Nail-down vs glue-down vs floating hardwood

The system is the floor

Homeowners shop species. Failed floors are almost never a species mistake — they are a fastening mistake. Nail-down, glue-down and floating are three different assemblies with three different failure modes, and the right one is decided by what is under the floor, not by what you want the floor to look like.

Specify the system from the subfloor up, then pick the oak. Doing it the other way round is how a solid 5-inch plank ends up glued to a slab that reads 88 percent relative humidity.

Nail-down: wood subfloor, solid or engineered

Cleats or staples through the tongue into a wood subfloor, 6 to 8 inches apart with two fasteners near each end of every board. It is the assembly most Toronto houses were built for, and it is the only one that suits 3/4-inch solid hardwood.

It needs a wood subfloor of adequate thickness — 5/8-inch minimum over joists at 16 inches on centre, and 3/4-inch is better. It also needs the subfloor flat within about 3/16 of an inch over 10 feet. A 1920s house with a hog in the middle of the room is a flattening job before it is a flooring job, and that work is real money that a quote which omits it will find later.

Failure mode: squeaks. Almost always underfastening, a subfloor that was never screwed down to the joists, or boards installed across a subfloor that was not flat.

Glue-down: concrete, radiant, and anywhere movement must be controlled

Full-spread adhesive under engineered board. It is the assembly for a condo slab, for anything over hydronic radiant, and for wide plank where seasonal movement has to be held rather than allowed.

The adhesive is not a commodity. Moisture-cure urethanes tolerate slab moisture that a water-based adhesive will not, and most manufacturers void the wear-layer warranty if their own moisture limit is exceeded — which is why the slab gets tested before anything is ordered, not after the boards are on site.

Failure mode: hollow spots and eventual delamination, from trowel notches that were wrong for the board, adhesive that flashed off before the board went down, or a slab that was wet.

Floating: fast, forgiving, and audibly a floating floor

Click-lock engineered board over an underlayment, fastened to nothing. It tolerates a less-than-perfect subfloor, it goes down quickly, and it can be lifted.

It also sounds like what it is. A floating floor over a slab has a hollow report underfoot that no underlayment fully removes, and in a big open room it moves as one sheet — which is why expansion gaps at every vertical surface are not optional and why a floating floor under a heavy kitchen island is a call-back waiting to happen.

Floating a floor over hydronic radiant to save money is the specific decision that voids most mill warranties. The heat has to conduct through an air gap it was never designed to cross.

How to read a quote

A quote that does not name the fastening system is not a quote for a floor, it is a price for boards. Ask which system, which adhesive or fastener, and what the subfloor preparation allowance is. Three numbers that look competitive can be three different assemblies.

If a contractor proposes floating over radiant, or solid 3/4-inch over a slab, or nail-down over a subfloor nobody has measured for flatness, the conversation is over — not because it will fail tomorrow, but because it will fail in year three and the warranty will not be there.