Subfloor for Laminate Flooring: Flatness, Plywood and Prep

Most laminate failures have nothing to do with the planks. Joints that peak, floors that click underfoot, and gaps that open in winter usually trace back to the surface underneath. A good subfloor for laminate flooring has to be flat, solid, dry and clean, and it can be plywood, OSB, concrete or even an existing hard floor as long as it meets those four tests.

Laminate is a floating floor. The planks lock to each other rather than to the subfloor, which makes the whole floor act like one thin sheet resting on a pad. That sheet bridges small imperfections, but it cannot bend over humps or sink into dips without stressing the locking joints. Most manufacturers ask for flatness within about 3/16 inch over 10 feet, or 1/8 inch over 6 feet, with no sudden changes in height.

Get the base right and laminate is one of the easiest floors to install. Skip the prep and even an expensive plank will squeak, flex and wear out early.

What a Subfloor for Laminate Flooring Needs

Before you buy flooring, evaluate the base against these requirements. The exact numbers vary by product, so always check the installation sheet in the box.

• Flat: Typically within 3/16 inch over 10 feet. Flatness matters more than level. A floor that slopes gently from one wall to another is fine; a floor with a dip in the middle is not.
• Structurally sound: No bounce, deflection or loose panels. Wood subfloors should be fastened tight to the joists.
• Dry: Wood should read within the moisture range the flooring maker allows, and concrete needs a moisture test and a vapor barrier.
• Clean: Free of debris, adhesive ridges, drywall mud and old nails that could press through the pad.

Laminate is also unforgiving of soft layers. Carpet and carpet pad must come out, and cushioned sheet vinyl may need to be removed or covered depending on the product rules.

Plywood, OSB and Concrete: How Each Performs

Plywood

Plywood in the 5/8 inch to 3/4 inch range is the classic wood subfloor and an excellent base for laminate. It holds screws well and is fairly stable when humidity swings. Thinner 1/2 inch plywood over joists at 16 inches on center often feels springy and may need an additional layer.

OSB

Oriented strand board at 23/32 inch is standard in newer homes and works fine under laminate when it is dry. Its weak spot is the panel edges, which can swell if they got wet during construction or from a leak. Run a straightedge across seams and sand or plane any ridges before you install.

Concrete Slabs

Concrete is flat and solid but carries moisture. Every slab, especially basements and slabs on grade, needs a 6 mil polyethylene vapor barrier or an underlayment with a built-in vapor barrier. Overlap seams by about 8 inches and tape them. Grind high spots and fill low ones before the barrier goes down.

Existing Hard Floors

Laminate can usually float over well-bonded ceramic tile, sheet vinyl or hardwood if the surface is flat. Deep grout lines may need a skim coat of patch so they do not telegraph through. If the old flooring is a 9 inch by 9 inch tile or sheet goods from before the 1980s, assume it could contain asbestos and leave it in place or have it tested before removing it.

Tools and Materials

• 6 to 10 foot straightedge or long level, plus a tape measure and marker
• Moisture meter for wood (pin or pinless), and a calcium chloride kit or in-situ probe for concrete
• Drill driver and subfloor screws
• Belt sander or concrete grinder for high spots
• Floor patch for shallow dips and seams
• Primer and self-leveling compound for larger low areas
• Vapor barrier film or a combination underlayment
• Shop vacuum, dust mask or respirator, safety glasses and gloves

How to Check and Level the Subfloor

1. Remove old soft flooring and fasteners. Pull carpet, pad and tack strip. Set or pull protruding nails and staples.
2. Tighten the wood subfloor. Drive screws into the joists every 6 to 8 inches along any squeaky or loose area. Screws hold better than the original nails and end most squeaks.
3. Map high and low spots. Lay the straightedge in several directions across the room and mark any gaps over 3/16 inch with a marker. Circle humps differently from dips.
4. Take down the high spots. Sand ridges at wood seams with a belt sander. On concrete, grind humps with a diamond cup wheel and use dust controls.
5. Fill shallow dips. Trowel a cement-based patch into small low spots and feather it out. Let it cure per the label before sanding it smooth.
6. Use floor leveling compound for larger areas. Prime the subfloor, set dams at doorways, mix the compound to the specified water ratio, and pour it into the low area. Spread with a gauge rake or smoother and let it find its level. Wood subfloors usually need metal lath or a specific primer system, so follow the product directions for wood.
7. Test moisture. Check wood with a meter and concrete with an approved test. Correct any problem before going further.
8. Clean thoroughly. Vacuum every bit of grit. A single pebble under a plank can become a click or a bump.
9. Install the underlayment. Roll out foam or combination pad per the flooring instructions, with vapor barrier where required.

Leveling subfloor for laminate is mostly about patience. A day spent with a straightedge and patch is cheaper than tearing up a finished room because the joints keep separating.

Moisture: The Hidden Subfloor Problem

Laminate has a fiberboard core that swells when it absorbs moisture, and swollen edges never fully recover. On wood subfloors, readings should generally sit within a few percentage points of the laminate itself, and many makers want the subfloor under about 12 percent. On concrete, follow the limits on the installation sheet and never skip the vapor barrier.

Crawl spaces deserve attention as well. Over a dirt crawl space, cover the ground with a sealed vapor barrier and confirm the space is vented or conditioned. Damp air rising from the crawl space can cup laminate from the bottom up even when the room itself looks dry.

Acclimate the planks in the room for 48 hours, or as directed, with the HVAC running at normal living conditions. That lets the boards settle near the moisture level they will live at.

Troubleshooting Common Laminate Subfloor Problems

• Clicking or hollow sound: Usually a dip under the floor. The plank flexes into the void with each step. Fix the low spot or the noise will continue.
• Peaking at joints: Often a hump, or too little expansion space at the walls. Check that the floor can float freely with the recommended gap at every wall and doorway.
• Gaps opening between planks: Can come from a flexing subfloor or big humidity swings. Tighten loose subfloor and keep indoor humidity steady.
• Swollen edges: A sign of moisture coming up through the slab or subfloor, or a spill that sat. Find and correct the source before replacing boards.
• Bounce over joists: Thin subfloor or long joist spans. Add a layer of 1/4 inch to 1/2 inch plywood screwed down, or reinforce the joists.

Cost to Prepare a Subfloor

Prep costs for a subfloor for laminate flooring depend on what you find once the old floor is up. In general terms:

• Screwing down and spot-patching a mostly flat subfloor: a modest cost for materials, often under $100 for an average room
• Self-leveling compound: roughly $1 to $3 per sq ft for DIY materials at shallow depths, and more when installed by a pro
• New plywood overlay: commonly $2 to $4 per sq ft installed
• Replacing water-damaged subfloor sections: varies with the extent of damage and access

When to Call a Pro

Call a flooring contractor when dips are deeper than about 1/2 inch, when large areas need self-leveling compound, or when the subfloor shows water damage or rot. A structural engineer or framing contractor should look at floors with noticeable bounce, sagging joists or cracked framing. If a plumbing leak caused the damage, have a licensed plumber fix it first, and treat any mold before new flooring goes over it. Wear a dust mask when sanding and a respirator rated for silica when grinding or mixing cement-based products, and use wet methods or a vacuum with a HEPA filter for cleanup.