Hardwood subfloor prep in Ontario houses

The part of the job nobody photographs

Every hardwood failure a Toronto shop gets called to fix is either moisture or flatness. Neither is visible in the finished floor photograph, which is exactly why subfloor preparation is the line most often thinned out of a competing quote.

It is also the line that cannot be added later. Once the floor is down, correcting the subfloor means taking the floor up.

Flatness: the number that governs

The working tolerance for most nail-down and glue-down hardwood is about 3/16 of an inch over a 10-foot span. Wide plank and glue-down assemblies are less forgiving than narrow strip, because a board that cannot flex has to bridge, and a bridged board telegraphs every void underneath it as a hollow spot or a squeak.

You find it with a straightedge and a light, not by eye. In a 1920s Toronto semi the usual pattern is a crown over a bearing wall and a dish in the middle of the room. In a 1990s Vaughan build it is more often local — a proud seam where two sheets meet, or a run of joists that dried differently.

High spots get sanded or planed. Low spots get filled with a cementitious patch that is rated for the assembly going over it. Neither is glamorous and both are cheaper than the alternative.

Moisture: measure, do not assume

Wood subfloors get a moisture meter reading, and the number that matters is not the flooring alone — it is the difference between the flooring and the subfloor. A gap wider than about 2 percent for narrow strip, or 1 percent for wide plank, means one of them will move relative to the other after installation.

Concrete slabs need a real test, not a plastic sheet taped down overnight. In-situ relative humidity probes drilled into the slab (ASTM F2170) are the current standard because they read the moisture that will eventually reach the surface, not just what is at the top today. A slab that reads dry in August can read very differently in March.

Below-grade and on-grade slabs also need a vapour retarder in the assembly. A basement floor in Etobicoke without one is a floor with a countdown on it.

Fastening the subfloor before fastening the floor

Squeaks that appear a year after a new hardwood floor are usually not the hardwood. They are the subfloor moving against a joist it was only ever nailed to. Screwing the subfloor down to the joists — properly, on a grid, before anything else happens — is an hour of work that removes the most common warranty call a flooring company gets.

In older houses it is also worth checking for a second layer. Plenty of Toronto floors have had a plywood underlay added over the original planks at some point, and the two layers are not always fastened to each other.

What good preparation looks like on paper

A written allowance for flattening, a stated moisture testing method, and a note on what happens if the readings come back outside limits. That last clause is the honest one: it says the price may change and explains exactly what would change it.

A quote with no subfloor line is not cheaper. It has simply moved the risk onto you.