Managing subfloor moisture takes three jobs before a new floor goes down. Find where the water comes from. Measure how much is in the slab or plywood, using a test the floor's maker accepts. Then match the barrier, or the floor itself, to that reading. Skip one and the new floor pays for it: planks cup or swell, glue lets go, and mold grows where nobody can see it.
On a concrete slab or basement floor, the question is how much moisture the concrete holds, never whether it holds any. The passing number depends on the floor. Among the makers we carry, solid hardwood needs a slab reading of 75% relative humidity or less, laminate needs under 80%, and rigid waterproof vinyl plank needs under 90%.
On plywood or OSB panels over a crawl space, a wood moisture meter reads the subfloor, and 12% or less suits hardwood and laminate. If the wood reads wet and nothing has leaked, look under the house. Plastic over the dirt, open vents and gutters that carry rain away keep a crawl space from soaking the floor above it.
If the subfloor just got wet, stop the water, take up the flooring over the wet spot and dry it within a day or two, before mold starts. The new floor waits until a meter shows the subfloor is back within that floor's limit.
Here is where to start, by what is under your floor.
| Your situation | What to check | What usually fixes it |
|---|---|---|
| Basement or ground-level concrete | RH probe or calcium chloride test | A floor and barrier matched to the reading |
| Wood subfloor over a crawl space | Meter on the plywood, then the crawl space | Ground plastic, open vents, drainage |
| Leak or flood under the floor | Meter readings until back to normal | Fast drying, then new flooring |
| New concrete in an addition | Its age, then an RH test | Time to dry; test before install |
| Floor already cupping or lifting | Where and when it happens | Stop the source before replacing |
Where subfloor moisture comes from
Subfloor moisture comes up from the ground, down from a leak or out of humid air. Homes around Huber Heights and Dayton sit on basements, slabs and crawl spaces, so the ground is the steady source here. Leaks and muggy summers are the surprises.
Concrete draws water up from the soil beneath it and releases it as vapor at the surface for as long as the slab exists. Some slabs were poured over a plastic sheet and some weren't. You can't tell which from the top, so a slab gets tested instead of judged by eye. Wet soil against the foundation, from a downspout that empties at a corner or ground that slopes toward the house, keeps the concrete wetter still.
In a crawl space, bare dirt feeds water into the air all year, and summer air brings in more. Leaks do their worst damage when they're slow, because a floor can hide them for months. A new slab, poured for an addition or a garage conversion, holds a lot of mixing water and gives it up slowly.
Signs of subfloor moisture problems
Subfloor moisture usually shows first in the floor on top, and each kind of floor shows it differently. Where and when the damage appears tells you the most: along outside walls, near plumbing, in a stripe from an appliance, or every summer and gone by winter.
Wood floors cup when the bottom of each board is wetter than the top. The edges rise and the middle dips, like a row of shallow troughs. Laminate swells at the edges, and its joints can peak where two planks push up against each other. Damage that comes every August and fades by January points below the house. Damage in a stripe points to a leak.
Floors that are glued down lift, bubble or curl at the seams, and some show adhesive oozing between pieces. Tile can come loose and sound hollow when you tap it. Gluing a lifting floor back down doesn't last. The moisture under it has to be fixed first, and then the flooring usually comes up and goes back down.
The concrete and the plywood give their own signs. Dark, damp patches on bare concrete, especially along the outside walls, mean water is coming through. A white, chalky crust on concrete or in tile grout is efflorescence. Moisture carries minerals up from the slab or the soil and leaves them behind as it dries, the way a glass of water leaves a white ring when it evaporates.
Plywood or OSB that feels soft or spongy underfoot, or panels that have swollen or started to come apart in layers, are breaking down. They need replacing before any floor covers them. In a carpeted basement, a musty smell is worth a look under the pad, one of the places mold hides.
How subfloor moisture is tested
Flooring makers accept two tests on concrete. The relative humidity (RH) probe test sets sensors into holes in the slab, and the calcium chloride test weighs the vapor coming off the surface. Plywood and OSB get a wood moisture meter. A taped-down square of plastic or a handheld concrete meter can show where the wet spots are, but neither can clear a slab for a new floor.
The relative humidity (RH) probe test
The RH probe test, listed on paperwork as ASTM F2170, reads the moisture inside the concrete as a percent. It measures the humidity of the air held in the slab, which is a different thing from the humidity in your room. Once a new floor covers the surface, that slab humidity is what the floor lives with.
Done properly, the test takes a few days. The room has to sit at normal living temperature and humidity for 48 hours first, so a basement being finished needs its heat or air running before testing. Holes go 40% of the way into a slab poured on the ground, three in the first 1,000 square feet and one more for each 1,000 after that. The sensors then sit for 24 hours before anyone reads them.
The calcium chloride test
The calcium chloride test, ASTM F1869, seals a dish of calcium chloride under a cover on the bare concrete. The salt pulls water out of the air, and weighing it afterward shows how much vapor the slab gave off, in pounds per 1,000 square feet over 24 hours. It measures the top of the slab at that moment. A slab that is wetter deeper down can pass it and still cause trouble once a floor covers it.
Testing a plywood or OSB subfloor
A pin moisture meter pushed into plywood or OSB reads its moisture content as a percent. Take several readings across the room, and look hardest near outside walls and plumbing walls. Under hardwood, the new boards get read too, because the gap between the subfloor and the flooring matters as much as either number.
Why a passing test has a season
A moisture test describes the slab on the day it was tested, in the spots that were tested. Concrete that tests dry can be wet at another time of year. Here, a slab tested in a heated, dry January can read higher after a wet April or a muggy August. So the test belongs close to the install date, and a reading just under the limit is a reason to fix the source first or choose a floor with more room.
Only some of these tests count when a maker looks at your floor.
| Test | What it tells you | Accepted by flooring makers? |
|---|---|---|
| RH probes (ASTM F2170) | Moisture inside the slab, as % RH | Yes; often the preferred test |
| Calcium chloride (ASTM F1869) | Vapor leaving the surface, in pounds | Yes |
| Plastic sheet taped down | Whether moisture is there at all | No; a first look only |
| Handheld concrete meter | Wetter and drier spots | No; shows where to test |
| Pin meter on plywood or OSB | Wood moisture content, % | Yes, for wood subfloors |
Acceptable moisture levels by floor type
No single moisture level is acceptable for every floor, because each maker sets its own. Among the floors we install, the concrete limit runs from 75% RH for solid hardwood to under 90% for rigid waterproof vinyl plank, and glued sheet flooring depends on its adhesive. Some makers also limit the concrete's pH, a measure of how alkaline the surface is. The figures come from the installation instructions for Bruce solid hardwood, Mohawk laminate and rigid vinyl plank, and Armstrong commercial sheet flooring.
| Floor | Slab limit, RH probe | Calcium chloride limit | Also required |
|---|---|---|---|
| Solid hardwood | 75% or less | 3 lb or less | A wood subfloor over the concrete |
| Laminate | Under 80% | Under 5 lb | 6-mil plastic under it, always |
| Rigid waterproof vinyl plank | Under 90% | 8 lb or less | Surface pH between 5 and 9 |
| Glued sheet flooring | 80% to 95%, by adhesive | Set by the adhesive | Surface pH 9 or less |
Hold a real reading up to the table and the choice gets simple. A basement slab at 85% is too wet for solid hardwood or laminate. It's within the limit for rigid vinyl plank, and for sheet flooring on a high-moisture adhesive, which tops out at 90% on a basement slab. A slab at 72% passes for all four. Whatever you choose, the limit that counts is the one printed for the floor you buy.
Wood subfloors have limits of their own. Under its solid hardwood, Bruce wants plywood or OSB at 12% or less and within 3 points of the hardwood boards, or 2 points for wider plank. The laminate in the table also stops at 12%, and the rigid vinyl plank allows up to 14%.
Tile shows slab moisture in its grout. Vapor rising through the concrete carries minerals that dry into a white crust between the tiles, so where a slab gives off vapor, a vapor-equalizing membrane under the tile is one way to handle it. Stretched-in carpet over a pad is the easy one and usually needs no moisture test, while glued carpet does.
Waterproof floors still need a dry subfloor
Yes, a waterproof floor can go over a basement slab, and it's often what we recommend there. Waterproof describes the plank, though, and the slab under it still has to pass its test. Mohawk's own instructions for its waterproof vinyl plank say the floor should not be used to seal a floor from moisture and can't stop mold or damage caused by it.
Water from above, like a tipped dog bowl or a wet boot, stays on top of a waterproof plank. Vapor from the slab collects underneath, in the dark, where it can't dry. Rigid vinyl plank has a more forgiving slab limit than laminate or solid hardwood, and most of the rigid-core vinyl we install is fully waterproof. That's why it's one of our top picks for Dayton-area basements, and our luxury vinyl installation page covers how we put it in.
If you want real wood downstairs, solid hardwood is generally not recommended below ground level. Engineered hardwood often can go in a basement when the moisture levels are acceptable. We test first and tell you honestly whether engineered wood or a waterproof wood-look floor is the safer choice, and our hardwood installation page explains how the two are built.
Moisture barriers for concrete and wood subfloors
Whether you need a moisture barrier depends on what the floor sits on and how it goes down. Laminate on concrete always goes over a plastic moisture barrier. Glued floors can't go over plastic, so the slab itself has to pass. Over plywood, the main defense is a crawl space with its dirt covered.
The 6-mil polyethylene sold for this job is what builders call a vapor barrier, because it stops almost all water vapor. Felt and asphalt-saturated building paper are vapor retarders. They slow vapor down but still let some through.
Over a concrete slab
Laminate on concrete needs an underlayment plus at least 6-mil plastic every time, unless the underlayment has a barrier built in. The seams get taped, and the sheet runs up the wall so the trim can cover it. Our guide to choosing underlayment for laminate covers the pads that come with a barrier attached. Under rigid vinyl plank, the plank's own instructions decide whether a sheet goes down, and the slab limit applies either way.
Glue-down floors skip the plastic, because the adhesive has to grip the concrete. Sheet vinyl, glued vinyl plank and glued engineered wood go straight onto the slab, so the slab has to meet the adhesive's limit or be treated until it does.
Over a plywood or OSB subfloor
Over a crawl space, the plastic goes on the dirt under the house. Under nail-down solid hardwood, the instructions call for asphalt-saturated felt or building paper over the plywood, and ordinary brown kraft paper and red rosin paper don't count as vapor retarders. Whether laminate also gets a sheet over the plywood is up to the plank's instructions, and we follow them.
When a concrete slab tests too wet
If the slab reads over the limit for the floor you picked, you have four ways forward. Which one fits depends on how far over it is and why, and they're worth weighing in this order:
- Stop the source and let the slab dry, then test again once the room is back at living conditions.
- Choose a floor whose limit the slab already meets. At 85%, that means rigid waterproof vinyl plank instead of laminate or solid hardwood.
- For a glued floor, use an adhesive rated for a higher reading. Sheet-flooring adhesives run from 80% to 95%, with 90% the ceiling on a basement slab.
- Apply a moisture mitigation system, a two-part resin that cures into a membrane on the concrete. It's made for slabs that test over the limit of a resilient floor or its adhesive.
Sealing is only a fix when the product is one of those systems, made and tested for the job. A floor primer isn't, and the Carpet and Rug Institute's installation standard says plainly that primers aren't designed to reduce moisture vapor. A high-moisture adhesive works in damp conditions too, but the same standard notes it doesn't act as a barrier.
Water that shows up as liquid is a different problem. Puddles after heavy rain, a wet line along the bottom of a wall or water through a crack means the foundation leaks. That's work for a basement waterproofing contractor before any floor goes down.
Keeping a crawl space from wetting the subfloor
A wood subfloor over a crawl space stays dry when the crawl space does. The hardwood and laminate makers we carry list the same basics in their installation instructions, and they're worth a look with a flashlight before any new floor goes in:
- Plastic over all of the dirt, at least 6-mil, with seams overlapped 6 to 8 inches and taped.
- Vents open, adding up to at least 1.5% of the crawl-space floor area. That's 15 square feet of vent for a 1,000-square-foot crawl space.
- At least 18 inches between the dirt and the bottom of the floor joists.
- Gutters and downspouts in place, and ground sloping away from the foundation.
Summer is when crawl spaces here cause the most trouble. Humid July air comes in through the vents and meets plywood cooled by the air conditioning above. When the plywood is colder than the dew point of that air, water condenses on it, the way a glass of iced tea sweats on the porch. The colder the house is kept, the worse it gets.
A floor with a sealed finish can't let that water out through the top, so the bottom of each board swells and the floor cups. Switching to a waterproof floor doesn't solve it, because a floor that lets nothing through leaves the plywood wetter all summer. The fix is under the house.
If the plywood already reads high, dry it before anything else. Cover the ground and the walls of the crawl space with plastic, run a dehumidifier down there, and keep taking readings until the wood is back within the new floor's limit.
What to do when a leak soaks the subfloor
A subfloor soaked by a leak has to dry fast. The EPA's guidance is to dry water-damaged areas within 24 to 48 hours to prevent mold, and after that, mold is likely under anything that stayed wet.
- Stop the water at the appliance or the main shutoff.
- Take up the flooring over the wet area. A floating floor traps water under it, and a soaked carpet pad holds it against the subfloor. Carpet that has grown mold may have to be thrown away.
- Dry the subfloor with moving air and dehumidifiers. For a large area, a water-damage restoration company has the equipment.
- Measure before you cover it. The plywood should read 12% or less under hardwood or laminate, or 14% or less under rigid vinyl plank.
- Replace any panels that are soft, swollen or coming apart in layers.
If mold has already started, the EPA treats a patch smaller than about 3 feet by 3 feet as a cleanup you can usually handle yourself. Anything bigger, or a lot of water damage, calls for more careful cleanup, and any contractor you hire for it should have experience with mold.
When water damage means a floor has to be replaced, our restoration and insurance team handles the replacement and works directly with your claim. We work with restoration contractors, insurance adjusters and property managers on those jobs.
Keeping moisture out after the new floor is down
Most moisture trouble after an installation starts with something that changed, like a clogged gutter, a new leak or a humid summer in a closed-up basement. Hold indoor humidity under 60%, and ideally between 30% and 50%. A dehumidifier running in a finished basement through the summer does most of that work.
Laminate's wood-fiber core swells and shrinks with the air around it, and our article on how humidity and the seasons affect laminate shows what that looks like through an Ohio year.
Keep rainwater away from the foundation with clear gutters, downspouts that empty well away from the house and soil that slopes away from the walls. A couple of times a year, check where slow leaks start: under the kitchen sink, behind the dishwasher and refrigerator, and around the water heater and washing machine.
What we check at your free measure
At A & B Flooring, we check for moisture before we install. It starts with your free in-home measure, where we look at your subfloor, your rooms and how you use them. As part of our subfloor prep, we check the slab or wood subfloor for moisture, flatten high and low spots, and address what we find so your floor sits flat and quiet.
New planks and boards rest in your home before they go down, so they adjust to your indoor temperature and humidity. If a room isn't ready for the floor you have in mind, we tell you what the reading means and which floors it's ready for.
Questions we hear about subfloor moisture
Does homeowners insurance cover a wet subfloor?
It depends on where the water came from. The Insurance Information Institute explains that a standard homeowners policy covers water that suddenly escapes from plumbing, like a burst pipe, but not water that seeps in from the ground, which insurers treat as maintenance. Flooding needs a separate flood policy. Your own policy decides, so call your agent as soon as you find the water.
How long does new concrete need to dry before flooring goes down?
At least a month, and up to three for some floors. The solid hardwood instructions we follow want concrete at least 30 days old before moisture testing even begins. Tile setters commonly wait 28 days, and Mohawk asks for 90 days of curing before its rigid vinyl plank goes down. After that, the test decides, not the calendar.
Can I test my own concrete slab for moisture?
Yes, for a first look. Tape an 18-inch square of clear plastic tight to the bare slab and leave it for a day. If the concrete under it has darkened, moisture is present, though that says nothing about how much. A handheld concrete meter does a similar job, showing the wetter and drier parts of the floor so you know where a real test should go. Neither gives a number a flooring maker will accept. That takes the RH probe or calcium chloride test, run by the standard.
Can subfloor moisture void a flooring warranty?
Yes. Installation instructions put damage from poor site conditions, a wet slab included, outside the maker's responsibility, and skipping the site conditions they set can void a warranty. Makers also ask for the moisture readings to be recorded and kept. The paperwork that comes with your floor is what counts, and we go through it with you.
Get your subfloor checked before the floor goes down
The cheapest moisture problem is the one found before a new floor covers it. Request a free estimate, and at the measure we'll go over your slab or subfloor with you and help you choose a floor it's ready for.
