How to prepare a subfloor for a floating floor
7 min read · Written by the WFA flooring team · Updated June 2026
Quick answer
A subfloor for a floating floor must be flat to within 3mm over any 3-metre span, structurally sound, clean, and dry. On concrete, test for moisture before laying. Fill low spots with self-leveller and grind down high points. Remove old soft floor coverings, allow flooring to acclimatise in the room for 24–48 hours, and leave a 10–12mm expansion gap around all fixed objects and perimeter walls.
Flatness
Why It Matters: Hollow spots cause clicks and joint damage
Pass / Fail Threshold: ≤3mm over any 3m span
Moisture (concrete)
Why It Matters: Trapped moisture causes swelling and mould
Pass / Fail Threshold: ≤75% RH or per product spec
Moisture (timber subfloor)
Why It Matters: Timber subfloor must be dry and stable
Pass / Fail Threshold: ≤14% moisture content
Old adhesive / residue
Why It Matters: Lumps create high spots under boards
Pass / Fail Threshold: Must be scraped or ground flush
Structural integrity
Why It Matters: Soft or springy areas cause joint failure
Pass / Fail Threshold: No flex, bounce, or rot
Cleanliness
Why It Matters: Debris lifts boards unevenly
Pass / Fail Threshold: Swept and vacuumed clean
Expansion gap
Why It Matters: Floating floors must move freely
Pass / Fail Threshold: 10–12mm at all fixed edges
| Check | Why It Matters | Pass / Fail Threshold |
|---|---|---|
| Flatness | Hollow spots cause clicks and joint damage | ≤3mm over any 3m span |
| Moisture (concrete) | Trapped moisture causes swelling and mould | ≤75% RH or per product spec |
| Moisture (timber subfloor) | Timber subfloor must be dry and stable | ≤14% moisture content |
| Old adhesive / residue | Lumps create high spots under boards | Must be scraped or ground flush |
| Structural integrity | Soft or springy areas cause joint failure | No flex, bounce, or rot |
| Cleanliness | Debris lifts boards unevenly | Swept and vacuumed clean |
| Expansion gap | Floating floors must move freely | 10–12mm at all fixed edges |
Why subfloor preparation matters so much
A floating floor — whether hybrid, laminate, engineered timber, or vinyl plank — is not glued or nailed down. Instead, the planks lock together and 'float' as one large sheet over the subfloor. This means any bump, hollow, or damp patch beneath the floor is directly felt through the boards above. Skipping subfloor prep is the single most common reason floating floors develop squeaks, clicks, raised joints, or buckle within months of installation.
Getting the subfloor right before your flooring arrives is genuinely worth the time. It protects your investment, keeps your warranty valid, and means the finished floor looks and feels exactly as it should.
Step 1 — Check and correct flatness (3mm over 3m rule)
The standard flatness tolerance for floating floors in Australia is no more than 3mm variation over any consecutive 3-metre span. Use a long spirit level or a straight piece of timber to identify high and low spots across the entire room. Mark any problem areas with chalk or a pencil.
High spots on a concrete slab can be ground down with an angle grinder fitted with a diamond cup wheel — hire one from your local hire shop if needed. Low spots, dips, and hollows are filled using a self-levelling compound (sometimes called self-leveller). Wholesale Floors Australia stocks self-leveller as part of its accessories range. Mix it according to the manufacturer's instructions, pour it into the low area, and allow it to cure fully — usually 24 hours — before proceeding. On timber subfloors (sheet flooring or floorboards), use a floor-grade filler or ply patch for low spots, and plane or sand down any high points or protruding nail heads.
Step 2 — Test for moisture, especially on concrete
Moisture is the enemy of any timber-based floating floor. Concrete slabs — particularly new pours or slabs in contact with the ground — can hold significant moisture even when they feel dry to the touch. The only reliable way to check is with a moisture test.
The most accurate method for concrete is an in-situ relative humidity (RH) probe test, which should read 75% RH or lower (check your chosen product's specification sheet, as some products allow slightly higher readings). A cheaper but less reliable option is a surface-mounted plastic sheet taped down for 24–72 hours — condensation on the underside indicates excess moisture. If moisture levels are too high, a moisture barrier or DPM (damp-proof membrane) is required before laying. Some underlay products include an integrated DPM — check the product specification carefully. For timber subfloors, aim for a moisture content of 14% or less, measured with a pin-type moisture meter. If your subfloor is significantly wetter or drier than the flooring you have purchased, allow more acclimatisation time before laying.
Step 3 — Remove old floor coverings and prepare the surface
Most floating floors cannot go over soft, compressible floor coverings such as carpet, cork tiles, or cushion-backed vinyl — these compress underfoot and cause joint damage over time. These must be removed completely, including the underlay and any foam backing. Strip carpet back to the gripper rods, remove the rods, and pull up the underlay. Old ceramic or porcelain tiles can sometimes remain in place if they are firmly bonded, fully intact, and within the 3mm flatness tolerance — but grout lines may need to be filled with a skim of self-leveller. Glued-down vinyl that cannot be lifted cleanly can often be skimmed over with self-leveller once it is flat and well-bonded. Sweep and vacuum the subfloor thoroughly before proceeding. A clean surface is essential, as even small pieces of grit or debris can cause localised high spots.
Step 4 — Acclimatise your flooring and leave expansion gaps
Floating floors expand and contract with changes in temperature and humidity. Before laying, bring the flooring boxes into the room where they will be installed and leave them flat for at least 24–48 hours (some engineered timber and laminate products recommend longer — check the installation guide). Keep the room at normal living temperature and humidity during this period. Do not store boxes on end or lean them against a wall.
When you begin laying, leave a 10–12mm expansion gap around the entire perimeter of the room — at walls, door frames, cabinetry, pipes, step-downs, and any other fixed object. This gap is hidden afterwards by skirting boards, scotia trim, or purpose-made transition trims and reducers. Wholesale Floors Australia stocks a full range of trims including end caps, reducers, and scotia to finish the edges neatly. Never close this gap by pushing skirting boards hard against the floor — the floor must be free to move. Bridging doorways between rooms also requires a suitable reducer trim to allow independent movement in each room.