How to Level a Subfloor for LVP or Laminate

Click-lock LVP and laminate are only as good as what is under them. Dips make planks flex at the joints, which causes clicking, hollow spots, and eventually broken locks. Humps make planks rock. The fix is usually not dramatic. Most floors need some sanding and a few low spots filled. Here is how to find the problems and fix each type.

Flat vs. level: what does your floor actually need?

People say "level," but flooring cares about flat. Level means not tilted. Flat means no dips or humps over a short distance. A floor can slope a half inch across a 15-foot room and still take LVP fine if that slope is smooth. A floor that is perfectly level overall can still have a 1/4 inch dip between two joists that ruins a click joint. Manufacturers write their requirement as a flatness spec, and meeting it is part of keeping your warranty.

How to check a subfloor for flatness

1. Clean the floor. Scrape off old adhesive ridges, staples and debris so you are measuring the floor, not the junk on it.
2. Use a straightedge. A 6-foot level, a 10-foot aluminum straightedge, or a straight piece of rip-cut plywood works. Lay it in several directions: along the joists, across them, and diagonally.
3. Measure the gaps. Slide a tape or a stack of washers under the low point. If you have a 10-foot straightedge and the gap is more than about 3/16 inch, that spot needs work. With a 6-foot straightedge, many products want 1/8 inch or less.
4. Check for humps. If the straightedge rocks, you are on a high spot. Mark its center.
5. Use a string line for big rooms. Stretch a mason's line across the room on equal-height blocks and measure down to the floor every couple of feet. A rotating laser does the same job faster.
6. Mark everything. Circle low spots in one color and high spots in another. That map becomes your work plan.
7. Walk it. In socks, feel for soft spots and listen for squeaks. Bounce on your heels between joists. Soft or bouncy areas are a structural question, covered below.

These are common industry figures. Your product's installation instructions are what count.

How to level a wood subfloor

Re-screw loose panels

Start here. Loose plywood or plank subfloor causes squeaks and can create small humps at seams. Find the joists, then drive subfloor screws through the panel into each joist, roughly every 6 inches along panel edges and every 8 to 12 inches in the field. Screws hold better than the original nails, which tend to back out over the years.

Sand down high seams

Plywood and OSB edges often swell or sit proud at the seams. A belt sander or a floor edger with coarse paper takes them down quickly. Check again with the straightedge as you go. In homes built before 1978, do not sand old painted surfaces or old resilient flooring residue without testing for lead and asbestos first.

Fill low spots with patch

For dips up to about 1/4 to 1/2 inch deep, use a cement-based floor patch rated for wood subfloors. Feather-finish products are made to trowel out to almost nothing at the edges. Build deeper fills in layers per the product's instructions, then screed with your straightedge and sand any ridges once it hardens. Patch does not add strength, so the subfloor under it must be solid.

Shim or plane the joists

When a dip or hump runs the length of a joist, the joist is the problem. Pull up the subfloor in that area, or work from the basement if you can reach it. Plane down a crowned joist with a power planer, or glue and screw tapered shims (or sister a straight 2x to the side) to bring a low joist up. Then reinstall or replace the subfloor.

Add a plywood overlay

Old plank subfloors with lots of small humps and gaps often take a 1/4 inch or thicker underlayment-grade plywood overlay best. Stagger the overlay seams away from the subfloor seams, leave a small gap between sheets, and fasten on a tight pattern per the plywood maker's schedule. An overlay gives you a uniform surface but raises the floor, so check door clearances and transitions first.

Self-leveler on wood

Pourable self-leveling underlayment can go over wood, but most products require a stiff floor, a specific primer, and often metal lath stapled down first. It is less forgiving than on concrete. Follow the manufacturer's wood-subfloor instructions to the letter or use patch and plywood instead.

How to level a concrete subfloor

1. Grind high spots. Use an angle grinder with a diamond cup wheel and a dust shroud connected to a HEPA vacuum. Concrete dust contains silica, so wear a properly fitted respirator.
2. Clean and repair. Remove paint, adhesive and loose concrete. Fill cracks and control joints with the patch your leveler system calls for. Do not pour leveler over moving cracks.
3. Test for moisture. Use a calcium chloride (MVER) test or in-situ RH probes (ASTM F2170). Leveler does not fix a wet slab. See LVP in basements.
4. Prime. This step is not optional. Primer keeps the slab from sucking water out of the leveler too fast, which causes weak, cracked or delaminating pours. Use the primer made for your leveler and let it dry per the label.
5. Dam and seal. Block doorways and floor drains with foam or wood dams. Seal cracks at the walls so leveler does not run into the basement or under walls.
6. Mix and pour. Measure water exactly. Too much water weakens the material. Mix with a drill and paddle, pour in a ribbon, and spread with a gauge rake or smoother. Work fast. Most levelers have only 10 to 20 minutes of working time.
7. Mind the pour depth. Many self-levelers are rated from a thin skim (around 1/8 inch) up to roughly an inch in one pour, and more with added aggregate. Thin feathered edges are the weak point. Check your product's minimum and maximum depth.

How long does subfloor leveler take to dry?

These are typical ranges. Drying time depends on the product, the thickness, the room temperature and airflow. Glue-down flooring usually needs the leveler drier than a floating floor does. Read the bag.

What drives the cost of subfloor leveling?

• Area and depth. A few low spots take one bag of patch. A whole room of self-leveler at half an inch takes many bags plus primer.
• Tear-out. Removing old tile, glued vinyl or multiple layers to get to the subfloor.
• Type of fix. Sanding and patching is cheap. Plywood overlay costs more. Joist work costs more again.
• Moisture problems. A wet slab needs its moisture source addressed or a mitigation system before anything else.
• Knock-on effects. Raising a floor can mean trimming doors, adjusting transitions and resetting toilets.
• Access. Joists you can reach from an open basement are much easier than ones behind a finished ceiling.

This is why flooring quotes vary so much. A quote that never mentions the subfloor either assumes a perfect floor or plans to add it later. For overall budgeting, see LVP installation cost in Connecticut.

When does leveling mean the floor needs structural repair?

Patch and leveler fix the surface. They do not fix framing. Get a contractor or structural engineer to look before new flooring goes down if you see:

• A floor that bounces noticeably or feels soft between joists
• A sag across the middle of a room, especially over a beam or a long joist span
• Cracked, notched or heavily drilled joists, or a settled support post in the basement
• Rot or water damage around tubs, toilets, exterior doors or sill plates
• Doors that stick or have uneven gaps, and cracks in walls above the sloped area

Old mill-era houses around Putnam, Danielson and Willimantic often have floors that settled over a century. Some of that is stable and can simply be flattened. Some is a beam or post that needs attention. Pouring leveler over a moving floor just cracks the leveler.

Common mistakes to avoid

• Skipping primer on concrete before self-leveler
• Adding extra water to make leveler flow easier
• Filling a dip without screwing down the loose panel that caused it
• Installing flooring before the leveler is dry enough
• Using foam underlayment to "hide" dips. It does not. The planks still flex.

When to hire a pro

Sanding seams and filling a few low spots is a reasonable DIY job. Call a pro for large self-leveler pours, joist work, a wet slab, or any sign of a structural problem. Our guides on how to install LVP and how to install laminate pick up where this one leaves off, and uneven floor heights between rooms covers what happens at the doorways.

Need it done for you?

We check the subfloor during a free in-home estimate and spell out the prep in a written fixed-price quote within 48 hours. Book an estimate and we will tell you what yours needs.