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Subfloor Flatness and Moisture: Why LVP Fails in Idaho Homes
Nearly every vinyl plank failure we are called to diagnose traces back to the substrate. Here is how to evaluate yours before anything gets installed.
Flat is not the same as level
A floor that slopes an inch across a room can install perfectly. A floor that is dead level but has a half-inch dip in the middle of a doorway will produce hollow spots, joint stress and eventually broken locking edges. Rigid plank cares about local deviation, not about global slope.
Manufacturers state this as a tolerance over a span. A very common figure is 3/16 inch of deviation over any 10 feet, with a tighter number for glue-down and for wide planks. You can check your own floor with a straightedge or a taut string: lay it down in several directions, and look for daylight beneath it.
Correcting what you find
Small isolated dips get feathered with a cementitious patch. Multiple deviations or a large area gets a poured self-leveling underlayment over a properly primed substrate — the priming step is not optional, and SLU poured over a dusty or unprimed floor will debond.
Humps are the opposite problem and are usually solved by sanding a plywood seam, removing a proud fastener, or grinding a concrete high spot. Filling around a hump instead of removing it wastes material and raises the floor unnecessarily.
Concrete moisture: test, do not assume
Concrete releases moisture for a long time, and a slab that looks and feels dry can still be well above the limit for an adhesive. The current industry method is in-situ relative humidity testing with probes placed in drilled holes per ASTM F2170, allowed to equilibrate, then read. Surface-only tests substantially understate the moisture available deeper in the slab.
If the reading exceeds what the flooring or adhesive manufacturer allows, the options are a moisture mitigation coating, a different assembly, or waiting. There is no adhesive that outmuscles a wet slab.
Wood subfloors and Idaho crawlspaces
On wood, we take moisture meter readings and compare them to the manufacturer's allowable range, and we look at the crawlspace, because that is where the water is coming from. A torn or missing ground vapor barrier, poor drainage or flood irrigation on the parcel will keep re-wetting the subfloor from below no matter what goes on top.
In much of Canyon County and around irrigated agricultural parcels, correcting the crawlspace is genuinely the highest-value thing you can do for the life of a new floor.
Squeaks and deflection
A squeak is movement between subfloor and joist, and it is fixed from above with structural screws driven into the joist before the new floor covers it. Once plank is down, that repair becomes far more invasive.
Excess bounce between joists is a structural issue, not a flooring one. Adding a thicker underlayment does not fix deflection; blocking, sistering or an added structural layer does.
Key takeaways
- Check local deviation with a 10-foot straightedge, not a level
- Prime before pouring self-leveling underlayment, always
- Test concrete with in-situ RH probes per ASTM F2170 before adhering anything
- Fix the crawlspace vapor barrier before installing over a damp wood subfloor
- Screw down squeaks and address deflection before the floor covers them