Before the New Floor Goes Down: How to Prep a Basement or Garage Concrete Slab

You've got 600 square feet of vinyl plank stacked in the basement, and the instructions in the box say the floor must be "flat within 3/16 inch in 10 feet," "free of adhesive residue," and "tested for moisture." Or the garage is finally getting a coating, and you're wondering what that stained, cracked slab needs first.

Most new-floor problems start with the concrete, not the flooring. Planks that click and flex, tile edges that catch your sock, vinyl that bubbles, a coating that peels in the tire paths: all of it traces back to prep. Here's how to check your slab, what each problem takes, and who should fix it.

Flat and Level Are Two Different Things

• ✓Flatness is how smooth the surface is over short distances: humps, dips, ridges, and steps.
• ✓Levelness is how close the floor comes to true horizontal across the room.

Most flooring specs are flatness specs. A basement floor can fall an inch across the room toward the floor drain and still be perfectly flat, and a floating plank floor won't care. A dead-level floor with a 1/4" dip in the middle will make planks flex and tile rock.

Some slope is there on purpose. The model building code requires a garage's parking area to slope so liquids move toward a drain or the main overhead door. Don't try to level that out before a coating. The coating follows the slab, so meltwater keeps running out the door.

How Flat Is Flat Enough?

Your product's installation instructions are the spec that counts, and following them is usually a condition of the flooring warranty.

Finding the High and Low Spots

With a straightedge and a pocketful of quarters

Use a 6-foot or 10-foot straightedge, like an aluminum screed or a long level. A 2x4 is almost never straight enough. Lay it in several directions, especially along the run of the planks. If it rocks, there's a hump under the middle. If light shows under the middle, there's a dip.

Now measure the gap. A U.S. quarter is 1.75 mm thick, a hair over 1/16". Two stacked quarters are a hair over 1/8", and three are a bit over 3/16". So if three stacked quarters slide under a 10-foot straightedge anywhere, you're outside a 3/16"-in-10-feet spec. If two slide under a 6-foot straightedge, you're outside 1/8" in 6 feet.

Mark what you find on the slab: circle the humps, X the dips, and write the gap beside each.

With a laser

A self-leveling line laser plus a tape measure maps a whole room. Shoot the beam across, then measure from the beam down to the floor every 2 feet. A bigger number means a lower floor.

Both ends match and the middle is 1/4" lower: a 1/4" dip across 8 feet. That's out of spec for a 3/16" plank floor, right at the limit for small tile, and well out for large-format tile. It needs filling.

Two other patterns to recognize. Readings that grow steadily, like 40", 40-1/4", 40-1/2", 40-3/4" at 4-foot steps toward the drain, mean a floor that's flat but sloped, and plank doesn't care. A sudden jump at a crack, like 40-1/16" on one side and 40-1/2" a foot away on the other, isn't a dip at all. A section of slab dropped.

Fixes, Problem by Problem

Humps and ridges: grind them down

Old patches, trowel ridges, leftover thinset, and raised crack edges get ground flush with a diamond cup wheel. Grinding concrete throws fine silica dust, so use a dust shroud hooked to a HEPA vacuum and wear a respirator. We grind garage floors as part of coating prep; on a basement flooring job, that's your flooring installer's work.

Shallow dips: patch or self-level

Small, shallow low spots get a cement-based patching compound, feathered out. Broad dips are what self-leveling underlayment is for, poured over a clean, mechanically roughened, primed floor that's done moving. Our guide to self-leveling concrete problems covers where it works and where it fails.

A section that sank: lift it

If one side of a crack sits lower, a section along a wall dropped, or the garage floor dips where it meets the apron, the slab lost the ground under it. Self-leveler poured over that hides the dip, but the void stays, and if the slab keeps settling, the overlay cracks with it.

Lifting fixes the cause. We drill a few 5/8" holes, about the size of a dime, and inject high-density polyurethane foam that fills the voids and raises the section in small, controlled increments. You can walk on it right away. Garage floors are the usual candidates. A basement slab that has truly dropped is less common and may have plumbing, drain tile, or a radon system under it, so send photos and we'll tell you whether it's a job for us.

Cracks: settle the movement, then fill

• ✓Stable hairlines get opened up, cleaned, filled with a rigid repair material, and ground flush for a coating. Tile installers often use a crack-isolation membrane instead.
• ✓Cracks that grow over a season, or line up with cracks in the foundation wall, need a structural engineer's look before anyone covers them.
• ✓Control joints on a coated floor can stay as a coated groove or get filled. Tile installers generally carry moving joints up through the tile.
• ✓Flexible caulk has to come out of any area you plan to coat, because coatings don't bond to it. See garage floor cracks.

Old adhesive: stop and check before you scrape

Carpet glue and modern vinyl adhesives can usually be scraped down; check your flooring's instructions for how clean the slab has to be.

Black, tar-like adhesive is different. In older homes, the black "cutback" mastic under old tile can contain asbestos, and so can the tiles. You can't tell by looking. The EPA says to treat suspect material as if it contains asbestos, not to sand, scrape, or otherwise disturb it, and to have samples taken by a trained, accredited professional. One large flooring maker's installation guide says to presume old cutback adhesive contains asbestos unless you're positively certain it doesn't. In Wisconsin, the Department of Health Services generally requires anyone whose work disturbs asbestos-containing material to be state-certified, so check with DHS before tackling it yourself.

Your options: get it tested, cover it in place with a floating floor if the installer approves, or hire a certified abatement company to remove it. It never gets ground off for a coating.

Paint, sealers, and oil

Drops of water that bead instead of soaking in mean something is on the surface. Failing paint and old sealers come off before a coating, and oil-soaked spots get degreased, then ground out or primed. See our guides to cleaning concrete floors and cleaning oil off concrete.

Moisture Testing: The Step Everyone Wants to Skip

Moisture rising through a slab can break down flooring adhesives, cup wood, feed mildew under vinyl, and make coatings blister or peel. Basement slabs sit on damp soil year-round, some older slabs have no plastic vapor barrier underneath, and spring snowmelt soaks the ground around Wisconsin foundations every year. (A garage floor that "sweats" on a humid spring morning is condensation, a different thing; our floor cleaning guide explains it.)

Limits vary a lot. One floating vinyl maker allows up to 8 pounds on the calcium chloride test or 90 percent relative humidity. One coating maker wants no more than 3 pounds or about 75 to 80 percent RH for its conventional primers, with specialty primers rated far higher. Read the sheet for your product.

Run the heat or air conditioning at normal living conditions for at least 48 hours before testing, and test more than one spot, including near outside walls. If the numbers come back high, start outside with downspouts, sump discharge, and the grade around the foundation, then ask your installer about moisture-rated adhesives or primers.

Garage Coating Prep: Grind, Don't Etch

Acid-etch kits are cheap and everywhere, and they're the wrong prep for most modern coatings:

• ✓The profile is too light. Acid typically leaves around CSP 1 to 2 on the industry's 10-step concrete surface profile scale, which suits only very thin coatings.
• ✓It leaves the real problems. Oil, curing compounds, and old sealers stay put.
• ✓It soaks the slab, which then has to dry out before coating.
• ✓The rinse water has to go somewhere, and northeast Wisconsin stormwater rules prohibit washing it into a storm drain.

Grinding opens the surface, removes what acid leaves behind, and gives the coating something to grip. On our coating jobs, the prep runs: lift any sunken section first, degrease, grind, fill cracks with a rigid material and grind them flush, check moisture, and remove failing paint or old coating. The joint between the apron and the garage floor gets a flexible sealant, outside the coated area.

A coating won't fix a slab that's still sinking, a basement water problem, or a floor that sweats in spring, and we'll say so before we quote if any of those apply.

Problem, Test, Fix, and Who

A Prep Order That Saves Doing Anything Twice

1. 1Clear the room and pull old flooring, after checking for suspect tile or black mastic.
2. 2Fix the water outside: downspouts, sump discharge, and grade.
3. 3Lift any sunken section, so everything after happens on a slab that's done moving.
4. 4Strip adhesive, paint, and oil.
5. 5Test for moisture after the HVAC has run at normal conditions for a couple of days.
6. 6Map flatness and grind the high spots.
7. 7Fill cracks, then patch or self-level the lows.
8. 8Vacuum everything, then lay the floor or coat it.