A finished floor is only as flat, dry, and stable as what sits underneath it. A concrete subfloor looks bulletproof, but slabs hold and release moisture for decades, develop cracks, and rarely show up flat enough for modern click-lock or glue-down flooring. Skip the prep and your beautiful new floor will telegraph every dip, cup at the seams, or trap moisture that grows mold within two years. This guide walks through the moisture testing, surface prep, leveling, and vapor-barrier choices that separate a 30-year install from a callback.
How Concrete Holds Moisture
Even a 20-year-old slab pulls moisture from the soil below through capillary action and releases water vapor upward. New slabs need 60 to 90 days of cure time per inch of thickness before flooring can go down, and that is just the starting point. Slabs in basements, slab-on-grade homes, and post-tension foundations all have different moisture profiles.
Vinyl, hardwood, and laminate manufacturers void warranties when the slab exceeds their moisture limits. Most LVP allows up to 5 lbs per 1,000 sq ft per 24 hours via calcium chloride or 75 to 80 percent relative humidity. Engineered hardwood is stricter — usually 3 lbs or 65 to 70 percent RH.
Calcium Chloride Testing
The calcium chloride (CaCl) test measures moisture vapor emission rate over 60 to 72 hours. You set a pre-weighed dish of anhydrous calcium chloride on a clean section of slab, seal it under a plastic dome, then weigh the dish again after the test period. The weight gain converts to pounds of moisture per 1,000 sq ft per 24 hours.
Run one test per 1,000 sq ft, with a minimum of three tests for any room. ASTM F1869 governs the procedure. Kits run $20 to $35 each from Calclok or Wagner. The downside of CaCl is it only measures the top half-inch of the slab — slabs can still release moisture from deeper down after the floor goes in.
Relative Humidity Probe Testing
RH probes (ASTM F2170) drill into the slab to 40 percent of its depth and measure the equilibrium humidity inside. This number predicts long-term moisture behavior much better than CaCl. Wagner Rapid RH and Tramex CMEXpert kits cost $200 to $400 for the meter plus $10 to $15 per disposable probe.
Drill the test hole, drop the sleeve, cap it, and wait 24 hours for equilibrium before reading. Three probes per 1,000 sq ft is the ASTM standard. RH testing is now the preferred method for most flooring manufacturers because it accounts for the entire slab thickness.
Surface Prep Before Anything Goes Down
The slab must be clean, sound, and flat. Walk the floor with a 10-foot straightedge and mark any spot where the gap exceeds 3/16 inch over 10 feet — that is the standard tolerance for most flooring. High spots get ground down with a rented concrete grinder ($65 per day plus $40 for a diamond cup wheel). Low spots get filled with self-leveler.
Strip any old adhesive, paint, sealer, or curing compound. Use a mechanical grinder, not just a chemical stripper, since residue stops new adhesive and self-leveler from bonding. Vacuum with a HEPA shop vac, then mop with clean water and let the slab dry fully before the next step.
Vapor Barriers and Underlayments
If your slab tests near or above the manufacturer’s moisture limit, add a vapor barrier. For floating floors, 6-mil polyethylene sheeting overlapped 8 inches and taped at seams gives basic protection. Better products include MP Global QuietWalk Plus, Roberts Super Felt with attached barrier, and Schluter DITRA-HEAT for tile.
For glue-down installs over a problem slab, you need a topical moisture mitigation system like Bostik Ultra-Set SingleStep2, MAPEI Planiseal MMS, or Koster VAP-I. These epoxy-based coatings handle slabs up to 100 percent RH and run $1.50 to $3 per sq ft installed. They are the only way to glue down hardwood or LVP over a wet slab without voiding the warranty.
Self-Leveling Underlayment
Self-leveler turns a wavy slab into a flat one. The workflow looks like this:
- Vacuum the slab clean and check for cracks wider than 1/8 inch (fill with epoxy crack filler first).
- Roll on the manufacturer’s primer (Mapei Primer L or Ardex P 51) and let it dry 1 to 3 hours.
- Set perimeter foam or backer rod to contain the pour at door thresholds and walls.
- Mix the leveler in 5-gallon batches with a paddle on a low-speed drill.
- Pour and spread with a gauge rake, then break the surface tension with a smoother.
Most levelers are walkable in 2 to 4 hours and ready for flooring in 16 to 24 hours. Plan on $1.20 to $2.50 per sq ft in materials for a 1/4-inch pour. Brands I trust include Mapei Ultraplan 1 Plus, Ardex K 15, and Henry 555.
Cracks, Joints, and Movement
Hairline cracks under 1/16 inch can usually be ignored under floating floors. Wider cracks need filler or stitching with carbon-fiber staples set in epoxy. Active cracks (those that move with seasons) require an isolation membrane like Schluter DITRA or Custom Crack Buster Pro to keep movement from telegraphing.
Honor every control joint and expansion joint in the slab. Filling them with epoxy or covering them with rigid flooring is the fastest way to get a cracked tile or a buckled plank one season later.
Frequently Asked Questions
How long should a new concrete subfloor cure before flooring?
Minimum 60 days for a 4-inch slab, 90 days for thicker slabs, and always confirm with a moisture test. Some manufacturers want 90 days regardless.
Do I need a vapor barrier over a concrete subfloor?
For below-grade slabs and slab-on-grade installs, yes. Above-grade slabs in dry climates may not need one if moisture tests pass.
Can I install hardwood directly on a concrete subfloor?
Engineered hardwood, yes — solid hardwood, not directly. Solid wood needs plywood subfloor sleepers or an engineered glue-down with topical moisture mitigation.
How flat does a concrete subfloor need to be?
Industry standard is 3/16 inch over 10 feet for most flooring, tighter (1/8 inch) for click-lock LVP and engineered hardwood.
What is the cheapest way to level a concrete subfloor?
Grind high spots and patch low spots with floor patch (under $15 per bag) for minor variations. Self-leveling underlayment is the only fix for rooms with multiple low spots.
