typical plywood subfloor in a 20-year-old house has 1/4 to 3/8 in. of hump at every joist lap and sag between. A typical basement slab has a 1/2 in. dish in the middle of the room where it was finished by hand.
How to check it yourself: a 10 ft straightedge, or a tight string line across the room, and a tape. Mark every spot over 3/16 in. with a lumber crayon. Count them. That count is the prep budget, and you want it before the quotes, not after.
The methods and what they cost
| Method | Per sq ft | What it fixes | Notes |
|---|---|---|---|
| Minor prep: screw down, fill, sand ridges | $0.75 | Squeaks, loose sheets, lap ridges, old nail heads, fastener holes, small dips with patch | Every plywood floor needs this. Priced on the whole room. |
| Grind concrete high spots | $2.00 | Humps, trowel ridges, old adhesive, paint | Dust. The installer needs a vacuum shroud and you need to be out of the house. |
| Self-levelling compound, average 1/4 in. | $3.00 | Dishes, slopes you want gone, a whole slab that is wavy, feathering to a flat plane | Needs primer, a sealed room, and a crew that pours the whole room in one go. Thicker than 1/2 in. costs more; the compound is the expensive part. |
| 3/8 in. plywood overlay | $2.75 | Old board subfloors, damaged plywood faces, a thin subfloor that needs stiffening for tile | Raises the floor 3/8 in. plus the flooring. Doors and transitions follow. |
| Replace damaged sheathing | $6.00 | Rot at the toilet, the dishwasher, the patio door; delaminated OSB | Priced on the area replaced, not the room. Usually a few sheets. |
| Uncoupling membrane over plywood, for tile | $3.50 | Not levelling; isolates tile from plywood movement | On top of prep, not instead of it. Tile costs. |
| Raised panel basement subfloor | $3.50 | A rough, cold or damp slab under a floating floor or carpet | Gives you a flat wood surface over a slab you do not want to grind or pour. Basement floors. |
Two rooms worked through
A 300 sq ft kitchen on a slab, getting 12×24 porcelain. The slab is dished 3/8 in. in the middle and has a trowel ridge along one wall. Grind the ridge and the high edges, about 60 sq ft at $2.00, $120. Self-level the room at $3.00, $900. Prep total $1,020. The tile job itself is 300 at $15.75, $4,725. The prep was 18% of the tile, and without it the setter either refuses the job or lays tile with lippage you can trip on. Large-format tile is where the 1/8 in. rule comes from.
A 400 sq ft upstairs hallway and two bedrooms, 1960s house, getting engineered hardwood. The subfloor is 1×6 boards on a diagonal, cupped and squeaking. Screw and sand at $0.75, $300. A 3/8 in. plywood overlay to give the nailer something to hold, 400 at $2.75, $1,100. Four door bottoms cut at $25, $100. Prep total $1,500 before a single plank, on a floor that will cost 400 at $10.75, $4,300, to lay. Skipping the overlay saves $1,100 and gets you a hardwood floor nailed into boards that move, which is a gaps and squeaks page waiting to happen.
Who pays
The homeowner, always, one way or another. The question is whether it is in the quote or in the change order. A good installer walks the room with a straightedge before he prices it, writes a prep allowance into the quote, and tells you it might move once the carpet is up. A bad one quotes the flooring, finds the subfloor on day one, and either does nothing about it or hands you a $1,500 change order while his crew stands in your kitchen. Neither of those is a surprise if you measured first.
In a new house the builder owns the subfloor and the flooring allowance assumes it is to spec. If your flooring installer finds a hump, that is a conversation with the builder before anything goes down, and the subfloor inspection checklist is how you document it with photographs. Under Tarion, a floor defect caused by a subfloor the builder supplied is the builder’s, but only if someone can show the subfloor was the cause, which is why the photographs matter.
- Ask for the prep as a separate line with a rate per square foot and an allowance. A quote with no prep line is a quote with a change order in it.
- Ask what the flatness spec is for the product you chose, and ask the installer to confirm the room meets it before he opens a box. If he cannot answer, that tells you something.
- Agree on who checks and when. The day the old floor comes up is the day the subfloor gets measured, and you want to be there with a phone.
- Do not let anyone feather self-leveller over a loose plywood floor. It cracks along every joist. Screw first, then pour.
If three quotes disagree about the prep, that is exactly what the quote comparison page is for.
Questions people ask
How much does it cost to level a floor in Ontario?
$0.75 a square foot to screw and sand a plywood subfloor, $3.00 for self-levelling compound at an average 1/4 in., $2.00 to grind a concrete slab, $2.75 for a 3/8 in. plywood overlay. A 300 sq ft room that needs compound is about $900 to $1,000.
How flat does a subfloor need to be for vinyl plank?
3/16 in. in 10 ft for most products, same as hardwood. Rigid SPC bridges a little better than flexible plank, but a dish deeper than that makes a floating floor flex at the joints until they break.
How flat does a subfloor need to be for tile?
1/4 in. in 10 ft for ordinary tile and 1/8 in. in 10 ft for anything with a side 15 in. or longer. The bigger the tile, the less it can follow the floor, and lippage is the result.
Can self-levelling compound go over plywood?
Yes, with the right primer and a lath or the manufacturer’s method, and only over plywood that is screwed down tight. Poured over a floor that moves, it cracks along the joists within a season.
Does the flooring installer level the floor?
Some do, some subcontract it, some refuse. Ask before you hire, and get the rate in writing. A tile setter will usually do his own; a hardwood crew will often do minor prep and nothing more.
Put the prep in the number
The flooring cost calculator carries every prep method on this page as a line item, so your number has the subfloor in it before the quotes arrive. And if the quotes already arrived and none of them mention the subfloor, the $99 review will.
Sources and further reading
- National Wood Flooring Association: the installation guidelines every hardwood warranty points to: subfloor flatness, moisture limits, acclimation, fastening schedules and the radiant-heat chapter.
- Tile Council of North America: the TCNA Handbook methods for tile over wood, concrete and in showers, and the ANSI lippage and grout standards a tile setter is held to.
- Tarion warranty coverage: what a new-home warranty covers for flooring gaps, cupping, cracked tile and squeaks in year one and year two, and the Construction Performance Guidelines it measures them by.
