Subfloor Preparation and Product Choice for Floor Leveling Compound

A self-leveling pour fails at the subfloor more often than in the bag. The compound bonds only to the surface beneath it, flows only within its own depth range, and cures only when the substrate does not steal its water or feed it air. Choose the product type and prepare the surface first; the bag count comes after.

What subfloor preparation does self-leveling compound need?

Clean the subfloor, confirm it is sound and dry, repair cracks and holes, then prime it with the primer made for your leveling product. Match the preparation to the subfloor material: concrete needs curing, contaminant removal, and often mechanical profiling; wood needs structural rigidity, joint filling, and usually metal lath; dense or coated surfaces need a bond-promoting primer. Skip any of these and the failure modes are predictable: pinholes from trapped air, weak bond from contamination, and cracks that follow movement the compound was poured across.

How do you prepare a concrete subfloor?

New concrete should be fully cured before a pour. Sika requires the substrate to be clean, sound, and fully cured at 28 days for SikaLevel-110 SL, and its primer data sheet requires at least 20 MPa (2,900 psi) compressive strength at 28 days with 1.4 MPa (200 psi) tensile strength. Check the data sheet of your own product for its curing requirement; 28 days is that product's rule, not a universal law.

Remove everything that breaks the bond: dust, oil, grease, paint, wax, laitance, curing compounds, and sealers. On commercial or heavily contaminated slabs, Sika specifies mechanical preparation by shot blasting or diamond grinding to an ICRI concrete surface profile of CSP 3 to 5. After aggressive mechanical prep on a very porous slab, Sika notes outgassing rises for about 48 hours and asks for 2 days before priming, so the primer is not applied over air still leaving the slab.

Fill cracks and holes with a compatible repair material and let it cure; SikaLevel-110 SL asks for crack repair at least 24 hours before the pour. Cut or form joints in the topping directly above every movement joint in the slab. ARDEX and Sika both state that leveling products must not be installed over structural or movement joints, because the crack will telegraph through. Dam the pour area with foam tape or caulk at perimeters, doorways, and penetrations so the fluid compound cannot escape into wall cavities or adjacent rooms.

How do you prepare a wood subfloor?

Wood moves, so the subfloor must be stiff, well fastened, and of exterior grade. ARDEX technical bulletins require a minimum of 3/4 in (18 mm) tongue-and-groove APA-rated Type 1 plywood for self-leveling underlayment, and a ProSpec specification section referencing TCNA F185 sets the same 3/4 in (19 mm) exterior-grade Type 1 plywood or OSB equivalent with maximum deflection of L/360. Re-nail any boards that move and fill open joints.

Sand the wood with a coarse abrasive to remove foreign matter and give the surface a mechanical profile, then vacuum up all dust. Where the product system requires it, fasten metal lath over the primed wood: the ProSpec specification calls for 3.4 lb per sq yd galvanized expanded diamond mesh with non-corrosive fasteners every 6 in, and a minimum 3/8 in (10 mm) of leveling compound measured from the highest point of the lath. Some products instead rely on polymer modification for wood substrates, so follow the data sheet of the product you bought.

What about tile, gypsum, or coated surfaces?

Well-bonded existing tile can take a leveling pour, but dense, non-porous surfaces need a bond-promoting primer that creates a keyed film for the compound to grip. Gypsum-based substrates have their own rule: the LevelQuik RS data sheet requires them to be solid, structurally sound, dry, and properly cured with compressive strength above 2,000 psi, and every gypsum surface must be sealed or primed before leveling.

Some substrates are simply not candidates. The LevelQuik RS data sheet states the product must not be bonded directly to hardwood, luan plywood, particle board, parquet, cushion or spongeback vinyl flooring, metal, fiberglass, or plastic. When the existing surface is one of these, the correct move is removal or a manufacturer-approved overlay, not a thicker pour.

Why does primer matter, and which one should you use?

Primer does three jobs. First, it seals the substrate so trapped air cannot migrate upward through the wet compound and leave pinholes and bubbles; CTS Cement names this as the reason primer is required over porous substrates. Second, it slows water loss from the fresh compound into absorbent concrete, which protects strength and flow. Third, it improves adhesion.

Match the primer to the substrate and the product. Porous concrete takes a diluted acrylic primer; dense surfaces take a bond-promoting primer; slabs with high moisture vapor readings take an epoxy primer. CTS Cement pairs its LevelFlor with an acrylic primer for most applications and its TXP epoxy primers where moisture vapor transmission runs high, up to 20 lbs. Use the primer from the same manufacturer as the compound. Let it dry to a continuous film before pouring; the LevelQuik data sheet notes the leveler goes down after the primer dries, normally within an hour between coats, while other systems specify longer.

How do you check moisture before pouring?

Start with the plastic sheet test: tape about 4 ft² of clear polyethylene to the slab per ASTM D 4263 and wait 24 hours. Sika's guidance is direct: if no moisture accumulates on the underside of the film, the slab is dry enough for that coating; if moisture appears, repeat the test until it does not. For specification-level work, test moisture vapor emission rate per ASTM F1869 and concrete relative humidity per ASTM F2170, then compare the readings against the limits in your product's data sheet and the requirements of the floor covering going on top.

There is no universal safe moisture number. Sika requires the substrate to meet the proposed floor covering's requirements, and epoxy primers exist precisely because some slabs read too high for a standard acrylic primer. Record the reading, the test method, and the limit you checked it against on the project record.

Which type of self-leveling product should you choose?

First decide whether you need an underlayment or a topping. Underlayments smooth a subfloor before a floor covering such as sheet vinyl, VCT, wood, ceramic tile, or carpet. Toppings are the finished floor, used in warehouses, retail, and institutional spaces, and they can receive dyes, stains, saw cuts, or polishing. LevelQuik RS and CTS LevelFlor both state they are not permanent wear surfaces; they are underlayments, and pouring one as a finished floor is a product-choice error.

Then choose the binder. Cement-based compounds reach higher compressive strength in quality formulations, tolerate short-term moisture ingress, and dry ready for covering faster at thicknesses above 3 mm, according to UZIN's technical comparison; high-quality products reach 30 to 40 N/mm². Calcium-sulfate-based compounds set practically stress-free with less cracking tendency but lose strength when they absorb water, so they belong in dry interiors. Under rubber coverings, whose high vapor-diffusion resistance traps residual moisture, UZIN recommends high-strength cement-based compounds. Let the substrate, the covering, and the exposure decide.

Check the product's depth range against your depth profile. The compound fits only when its minimum pour depth is at or below your depth over the high point and its maximum covers your deepest low spot. CTS LevelFlor applies neat up to 2 in, extends to 5 in with pea gravel, and can be troweled to a feather edge; CTS Concrete Leveler requires a minimum of 1/8 in over the highest point. If the low spot exceeds the neat maximum, you need the extended version or a different product, not a deeper single pour. The room measurement guide shows how to build the depth profile this check needs.

Two more data-sheet checks close the choice. Confirm interior versus exterior use: LevelQuik RS is rated for interior dry areas plus limited covered exterior areas such as entryways and porches, not open exterior slabs. Confirm working time against your crew size: LevelFlor gives about 15 minutes of flow life and up to 30 minutes of working time at 70°F, and SikaLevel-110 SL about 30 minutes at 68°F, with higher temperatures cutting both figures.

Which preparation mistakes ruin a pour?

The repeat failures are all preparation failures. Pouring without primer leaves pinholes from substrate air and a weak bond. A dusty or contaminated surface is a bond breaker no primer can fully fix. Unfilled cracks and holes let the fluid compound drain through and telegraph as reflective cracks. Bridging a movement joint guarantees a crack where the slab wants to move. Pouring over an untested damp slab pushes the moisture problem into the floor covering. Using an underlayment as a finished floor asks a smoothing layer to do a topping's job. Exceeding the product's depth range, or mixing primer and compound from different manufacturers, voids the system the data sheet tested.

Safety and professional scope

Grinding or shot blasting concrete produces dust that requires extraction and respiratory protection under local occupational rules. Large pours, structural movement, active moisture intrusion, and commercial specifications belong with a qualified installer or engineer. Quantity planning does not replace structural design, project drawings, specifications, permits, codes, or a qualified professional.