Geotextile Overlap Calculator — Fabric Area, Seams and Extra Material
Calculate the additional geotextile required when adjacent fabric sheets or rolls overlap. The tool separates the actual project area from material consumed by overlap at side and end joints.
Geotextile layout
Overlap sensitivity
Overlap consumes fabric even though it does not increase the covered ground area. This table shows how the planning area changes when the side overlap assumption changes while the other inputs remain fixed.
| Side overlap | Strip count | Planned area |
|---|
How overlap affects quantity
The net project area is simply:
When fabric is installed in strips, every shared seam consumes an additional overlap width. End joints create a similar effect along the length of each strip. The finished material requirement therefore depends on the installation layout, not only the site area.
Why net area and purchased area are different
Twenty square metres of ground does not necessarily mean twenty square metres of purchased fabric. Overlap, trimming and roll-cut patterns can make the purchased area larger than the covered surface.
Keeping these numbers separate helps with tender comparisons and material reconciliation because an apparent “excess” can be explained by the installation geometry rather than unexplained waste.
Worked example
A 30 m × 12 m area is 360 m². With 4 m wide fabric and a 300 mm side overlap, three strips are required across the width. With a 500 mm end overlap, each strip also requires additional run material where the roll cannot cover the full 30 m continuously. The calculator reports that layout explicitly before adding the cutting allowance.