How do you lay laminate flooring on concrete?

Moisture test

ASTM F2170in-situ relative humidity probes

Vapor barrier

6 mil polymanufacturer-specified

Expansion gap

1/4–3/8 inmanufacturer-specified

Acclimation

48 hrscheck the product data sheet

Short version

• Run a moisture test before you buy anything. Slab moisture is what destroys laminate over concrete.
• Grind the high spots and fill the low spots until the slab meets the flatness tolerance in the instructions.
• Put down a vapor barrier, or an underlayment with one built in, and tape the seams.
• Leave an expansion gap at every wall, pipe, and door frame. The floor has to move.
• Cover the gap with baseboard or shoe molding fastened to the wall, never to the floor.

How do you test a concrete slab for moisture before laying laminate?

Moisture testing is the step that decides whether the job succeeds. Concrete slabs release water for months or years, and a slab that feels bone dry can still push enough vapor into a floating floor to swell plank edges, lift the finish, and grow mold under the underlayment.

Two ASTM methods cover this. ASTM F2170 measures relative humidity with probes drilled into the slab, which reads conditions at depth and is the method most manufacturers now require. ASTM F1869 uses anhydrous calcium chloride to measure moisture vapor emission from the surface. ASTM F710 is the standard practice for preparing concrete floors to receive resilient flooring and covers the broader prep sequence.

The pass or fail number comes from the flooring manufacturer, not from the test method, and it varies between products. Read the installation instructions for the exact limit, because installing over a slab that exceeds it usually voids the warranty.

A plastic sheet taped down overnight is a rough field check, not a test. If condensation forms underneath, you certainly have a moisture problem, but a dry sheet does not prove the slab passes.

How flat does the concrete need to be?

Flatness matters more than level. A floating floor can sit on a sloped slab without trouble, but it cannot bridge dips and humps. Where it spans a low spot, the click joints flex with every footstep, and eventually they wear, separate, and start to creak.

Manufacturers publish their own tolerance, commonly expressed as a maximum deviation over a 10-foot span, with roughly 3/16 inch being a widely used figure. Check the instructions for your product rather than assuming.

Find the problems with a long straightedge or a 6-foot level dragged across the slab in several directions. Mark every gap you can slide a pencil under.

Grind down the high spots with a concrete grinder fitted with dust collection. Fill low spots with a cement-based patching compound or a self-leveling underlayment, following the mixing and thickness limits on the bag. Let any patch cure fully before covering it; fresh patching material carries its own moisture.

Sweep and vacuum thoroughly at the end. A single pebble left under the underlayment telegraphs through as a hard bump underfoot.

Do you need a vapor barrier under laminate on concrete?

Yes, a vapor retarder goes between the slab and the laminate on every concrete installation. Concrete is permeable, and the barrier is what keeps slab moisture from reaching the plank cores.

A 6-mil polyethylene film is the common choice, run up the wall a couple of inches at the perimeter and lapped and taped at every seam. Many underlayments sold for concrete come with the film already bonded, which saves a step. Do not install two separate vapor barriers, though, because moisture trapped between them has nowhere to go.

The underlayment itself does a different job: it cushions the floor, evens out minor surface texture, and cuts the hollow sound of planks over a hard slab. Foam and felt products vary in thickness and acoustic rating, and some flooring ships with a pad already attached to each plank. Adding a second pad under pre-attached padding makes the floor too soft and stresses the joints.

Note that this film is not the same thing as the under-slab vapor retarder specified during construction, which is a heavier product installed beneath the concrete before it is placed.

How do you install the laminate over the slab?

Start by letting the flooring acclimate in the room where it will be installed, commonly for about 48 hours, with the boxes laid flat and the room at normal living temperature and humidity. Confirm the period in the instructions.

Plan the direction before you open a box. Running planks parallel to the longest wall or toward the main light source usually looks best, and it helps to measure the room width against the plank width so you do not finish with a sliver-width final row.

Lay the barrier and underlayment, then start the first row along the straightest wall with spacers holding the expansion gap. Cut the tongue off the boards facing the wall so they sit flat. Click each row into the last, stagger end joints from row to row so seams never line up, and keep offcuts to start the next row.

Work around pipes by drilling a hole slightly larger than the pipe and cutting a wedge out to the edge, then gluing the wedge back after the plank is in place. Undercut door casings and slide planks beneath them rather than cutting the plank to fit around them.

Finish by pulling the spacers and installing baseboard or shoe molding, fastened into the wall only. Anything screwed or nailed through the laminate pins the floor and causes buckling.

Can you put laminate on a basement floor?

Basement slabs are the highest-risk application, and many laminate products are rated for below-grade use as long as the slab passes the moisture test. Rating and warranty language both come from the manufacturer, so read them before buying.

Below-grade slabs carry more moisture than slabs above grade because they sit against damp soil, often without an under-slab vapor retarder if the house is older. Test in several locations, since moisture varies across a basement floor, and test near exterior walls where it is usually worst.

Fix the water source before installing anything. Grading that slopes toward the house, downspouts discharging at the foundation, and a slab that darkens after heavy rain all point to problems no vapor barrier will solve. Where the slab shows active seepage or efflorescence, deal with drainage first.

Where testing fails, the options are a moisture mitigation coating applied to the slab, a subfloor panel system that creates an air gap, or a flooring type better suited to damp conditions.

Common mistakes

• Skipping the moisture test — slab vapor swells plank edges and voids the warranty, and the damage only appears months after the job looks finished.
• Installing over an uneven slab — unsupported spans flex until the click joints separate, leaving gaps and creaks that cannot be repaired in place.
• Leaving no expansion gap — the floor grows against the walls and buckles into a ridge across the room.
• Fastening baseboard or transitions through the flooring — a single nail through a plank locks the floor and forces it to buckle elsewhere.
• Doubling up on padding — pad over pre-attached pad makes the floor spongy and overloads the joints.