Why basements are different
Concrete wicks moisture from the ground continuously, and below-grade rooms sit in cool, damp soil with little air exchange. A floor laid directly on a slab without a vapor barrier takes on that moisture; laminate swells, carpet pad molds, and glued products release. Tacoma's North End basements are a century old; Lakewood's and University Place's daylight lower levels were finished decades after they were poured; garage slabs were never meant to be lived on.
The right floor
Rigid-core vinyl plank over a vapor barrier is the standard. It is waterproof, stable in humidity, softer and warmer than tile, and floats so it can be lifted if the worst happens. Loose-lay vinyl is an option where occasional moisture is still possible. Laminate is avoided below grade; carpet is the most vulnerable choice there is.
Testing the slab
Products specify a maximum slab moisture level and a test method, either calcium chloride or in-slab relative humidity probes. Installers run the test the product requires and look for efflorescence, staining and past water lines. Below the limit, the floor goes in over a vapor barrier. Above it, a moisture mitigation coating on the slab or addressing the water source comes first, and the installer tells you plainly.
Leveling
Slabs are rarely flat. High spots are ground, low spots and cracks filled with self-leveling compound, and garage slabs that slope toward the door are built up. Flatness within the product's tolerance is what keeps the click joints locked and the floor quiet.
Vapor barrier and underlayment
Six-mil polyethylene taped at every seam and turned up the wall behind the baseboard, then the underlayment the product specifies, or a combined vapor-barrier underlayment where the manufacturer allows it. The small cost that makes the floor last.
Stairs and transitions
Basement stairs get matching nosings; the transition at the top of the stairs to the main-floor material and any slab-to-wood changes are planned so heights work and the floor can move.
On the quote
Slab testing, mitigation if required, leveling as measured, vapor barrier and underlayment, the product by name, transitions, stairs, trim and labor, approved in writing before anything is ordered.
