The flatness standard, and what it means in practice
Most rigid-core LVP manufacturers specify a flatness tolerance in the range of 3/16 inch over 10 feet, with a tighter number over shorter spans. That is a real measurement made with a straightedge, not a judgment call made by eye.
In practice, almost no existing floor in a Treasure Valley home meets that number without some work. Plywood subfloors sag slightly between joists and crown over them. Slabs have trowel ridges, control joints, and the occasional low spot near a drain. Older homes have settled. The prep scope is whatever it takes to bring the actual surface within the actual tolerance.
Moisture testing on slab-on-grade
A lot of Treasure Valley housing — daylight basements, garage conversions, and slab-on-grade builds across Meridian, Nampa, and Kuna — puts LVP directly over concrete. Concrete always carries some moisture, and moisture vapor moving up through a slab is the mechanism behind adhesive failure, trapped odor, and cupped glue-down plank.
The relevant test depends on the product's requirement: relative humidity probes set into the slab per ASTM F2170, or a calcium chloride test per ASTM F1869. Newly poured slabs need extended cure time regardless of what the reading says on day thirty. Where readings exceed the product's limit, the answer is a moisture mitigation system or a different product — not a heavier underlayment.
The correction toolkit
Different substrate problems call for different fixes, and using the wrong one wastes the prep budget.
- Low spots and dips: cementitious patching compound feathered to the surrounding surface
- Large or complex low areas: self-leveling underlayment poured over a primed substrate
- High spots, trowel ridges, and thinset: diamond grinding with dust extraction
- Squeaky or loose plywood: refastening to the joists with screws rather than nails
- Deteriorated or water-damaged panels: cut out and replace with matching-thickness sheathing
- Slab cracks: assess whether they are static shrinkage cracks or active movement before filling
Why prep is quoted separately
We break prep out as its own line rather than hiding it in a per-square-foot install rate. A flat, sound slab in a newer Meridian build needs very little. A 1950s Boise Bench house with three layers of old flooring, a patched-in kitchen addition, and a settled hallway needs a great deal. Quoting both at the same rate would mean overcharging one homeowner to subsidize the other.
You get the measurements we took and what we propose to do about them, so the number is traceable to something real.
Questions we get about subfloor prep
- Do I really need subfloor prep if my floor looks flat?
- Flat to the eye and flat to a 10-foot straightedge are different things. A quarter-inch dip across a room is invisible standing up and very audible underfoot once a floating floor sits on top of it. We measure rather than estimate.
- How much does subfloor prep add to a project?
- It varies too much to quote generically — it depends entirely on the readings we take on site. We give you the flatness survey and a specific prep scope with the estimate so you can see what is driving the number.
- Can underlayment fix an uneven subfloor?
- No. Underlayment is typically 1 to 2 millimeters of foam or an attached pad. It softens sound and handles very minor texture. It does not fill dips, and using it that way just moves the problem into the locking edge of the planks.
- How long does self-leveling underlayment take to cure?
- Cure time depends on the product, pour depth, and ambient conditions. Manufacturers publish a walkable time and a separate flooring-installation time, and the second is what governs the schedule. We plan the install date around it rather than rushing it.