How to check a subfloor before new flooring yourself: step by step
Half a day that decides whether your new floor lasts. More than 1/8" over 6 ft needs leveling.
Difficulty 2 of 5 · about half a day · careful
Stop here
Photo of your room, the look you pick, then this guide with the materials counted for your room.
What you need
- Clear plastic sheeting for the moisture test, 2 ft x 2 ft pieces
- Duct or house-wrap tape (seals the moisture test)
- Subfloor screws, 2 1/2"
- Floor patch / repair compound
- Chalk or a lumber crayon (marking the low spots)
Tools
- 6 ft straightedge or long level — the one tool this task is about
- Bright flashlight or work light (you read the gap by the light under the straightedge)
- Tape measure and a set of coins or feeler gauges to measure the gap
- Shop vacuum
- Pry bar and a scraper (lifting old flooring and staples)
- Drill/driver (screwing down squeaks)
- Moisture meter for wood subfloors — $20–30/day. Optional — the plastic-sheet test is the free version for concrete.
Step by step
Step 1 of 12
Strip the room down to bare subfloor
Pull the carpet and pad, the tack strip, staples, and any loose flooring. One layer of old glued-down vinyl can stay if it is stuck tight everywhere — just do not sand it.
Step 2 of 12
Vacuum until the floor is clean
Sweep first, then vacuum the whole floor including the perimeter and corners. A single pebble under a straightedge gives you a false reading, and later it is a click underfoot forever.
Step 3 of 12
Identify what you are standing on
Plywood or OSB over joists, or a concrete slab? Tap with a knuckle: hollow and slightly springy is wood, dead solid is concrete. The moisture test and the leveling products differ for each.
Step 4 of 12
Lay the 6 ft straightedge and read the light
Set the straightedge flat on the floor and look at the gap underneath with a flashlight behind it. If light shows through, slide a coin or feeler gauge in: more than 1/8" (3 mm) anywhere along those six feet is out of tolerance.
Step 5 of 12
Walk the straightedge across the whole room
Move it in a grid — five or six positions lengthwise, the same crosswise, then two diagonals, and one straight through each doorway. Overlap each position by about half the straightedge so nothing is missed.
Step 6 of 12
Circle and label every out-of-tolerance spot
Draw a chalk circle around each low or high spot and write the measured gap inside it. Write down the single worst number in the room — that is what decides whether you patch, grind, or pour self-leveler.
Step 7 of 12
Concrete: tape down the plastic-sheet test
Tape a 2 ft x 2 ft square of clear plastic flat to the bare slab and seal all four edges so no air gets in. Put one square in the middle of the room and one near an exterior or below-grade wall.
Step 8 of 12
Leave it 24 hours, then read it
Pull the tape and look: dry plastic and a dry slab means you can proceed. Condensation on the underside, a dark patch on the concrete, or standing water means stop — get a specialist to test and seal the slab before anything covers it.
Step 9 of 12
Wood subfloor: hunt the squeaks and screw them down
Walk the whole floor slowly in stocking feet, shifting your weight, and mark every noise. Find the joist under each mark and drive a 2 1/2" subfloor screw through the plywood into it, a couple of inches from the existing nail.
Step 10 of 12
Look for soft spots, stains, and movement
Press hard with your heel near every wall, under every window, and around the toilet and dishwasher. Spongy plywood, a dark stain, or a musty smell means water — stop and find the source before you go any further.
Step 11 of 12
Patch the small dips and knock down the small humps
Trowel floor patch into gouges, seams, and dips smaller than a dinner plate and let it dry per the label. Grind or sand down small high spots on concrete — but only if the slab has no old vinyl or glue on it.
Step 12 of 12
Re-check with the straightedge and decide what is next
Go over every spot you patched. If nothing exceeds 1/8" over 6 ft you can go straight to the floor; if anything still does, the next task is self-leveling underlayment.
Quality check
- The straightedge was run lengthwise, crosswise, diagonally, and through every doorway
- No gap anywhere exceeds 1/8" (3 mm) over 6 ft, or the spot is written down for leveling
- On concrete: the 24-hour plastic-sheet test came back dry
- No squeaks left when you walk the room
- No soft, spongy, or stained areas
- Floor is vacuumed clean and patches are dry
Common mistakes
- Judging flatness by eye or with a 2 ft level. A floor can look perfect and still roll 1/4" over six feet, which is what breaks click-lock joints. Only a 6 ft straightedge tells you. This is mistake #1 in this task.
- Skipping the moisture test on a slab. Concrete on grade can push moisture up for years. Flooring goes down, the moisture has nowhere to escape, and you get cupping, mold, and a failed warranty. The test costs a piece of plastic and one day.
- Checking in one direction only. A floor that reads flat along the room can roll badly across it. Check lengthwise, crosswise, and diagonally, and always check right across every doorway.
- Sanding or grinding old vinyl or the black glue under it. In a building from before 1980 that material and its adhesive can contain asbestos, and sanding it puts fibers in the air through the whole apartment. Do not sand, grind, or smash it — test first.
- Pouring leveler over a squeak. The squeak is the subfloor moving against a joist or a nail. Leveler adds weight, not fastening, and the noise comes back through your new floor. Screw it down first.
- Covering a soft or stained spot. Spongy plywood or a water stain means an old or active leak. Putting a new floor over it hides the problem until it is a much larger repair. Find the source before you go further.
Draft guide written from standard practice, not yet reviewed by a licensed pro. Follow product labels where they differ.
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