- Rigid plank needs a flat slab, often within 3/16 inch over 10 feet.
- Slab moisture is measured with standard ASTM tests, and makers set limits.
- Many makers allow an extra 6 mil poly vapor retarder on concrete.
- Keep the AC running normally before, during and after installation.
Why the slab matters
Most houses in the Palm Beaches and on the Treasure Coast sit on a concrete slab poured on the ground. Condo floors above the first level are concrete too. That’s good news for vinyl plank. Concrete doesn’t flex or squeak, and a waterproof SPC plank doesn’t mind the moisture a slab gives off, within limits.
Those limits are the point of this guide. A rigid plank is only as good as the surface it rests on. If the slab has dips, the locking joints flex with every step and can wear or separate. If it’s giving off more moisture than the maker allows, the space under the floor stays damp. Neither problem shows up on installation day. Both show up months later. So the work that matters most happens before the first plank goes down.
Flatness: the number that decides everything
Every rigid-core maker publishes a flatness tolerance. The COREtec Stone installation guide, as one example, allows no more than 3/16 inch of variation in 10 feet, or 1/8 inch in 6 feet. That’s tighter than it sounds. A slab that looks fine to the eye can have a dip near a doorway or a ridge where two pours met.
We check flatness with a long straightedge across each room, paying attention to the places where problems usually hide:
- Where old tile came up and left thinset ridges behind.
- Along old carpet tack strips, which leave nail holes and small humps.
- At the junction of an addition or an enclosed porch and the original house.
- Near sliders, where the slab often slopes slightly toward the outside.
- Over old repair patches and plumbing trenches.
High spots are ground down. Low spots are filled with a cement-based patch or self-leveling compound. Larger areas may need a pour of leveler across the whole room. This prep is part of our installed estimate, and it’s one of the main things that moves a job within the price range.
Moisture: how slabs are tested
Concrete is porous. Moisture moves up from the ground through the slab and into the room. A slab near the coast, a slab over a high water table or one poured without an effective vapor retarder can give off a lot of it. Makers of vinyl plank require testing on concrete for that reason, and they set limits for the two standard tests.
- Calcium chloride test (ASTM F1869). A dish of calcium chloride sits under a sealed dome on the slab and measures how much moisture it absorbs. COREtec Stone sets a maximum of 8 pounds per 1,000 square feet in 24 hours.
- In-slab relative humidity (ASTM F2170). A probe goes into a hole drilled into the concrete and reads the humidity inside it. COREtec Stone sets a maximum of 90%.
ASTM notes that excess slab moisture after a floor is installed can cause failures, including microbial growth, and that a test result describes the slab only at the time and in the places it was tested. That’s why testing happens close to installation, in more than one spot, and why a slab that recently got wet from a leak needs time before it’s tested at all.
When a vapor retarder goes down
Many rigid planks come with an attached pad, and many makers also allow an extra sheet vapor retarder under the floor on concrete. The COREtec Stone guide allows an added layer of 6 mil poly film or an equal vapor retarder with a perm rating of 0.1 or less. The perm rating measures how much water vapor passes through; lower means less.
We follow the maker’s instructions for the exact product you choose. Where a sheet retarder is used, the seams are overlapped and taped, and it runs slightly up the wall behind the baseboard. It’s cheap insurance on a ground-floor slab. It doesn’t fix a slab that’s actively wet, though. If readings are over the limit, the source of the water gets found and fixed first.
Cracks, leaks and old adhesive
Hairline shrinkage cracks are normal in a slab and a floating floor bridges them without trouble. Cracks that are wide, uneven on each side or growing are a different matter and need a look from the right trade before any floor covers them.
A slab that stays damp in one spot, or a water bill that climbs for no clear reason, may point to a leak in a supply line under the slab. That’s a plumbing repair, and the floor comes after. Our guide on slab leaks and burst supply lines covers what happens to the floor.
Old adhesive and mastic under carpet pad or tile deserve care too. Have older tile and mastic tested before anyone tears it out. The EPA advises against sanding or leveling flooring that may contain asbestos. See older floor tile and mastic.
Acclimation and the AC
Vinyl plank moves slightly with temperature. The COREtec Stone guide asks for storage and installation between 55 and 85 degrees Fahrenheit, with 48 hours of acclimation if the boxes were kept outside that range, and it asks that the room stay in that range afterward. In a Florida house that means one simple thing: the AC should be running normally before, during and after the job.
That’s worth planning for in a seasonal home. If the house has been closed with the thermostat set high, turn it down to normal living temperature a couple of days ahead. Leave the boxes inside the house, not in a hot garage or car. The same guide calls for a 1/4 inch expansion gap at the walls for most rooms and 1/2 inch for very large areas, hidden under the baseboard or quarter round.
Over existing tile, or down to the slab
Many Florida homes have ceramic tile already bonded to the slab. Some makers allow rigid plank over sound, flat tile, with their own rules for how wide and deep the grout lines can be. That can save the removal. Loose or cracked tile, hollow spots or a floor that’s too high at the doors mean the tile comes out. We confirm which way your floor goes at the measurement. Read vinyl plank over ceramic tile for the details, and tile removal and replacement for the removal side.
How we handle a slab job
At the free in-home measurement and design consultation, our installers measure each room, check flatness, look at the transitions at sliders and doors, and note anything that needs testing or repair first. The estimate spells out the prep. As an estimate, waterproof vinyl plank runs $5 to $12 per square foot installed, before your discount, and a 200 square foot room is roughly $1,200 to $1,900 before the discount.
On installation day the crew removes the old floor, preps the slab to the maker’s tolerance, lays any vapor retarder the product calls for, and installs the plank with its expansion gap and trim. Most installs are completed in a single day once your materials arrive. See luxury vinyl plank installation for the full process, and the best flooring for Florida humidity for the bigger picture.
Sources: COREtec Stone installation guide (flatness, slab moisture, temperature) · ASTM F2170: relative humidity testing in concrete slabs · EPA: protect your family from asbestos exposure.