Subfloor Preparation Guide: Getting the Foundation Right

Updated April 2026 · By the FlooringCalcs Team

The subfloor is the foundation of every flooring installation, and problems here cause problems with every flooring type installed over it. A hollow spot in laminate, a cracked tile, a creaky hardwood plank — these failures almost always trace back to inadequate subfloor preparation. The hour spent properly preparing your subfloor before installing new flooring prevents hundreds of dollars in failures and callbacks later. This guide covers subfloor assessment, repair, and preparation for each major flooring type.

Assessing Your Existing Subfloor

Start by identifying your subfloor material. Homes built on concrete slabs have a concrete subfloor. Homes with basements or crawl spaces typically have plywood or OSB (oriented strand board) subfloors over joists. Older homes may have diagonal board subfloors. Each type has different preparation requirements.

Check for three things: flatness, structural integrity, and moisture. For flatness, lay a 6-foot straightedge on the floor and look for gaps. Most flooring requires the subfloor to be flat within 3/16 inch over 6 feet. For structure, walk the floor and note any springy or squeaky areas. For moisture, use a moisture meter or the plastic sheet test on concrete.

• Identify material: concrete, plywood, OSB, or board
• Flatness: within 3/16 inch over 6 feet for most flooring
• Structure: no soft spots, excessive bounce, or squeaks
• Moisture: below acceptable levels for your flooring type
• Clean: free of adhesive residue, paint, drywall compound, and debris

Leveling and Flattening

High spots on plywood subfloors can be sanded down with a belt sander. High spots on concrete can be ground down with a concrete grinder. Low spots are filled with self-leveling compound (for areas larger than 1 square foot) or floor patch compound (for small divots and imperfections).

Self-leveling compound costs $30 to $50 per bag and covers approximately 20 to 50 square feet at 1/4 inch depth. For large areas, this is the fastest way to create a flat surface. The compound is mixed to a pourable consistency, spread over the floor, and levels itself through gravity. It sets in 4 to 6 hours and can be walked on in 24 hours.

• High spots on plywood: belt sander
• High spots on concrete: concrete grinder
• Low spots: self-leveling compound or floor patch
• Self-leveling compound: $30-$50 per bag, covers 20-50 sq ft at 1/4 inch
• Allow 24 hours cure time before installing flooring

Fixing Squeaks and Structural Issues

Squeaky subfloors are caused by plywood rubbing against nails or joists. The fix is to drive screws through the subfloor into the joists, pulling the plywood tight. Use 2-1/2 inch screws every 6 inches along each joist in the squeaky area. This eliminates the movement that causes the noise.

Soft or damaged areas require more extensive repair. Cut out the damaged section back to the center of the nearest joists on each side. Install blocking between joists if needed, then screw in a new piece of plywood matching the original thickness. For widespread structural issues (excessive bounce or deflection), adding another layer of plywood or stiffening joists with sister boards may be necessary.

Moisture Management

Concrete subfloors transmit moisture vapor from the ground. Most flooring manufacturers specify a maximum moisture level, typically 3 to 5 pounds per 1,000 square feet over 24 hours (calcium chloride test) or 75 to 80 percent relative humidity (in-situ probe test). Exceeding these levels voids warranties and causes flooring failure.

Moisture mitigation options include moisture barrier underlayment (6-mil polyethylene sheeting), moisture-blocking primers and sealers applied to concrete, and specialized moisture-mitigating membranes for severe cases. Plywood subfloors over crawl spaces need vapor barriers on the ground below and adequate ventilation to prevent moisture accumulation.

Preparation by Flooring Type

Tile requires the flattest subfloor (within 1/8 inch over 10 feet) and a rigid substrate. Plywood subfloors for tile should be at least 1-1/8 inches total thickness with a cement board or uncoupling membrane layer. Hardwood requires a clean, dry plywood subfloor and typically accepts slightly less flatness than tile.

Floating floors (laminate, vinyl plank) are the most forgiving, accepting flatness within 3/16 inch over 6 feet. They can float over most subfloor types with appropriate underlayment. Carpet is the most forgiving of all, conforming to minor subfloor imperfections. However, significant dips or humps will telegraph through carpet and feel uneven underfoot.

• Tile: 1/8 inch over 10 feet, rigid substrate, cement board layer
• Hardwood: 3/16 inch over 6 feet, dry plywood, nail or glue down
• Laminate: 3/16 inch over 6 feet, any clean subfloor with underlayment
• Vinyl plank: 3/16 inch over 6 feet, smooth and level surface
• Carpet: most forgiving, but lumps will show through