Understanding your result
For a sheet barrier the headline is the rolls of poly, with the length of sheet and the overlap beneath it; for a liquid barrier it is the gallons. The table shows the strips, the sheet bought, the rolls, the seams and the tape, or the coats and the liquid.
Strips run along the room’s length, each the room length plus the turn-up at both ends. Their number is the room’s width, less one overlap, divided by the roll width less the overlap, because each strip after the first loses an overlap’s width to the one before. The sheet bought is the strips times their length, in the roll width, and the rolls follow from the roll length. Seams are one fewer than the strips, each a room length long, and the tape covers them plus some for patching around pipes and posts.
The liquid option is the area times the coats divided by the coverage per coat. It is for glue-down floors, where the adhesive must reach the slab. For floating floors where the underlayment has a built-in film, a separate sheet is not needed; the underlayment calculator covers those products, and the laminate calculator notes when a barrier is required. A screed laid over a membrane is on the floor screed calculator.
How we calculate this
The overlap of 6 in and the turn-up of about 3 in are the figures most flooring makers print for poly under floating floors; the turn-up is trimmed after the baseboard is on. The 10, 15 mil and ASTM E1745 references are for vapor retarders under slabs and in crawl spaces, which follow the slab and building code requirements. The subfloor prep guide explains moisture testing and where barriers belong, and the underlayment guide covers combined underlayment-and-barrier products.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Sheet | 6 mil polyethylene, 10 × 25 ft roll | Typical flooring maker requirement under floating floors on concrete |
| Overlap | 6 in (15 cm), taped | Flooring maker installation instructions (6 to 8 in) |
| Turn-up | 3 in (7.5 cm) | Common practice, trimmed after trim is fitted |
| Under-slab retarder | at least 10 mil, ASTM E1745 | ACI 302.2R guidance and common specifications |
| Moisture tests | ASTM F1869 (calcium chloride), ASTM F2170 (relative humidity) | The tests flooring warranties refer to |
| Liquid barrier | 200 sq ft per gallon per coat, 2 coats | Typical data sheets range from 100 to 300 sq ft per gallon |
Assumptions last reviewed October 8, 2026.
The calculator assumes a rectangular room and strips running the long way; an irregular room is covered with more seams and more tape. It does not test the slab or decide whether a barrier alone is enough; a slab with high readings may need a moisture mitigation system rated for the numbers measured. It also does not cover crawl space encapsulation beyond the sheet quantity.
Two worked examples
A 24 × 18 ft basement under laminate
6 mil poly in 12 × 50 ft rolls, 6 in overlap, 3 in turn-up, 180 ft tape.
- Strips: (18 − 0.5) ÷ (12 − 0.5) = 1.5, rounded up to 2 strips, each 24 + 0.5 = 24.5 ft
- Sheet: 2 × 24.5 = 49 ft of 12 ft roll, 588 sq ft; 1 roll
- Seams: 24 ft; tape: one roll
One 50 ft roll with a foot to spare. The 588 sq ft of sheet for a 432 sq ft floor is the overlap and the turn-ups; ordering by floor area would have bought a 10 × 25 roll and been short.
350 sq ft of slab under glue-down vinyl
Liquid barrier at 150 sq ft per gallon per coat, two coats.
- Liquid: 350 × 2 ÷ 150 = 4.67 gallons
Five gallons, applied to a clean, open slab that may need grinding first. The barrier and the vinyl adhesive should come from the same maker’s system, or at least be approved together.
Where to find your inputs
Room. Wall to wall over the slab.
Roll size. On the roll: width and length, often 10 × 25, 10 × 100 or 12 × 50 ft.
Overlap and turn-up. In the flooring’s installation instructions.
Liquid coverage and coats. On the barrier’s data sheet, for your slab’s moisture reading.
Common mistakes
- Ordering poly by the floor area. Overlaps and turn-ups add a quarter or more.
- Untaped seams. A barrier with open seams leaks vapor at every one.
- Plastic under a glued floor. The adhesive needs the slab. Use a liquid system.
- Barriers on both sides of a wood subfloor. It traps moisture in the wood.
- No test. A barrier rated for the wrong moisture level fails. Test the slab.
- Doubling up. A film-backed underlayment and a separate poly sheet are rarely both needed.
Questions people ask
- Do I need a vapor barrier under laminate on concrete?
- Yes. Laminate, engineered wood and most floating floors on a concrete slab need a vapor barrier, either a 6 mil polyethylene sheet with 6 to 8 inch taped overlaps or an underlayment with a bonded film. Over a wood subfloor above a heated space, no barrier is needed.
- How much poly do I need for a floor?
- More than the floor area, because the strips overlap 6 to 8 inches and turn up the walls a few inches. A 24 × 18 ft room in a 12 ft wide roll takes two strips 24.5 ft long, 49 ft of roll and 588 sq ft of sheet, for a 432 sq ft floor.
- Is 6 mil poly thick enough?
- Under floating floors on a slab, 6 mil is what most flooring makers specify. Under a new slab, a heavier vapor retarder of at least 10 mil meeting ASTM E1745 is the usual specification, and 15 mil under moisture-sensitive floors. Crawl space ground covers are often 10 to 20 mil.
- Can I put plastic under a glued floor?
- No. A glue-down floor bonds to the slab, so a sheet barrier would leave nothing to glue to. Use a liquid moisture barrier or a moisture-tolerant adhesive system designed to work with the flooring adhesive.
- How do I know if my slab has a moisture problem?
- Test it. A calcium chloride test (ASTM F1869) measures vapor emission and an in-situ relative humidity test (ASTM F2170) measures moisture in the slab; the flooring warranty gives a limit for one or both. Taped-down plastic sheets show a problem but not how big.
- Should there be a vapor barrier under a wood subfloor?
- Not directly under it, and never on both sides of it. Over a crawl space, the barrier goes on the crawl space ground, lapped and sealed. A barrier on both sides of a wood subfloor traps moisture in the wood.