Waterproofing Overlap Calculator
Calculate how membrane overlaps reduce effective coverage and estimate the extra waterproofing material needed for a defined sheet layout.
Membrane and overlap dimensions
Overlap-adjusted membrane requirement
Why overlap changes membrane quantity
A membrane roll has a nominal area based on its full width and length. Adjacent sheets cannot normally be counted as fully effective waterproofed area when a portion is intentionally covered by the next sheet.
Detailed strip model
The calculator estimates the sheet count required across the target area, then models the effective repeating width and length after overlaps. It is intentionally more geometric than a simple blanket percentage.
Effective sheet length ≈ sheet length − end overlap where end joints occur
Worked example
With 1 m wide sheets and a 100 mm side overlap, each subsequent adjacent strip contributes roughly 0.9 m of effective width. A 150 mm end overlap similarly reduces effective length where sheet ends are joined.
Why layout matters
Changing the orientation of the sheet can change the number of joints and therefore the material consumed. This is why a detailed installation drawing can be more accurate than multiplying total area by a generic allowance.
Limitations
This model assumes repeating rectangular sheets. It does not model every corner, penetration, upstand, drain or irregular cut. Final quantities should follow the actual membrane layout.
Frequently Asked Questions
Does overlap reduce effective width?
Yes.
Should overlap be a fixed percentage?
No. Use the specified dimensional overlap whenever available.
Can sheet orientation matter?
Yes.
Does this design the seam?
No.