What Flooring Can Go Over a Concrete Slab? Glue-Down vs Floating, Moisture Testing, and What to Buy

Quick answer: Luxury vinyl plank and engineered hardwood can both go directly over a concrete slab, as long as the slab tests within the product's moisture limit: up to 90% relative humidity by ASTM F2170 or 8 lbs per 1,000 sq ft per 24 hours by ASTM F1869 for COREtec Pro rigid-core vinyl,^{Source 1} and 75% or 3 lbs for Anderson Tuftex engineered hardwood.^{Source 2} Both guides require a slab flat within 3/16 inch in 10 feet (or 1/8 inch in 6 feet), and the hardwood guide says a new slab must cure at least 30 days before moisture testing even begins.^{Source 1Source 2} Solid hardwood is the one to leave off the list below grade unless its manufacturer recommends otherwise.^{Source 3}

A concrete slab comes up often when customers talk with us, and it usually arrives with the same question: should the floor be glued down or floated? The honest answer is that the slab decides more than the floor does. Its moisture reading, its flatness and its age determine which floors qualify and which installation methods are open to you.

This guide is about slabs under the main living areas of a home: slab-on-grade houses, ground-floor kitchens and family rooms, and additions poured on new concrete. If your project is a basement, our best flooring for basements guide covers below-grade rooms in depth, and we won't repeat it here.

We sell engineered hardwood from Shaw and Anderson Tuftex and luxury vinyl plank from Shaw and COREtec as an authorized dealer. Every threshold below comes from a manufacturer installation document, the National Wood Flooring Association (NWFA), ASTM International, or our own live catalog.

Flooring over concrete at a glance

The table pulls together the limits from COREtec's Pro SPC installation guide,^{Source 1} Shaw's residential resilient direct-glue guide,^{Source 4} Anderson Tuftex's engineered hardwood guide^{Source 2} and NWFA's current installation guidelines.^{Source 3} Each row is explained in the sections that follow.

Can luxury vinyl plank go directly on concrete?

Yes. COREtec's installation guidelines for its Pro rigid-core (SPC) products approve two methods, floating and glue-down, and allow both on grade, above grade and below grade.^{Source 1} Once the slab passes testing, the slab itself is the subfloor.

Our catalog shows how flexible vinyl is over concrete. All 466 active luxury vinyl plank products we carry (each color counted separately) list floating as an installation method, and 409 of them, or 87.8%, also list direct glue. The other 57 are floating-only, so check the installation line on the product page before you plan a glue-down.

Every one of those planks also comes with a pad bonded to the back. All 215 active Shaw vinyl products we carry show an attached pad, most often EVA, and all 251 active COREtec products do, 232 of them with attached cork. That pad cushions the floor. Whether it also counts as moisture protection over a slab is a separate question, and it's answered in the vapor retarder section below.

Two slab details apply specifically to vinyl. COREtec's Pro guide says the concrete pH should sit between 7 and 10.^{Source 1} Shaw's resilient direct-glue guide names ASTM F710, the standard practice for preparing concrete floors to receive resilient flooring, as the prep standard for new and existing slabs, including how alkalinity is tested.^{Source 4} If you're still choosing between rigid cores, our SPC vs WPC guide explains the difference.

Can engineered hardwood go on concrete, on grade and below grade?

Yes. NWFA's current installation guidelines say engineered wood floors — what this means can be installed above grade, on grade and below grade,^{Source 3} and NWFA's concrete subfloor bulletin says solid and engineered floors alike can go over concrete that has been properly prepared for wood.^{Source 5} Solid wood is the exception below grade: the same guidelines say solid flooring should not be installed there unless its manufacturer recommends otherwise.^{Source 3} Every hardwood floor we sell is engineered. Our solid vs engineered hardwood guide covers how the two are built.

"Below grade" has a precise meaning. NWFA's current guidelines define it as any portion of the subfloor where wood flooring is to be installed that sits 3 inches or more below ground level.^{Source 3} Because the definition covers any portion of the subfloor, check the ground level along every side of the room, not only at the entrance.

Every engineered hardwood product we carry lists glue-down as an approved method. All 315 active engineered hardwood products in our catalog include glue among their installation methods, and 279 of them (88.6%) also list floating: 184 of 190 Shaw hardwood products and 95 of 125 Anderson Tuftex products. The 36 that don't list floating are 27 glue-only products plus 9 Anderson Tuftex products rated for nail, staple or glue.

NWFA's guidelines have separate installation chapters for nail-down, glue-down and floating floors, and they state that a manufacturer's recommendations for a specific product should always supersede NWFA's own.^{Source 3} Your product's own installation guide is the final word.

Concrete strength matters for wood, too. NWFA's concrete subfloor bulletin calls for a slab of at least 3,000 psi,^{Source 5} and its installation guidelines name 3,000 psi as the minimum for most standard wood floor installations, glue-down included.^{Source 3} If nobody can tell you what type of concrete you have, NWFA's guidelines say to run a nail forcefully across the surface. If it leaves an indentation, the slab needs a sealer or densifier compatible with the adhesive, and the adhesive manufacturer should tell you which one.^{Source 3} For glue-down over lightweight concrete, the same guidelines say to check with the adhesive manufacturer for approved installation methods.^{Source 3}

Glue-down or floating over a slab: which do you need?

For most floors we carry, both methods are approved, so the choice comes down to three things: the methods your exact product lists, what your slab tests at, and whether the concrete is strong enough to bond to. Here's how each one plays out.

When the product or the slab decides for you

• The product is floating-only. 57 of our active vinyl products list floating and no glue option. Float them.
• The product doesn't list floating. 36 of our active engineered hardwood products don't. Glue them.
• The slab is lightweight concrete. Anderson Tuftex's engineered hardwood guide allows only the floating method over lightweight concrete of 100 pounds or less per cubic foot, and only for 5-ply products 3 inches or wider.^{Source 2} For glue-down wood over lightweight concrete, NWFA's guidelines say to check with the adhesive manufacturer for applicable installation methods.^{Source 3}

What floating over a slab involves

A floating floor — what this means isn't bonded to the concrete, so what goes between the two matters. COREtec's Pro guide recommends a 6-mil poly film underlayment for floating installations over concrete, and says an additional vapor retarder rated 0.1 perm or less may be added for extra protection.^{Source 1} COREtec also says its planks and tiles don't need to acclimate before installation, as long as they go down in a climate-controlled room.^{Source 6}

Floating engineered hardwood has its own rules. Anderson Tuftex's guide limits floating to products 3 inches or wider, requires 6-mil polyethylene over concrete on grade or below grade, and says a 2-in-1 floating floor underlayment may be used, following that underlayment manufacturer's instructions.^{Source 2} We carry two underlayments that combine foam with an attached vapor barrier in a single roll, both described as ideal for floating installations over concrete. Details are in the vapor retarder section.

What gluing down over a slab involves

A glued floor is bonded straight to the concrete surface, so the slab's chemistry matters to the adhesive. Shaw's resilient direct-glue guide says all concrete substrates should be tested for internal relative humidity by ASTM F2170, allows calcium chloride tests in addition, and points to the adhesive's own information for the acceptable moisture limits. It says pH readings below 7.0 or above 10.0 affect resilient flooring and adhesives negatively.^{Source 4}

The same guide asks for the flooring and adhesive to acclimate in the room for at least 48 hours, and for the permanent HVAC to run between 65°F and 85°F before, during and after installation.^{Source 4} One section says 72 hours of HVAC run-time beforehand and another says 7 days. When a document gives two figures, plan on the longer one.

For glued hardwood, four adhesives in our catalog carry moisture-barrier language, covered below. We would still test the slab and keep the readings when using one, for the warranty reasons explained later in this guide.

How do you test a concrete slab for moisture?

Use ASTM F2170 in-situ relative humidity probes, ASTM F1869 calcium chloride kits, or both, on a slab that has cured long enough to test, then compare each reading with the limit your product's installation guide sets for that specific test. The two tests measure different things, so a relative humidity percentage and a calcium chloride figure are never interchangeable.

ASTM F2170: probes inside the slab

ASTM describes F2170 plainly: a test method for determining relative humidity in concrete floor slabs using in situ probes, covering the quantitative determination of percent relative humidity.^{Source 7} It's the test Shaw's resilient direct-glue guide calls for on all concrete substrates.^{Source 4}

ASTM's own summary of the standard carries a caution worth taking seriously: the results indicate the slab's moisture condition only at the time of the test and only in the specific locations tested.^{Source 7} That's why the date and the probe locations belong in your records next to the numbers.

ASTM F1869: calcium chloride at the surface

F1869 measures the moisture vapor emission rate from a bare concrete floor, expressed as pounds of moisture per 1,000 square feet over 24 hours. It applies to below-grade, on-grade and above-grade (suspended) bare concrete floors.^{Source 8}

ASTM also lists where F1869 should not be used: on gypsum concrete or lightweight aggregate floors, over coatings on concrete or reactive penetrants, and over patching or leveling compounds.^{Source 8} If you're using calcium chloride kits, test the bare slab before any patching or leveling goes down.

What moisture limits do manufacturers set?

The limits vary a great deal by floor type, and if you're choosing between vinyl and wood, this gap is the most useful comparison in the guide. The figures come from COREtec's Pro SPC guide,^{Source 1} Anderson Tuftex's engineered hardwood guide^{Source 2} and Shaw's resilient direct-glue guide.^{Source 4}

Two notes on reading those documents. COREtec's guide states both of its limits in a single sentence and places the per-1,000-sq-ft-in-24-hours unit right after the 90% figure.^{Source 1} That unit belongs to the 8-lb calcium chloride figure, since F1869 is the test reported in pounds per 1,000 square feet over 24 hours;^{Source 8} the 90% figure is a plain relative humidity percentage.

Shaw's resilient guide gives two different pH statements in two different sections: one says pH must not exceed 10.0 and that readings below 7.0 cause problems, and another says between 5 and 10.^{Source 4} A reading between 7 and 10 satisfies both statements, and it matches COREtec's range too.

Is a plastic sheet taped to the slab good enough?

Not for these documents. Every limit above is written as a percentage from F2170 probes or a pounds figure from F1869, and a sheet of plastic taped to the floor produces neither number. If the reading is what the installation guide measures against, the reading is what you need.

Who runs the test, and should you trust the result?

Shaw's resilient guide is direct about responsibility: running the tests may not be the flooring installer's job, but making sure they were done and that the results are acceptable before installation is.^{Source 4} Anderson Tuftex's hardwood warranty goes further and assigns moisture testing, including relative humidity, pH and calcium chloride, to the installer.^{Source 9}

Whoever runs it, ask for the results in writing. Because both ASTM methods produce a number, you never have to rely on someone saying the slab looks fine. Our guide to finding a flooring installer covers the questions worth asking up front.

Keep your readings with your warranty papers

This is the advice we give every customer with a slab: test it, and keep the results. A useful record includes:

1. The test method (ASTM F2170, ASTM F1869 or both) and the reading from each location
2. The date of each test, plus a sketch or photo showing where each probe or kit sat
3. The concrete pH reading
4. The flatness check, including any spots that were ground down or patched
5. The product name, style and color number, and the installation method used
6. Who ran the tests, and the vapor retarder or moisture-barrier adhesive used, if any

Store it with your receipt and a copy of the installation guide for your exact product.

Do you need a vapor retarder, and does an attached pad count?

Often you do, and no manufacturer document we found treats an attached pad as a substitute. Part of the confusion is that two different layers get called a vapor retarder, and they sit in different places.

The one under the slab. Shaw's resilient direct-glue guide says on-grade and below-grade slabs must have an effective vapor retarder directly under the concrete.^{Source 4} That layer goes in when the slab is built. It isn't something you add when the floor goes down.

The one on top of the slab. This is a membrane laid over existing concrete before the flooring. COREtec's Pro guide recommends 6-mil poly film for floating installations over concrete.^{Source 1} NWFA's concrete subfloor bulletin strongly recommends a vapor retarding membrane rated 0.13 perm or less, such as polyethylene film, for wood flooring over concrete.^{Source 5} Anderson Tuftex requires a proper vapor retarder whenever F2170 readings come in above 75%, and 6-mil polyethylene under floating installations over concrete on grade or below grade.^{Source 2}

About the attached pad. All 251 active COREtec products we carry have an attached pad, yet COREtec's Pro SPC guide still recommends a 6-mil poly film for floating over concrete.^{Source 1} We couldn't find a Shaw or COREtec document that says an attached pad can stand in for that film, so we treat the pad as cushioning and plan the poly as its own layer, unless your product's guide says otherwise. Our attached underlayment explainer covers what the pad does do.

The slab layers in our catalog

• 6-mil poly film: US101 6 Mil Black Poly, $18 for a 100 sq ft roll, and US160 6 Mil Poly Jumbo, $69 for a 400 sq ft roll. Both are described as recommended in the manufacturer install guidelines for floating installations over concrete, and not for use over wood subfloors.
• Seam tape: SL101 Selitac Tape, $10 for a 1.96 in. by 164 ft roll that seals about 600 sq ft. Its description notes that film seams must be taped over concrete per the install guidelines.
• Foam plus vapor barrier, for floating hardwood: 166SM Shaw 2 In 1 Pro and 104AU Anderson Tuftex 2 In 1 Pro 200, $39 each for a 200 sq ft roll.
• Moisture-barrier adhesives, for glued hardwood: Shaw HS105 and Anderson Tuftex A35MC (both 35MC, $199 to $253 for a 4-gallon pail covering 120 to 140 sq ft), plus Shaw 111SA and Anderson Tuftex 124AN (both 3 In 1 Ultra, $199). HS105 and A35MC are described as best for concrete slabs.

How flat does a concrete slab need to be?

Within 3/16 inch over 10 feet, or 1/8 inch over 6 feet. That same tolerance appears in COREtec's Pro guide,^{Source 1} Anderson Tuftex's engineered hardwood guide,^{Source 2} Shaw's resilient direct-glue guide^{Source 4} and NWFA's concrete subfloor bulletin, which expresses it as a 6-foot or 10-foot radius.^{Source 5}

Shaw's guide adds a second way to state the spec: overall F-Number values of FF 36 and FL 20 may be appropriate for resilient floor coverings.^{Source 4}

Order matters when a slab needs both testing and patching. Because ASTM excludes patching and leveling compounds from the calcium chloride test,^{Source 8} run any F1869 kits on bare concrete before the low spots get filled.

How long does a new concrete slab need to cure?

At least 30 days before moisture testing can begin, according to Anderson Tuftex's engineered hardwood guide.^{Source 2} Thirty days is when you're allowed to start testing, not when the slab will pass. We couldn't find an NWFA or manufacturer figure that predicts how long a new slab takes to dry to a passing reading, so the test result, not the calendar, tells you when the slab is ready.

Shaw's resilient direct-glue guide adds two curing details for new concrete: wet curing for 7 days is its preferred method, and curing compounds should be removed 28 days after the concrete is placed so it can begin drying.^{Source 4}

If the new slab has radiant heat, Shaw's guide calls for the system to run for at least 2 weeks before installation to reduce residual moisture. It should be turned down to 65°F three days before installation, then raised gradually in 5°F steps afterward, never above 85°F.^{Source 4} Our radiant heat flooring guide covers which floors pair well with it.

What happens to your warranty if you skip moisture testing?

You give up the proof that the floor went down according to the manufacturer's guidelines, and moisture is where the warranty language is most direct.

• Anderson Tuftex hardwood: the limited residential warranty says the floor must be installed according to Shaw's installation guidelines, that damage caused by improper installation methods isn't covered, and that the warranty does not apply to improperly installed flooring. It also assigns moisture testing to the installer.^{Source 9}
• COREtec Pro SPC vinyl: the installation guide says a failure to mitigate moisture may cause the flooring to fail and may void the warranty, and that Shaw Industries does not warrant damage to floor covering caused by moisture-related issues.^{Source 1}

So the warning isn't just something installers say. It comes straight from the manufacturers' own documents. A dated set of readings within your product's limits is the clearest record you can have that the slab was ready.

Which floor should you choose for your slab?

Choose rigid-core luxury vinyl plank if:

• Your slab reads above 75% relative humidity or 3 lbs, but within the limits in your vinyl's guide. COREtec Pro allows up to 90% and 8 lbs.^{Source 1}
• You want floating available on every option you look at. All 466 active vinyl products we carry list it.

Choose engineered hardwood if:

• Your slab reads 75% relative humidity or less and 3 lbs or less, or you're prepared to add a proper vapor retarder above 75%.^{Source 2}
• You want real wood on a slab at any grade. NWFA's guidelines say engineered wood can be installed above, on and below grade, while solid wood shouldn't go below grade unless its manufacturer recommends it.^{Source 3}

Choose glue-down if:

• Your product doesn't list floating, as with 36 of our active engineered hardwood products.
• You're gluing vinyl to a slab that meets Shaw's direct-glue requirements, including F2170 testing and a pH between 7 and 10.^{Source 4}

Choose floating if:

• Your product is floating-only, as with 57 of our active vinyl products.

Skip solid hardwood on any slab that sits below grade, unless its manufacturer recommends it. For a full side-by-side of the two floors we recommend, see our LVP vs engineered hardwood guide.

Your slab checklist before install day

1. Confirm the grade. Any part of the subfloor sitting 3 inches or more below ground level counts as below grade.
2. Check the slab's age. A new slab needs at least 30 days before testing starts.
3. Test and record. ASTM F2170, ASTM F1869 or both, with dates and locations written down.
4. Check pH. For vinyl, aim for 7 to 10.
5. Check flatness. 3/16 inch in 10 feet or 1/8 inch in 6 feet, and test before you patch.
6. Match the method to the product. Use only the methods listed for your exact style and color.
7. Plan the vapor retarder. For floating vinyl, 6-mil poly with taped seams. For floating Anderson Tuftex hardwood, the 6-mil poly its guide requires over on- and below-grade concrete. For glued hardwood, consider a moisture-barrier adhesive. For Anderson Tuftex hardwood, a proper vapor retarder whenever F2170 readings top 75%.
8. Condition the room. For glue-down vinyl, Shaw's guide calls for HVAC between 65°F and 85°F and 48 hours of acclimation for the flooring and adhesive.
9. File everything. Readings, photos, product details and the installation guide, kept with your warranty papers.

A slab is one of the most predictable subfloors you can have once you know its numbers. Get the readings first, then choose the floor that fits them, and order a few samples to see how they look in your own light before you commit.