Subfloor preparation and levelling

Does my subfloor need levelling before new flooring?

Your subfloor needs levelling if it is not flat within about an eighth of an inch over six feet, which is the tolerance most floating floor manufacturers specify. Flatness is not the same as level: a floor can slope gently across a room and still be perfectly acceptable, but humps and dips will cause a floating floor to flex at the joints and eventually fail. Levelling costs $1 to $4 per square foot over the areas that need it, using self-levelling compound on low spots or grinding on high ones, and on older homes a plywood overlay is sometimes more economical. This is the single most common reason two quotes for the same room differ by thousands of dollars: one installer has priced the preparation and the other has not.

Flat vs level

Level means horizontal. Flat means without humps and dips. Floating floors care about flatness only, which is why a century home with a gently sloping floor can still take a new vinyl or laminate floor without structural work. Tile is the exception: large-format tile needs both flatness and minimal deflection.

How it is checked

With a six-foot straightedge laid across the floor in multiple directions and a tape measure or shims to record gaps. Any installer quoting a floating floor should do this. If nobody has put a straightedge on your floor, the quote is an estimate rather than a price.

Methods and costs

Self-levelling compound fills low areas and costs $1 to $3 per square foot over the affected zone. Grinding removes high spots in concrete, $2 to $4. Plywood overlay adds a new flat surface over an uneven wood subfloor, $2 to $4, and also raises the floor height which affects doors and transitions.

Common causes of uneven subfloors

Settlement in century homes. Additions built at a different height. Plank subfloors with cupped or gapped boards. Previous flooring removed leaving adhesive ridges. Concrete slabs poured out of tolerance. In Winnipeg, Regina and Saskatoon, seasonal clay movement adds an ongoing component.

Squeaks and fasteners

A subfloor that squeaks will still squeak under a new floor. Refastening the subfloor to the joists with screws while it is exposed is inexpensive and is the only practical time to do it. Ask for it explicitly if your floors are noisy.

Common questions

Why is levelling not included in some quotes?

Because some installers quote a base price and bill preparation as extra once they see the subfloor, while others inspect first and include it. Neither is dishonest, but they are not comparable. Ask every installer whether their quote includes subfloor preparation and what happens if more is needed.

Can I install over a sloping floor?

Yes, if it is flat. A gentle consistent slope in an older home is not a problem for a floating floor. What matters is whether a six-foot straightedge shows gaps greater than an eighth of an inch. Sharp transitions and localised dips must be corrected.

Will new flooring fix my squeaky floor?

No, and this catches people out. The squeak comes from the subfloor moving against joists or fasteners. Fixing it means screwing the subfloor down while it is exposed, which is only practical during a flooring replacement. Raise it with your installer before the new floor goes down.

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