Boise Subfloor Prep and Moisture Testing

Why prep decides the result

A new floor can only be as good as what it sits on. Most of the complaints people have about a flooring job a year later (a soft spot, a squeak, joints peaking, tile cracking along a line, adhesive letting go) come from a subfloor problem that was covered instead of fixed.

Prep is also where quotes differ the most. One installer prices a full day of grinding, patching and screwing down loose panels. Another prices none, because they haven't looked yet or because they plan to find out on install day. Neither number means much until you know what's under your current floor and what condition it's in.

Slabs and wood subfloors around the valley

Treasure Valley homes sit on a mix of foundations. You'll find concrete slab-on-grade, full or partial basements with slab floors, and wood-framed floors over a crawlspace, sometimes all within one neighborhood and occasionally within one house. Each subfloor has its own failure modes.

Concrete slabs

Concrete is stiff, which tile loves, but it's rarely perfectly flat and it can move moisture up from the ground below. A slab can feel dry to the touch and still deliver enough vapor to break down an adhesive or trap moisture under a vinyl floor. Cracks matter too: a hairline shrinkage crack is common, while a crack with one side higher than the other suggests movement worth asking about before covering it.

Wood subfloors

Plywood or OSB panels over joists are the norm on framed floors. The usual problems are loose panels (the source of most squeaks), seams that have swelled where water once got in, and dips between joists. Over a crawlspace, the crawlspace conditions matter: a wet crawlspace with no vapor barrier puts moisture into the framing above, and a wood floor on top will tell you about it.

Flatness, measured instead of guessed

Flat and level are different things. Level means horizontal. Flat means no humps or dips over a given distance. Floors don't have to be level (most older floors aren't), but they do have to be flat within the tolerance the flooring maker publishes.

Tolerances are commonly written as a maximum deviation over a distance, something like 3/16 inch over 10 feet, and they vary by product. Click-lock LVP and large-format tile tend to be the strictest. The check itself is simple: a long straightedge laid across the floor in several directions, with the gaps measured and the high and low spots marked.

Fixing it means grinding high spots on concrete, filling low spots with a patching compound or a self-leveling underlayment, and on wood, planing seams, adding screws and sometimes replacing damaged panels. Self-leveler needs a primer, a properly dammed pour and drying time before anything goes on top, so it affects the schedule as well as the price.

Moisture testing, slab and wood

On concrete

Two test methods come up most often, and they measure different things:

• In-situ relative humidity (ASTM F2170). Small holes are drilled to a set depth in the slab, probes are sealed in, and after an equilibration period the humidity inside the concrete is read. Many flooring and adhesive makers now specify this method and a maximum RH.
• Calcium chloride (ASTM F1869). A dish of calcium chloride sits under a sealed dome on the slab surface and is weighed before and after, giving a moisture vapor emission rate. Some manufacturers still list a maximum for it.

The plastic sheet taped to the floor (ASTM D4263) shows whether moisture is obviously coming through, and it's a quick sanity check, but most manufacturers don't accept it as the test of record. Handheld concrete meters are useful for mapping wetter and drier areas, not for meeting a warranty requirement.

On wood

For wood subfloors and wood flooring, a pin or pinless moisture meter reads moisture content. What matters is the comparison: the subfloor against the flooring, and the flooring against what's expected for the home. National Wood Flooring Association guidance has long called for the two to be within a few points of each other, tighter for wide planks than for narrow strips. Check the current numbers in the flooring maker's instructions.

Which test applies to your floor?

A prep day, step by step

Every job is different, but a thorough prep usually runs in roughly this order once the old floor is out. The removal and timeline guide covers the tear-out itself.