Installing SPC over a Las Vegas concrete slab

Why test moisture under a waterproof floor?

The SPC plank will not be harmed by moisture. Everything else might be. Concrete can keep releasing water vapor long after it is poured, and a slab on grade can pull moisture up from the soil below. Under a floating floor that vapor has nowhere to go. It can feed mold and odor on the slab surface, attack pads and adhesives, and void the flooring warranty. That is why manufacturers ask for a moisture test even for waterproof products, and why many require a plastic vapor retarder over concrete.

The two standard slab moisture tests

Which test applies, and the limits that count as a pass, come from the flooring manufacturer. As one worked example, a national manufacturer's rigid-core SPC guide says concrete must not exceed 5 lbs per 1,000 sq ft per 24 hours (calcium chloride) and/or 95% relative humidity (in-situ probe), and it requires a 6 mil polyethylene film as a moisture barrier over concrete for floating installs. Your product's limits may be different, so use its guide. Vegas Flooring Outlet offers pre-installation slab moisture testing as part of flooring projects.

Flatness: the rule most DIY installs break

Click-lock joints are machined to tight tolerances. If the slab dips or humps, the planks flex at the joints every time someone walks over the low spot, and the locks can wear, gap or break. Manufacturers state flatness as a maximum deviation over a distance. The same example guide asks for a subfloor flat within 3/16 inch in 10 feet, or 1/8 inch in 6 feet. Check with a long straightedge in several directions; high spots are ground down and low spots filled with a cement-based patch or self-leveling underlayment before any plank goes down.

Cracks

Hairline shrinkage cracks are common in slabs and usually are not a problem for a floating floor once they are filled flush. Cracks that are wide, that have one side higher than the other, or that keep growing point to movement in the slab. Get those looked at by a qualified contractor before you cover them, because a new floor will telegraph them.

Going over existing tile: when it's OK

Floating SPC can often go right over old ceramic or porcelain tile, which saves the noise, dust and cost of tear-out. It works when:

• Every tile is solidly bonded (no hollow sounds, cracked or loose tiles)
• Grout joints and uneven edges are filled and smoothed with a cement-based patch, so the grout lines do not show through
• The tile surface is flat within the product's tolerance
• The extra height still works at doors, appliances, stairs and where it meets other floors

Tear-out is the better call when tiles are loose or tented, the floor is out of flat, the added height causes problems, or the tile sits on a moisture issue you need to see. Guides also list surfaces you cannot float over, such as carpet, cushion-backed vinyl and another floating floor.

Older homes: very old resilient flooring and black "cutback" adhesive can contain asbestos. Do not sand, grind or scrape them dry. Have them tested and handled by professionals.

A good slab-prep sequence

1. Remove baseboards, old flooring as needed, and any adhesive residue the guide calls for.
2. Moisture test with the method the manufacturer requires.
3. Check flatness, then grind high spots and fill low spots.
4. Fill cracks and grout lines; sweep and vacuum thoroughly.
5. Lay the vapor retarder if the guide calls for one, then the floor.

Want the prep done for you? See how Vegas Flooring Outlet handles installation.