Installing Vinyl Plank Flooring Over Concrete: Preparation & Moisture

Concrete slabs act like a dense sponge, drawing moisture from the ground below and releasing it slowly through vapor evaporation. In ground-level or basement slabs, unmitigated vapor pressure under a non-breathable vinyl surface creates a moist microclimate. This moisture can foster mildew growth beneath underlayment pads or breakdown adhesives if a glue-down system is used.

Professional installation protocols require assessing concrete slab moisture prior to plank placement:

• Vapor Barrier Membrane: A continuous sheet of 6-mil (0.006 inch) virgin polyethylene film should be laid over raw concrete slabs with seams overlapped by 6 to 8 inches and taped securely.
• Attached Pads: Even if LVP planks have pre-attached acoustic foam backing, an underlying poly film is required to act as the moisture barrier unless the manufacturer explicitly specifies otherwise.

Floating click-lock planks depend on a smooth foundation. Subfloor prep steps include:

1. Grinding: High spots, trowel ridges, or mortar droppings on the slab must be mechanically ground flat.
2. Filling & Patching: Deep cracks, spalls, or control joints are filled with cementitious patching compound.
3. Self-Leveling: Low areas exceeding 3/16 inch variation across a 10-foot span are leveled with self-leveling underlayment to avoid hollow sound or seam flex underfoot.