Trench From the drain detail

Drain Length m

Trench Width cm

Depth of Stone to Wrap cm

Overlap at the Top cm

How far the two edges of the fabric lap over the stone; the drain detail sets it.

Pipe Diameter (0 = No Pipe) cm

Stone Bulk Density t/m³

From the quarry; clean single-size stone is lighter than graded fill.

Geotextile Roll From the roll label

Roll Width m

Roll Length m

Lap at a Join along the Run cm

Cutting & Damage Allowance %

Rolls to Order

— rolls

Strip length with joins and allowance, shared between the strips one roll gives, rounded up

Take-Off

Strip Width: — cm
Strips Slit from One Roll Width: — strips
Joins along the Run: — joins
Strip Length incl. Joins: — m
Roll Length Used incl. Allowance: — m
Geotextile Laid: — m²
Drainage Stone: — m³
Drainage Stone by Mass: — t

The trench is taken as rectangular, wrapped on both walls and the floor, with the two edges lapped over the stone. Widths, depth, overlap, laps and the stone are the drain detail's; the tool has no figure for which geotextile to use. When the roll is wider than two strips it is counted as slit into whole strips, each using the same roll length; if the strip is wider than the roll, strips and rolls show zero: the roll is too narrow for the wrap. Stone is the wrapped volume less the pipe. **This is a quantity take-off; the geotextile's filtration properties against the soil are a separate check.**

French Drain Geotextile Wrap: Strip Width, Rolls, Fabric Area & Drainage Stone — free, with the formula and a worked example, at Textile School.