Is Your Subfloor Ready for LVP? Flatness and Moisture, Explained
· 5 min read
When a floating LVP floor goes wrong a year or two later, the plank is rarely the culprit. Clicking seams that open, a soft spot that creaks, a musty edge along the wall: the cause is usually the surface underneath.
That's why a good installer spends time on the subfloor before opening a box. Here is what gets checked, what it means, and what you can do ahead of the visit.
Why flatness matters for a floating floor
A rigid-core floor floats: the planks lock to each other, not to the subfloor. Over a dip, the locked planks bridge the gap. Walk across it repeatedly and the joint flexes, and over time that flexing can loosen the click connection or leave a visible gap.
A high spot is the opposite problem. The plank rests on the ridge and can rock or telegraph a ridge line, especially in raking light from a window.
Rigid cores forgive small imperfections, but they aren't a substitute for prep. Every manufacturer publishes its own flatness tolerance, and installation instructions for the specific product are the standard to meet. Partek's own tolerance for our line is something we confirm at measure, not something to guess from a blog post.
How flatness is checked
No special equipment is needed to understand the idea. A long straightedge, typically six to ten feet, is laid on the floor in several directions. Any gap under it is measured. Where the gap exceeds the tolerance, the spot is marked for correction.
You can do a rough version yourself: lay a long, straight board on the floor and slide a coin or pencil underneath. A wobble or a visible gap tells you where to point the installer.
- Low spots are filled or leveled, usually with a patching or leveling compound on concrete.
- High spots are ground or sanded down on concrete; on plywood they may be sanded or the panel re-fastened.
- Squeaks and movement in a wood subfloor are often loose panels that get screwed down before flooring goes over them.
Moisture: the check that's easy to skip
Waterproof plank doesn't make a wet subfloor safe. Moisture rising from a slab can collect under a floating floor, where it can cause odor and mildew, and it can also void a manufacturer's warranty when the installation instructions weren't followed.
On concrete, moisture is tested. Common methods include in-slab relative humidity probes and calcium chloride tests, and each flooring manufacturer states the limits it accepts. A quick plastic-sheet test (tape a square of clear plastic to the slab for a day or two and look for condensation) is a useful warning sign, though it isn't a substitute for a proper test.
On wood subfloors, the concern is different: a plywood or OSB subfloor over a damp crawlspace, or one that has seen a past leak, can hold moisture and swell or soften. Discoloration, softness, or a musty smell are reasons to stop and investigate first.
What to expect in an older DMV home
Older houses across Maryland, DC, and Northern Virginia tend to have subfloors that have settled and moved over decades: slightly crowned joists, patched sections from old plumbing work, or a mix of materials where an addition meets the original house. Basements bring their own question: is the slab dry in every season, not just on the day someone looks? Our basement flooring page covers that room in more depth.
None of this is a reason to worry. It's simply the reason the subfloor gets looked at during the measure and not on install day, when surprises are more expensive.
What prep can add to a project
Minor prep, such as clearing debris, setting a few screws, or scraping high spots, often folds into the job. Larger correction (leveling compound over a big area, moisture mitigation, or removing tile or hardwood) is separate work that may be priced separately. That is why a quote that is silent on the subfloor deserves a question. We break down common LVP installation mistakes and what actually needs to happen before install day in companion articles.
How to prepare for the measure
A few small steps make the subfloor conversation faster and more accurate:
- Tell us about any past leaks, flooding, or squeaky areas.
- Note where floors feel bouncy, sloped, or uneven underfoot.
- If you have a slab, mention whether the room has ever smelled musty or shown dampness.
- Clear a path to the areas you're unsure about so they can be checked.
Frequently asked questions
+ Can LVP go over an uneven floor?
Within the manufacturer's tolerance, yes. Beyond it, the floor should be corrected first, because flexing over dips can loosen the click joints over time.
+ Do I need a moisture test if my LVP is waterproof?
On concrete, usually yes. Waterproof plank protects the plank itself, but it doesn't stop moisture vapor from the slab from collecting under a floating floor. Manufacturers also set moisture limits their warranties rely on.
+ Can LVP be installed over existing tile?
Sometimes, if the tile is sound, flat, and the grout lines are filled or smooth enough not to telegraph. It's judged case by case at measure.
+ Who pays for subfloor prep?
Minor prep may be part of the job, while larger correction is scoped and quoted separately once we've seen the floor. We'd rather tell you at measure than surprise you later.
