How to Lay Hardwood Floor Over Concrete: Test, Prep, Install

You can lay hardwood over concrete, but only after the slab passes a moisture test. The NWFA’s 2025 guidelines set the in-slab humidity line at 80% RH, though many flooring warranties are stricter, so your manufacturer’s number rules. Skip testing and the boards can cup or buckle and the warranty can fail. This guide covers which tests to run, which method fits solid versus engineered wood, and how to prep and install.

Test the Slab Before You Buy Any Flooring

Concrete is never fully dry. It holds water from the mix, from the ground below, and from the air above, and a slab that tested fine in a dry August can read high after a wet spring. That is why moisture testing is step one, before you choose flooring or order a single box.

One caution first: a test only describes the slab on the day you ran it. The NWFA notes that without an effective vapor retarder directly under the slab, readings tend to rise once the floor covers it, because ground moisture keeps moving up. Treat a passing result on an older slab with no known retarder as a reason to add a retarder anyway, not as permission to skip one.

Which Moisture Test to Use and What the Numbers Mean

Four methods appear in the NWFA guidelines. Only the first two give you a pass-or-wait number for most homes. Here is how they compare, using the NWFA’s own limits for wood flooring.

Test	What it measures	NWFA action line	How it is run
ASTM F2170 (in-situ RH probe)	Relative humidity inside the slab	Over 80%: wait, or add a Class I retarder	3 tests in the first 1,000 sq ft, 1 per extra 1,000 sq ft; holes at 40% of slab depth for slab-on-grade; sleeves capped at least 24 hours
ASTM F1869 (calcium chloride)	Pounds of water emitted per 1,000 sq ft per 24 hours	Over 3 lb: wait, or add a Class I retarder	3 kits in the first 1,000 sq ft; 60 to 72 hours sealed; surface ground lightly first
ASTM D4944 (calcium carbide)	Moisture as a percent of the sample’s dry mass	Over 2.5%: Class I retarder; over 4% may be unacceptable	Concrete is sampled and pulverized, so it is rarely a DIY test
ASTM D4263 (plastic sheet)	Whether capillary moisture is present	None. Not a go or no-go test	18 x 18 in sheet of 4 mil poly taped down for 16 hours, 1 per 500 sq ft

The 75% myth. Many articles state 75% RH as the hardwood limit. That figure does show up in some manufacturer instructions and in ASTM F2170 guidance, but the current NWFA guideline text uses 80% as the line where you should wait or add a retarder. The practical rule: use whichever limit your flooring and adhesive manufacturers print, since that is what a warranty claim will be judged against. Read both documents before the test, not after.

NWFA limits for a slab under wood flooring: calcium chloride over 3 lb/1,000 sq ft/24 hr, or in-situ RH over 80%. Source: NWFA Wood Flooring Installation Guidelines, 2025

Two more details trip people up. The test room should sit at normal living conditions for 48 hours beforehand, and calcium chloride kits are not valid on lightweight concrete, where ASTM disallows them and RH probes are the right tool. A handheld electrical meter (ASTM F2659) is useful for mapping damp zones across the slab, but the NWFA says it should not make the go or no-go call on its own.

Pick the Right Method for Your Hardwood

Solid and engineered flooring do not have the same options over concrete. The deciding fact most guides skip is that solid wood is not allowed to float unless the maker says so.

Flooring	Glue-down to slab	Floating	Nail-down over plywood subfloor
Solid hardwood	Only if flooring and adhesive makers both approve	Never, unless the manufacturer recommends and warrants it	Yes, the usual route
Engineered hardwood	Yes, with the adhesive maker’s moisture and trowel specs	Yes, the only wood flooring the NWFA allows to float	Yes, if the maker allows it

Solid wood moves too much with the seasons to hold up floated, which is the NWFA’s stated reason. If you want the least prep and a floating or glue-down floor, engineered is the realistic choice. If you want solid boards, plan on a plywood subfloor system, covered below.

Prepare the Slab: Flatness, Joints, and Dust

Wood flooring does not need a level slab, but it does need a flat one. The NWFA tolerance is 1/8 inch in 6 feet or 3/16 inch in 10 feet, unless the flooring manufacturer specifies something tighter. Check it with a straightedge, string line, or laser, and rotate the straightedge 180 degrees at each spot so you do not trust one reading.

• High spots: grind them down with an angle grinder fitted with a dust shroud and a diamond cup wheel.
• Low spots: fill with a patching compound or self-leveler that the adhesive or flooring maker approves. Test moisture first, since the NWFA says moisture must be assessed before applying patches or levelers.
• Strength for glue-down: glue-down or mechanically anchored panels call for slab compressive strength of at least 3,000 psi.
• Contaminants: sealers, curing compounds, paint, and old adhesive can block bond or hide moisture, so remove them before testing and gluing.

Do not cover moving joints. Control, isolation, and expansion joints in the slab are meant to move. The NWFA says wood flooring secured to the slab should not bridge them without a breaking point, and the joints should not be filled with underlayment. In practice that means a transition strip or expansion gap above the joint; our guide to transitioning hardwood floors between rooms covers the hardware.

Silica dust is a real hazard. Grinding concrete and mixing leveler release respirable crystalline silica, which the NWFA links to silicosis and lung cancer with long exposure. Use a shrouded grinder on a dust extractor, a HEPA vacuum between passes, never dry sweeping or compressed air, and a respirator suited to silica. For a one-room DIY grind, consider renting a grinder with built-in dust collection or hiring the grinding out.

Plywood Subfloor Systems Over Concrete

A plywood or OSB layer lets you nail or staple a solid or engineered floor and puts wood under the flooring that behaves like wood. The NWFA describes three systems. All of them sit over a Class I vapor retarder, and the NWFA says to always plan for one on-grade and below-grade.

System	Panels	Attachment	Key details
Floated double layer	Two layers, each at least 11/32 in, 4 x 8 sheets	Bottom layer loose; top layer fastened 6 in on edges and 12 in grid in the field with ring or screw shank nails or screws	Offset top seams at least 4 in; 1/16 to 1/8 in gaps; 3/4 in at walls
Single layer, glued	At least 23/32 in, cut to 2 x 8 or 4 x 4 ft	Elastomeric adhesive over a compatible liquid-applied Class I retarder	Kerf the back 3/8 in deep on a 12 in grid; 1/8 in gaps; 3/4 in at walls
Mechanically anchored	At least 23/32 in, full sheets or cut	Powder-driven pins, screws, or other fasteners made for concrete	Seal each penetration with the retarder maker’s mastic; 1/8 in gaps; 3/4 in at walls

Pressure-treated plywood is out unless it was kiln-dried after treatment and conditioned, and the NWFA says to pick flooring fastener lengths that stop short of the vapor retarder. Once the subfloor is down, the nailing schedule, layout, and gap specs follow the same rules as any wood subfloor, which our hardwood nail-down guide walks through.

Step-by-Step: How to Lay Hardwood Floor Over Concrete

1. Condition the space: keep the room at normal living temperature and humidity. The NWFA general range for solid flooring is 30 to 50% RH and 60 to 80 F, and the flooring maker’s range wins if it differs.
2. Test the slab: run F2170 or F1869 tests at the counts above and write down every reading with the date.
3. Decide on moisture control: if any reading is over the limit, wait and retest, or install a Class I vapor retarder compatible with your adhesive.
4. Flatten and clean: grind high spots, fill low spots, and remove sealers and contaminants. Re-check flatness.
5. Lay the subfloor or underlayment: follow the system you chose, keeping the 3/4 in expansion space at walls and obstructions.
6. Check wood moisture: measure boards from several bundles with a meter set for the species. The USDA Forest Products Laboratory average interior moisture content is about 8% (6 to 10%) in most of the US, but your flooring maker’s target is the one that counts.
7. Start the first row straight: snap a line parallel to the longest wall and keep the expansion gap the flooring maker specifies at every wall.
8. Install the field: nail or staple to the plywood, trowel adhesive with the notch the adhesive maker specifies, or click and glue a floating floor as the product allows. Stagger end joints and work from several boxes.
9. Finish the edges: install base and shoe molding that covers the gap without pinning the floor down, and add transitions at doorways. If the floor ends at a stair, see our guide to installing hardwood flooring on stairs.

Common Mistakes That Ruin Hardwood Over Concrete

• Relying on the plastic sheet test as a pass.
• Testing once in one corner instead of at the required count and near exterior walls.
• Trusting the slab’s age instead of readings.
• Floating solid hardwood without written approval from the manufacturer.
• Using fasteners long enough to puncture the vapor retarder.
• Bridging a slab control joint with continuous flooring and no break.
• Grinding or sweeping dry concrete dust with no extraction.

“Never install a wood floor or underlayment over a known moisture condition.”

NWFA Wood Flooring Installation Guidelines, 2025

Frequently Asked Questions

Can you nail hardwood directly into a concrete slab?

Not with ordinary flooring nails. Over concrete, nail-down floors go on a plywood or OSB subfloor system, and that subfloor is either floated, glued, or mechanically anchored with fasteners made for concrete. Match the flooring fastener length to the subfloor thickness so nothing pierces the vapor retarder below.

Do you need a vapor barrier under hardwood on concrete?

Often yes. The NWFA says to install a Class I impermeable vapor retarder when calcium chloride readings exceed 3 pounds, RH exceeds 80%, or calcium carbide readings exceed 2.5%. On and below grade, it recommends a Class I retarder every time, because slab moisture changes over the years.

Can engineered hardwood go over concrete without plywood?

Yes, if the manufacturer allows it. Engineered flooring can be glued down or floated directly over a flat, dry, tested slab, and it is the only wood flooring the NWFA allows to be floated. Use the underlayment, adhesive, and moisture limits written in that manufacturer’s installation instructions.

How long should a new slab cure before installing hardwood?

There is no safe calendar date. The NWFA says moisture testing of a drying slab may begin 30 days after placement, but readings, not the age of the concrete, decide when you can install. Slabs also gain moisture later if the vapor retarder under them is damaged or missing.

Is the plastic sheet test enough for hardwood over concrete?

No. The plastic sheet method (ASTM D4263) only shows whether capillary moisture is present. The NWFA says it should not be used to decide whether a floor can be installed. Use an in-situ RH test or a calcium chloride test to get a number you can compare to a limit.

Hardwood over concrete is a testing and preparation job first and an installation job second. Run the right moisture test, fix flatness, choose a method your flooring allows, and keep documentation of every reading. For a wider look at species and floor types, see our wood flooring guide, and once the floor is down, our engineered hardwood care guide keeps it looking right.