LVP guide

LVP Subfloor Requirements

LVP is thin and, when floating, unattached. It follows the surface underneath it very closely — which is why the subfloor decides how the finished floor performs.

What the subfloor has to be

Clean — no dust, grit, adhesive residue, paint drips, drywall mud or fasteners standing proud.
Structurally sound — no soft, delaminated, rotted or water-damaged sheathing.
Stable — no deflection, movement or squeaks underfoot.
Flat enough for the specific product you are installing.
Dry, and within the moisture limits your product sets for that substrate.
Free of anything that interferes with plank sitting flat and locking together.

Flat is not the same as level

This distinction causes more confusion than any other subfloor topic. Level means horizontal — parallel to a level line. Flat means free of humps and dips across its own plane. A floor can slope noticeably across a room and still be perfectly flat, and a dead-level floor can be full of high and low spots.

Floating LVP cares about flatness. A consistent slope is generally tolerable; a ridge at a plywood seam or a dip in the middle of a room is not, because that is where the floor flexes and the joints work loose.

How flat is flat enough

Manufacturers state a flatness requirement, usually as a maximum variation over a given distance — a specific fraction of an inch measured across a specified span. The values differ between products, and the product requirement is what controls. FloorGauge does not publish a universal tolerance, because there isn't one.

Find the number in your installation instructions before you evaluate the floor, so you are measuring against the requirement you actually have to meet.

Checking the floor

Use a long, genuinely straight straightedge — a quality level, an aluminum straightedge or a jointed screed. Lay it across the floor in several directions and positions, including across plywood seams, along walls and diagonally. Light under the middle means a low spot; rocking on the center means a high spot. Mark what you find directly on the subfloor with chalk or a marker, then measure the worst deviations against your product's span requirement.

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.

Wood subfloors

Plywood and OSB need to be well fastened to the joists, with no movement and no loose panels. Re-screw the field where it flexes, set or remove proud fasteners, and address squeaks now. Seams between panels are the usual source of ridges and gaps — sand or grind down high edges and fill low seams with a suitable patch.

Damaged, soft or delaminated sheathing gets replaced rather than patched over. Check what your product requires in terms of panel thickness and span, especially if you are going over an older or sagging floor system.

Concrete substrates

Concrete brings three separate questions: flatness, surface condition and moisture.

Flatness — trowel ridges, control joints, patched areas and slab curl at the edges are common. Grinding handles high spots; a leveling compound handles broad low areas.
Surface condition — old adhesive, curing compounds, sealers and paint can interfere with patch, leveler or underlayment bonding. Check what your prep products require of the surface.
Moisture — slabs can pass vapor indefinitely, including on-grade and below-grade slabs that feel dry. Manufacturers set the test method and the acceptable result, and may require a specific vapor retarder. Test rather than assume.

Installing over existing flooring

Some LVP products allow installation over certain existing hard surfaces; others prohibit it outright, or prohibit going over cushioned vinyl, over carpet, or over more than one existing layer. If yours allows it, the existing floor still has to meet every requirement above — well bonded, flat, stable and clean. Tile grout lines can telegraph through thin plank and may need filling first.

Matching the fix to the problem

Common subfloor conditions and the usual remedy
Condition	Typical approach
Small dips, seams, fastener holes, minor voids	Cement-based floor patch, feathered out.
Trowel ridges, high seams, adhesive residue, slab high spots	Grinding or sanding down to the surrounding plane.
Broad low areas or a generally uneven slab	A self-leveling underlayment poured over prepared substrate.
Movement, squeaks, loose panels	Refasten to the framing before any surface work.
Soft, rotted or delaminated sheathing	Cut out and replace the affected area.
Failing moisture test on concrete	Follow the flooring manufacturer's required mitigation for that product.

Prep sequence matters: grind the high spots first, then patch or level the lows, then re-check with the straightedge. More detail on each approach is in FloorGauge floor prep.

Planning a self-leveler pour?

Estimate bags from your area, average pour depth and your product's stated yield.

Open the Self-Leveler Calculator

What the subfloor will not fix itself

Underlayment does not correct flatness, and a thicker plank does not span a dip. If the floor fails your product's flatness requirement, the floor gets corrected — that is the only remedy that holds up over time, and the only one a warranty claim will respect.