Engineered or Solid Hardwood on a Concrete Slab?
On a concrete slab, engineered hardwood is the practical choice. It can be glued directly to the slab or floated over an underlayment once the concrete passes moisture testing. Solid hardwood is normally nailed or stapled, so it needs a wood subfloor built over the slab first, which adds height, cost and another moisture barrier. Either way, the slab has to be dry, flat, clean and tested, and the flooring and adhesive manufacturers' limits come first.
Why concrete changes the decision
Concrete is porous and holds water from the day it is poured, and slabs poured on grade can keep picking up moisture from the ground below. That moisture moves up into whatever sits on top. Wood responds to moisture by swelling, so a floor laid on a damp slab can cup, gap or lose its bond. That is why the National Wood Flooring Association (NWFA) treats moisture testing and a vapor retarder as the core of any wood-over-concrete job, and why the flooring and adhesive manufacturers set their own limits.
Engineered vs. solid over concrete
Engineered hardwood has a real hardwood wear layer glued over a cross-layered core. The construction resists swelling and shrinking far better than a single piece of solid wood, which is what makes direct glue-down and floating installs possible on concrete. Solid hardwood is a single piece of wood and is designed to be nailed or stapled. You can't nail into concrete, so solid needs a plywood subfloor installed over the slab first, using one of the methods in the NWFA installation guidelines. That raises the floor, which affects doors, appliances and transitions to other rooms.
| Engineered, glue-down | Engineered, floating | Solid over a wood subfloor | |
|---|---|---|---|
| How it's attached | Full-spread adhesive glued to the slab | Boards joined to each other over an underlayment; not attached to the slab | Plywood built over the slab, then boards nailed or stapled |
| Feel underfoot | Solid and quiet | Can sound hollower; underlayment helps | Like a traditional wood floor |
| Moisture protection | Adhesive or sealer approved by the adhesive maker | Vapor retarder under the underlayment | Vapor retarder under the plywood |
| Added height | Least | Underlayment thickness | Most |
| Best for | Most single-family slabs | Some condos, or where adhesive isn't suitable | Owners set on solid boards with room to build up |
Moisture testing, explained
NWFA guidelines say new concrete should be at least 30 days old before you test it, and the result is only a snapshot of the slab on the day of the test. The common tests are:
- In-situ relative humidity probes (ASTM F2170). Probes are set into holes drilled in the slab and read after they stabilize. NWFA cites 75 percent relative humidity as a typical limit; above that, a vapor retarder is required or the slab needs more time to dry.
- Calcium chloride test (ASTM F1869). This measures moisture emitted from the surface over a set period. Many manufacturers cap it at 3 pounds per 1,000 square feet in 24 hours for glue-down floors.
- Moisture meters. These are useful for finding wet areas but, per NWFA, are a survey tool only and not an acceptance test.
The manufacturer's limit for your specific flooring and adhesive is the one that counts. If the slab tests too wet, the usual options are a moisture-control sealer or membrane approved by the adhesive maker, a floating install with a vapor retarder, or waiting for the slab to dry.
Flatness and condition
Wood can't bridge dips or rock over humps. NWFA's tolerance for concrete under wood floors is 1/8 inch in 6 feet or 3/16 inch in 10 feet, unless the manufacturer specifies otherwise. High spots are ground down and low spots filled with a leveling compound. The concrete should also be sound, at least 3,000 psi, with no old adhesive, paint, sealers, wax, cracks or hollow spots that could weaken the bond. For floating floors and plywood subfloors, NWFA strongly recommends a vapor retarder rated 0.13 perm or less, such as 6-mil polyethylene.
Picking an engineered product for a slab
- Check the installation methods the manufacturer approves; not every engineered floor is rated for glue-down or floating.
- Choose the wear layer for the life you want. Our European white oak comes in 2, 4 and 6 mm wear layers and 1/2" to 3/4" overall thickness.
- Match the adhesive system the flooring maker specifies, including any moisture-control layer.
For how our crews handle testing, leveling and acclimation on site, read our slab installation process. In Encino, for example, we glued down about 600 square feet of Provenza Antico Relic over a slab in about two days. To compare products, start with our engineered European white oak or the full range of engineered and solid floors.
See the boards before you decide
Bring your slab questions to someone who measures them. Visit our showroom at 21030 Victory Blvd, Woodland Hills, or book an in-home consultation. We bring flooring samples to your home. Call (818) 963-0878. Glamour Flooring is a family business serving the area since 2007, a licensed flooring contractor (CSLB #1005283) that installs the floors we sell.
Frequently asked questions
Can you install solid hardwood directly on concrete?
Not directly in most homes. Solid hardwood is nailed or stapled, so it needs a plywood subfloor built over the slab with a vapor retarder underneath. Engineered hardwood can be glued directly to the slab or floated.
How long does new concrete need before hardwood goes in?
NWFA guidelines say new concrete should be at least 30 days old before moisture testing, and the flooring goes in only after it passes the manufacturer's limits. Many slabs need longer than 30 days to dry.
What moisture level is acceptable for hardwood on a slab?
It depends on the flooring and adhesive manufacturer. Common limits are 75 percent relative humidity by ASTM F2170 probes, or 3 pounds per 1,000 square feet in 24 hours by the ASTM F1869 calcium chloride test.
Is glue-down or floating better on a concrete slab?
Glue-down feels more solid and quiet and adds the least height. Floating doesn't bond to the slab and suits some condos and slabs where adhesive isn't a fit. The right choice depends on the product, the slab test and any HOA sound rules.
What happens if my slab fails the moisture test?
The usual options are a moisture-control sealer or membrane approved by the adhesive manufacturer, a floating floor over a vapor retarder, or giving the slab more time to dry and testing again.
Do you test the slab before quoting?
Yes. We test the slab for moisture and check flatness before we quote, and any leveling or moisture work is listed on the written quote. Call (818) 963-0878 to book.