SettingsSettings for this calculationUS
Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.
The overall width of the slab area being covered by the vapor barrier.
Measure across the direction the vapor barrier sheets are rolled out, so this width determines how many sheets are needed side by side.
The width of a single vapor barrier roll or sheet.
Check the manufacturer's roll width — a common under-slab poly roll is 20 ft (about 6.1 m) wide.
The overall length of the slab area, running the direction of each seam.
Each seam between adjacent sheets runs the full length of the slab, so this sets the length of tape needed per seam.
Extra tape to allow for overlaps, patches, and cut waste.
5% is a typical starting point for a straightforward layout; increase it for slabs with many penetrations or an irregular perimeter.
Seam tape needed
154.3 ft
- Number of seams
- 3
They open the calculator with your figures already in it
Under-Slab Vapor Barrier Seam Tape Calculator: 154 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Show calculation logicHide calculation logic
How this was calculated
Formula source(s)
- Number of seams = ceil(slab width ÷ sheet width) − 1; total seam tape = seams × slab length × (1 + waste allowance), the standard method for taping overlap seams across a wide under-slab vapor barrier installation
Inputs used
- Slab Width
- 66 ft
- Vapour barrier sheet width
- 20 ft
- Slab Length
- 49 ft
- Waste Allowance (%)
- 5
Intermediate steps
- Number of seams
- 3
What this calculation does not cover
- The sheet count divides the slab width by the full roll width, so nothing is deducted for the side lap where adjacent sheets overlap each other. Laps are typically specified somewhere between 150 mm (6 in) and 300 mm (12 in) depending on the barrier manufacturer and the project specification, and once that is taken off the effective coverage of each sheet a wide slab can need one more sheet, and one more seam, than this calculation reports. Enter the lapped coverage width rather than the printed roll width if you want the lap counted.
- Only the longitudinal seams between sheets laid side by side are counted, and each is taken as running the full slab length in one unbroken run. If the roll is shorter than the slab, every run also carries an end lap partway along it, and there is no roll-length input here, so that transverse tape is absent from the total.
- The figure is seam tape between sheets and nothing else. Sealing the barrier to the footing, the foundation wall or the slab edge, and taping around pipes, conduit, column bases and other penetrations are all excluded and have to be estimated separately. On the default 20 m by 15 m slab the perimeter alone is 70 m against the 47.25 m of seam tape returned, so the excluded work can be the larger quantity.
- Which dimension you enter as the width decides the answer, and the calculator does not compare the two lay directions for you. Running 6.1 m sheets across the 20 m side of a 20 m by 15 m slab gives three seams and 47.25 m of tape, while running them across the 15 m side gives two seams and 42 m. The geometry also assumes one plain rectangle, so an L-shaped or stepped slab has to be broken into rectangles and the results added.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Part of a bigger job
Or plan the space itself: measure it once, doors and windows included, and this figure comes back worked out on that space, with what goes with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Number of seams = ceil(slab width ÷ sheet width) − 1; total seam tape = seams × slab length × (1 + waste allowance), the standard method for taping overlap seams across a wide under-slab vapor barrier installation
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
Tools and safety for this job
To lay the damp-proof membrane. Generic types, no brands, no prices.
Protection this work requires
- Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
- Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.
Show the 2 tools this job needsHide tools
Essential
Tape measure
Utility knife
Also needed as materials: replacement blades.
what each concrete tool is for, and the spec that decides which to buy where one does.