Subfloor preparation: flat, dry, sound and clean before any floor

Four tests before any floor

Flooring makers and installation standards agree on what a subfloor must be: flat, dry, structurally sound and clean. Each new floor’s instructions give the limits, and the warranty depends on meeting them. Check all four before buying the floor, because fixing any of them adds materials, time and sometimes height.

Flat

Flatness is measured with a straightedge, 6 or 10 ft long, laid across the floor in several directions. Slide shims or a tape under the gaps. The limit depends on the floor going down:

The tile limits come from the TCNA Handbook; the others from common manufacturers’ instructions, which vary. Flatness is not the same as level: a floor can slope gently across a room and still be flat. Most floors only need to be flat.

Fixes. Grind or sand high spots. Fill low spots with a patching compound if they are small, or pour a self-leveling compound over a larger area. The self-leveling compound calculator counts the bags, and the leveler guide explains the depth.

Dry

Moisture from a concrete slab or a damp crawl space ruins wood and laminate, lifts glued vinyl and breaks tile bonds. Test a slab with the method the flooring maker specifies, usually a calcium chloride test or an in-slab relative humidity probe, and compare the result with the limit in the instructions. Test a wood subfloor with a moisture meter: for wood flooring, the National Wood Flooring Association asks for the subfloor and the flooring to be within 4 percentage points of each other for strip flooring and within 2 points for wide plank.

Fixes. Over concrete, a 6 mil polyethylene vapour barrier under a floating floor, or a moisture-mitigation primer under a glued one. Over a crawl space, a ground cover of polyethylene in the crawl space itself. The moisture barrier calculator counts the rolls with overlaps.

Sound

A floor that bounces, flexes or squeaks will crack tile and grout, break click joints and squeak forever under wood. The structure under it is the joists and the subfloor panels. Most floors need a subfloor of at least 23/32 inch (3/4 inch nominal) plywood or OSB on joists at 16 inches on centre; tile needs a stiffer floor than other finishes.

Fixes. Screw loose panels to the joists, which stops most squeaks. Replace damaged or delaminated panels. Add a layer of underlayment plywood for tile or glue-down vinyl. Sister or block joists that bounce.

The subfloor panels calculator counts sheets, screws and adhesive, and the floor joists calculator counts the framing if it has to be rebuilt.

Clean

Old adhesive, paint, drywall mud, wax and dust stop thinset, glue and leveler from bonding. Scrape and vacuum the floor; on concrete, grind or shot-blast coatings off. Remove old carpet staples and tack strip completely. Before scraping old resilient flooring or its black adhesive, have it tested: flooring and adhesives made before the mid-1980s often contain asbestos.

Height

Every layer added under the floor raises it. A new layer of plywood, a pour of leveler or a thicker floor can leave a step at the doorway to the next room, stop a door from opening, or put the floor out of line with a dishwasher opening. Add up the heights of each layer before you start. Tile on cement board is about 7/8 inch; a 6 mm vinyl plank with its pad is about a quarter inch. The difference between the two at a doorway is more than half an inch and needs a reducer or a build-up. The floor height and transition calculator works out the difference and the strip to use.

Order of work

1. Remove the old floor and its fasteners.
2. Fix the structure: loose panels, squeaks, bounce.
3. Test for moisture and fix the source.
4. Check flatness and level the low spots.
5. Clean.
6. Add the underlayment, membrane or barrier the new floor requires.

The underlayment guide covers the last step, and the large-format tile guide covers the strictest flatness requirement.