Renovation guides

How to check a subfloor before new flooring yourself: step by step

Quick review by a licensed contractor on Oct 7, 2026. This is not a certification. Follow product labels where they differ.

Half a day that decides whether your new floor lasts. Most plank floors allow about 3/16" in 10 ft (1/8" in 6 ft), tile under 15" about 1/4" in 10 ft, and large-format tile only 1/8" in 10 ft. Your flooring's install guide is the number that counts.

Difficulty 2 of 5 · about half a day · careful

Photo of your room, the look you pick, then this guide with the materials counted for your room.

Step by step

1. 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 under a floating plank floor if it is stuck tight everywhere and your flooring's instructions allow it — just do not sand it. Tile and glue-down floors need it removed or covered per their instructions.

2. 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.

3. 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.

4. 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 and write the gap down. Compare it with your flooring's install guide: for most plank floors, more than 1/8" (3 mm) under a 6 ft straightedge is out of tolerance. Large-format tile is tighter, about 1/8" in 10 ft — check it with a 10 ft straightedge or a tight string line.

5. 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.

6. 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.

7. 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.

8. Step 8 of 12

   Leave it 24 hours, then read it

   

   Pull the tape and look. Dry plastic and a dry slab is a good sign, not a pass: many flooring instructions require a calcium chloride or in-slab RH test with a number, and a 6 mil plastic vapor barrier over concrete even when the slab tests dry. 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.

9. 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. Keep screws on the joist line, and use a length that goes through the subfloor plus about 1-1/4" into the joist, no more — pipes and wires run between and under the joists.

10. 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.

11. 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 small high spots on concrete only with a grinder whose dust shroud is hooked to a HEPA vacuum, wearing a fitted N95 or better — grinding concrete makes silica dust, so never dry-grind without dust collection. And only if the slab has no old vinyl or glue on it.

12. Step 12 of 12

    Re-check with the straightedge and decide what is next

    

    Go over every spot you patched. If nothing exceeds your flooring's flatness spec 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 exceeds your flooring's flatness spec (1/8" over 6 ft for most plank, tighter for large tile), or the spot is written down for leveling
    • On concrete: the 24-hour plastic-sheet test came back dry, and any calcium chloride or RH test your flooring requires is done
    • No squeaks left when you walk the room
    • No soft, spongy, or stained areas
    • Floor is vacuumed clean and patches are dry

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