Why a waterproof plank still needs protection
Waterproof describes the plank. It means a spill on top won't swell the core. It says nothing about water vapor rising out of the concrete underneath. Concrete can keep giving off moisture well after it is poured, and a plank floor holds that moisture against the slab.
The trouble shows up underneath. The Montserrat SPC guide says preventing subfloor moisture limits the growth of harmful bacteria, mold and mildew. ASTM, the standards body, lists debonding, peaking, deterioration of the flooring and microbial growth as the failures excess slab moisture can cause after a floor is installed.
What is already under a Florida slab
The Florida Building Code requires a vapor retarder under slabs poured on the ground. Section R506.2.3 calls for 6-mil polyethylene, or an approved equivalent, with joints lapped at least 6 inches, placed between the slab and the ground beneath it. Section 1907.1 has a similar rule.
That sheet is under the concrete, where nobody can inspect it once the slab is poured. It can be torn during construction, and it does nothing about moisture already in the concrete. So flooring makers still ask for testing and, for floating floors, a film on top of the slab.
How the slab gets tested
Testing matters most on older homes where tile or carpet has just come up. Nobody has looked at that concrete in years. Test after tear-out, not before, so the readings reflect the bare slab the new floor will sit on.
There are two common tests. ASTM F1869 uses calcium chloride to measure how much moisture the slab gives off over 24 hours. ASTM F2170 places probes into the concrete and reads the relative humidity inside it. ASTM notes that either result is a snapshot. It shows the condition only on the day and in the spots tested.
- New concrete needs time. The Montserrat guide asks for at least 60 days of curing before testing.
- Mohawk makes testing the installer's responsibility on old and new slabs alike, and asks that it be documented before installation.
| Maker guide | F1869 limit | F2170 limit | pH |
|---|---|---|---|
| COREtec Pro SPC (floating or glue) | 8 lbs | 90% RH | 7 to 10 |
| Mohawk LVT glue-down | 8 lbs or less | no more than 90% RH | 7 to 10 |
| Montserrat SPC (floating) | 8 lbs | 90% RH | up to 9 |
When the poly film goes down
For floating floors on concrete, COREtec Pro recommends a 6-mil poly film underlayment. Its instructions call for rolling the film out parallel to the starting wall, running it about 2 inches up the wall, overlapping each run by at least 4 inches, and taping the seams with clear tape. The excess is trimmed after the floor is in. The Montserrat guide goes further and requires a 6-mil film when a slab tests outside its limits.
The film ends up hidden. It runs up the wall a couple of inches during installation, gets trimmed after the planks are down, and the baseboard or quarter round covers the edge along with the expansion gap.
Many SPC planks come with a pad already attached. That pad is for comfort and sound. It is not a vapor barrier. COREtec Pro allows a second underlayment under an attached-pad plank in homes, but only up to 3 mm thick, so stacking thick foam isn't an option.
Glue-down floors play by different rules
A glued floor can't sit on plastic. The adhesive has to bond to the concrete. Mohawk states that its acceptable moisture readings apply only when its approved adhesive is used. If a slab reads too wet for the adhesive, the fix is a moisture-control product approved by the adhesive maker, or a different installation method. Our floating vs glue-down guide covers that choice in more detail.
Keeping the slab dry after the floor goes in
Moisture control doesn't end on install day. The Montserrat guide asks for working heat and air conditioning, with room temperature between 65 and 80 degrees and relative humidity between 35 and 55%, starting 48 hours before installation and for the life of the floor. In a Florida house, that mostly means running the AC and not letting the house sit closed and hot for months.
If the slab tests high, there are options. The Montserrat guide requires film under its floating floor when readings fall outside its limits. For glued floors, the fix is whatever moisture-control product the adhesive maker approves, or switching to a floating install. What doesn't work is skipping the test and hoping. A floor that fails from below usually fails slowly, and by the time you notice, the damage is done.
Slab testing and film are part of the prep on every luxury vinyl plank installation we do. If a slab reads wet, we tell you before any plank goes down.
Questions People Ask
Do you need a vapor barrier under vinyl plank on concrete?
For floating vinyl plank, usually yes. COREtec Pro recommends 6-mil poly film over concrete with the seams taped, and the Montserrat guide requires it when a slab tests outside its moisture limits. Glue-down floors don't use film. They rely on the adhesive's own moisture limits and, where needed, an approved moisture-control product.
Isn't the attached pad on my plank a moisture barrier?
No. An attached pad is there for comfort and sound. It won't stop water vapor from the slab. Film is a separate layer under the floor. COREtec Pro allows a second underlayment under attached-pad planks in homes, but it can't be thicker than 3 mm.
My slab already has a vapor barrier under it. Isn't that enough?
Florida code requires 6-mil polyethylene under slabs on the ground, but it sits beneath the concrete, where it can't be checked once the slab is poured. It also doesn't dry out moisture already inside the concrete. That's why flooring makers still require testing and recommend film on top.
What moisture reading is too high for vinyl plank?
It depends on the product. The COREtec Pro, Mohawk and Montserrat guides we cite set the same limits: 8 pounds by the ASTM F1869 calcium chloride test, or 90% relative humidity by ASTM F2170 probes. Above that, the maker's instructions for extra protection or treatment apply.
Who is responsible for testing the slab?
The installer. Mohawk's glue-down guide puts the job of testing old and new concrete on the flooring contractor and installer, and asks that the results be documented before installation. You shouldn't have to arrange it yourself. Ask any installer you're considering how and where they test before the floor goes down.
Can mold grow under vinyl plank on a slab?
It can if moisture gets trapped. The Montserrat installation guide ties subfloor moisture prevention to limiting harmful bacteria and mold. ASTM lists microbial growth among the failures caused by excess slab moisture. Testing first, using film under floating floors, and keeping the AC running are the main defenses.
Sources
- COREtec Pro residential installation guidelines for SPC products: https://pdmsview.shawinc.com/USFloors/COREtec-Pro/Installation/Residential-Installation-Guidelines-for-SPC-Produc
- Montserrat rigid core vinyl (SPC) installation and maintenance guide: https://images.thdstatic.com/catalog/pdfImages/5a/5a9c1ad8-176c-4d32-9bda-225bbe6d4c45.pdf
- Mohawk residential installation guide, LVT click and glue-down: https://mohawk.blob.core.windows.net/mohawkflooring/pdf/install/Unidrop_LVT_Installation_Instructions.pdf
- ASTM F2170, relative humidity in concrete floor slabs: https://store.astm.org/f2170-19a.html
- City of Pensacola Building Inspections memo quoting Florida Building Code 1907.1 and R506.2.3: https://www.cityofpensacola.com/DocumentCenter/View/28130/Vapor-Barrier-in-Footings-Policy-