How do you level a floor with plywood? Levelling floor with plywood over boards, subfloors and concrete

Levelling floor with plywood: when does it work?

Levelling a floor with plywood works when the floor under it is sound and within the required flatness: it can smooth minor gaps and board edges but cannot cure structural sag. Use only a panel rated for the chosen underlayment duty and covering. For tile, the substrate and backer layers must follow an approved assembly.

What plywood cannot do is make a sloping or sagging floor level. A thin sheet follows the shape under it. If the floor drops 15 mm from one wall to the other, an overlay screwed to it drops 15 mm too. That kind of fault needs packers, sistered joists or a levelling compound before the plywood goes down.

Three questions decide the method:

1. Is the structure sound? No springy joists, rot, or loose boards that cannot be fixed.
2. How uneven is it, in millimetres? Measure with a straightedge and a level, not by eye.
3. What finish goes on top? Tile needs a stiffer, flatter base than vinyl or carpet.

Our pillar on plywood for subfloors covers the structural layer below; this guide covers getting the surface flat.

How uneven is your floor? Choose the method by millimetres

Measure the unevenness first, then choose the method. Lay a long straightedge or a straight timber across the floor in several directions and measure the biggest gap under it with feeler gauges or a ruler; check the overall slope with a spirit or laser level.

A useful rule: plywood levels the surface, packers and joists level the structure, and compound fills hollows. Most old timber floors need a little of each.

How to level a floor with plywood, step by step

To level a floor with plywood over sound floorboards, fix loose boards, mark services, remove high spots and correct low areas before laying the overlay. Acclimatise the sheets, stagger the joints and fix them securely. Check the finished surface against the floor covering manufacturer’s flatness requirements.

1. Clear and inspect. Lift the old covering, check every board, and screw down loose boards into the joists. Replace any that are split or rotten.
2. Find the services. Mark pipe and cable runs on the boards so no screw goes through them.
3. Plane or sand the high spots. Punch nail heads below the surface first, then plane or sand proud board edges.
4. Fill or pack the low spots. Fill dips of a few millimetres with a floor leveler made for wood, or lift boards and pack the joists for deeper ones.
5. Acclimatise the plywood. Leave the sheets flat in the room for at least two days.
6. Lay out the sheets. Run the long side across the boards, start from the straightest wall, stagger end joints and keep sheet joints off board joints.
7. Fasten to the selected system. After marking pipes and cables, choose nail/screw type, length, spacing and joint treatment from the rated underlayment and floor-covering instructions. APA resilient underlayment shows 1/32 in butt-joint gaps; other structural panel systems may differ.
8. Check and finish. Check with the straightedge, sand any lipping at joints, fill screw heads if the finish needs it, and vacuum before the floor covering goes down.

Our guide on how to install a plywood subfloor covers the same steps for a new floor on joists.

How to level a subfloor with plywood: packers, shims and sistering

How to level a subfloor with plywood when the joists are out of level: correct the joists or battens first, then lay the plywood. The overlay or new subfloor sheet then sits on a flat plane.

Three ways to correct the structure:

• Packers and shims. Lift the old boards and fix tapered timber packers or plastic shims on the low joists until a string line or laser shows them level. Glue and screw packers so they cannot creep.
• Sistering. Fix a new, straight joist alongside a sagging one, set level and bolted or screwed through. This both levels and stiffens.
• Planing. Plane the tops of high joists down, where the joist depth allows it.

On a concrete or solid base, the same job is done with treated battens laid on packers. Once the joists or battens are in one plane, an 18 mm or thicker subfloor sheet spans them; our guide to subfloor plywood thickness gives the sheet by joist spacing.

What thickness plywood over floorboards?

The plywood thickness depends on the subfloor, intended finish, underlayment grade and approved floor assembly. APA E30X separates resilient-finish underlayment from TCNA ceramic-tile floor systems in separate tables; see the APA plywood underlayment guide.

Every millimetre raises the floor: check door clearances, thresholds and the step to the next room before choosing thicker sheets. Our thin plywood at 3–5 mm is always 5-ply; the ply count for 6 mm panels is set per order.

How to lay plywood over floorboards and fix it

Lay approved plywood underlayment over stable boards with staggered joints and the rated fastening schedule. APA E30X Table 17 distinguishes smooth versus lumber/uneven substrates and lists ring- or screw-shank nails; it does not establish a universal screw grid for all plywood products.

APA calls for approved ring- or screw-shank nails for its underlayment; flush heads, offsets and proper joint finishing matter more than a generic statement that one fastener always works best. An approved screw assembly may differ. See APA underlayment installation guidance and check for pipes and cables before fixing.

Floor leveler for plywood: when should you use compound?

Use a floor leveler for plywood when the dips are too deep for the sheet to bridge but too shallow to pack, or when the finish needs a flatter surface than the plywood alone gives. The compound must be one made for timber subfloors: flexible, often fibre-reinforced, and applied over a primer.

Product-specific example: ARDEX Liquid BackerBoard is designed for specified interior wood subfloors and explicitly does not require metal lath. Other products have different requirements; verify the exact datasheet rather than applying this table to every cement compound:

How to use it on wood:

1. Confirm that the existing structure and plywood meet the chosen compound and floor-covering system’s stiffness requirements; fix loose and flexing areas before pouring. Do not extrapolate a deflection limit from a single joist-span example.
2. Seal the joints and gaps so the compound does not run into the void below.
3. Prime the plywood to the compound maker’s instructions, and fix lath or fibre reinforcement if the maker asks for it.
4. Mix and pour at the depth the maker allows, feathering into the high areas.
5. Let it cure for the stated time before laying the floor covering; check the flatness target below before the finish goes on.

A cement compound made only for concrete is the wrong product on wood; it does not tolerate the small movement of a timber floor. For slopes larger than the compound’s depth limit, correct the structure instead.

What flatness do flooring makers ask for?

Flooring makers state flatness as a gap under a straightedge. For wood flooring, the National Wood Flooring Association asks for a subfloor flat to 3/16 in in 10 ft (about 5 mm) or 1/8 in in 6 ft (about 3 mm) for glue-down and short-fastener nail-down. Nail-down with fasteners of 1.5 in or longer allows 1/4 in in 10 ft or 3/16 in in 6 ft. Tile and resilient flooring makers set their own figures, so read the data sheet before the sheets go down.

Plywood overlay sheets with a tolerance of ±0.5 mm limit thickness variation, but adjoining sheets still need to be checked for lipping.

How to level a concrete floor with plywood

To level a concrete floor with plywood, fix a damp-proof membrane first, then lay treated battens on packers until they are level, and fix plywood on top. For small dips on an otherwise flat slab, a floating plywood floor on a levelling underlay works; deep hollows are better filled with a cement-based self-levelling compound. In England, Approved Document C is the building-regulation guidance on site preparation and resistance to moisture.

Concrete brings moisture from below, so the order matters:

• Membrane: polythene or a liquid damp-proof membrane over the slab, lapped up the walls.
• Battens: treated timber, packed level with plastic shims, fixed to the slab.
• Plywood: exterior-grade sheets with a water-resistant bond, screwed to the battens with joints on a batten.
• Ventilation: leave the batten void ventilated or filled with insulation as the design needs.

Our guide to plywood subfloors over concrete covers basements and the moisture checks in full.

How to level a bathroom floor with plywood

To level a bathroom floor with plywood, repair and level the joists or boards first. Choose the thickness, backer or uncoupling layer, waterproof membrane and fixing pattern as one approved wet-room system; a blanket 15 mm overlay does not establish stiffness or waterproofing. The US EPA advises drying water-damaged areas and items within 24-48 hours to prevent mold growth.

Bathrooms add two loads to the job: tile, which needs a stiff base, and water, which finds every open edge.

• Bond: melamine-fortified or phenolic WBP plywood, not interior UF-bonded sheets.
• Stiffness: a thicker overlay, screwed at close centres, so the floor does not flex under the bath or shower tray.
• Sealing: seal all cut edges and joints before the membrane goes on.
• Membrane or backer: a tanking membrane or cement backer board between plywood and tile.

Our guide to bathroom subfloors covers wet-room build-ups.

Levelling a plywood floor for laminate, vinyl or tile

Levelling a plywood floor that is already down follows the same rule: find the fault, correct it at its cause, then smooth the surface for the finish.

Laminate on its underlay tolerates small undulations but not steps; sheet vinyl and LVT show every bump and need the flattest surface; tile needs both flatness and stiffness. Check the flooring maker’s flatness tolerance before you start and measure against it.

Why does calibrated plywood thickness matter for a level floor?

A level floor needs every sheet to be the same thickness, because two neighbouring sheets of different thickness leave a step at the joint. Under vinyl or LVT that step shows through as a line; under tile it can crack the grout.

Our panels are pressed to known tolerances (quality and certificates): commercial and exterior panels to ±0.5 mm, sanded furniture panels to ±0.2–0.3 mm and down to ±0.2 mm, with ±2 mm on length and width. Furniture-grade panels are pressed twice, the core first and calibrated, then the faces, so the thickness is set before the face goes on.

On site, measure a few sheets with a caliper before laying and sort out-of-tolerance panels. The factory length/width tolerance is not a suitable substitute for checking joint and perimeter gaps, which must follow the specified subfloor and finish installation instructions.

Which plywood for levelling a floor?

Choose a grade approved for the actual floor covering and underlayment duty, not just a water-resistant glue line. APA cautions that ordinary C-D Plugged is not a substitute for rated plywood underlayment under resilient finishes because face and inner-ply dent resistance differ. An exterior/CDX-type substrate is not automatically a certified APA Underlayment or a tile-ready assembly; see APA underlayment selection guidance. Under the US standard PS 1-22, underlayment is plywood produced with touch-sanded or sanded faces.

What we supply for floor levelling and subfloor work:

Our exterior plywood page lists bonds and faces, not an APA underlayment certification. Obtain the floor finish manufacturer’s substrate approval and assembly instructions before selecting a sheet for installation.

What mistakes should you avoid when levelling a floor with plywood?

The common mistakes are laying plywood over a floor that is sloping or weak, and fixing it too loosely. Avoid these:

• Overlaying a sag. Plywood follows the floor; correct the joists first.
• Wrong fastener schedule. A generic wide nail or screw grid can leave movement under the selected underlayment grade.
• Wrong gaps. Joint separation and filling depend on the subfloor/underlayment and finish system, not one figure for all panels.
• Joints on board joints. The overlay then moves with the boards beneath.
• Interior-bonded plywood in wet rooms. Use a water-resistant bond.
• Screwing into services. Mark pipes and cables first.
• Skipping acclimatisation. Sheets laid straight from a cold store move later.

To quote plywood for a floor levelling or subfloor project, send the thickness, bond, sheet size, quantity and destination port through the exterior plywood page. We reply within hours.