Floor leveling compound is a cement-based underlayment poured over a rigid, primed wood subfloor to fill low spots and create a flat surface for finished flooring.
Pour a bag of self-leveling underlayment over a bouncy wood floor and it will crack along every seam within a season. The material works only when the wood underneath it is rigid, dry, and prepared to the manufacturer’s spec. Get that part right and a 1/4-inch skim coat turns a wavy subfloor into a tile-ready plane; get it wrong and you have paid for a decorative crack.
What Floor Leveling Compound Actually Is
Floor leveling compound — also sold as self-leveling underlayment, or SLU — is a hydraulic cement and polymer powder that you mix with water and pour. It flows into low areas under its own weight, gets guided with a gauge rake or smoothing tool, and cures into a hard, rigid layer.
ARDEX describes its K 60 ARDITEX as a Portland-cement and hydraulic-cement latex compound with adhesion, flexibility, and moisture resistance. Despite the “self-leveling” name, it does not level itself. You still spread it, and you still move it into the low spots. It simply does the flattening work with far less effort than a trowel and patch job.
The cured layer does two jobs: it corrects surface unevenness, and it gives tile, resilient flooring, or wood flooring a suitable plane. The finish flooring manufacturer’s flatness tolerance is the number that matters, not the compound’s own claims.
How It Works on Wood Subfloors
On wood, the compound bonds to a primed and reinforced surface rather than bare boards. Manufacturers impose stricter prep requirements on wood than on concrete because wood moves — it expands, contracts, vibrates, and deflects under load.
MAPEI sets L/360 as the maximum allowable deflection for the complete flooring system, and warns that excessive subfloor movement causes delamination. Moisture problems have to be corrected before installation, and the wood must stay dry during and after curing. Skip either step and the bond fails from underneath.
The substrate rules are specific. MAPEI’s SLU guidance calls for two layers of exterior-grade plywood, each at least 5/8 inch, for a total minimum of 1-1/4 inches. It specifies Group 1 exterior-grade plywood, CC-plugged or better, installed smooth side up with adjacent edges no more than 1/32 inch out of plane.
ARDEX K 60 accepts solid hardwood, minimum 3/4-inch tongue-and-groove APA-rated Type 1 exterior-exposure plywood, or an approved OSB equivalent. ARDEX requires the wood to be code-compliant, rigid, securely fixed, and free of undue flex, with loose boards refastened.
Some materials are flatly prohibited. MAPEI bans its SLUs over particleboard, Lauan plywood, presswood, bamboo flooring, and glue-down engineered or solid wood. OSB is not universally approved either — MAPEI allows it only when the specific primer and SLU list it as acceptable.
| Preparation Step | What It Involves | Why It Fails Without It |
|---|---|---|
| Stabilize structure | Repair framing, replace damaged wood, refasten loose boards | Compound cracks over a moving floor |
| Fix moisture | Repair leaks, keep wood dry during cure | Bond breaks; wood swells under the pour |
| Clean to bare wood | Sand, vacuum; no solvents, strippers, or cleaners | Wax, finish, and dust act as bond breakers |
| Fill joints and defects | Fill open joints; patch low spots, holes, cracks | Compound drains into gaps and puddles |
| Prime | Manufacturer’s primer for the specific SLU | Compound cures too fast and shells off |
| Reinforce | Galvanized metal lath or approved mesh system | Wood movement telegraphs through the layer |
| Pour and cure | Stay within the data sheet’s thickness and time limits | Over-thick pours and drafts cause uneven set |
If you are ready to pick a product, our tested roundup of leveling compound for wood subfloors compares the options that actually list wood as an approved substrate.
Where the Installation Usually Goes Wrong
Most failures trace back to one of three mistakes, and none of them are about the compound itself. The first is treating leveling compound as a structural fix. MAPEI and ARDEX both require deflection and loose boards to be repaired before any pour, and neither product is approved over wood weakened by flooding, fire, chemicals, weather exposure, or demolition damage.
The second is skipping preparation steps: no primer, no lath, or a pour over dust, wax, finish, or adhesive residue. ARDEX directs installers to sand to bare wood when necessary and vacuum thoroughly. The third is exceeding the product’s limits — thickness, temperature, working time, or cure. ARDEX K 60 caps installation over wood at a maximum 1/4 inch, while MAPEI’s hydraulic-cement specification lists products running from 1/4 to 1 inch.
Drafts are an underrated problem. MAPEI warns that open doors, windows, and active HVAC airflow during placement and cure cause uneven drying. ARDEX guidance sets a maximum variance of 1/4 inch in 10 feet for applicable flooring preparation, unless the flooring maker requires tighter. Check that tolerance against your finish flooring before you pour, not after.
Can You Use It Over Any Wood Subfloor?
No — the subfloor has to be on the specific product’s approved substrate list. Solid hardwood, the right exterior-grade plywood, and some approved OSB equivalents qualify. Particleboard, Lauan, bamboo, and glued-down wood flooring do not.
Unstable dimensionally sensitive materials are excluded because they swell and shrink with humidity, and a rigid cement layer bonded to a moving surface will fail. Verify the exact substrate and thickness in the current data sheet for your chosen product before you buy, because approval varies by primer and by SLU.
Common Questions
Does self-leveling compound level a floor by itself?
No. It flows and flattens far more easily than a patched skim coat, but you still spread it with a gauge rake and guide it into low spots. The name describes its flow behavior, not an automatic result. Flatness depends on how well you place and work the material.
Can you pour leveling compound directly onto old plywood?
Only after preparation. The plywood must be rigid, securely fastened, clean, dry, and within the manufacturer’s thickness and deflection limits. Most wood installations also require a compatible primer and galvanized metal lath or another approved reinforcement system before the pour. Bare, dusty plywood will not hold the bond.
Is floor leveling compound the same as concrete?
No. It is a cementitious underlayment, but it is formulated with polymer binders for flow, adhesion, and flexibility rather than structural strength. MAPEI’s specification lists compressive strengths from 4,000 to 6,000 psi at 28 days depending on the product, and none of it replaces the structural work of fixing a weak subfloor.
Sources
- ARDEX Americas. “ARDEX K 60 ARDITEX Technical Data” Supports wood substrate approvals, the 1/4-inch maximum over wood, and cleaning requirements.
- ARDEX Americas. “ARDEX 740 Technical Data” Supports primer selection and flatness tolerance guidance.