Floor prep guide

How to Prepare a Subfloor for Flooring

Expose the substrate where the old floor comes out, clean it, confirm it is sound and stable, then measure what is actually there before deciding on any correction.

The short answer

Subfloor preparation is an inspection job before it is a repair job. You clean and expose the surface, find out what condition it is in, and only then decide whether anything needs to be ground down, filled, fastened or replaced.

The common mistake is to start with a solution — "we'll pour leveler" — before the floor has been looked at. A floor that moves, a floor with a high spot and a floor with a broad dip are three different problems, and only one of them is solved by adding material.

The preparation sequence

Evaluate the existing floor, and remove it where removal is the right call.
Expose the substrate in every area where the old floor is coming out.
Clean the floor — debris, dust, residue, anything loose.
Inspect the structural condition of what you have exposed.
Check for movement, deflection, loose panels and damaged areas.
Check flatness across the room in several directions.
Identify and mark high spots.
Identify and mark low spots.
Evaluate moisture where the substrate and product call for it.
Carry out the repairs and preparation the conditions call for.
Clean again — prep work makes its own dust and debris.
Recheck the floor before any flooring is opened.

What to check once the substrate is exposed

Substrate conditions worth checking before anything is corrected
Condition	What you are looking for
Cleanliness	Dust, grit, drywall compound, paint, adhesive residue and anything that will not stay put. Prep products and flooring both depend on a clean surface.
Fasteners and protrusions	Proud staples, nail heads, screw heads, tack strip remnants and hardened adhesive ridges. Each one is a localized high spot.
Loose panels and movement	Panels that flex or shift underfoot, gaps at seams, and areas that feel different from the rest of the floor.
Squeaks	Not a finish problem — usually a symptom of movement between panels, fasteners or framing. Worth understanding before it is covered up.
Damaged plywood or OSB	Delamination, swelling at seams, soft or stained areas, or panels that have been wet at some point.
Concrete defects	Cracks, spalling, crumbling or dusting surfaces, and previously coated or sealed areas.
High spots	Seam ridges, humps along old walls, trowel ridges in concrete and framing high points.
Low spots	Depressions that let a straightedge show daylight underneath, often near perimeters, drains and former partitions.
Moisture	Substrate conditions that call for evaluation, especially over concrete, over crawl spaces and below grade.
Old adhesive and residue	What is bonded, what is loose, and whether the remaining residue is compatible with the prep products you plan to use.
Finished height	What adding preparation material does to doors, appliances, transitions and adjoining floors.

No single method corrects all of these. A wood subfloor and a concrete slab respond differently to the same product, and what is appropriate for one is sometimes prohibited on the other.

Checking flatness

A straightedge laid across the floor shows the variation: light under the middle is a low spot, rocking on a ridge is a high spot.

Flatness is about variation across the surface, not about whether the floor is horizontal. Pull a straightedge across the room in several directions, including diagonals, and mark where it rocks and where it shows daylight underneath.

The variation you are allowed to leave behind is set by the flooring manufacturer for the product you bought — tolerances differ between products and between flooring systems. The difference between the two measurements is explained in flatness vs. level.

Deciding what the floor needs

Use what you actually found. These are starting points for a decision, not a diagnosis of your floor.

Condition-based prep decisions
If you find	What to consider
A high spot	Investigate the cause first. Depending on the substrate, grinding, sanding or removing the protrusion may be appropriate. Correcting a hump sometimes removes the need to fill everything around it.
A low spot	A patching compound or an appropriate leveling product may be suitable, depending on depth, area and what the product permits over that substrate.
Movement or deflection	Correct the structural or fastening issue. Patch and leveler are not designed to hold a moving substrate together, and a rigid layer over a flexing floor tends to crack.
A moisture concern	Evaluate it according to the requirements set by the flooring, adhesive and preparation products before anything covers the floor.
Damaged substrate	Determine whether repair or replacement of the affected area is appropriate before installation, rather than bridging it.

Correcting what you found

Once the condition is understood, the correction usually falls into one of three categories, and jobs often use more than one.

Remove material: grinding, sanding or scraping high areas, ridges and residue where the substrate allows it.
Add material: patching localized dips, or a leveling product across broader low areas — see floor patch and self-leveling compound.
Repair or refasten: securing loose panels, replacing damaged sections, and addressing the cause of movement.

The two add-material methods are covered in detail in floor patch and low spots and self-leveling compound.

Recheck before you install

Prep work changes the floor, so the floor gets measured again. A patch can dry with a ridge at its edge, grinding can leave a shallow scoop, and a leveler pour can stop short of a corner.

Vacuum again — patch dust and grinding dust are the most common contamination on a prepped floor.
Re-run the straightedge over every area you corrected, and across the corrections in more than one direction.
Feel the edges of patched areas for ridges left where the product stopped.
Confirm curing or drying requirements for any preparation product have been satisfied before flooring goes over them.
Recheck the finished height at doorways, appliances and transitions.

Product instructions set the numbers

Flatness tolerance, acceptable substrates, moisture requirements, primer, minimum and maximum thickness, mixing, working time and how long a preparation product must cure before flooring goes over it are all set by the manufacturers of the flooring and the prep products. Where those instructions and general practice disagree, the instructions are what your warranty is written against.

For product-specific substrate requirements under vinyl plank, see LVP subfloor requirements.