Subfloor Prep for LVP and Other Floors: Flat, Clean, Dry and Solid

Ask any experienced flooring installer what makes a floor look great and last, and you'll hear the same answer: the subfloor. The finish floor gets all the attention, but it can only be as good as what's under it. Most flooring complaints, like bumps, squeaks, gaps, hollow spots and broken click joints, trace back to subfloor problems.

The four requirements

Flooring manufacturers word it differently, but they all want a subfloor that's:

• Flat within a specified tolerance.
• Structurally sound, without excessive bounce or loose panels.
• Clean, free of dust, debris, old adhesive, paint, wax and oil.
• Dry, within moisture limits for the flooring and adhesive.

Flat is not the same as level. A floor can slope gently and still be flat. Flooring cares about flatness: no humps, dips or ridges over a short span.

Measuring flatness

Manufacturers typically specify flatness as a maximum gap under a straightedge over a set span, for example a fraction of an inch over 6 or 10 feet. Check the installation instructions for your exact product. To measure:

1. Lay a long, straight level or straightedge on the floor in several directions.
2. Look for gaps under it and measure them with a tape or feeler gauges.
3. Slide it along and mark high spots and low spots with chalk or a marker.

Glue-down LVP is the most demanding because it's thin and flexible and follows every irregularity. Small ridges show as lines in raking light. Click LVP bridges very small dips but still needs to meet the spec to protect the locking joints.

Wood subfloors

Most homes in our area have plywood or OSB subfloors over wood joists, in some older homes over plank subfloors.

Fix the structure

• Squeaks and loose spots: screw the subfloor to the joists with flooring screws. Nails work loose; screws hold.
• Bounce: excessive deflection can crack tile and stress click joints. Sistering joists or adding blocking may be needed for tile; ask a pro.
• Damaged panels: replace water-damaged, delaminated or soft areas.

Flatten it

• High spots at panel seams and joists can be planed or sanded down.
• Low spots can be filled with a patching compound rated for wood subfloors, feathered smooth.
• Old plank subfloors often need a layer of underlayment-grade plywood over them for glue-down floors and tile.

Check moisture

Use a pin moisture meter. Wood subfloors that are too wet will shrink later, opening gaps and causing movement. See moisture testing.

Concrete subfloors

Basements and slab-on-grade homes have concrete. Concrete is strong but rarely perfectly flat, and it brings moisture into the picture.

• Grind high spots and remove ridges, trowel marks and old adhesive with a grinder fitted with a dust collector.
• Fill cracks and low spots with a cement-based patch. Note that moving cracks may reappear through new flooring; cracks that keep moving need evaluation.
• Self-leveling underlayment may be used for larger areas that are wavy. It requires proper priming, mixing, pouring and dam work.
• Test moisture before choosing products; it's often the deciding factor.

Existing floors

Some LVP products allow installation over existing well-bonded sheet vinyl, VCT or tile, if the surface is flat, sound and properly prepared. Grout lines in tile may need to be filled so they don't telegraph through. Old floors can contain asbestos in some vinyl and adhesives; don't sand or grind suspect materials without testing. Carpet and pad always have to come out; see carpet removal.

Subfloor prep step by step

1. Remove the old flooring, tack strips, staples and nails down to the subfloor.
2. Inspect the subfloor for damage, rot, loose panels and squeaks, and repair or replace as needed.
3. Screw down loose areas and set any protruding fasteners.
4. Test moisture in wood and concrete with the right methods for each.
5. Check flatness with a long straightedge in several directions and mark highs and lows.
6. Sand, plane or grind the high spots.
7. Fill low spots, seams and cracks with a patch rated for the subfloor.
8. Skim coat or use self-leveler where large areas are out of tolerance, if needed for the product.
9. Undercut door casings and jambs so new flooring can slide underneath.
10. Vacuum thoroughly and wipe the surface so it's completely clean before installing.

Why prep costs what it does

Subfloor prep is labor, and it's hard to see in the finished room. It can be tempting to skip it to save money. But a glue-down floor over an unprepared subfloor will show every flaw permanently, and a click floor over a wavy subfloor can develop failing joints. A good estimate lists subfloor prep separately so you know what you're paying for and why.