Subfloor Preparation Best Practices
Why subfloor preparation is the most important step in any flooring installation — leveling, moisture testing, and substrate requirements for every flooring type.
The Foundation of Great Flooring
90% of flooring failures are caused by improper subfloor preparation, not defective materials. A perfectly installed floor on a poorly prepared subfloor will telegraph every imperfection, develop squeaks, and may void your material warranty.
Step 1: Moisture Testing
Moisture testing is mandatory before any flooring installation. Each flooring type has specific moisture tolerance thresholds:
For concrete slabs, use the calcium chloride test (ASTM F1869) or relative humidity probe test (ASTM F2170). For wood subfloors, use a pin-type moisture meter. Test in multiple locations — moisture levels can vary significantly across a floor.
Step 2: Leveling
Most flooring manufacturers require subfloors to be flat within 3/16 inch over a 10-foot span. Use a long straightedge or laser level to identify high and low spots.
Self-leveling compound handles depressions up to 1 inch deep. Mix according to manufacturer instructions and pour while fluid — it finds its own level. Cost: $30-$50 per 50-lb bag, covering approximately 50 sq ft at 1/8 inch thickness.
Grinding addresses high spots on concrete. Rent a concrete grinder ($50-$100/day) for small areas or hire a professional for larger corrections.
Step 3: Substrate Requirements
Over plywood: Minimum 3/4 inch total thickness. Secure loose panels with screws (not nails) every 6 inches. Replace any water-damaged or delaminated sections completely.
Over concrete: Must be fully cured (28+ days for new concrete). Apply moisture barrier if testing reveals elevated moisture levels.
Over existing flooring: Some flooring can be installed over existing floors, but this raises floor height (affecting door clearances and transitions) and may mask underlying problems. Removal is generally recommended.
Common Mistakes
Skipping moisture testing, using floor patches that are incompatible with your adhesive, and failing to remove all tack strips and staples from previous flooring installations are the most common professional errors.