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Under-Slab Vapor Barrier Seam Tape Calculator

Calculate the seam tape needed to seal overlapping vapor barrier sheets beneath a concrete slab.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
49.21 ft65.62 ft
Schematic, drawn to the proportions you entered — not to scale on screen.
Who is doing the work?

Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.

Seam tape needed

155.02 ft

High confidence
Number of seams
3

With the figures above, the seam tape needed comes to 47.3. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

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.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • 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

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Frequently asked questions

Why is the seam count one less than the number of sheets?
Tape is only needed at the overlaps between adjacent sheets. Covering the slab width with N sheets creates N − 1 seams between them, so seams = ceil(slab width ÷ sheet width) − 1.
What sheet width should I use?
Use the actual roll width of your vapor barrier product — a common under-slab polyethylene roll is about 6.1 m (20 ft) wide, but check your specific product's roll width.
Does the waste allowance cover tape for patches or penetrations?
No — this calculates tape for the straight overlap seams across the slab width only. Additional tape for patches, pipe penetrations, or sealing the perimeter to the foundation wall should be estimated separately.