Flooring Guides · Hardwood
Engineered Hardwood Over Concrete: Glue-Down or Floating?
Many Los Angeles homes, condos and ground-floor rooms sit on a concrete slab. Engineered hardwood is the usual choice there, but the slab decides a lot: how much preparation the floor needs, which installation methods the manufacturer allows, and what the project costs. Here is what to understand before you compare quotes.
By Los Angeles Hardwoods ·
Why the slab matters
Wood moves with moisture; concrete does not. A slab can also hold water from the ground, plumbing, irrigation or the original pour for a long time. Getting hardwood to last on concrete comes down to three things done in order: check the moisture, make the slab flat enough, then use an installation method the flooring manufacturer approves for that product.
That is why two quotes for "the same floor" can differ so much. One may include testing, grinding and leveling, and the other may assume the slab is ready.
Engineered, not solid, in most cases
Engineered hardwood has a real wood top layer bonded to a cross-layered core. The core keeps the plank more stable as humidity changes, which is why most engineered floors can be glued to a slab or floated over it. Most solid hardwood is not approved for gluing directly to concrete; it typically needs a wood subfloor built on top of the slab, which adds height that doors, stairs and appliances may not allow. Learn more about engineered hardwood and European white oak.
Step 1: moisture testing
Manufacturers publish moisture limits for their floors and adhesives and usually require a test result to keep the warranty valid. The two common methods are:
- In-slab relative humidity (ASTM F2170). Probes are set into holes drilled into the slab at a depth based on its thickness, then read after they stabilize. This is the method the industry increasingly relies on because it shows what the slab will settle at once it is covered.
- Calcium chloride (ASTM F1869). A dish under a sealed dome measures how much moisture leaves the surface over a set period.
The result decides the next step. Within limits, installation can go ahead with the specified adhesive. Above limits, the options may be a moisture-control adhesive or membrane approved by the manufacturer, or more drying time. A test that is skipped is the most common reason a floor on concrete fails later.
Step 2: flatness
Every manufacturer sets a flatness tolerance. The National Wood Flooring Association's guideline for wood is no more than 3/16 inch of variation within a 10-foot radius, or 1/8 inch within 6 feet, and some products are stricter. Wide planks and glue-down installations are less forgiving than narrow planks and floating floors.
High spots and old adhesive ridges are ground down. Low spots are filled with a cement-based patch or a self-leveling underlayment that is compatible with the adhesive. Old flooring, tack strip and glue have to come off first. More on floor removal and subfloor preparation.
Step 3: glue-down or floating
Full-spread glue-down. The planks are set into adhesive troweled over the whole slab. It gives the most solid feel and the quietest step, and the floor stays low, so transitions to tile or stone can be close to flush. It asks more of the slab: flatness and moisture have to be right, and the adhesive, labor and cure time add cost. Traffic is usually restricted for a period the adhesive maker sets.
Floating. The planks lock or glue to each other, not to the slab, and sit on an underlayment. It can be faster, it can go over some surfaces a glued floor cannot, and the underlayment can include a moisture barrier. The trade-offs: a hollower sound underfoot, a slightly springier feel, and the need for expansion space at walls and transitions. Not every engineered product is approved for floating, and some limit plank width or room length.
Two practical rules: use the method the manufacturer approves for that exact product, and choose it before ordering, because the method changes the adhesive or underlayment, the transitions and the estimate.
How to choose for your home
- Condo or upstairs neighbors below? Sound rules may decide for you. Many associations require an acoustic underlayment or membrane with a tested rating. See our guide to condo and HOA flooring rules.
- Want the most solid feel and low transitions? Glue-down, if the slab can be made flat and the moisture reading allows it.
- Slab with minor moisture or an uneven surface you want to bridge? A floating floor over the right underlayment may be the better fit, if the product allows it.
- Stairs or tile next to the new floor? Plan floor height and transitions together. Read about hardwood stair installation.
What we look at during the estimate
We come to your home with samples and measure every room. On a slab we look at what is on it now, how flat it is, where transitions fall and whether stairs or baseboards are part of the job, and we talk through glue-down and floating for the floor you like. The written quote spells out material, preparation and installation. For pricing, call (323) 498-3735 or book a free in-home estimate. Read more about our hardwood flooring installation service.
Common questions
Can solid hardwood go directly on concrete?+
Usually not glued straight to the slab. Solid hardwood moves more with moisture, so on concrete it normally needs a wood subfloor system built over the slab, which raises the floor height. Engineered hardwood is designed for slabs and is the more common choice. The manufacturer's instructions for the specific product decide what is allowed.
Is glue-down or floating better on a slab?+
Neither is better in every home. Glue-down feels and sounds more like a traditional wood floor and keeps transitions low, but needs a flatter slab and more preparation. Floating can be quicker and can go over some surfaces a glue-down floor cannot, but it sounds hollower underfoot and needs room to expand at walls. The product you choose may only be approved for one method.
Does a concrete slab need a moisture test before hardwood?+
Yes. Every slab holds some moisture, and manufacturers set limits for their floors and adhesives. They typically require a test, such as in-slab relative humidity probes or a calcium chloride test, to keep their warranty valid. The reading decides whether you need a moisture-control adhesive, a membrane or more drying time.
What if the slab is not flat?+
High spots and ridges are ground down and low spots are filled with a patch or self-leveling underlayment until the floor is inside the manufacturer's tolerance. Adhesive should not be used to fill dips. How much work this takes is one of the biggest variables in the estimate.