French Drain Geotextile Wrap: Strip Width, Rolls, Fabric Area & Drainage Stone

The formula

This is the expression the tool evaluates. Every term is named underneath, with the unit it must be supplied in.

How the result is derived

Step by step, from the values you type to the figure on screen.

1. The 10 inputs are read from the form on every keystroke: Drain Length, Trench Width, Depth of Stone to Wrap, Overlap at the Top, Pipe Diameter (0 = No Pipe), Stone Bulk Density, Roll Width, Roll Length, Lap at a Join along the Run and Cutting & Damage Allowance.
2. Each value is checked against the accepted range in the input table below. A value outside its range stops the calculation rather than producing a misleading figure — the results blank out and a message appears.
3. The validated values are substituted into the expression above, which resolves Rolls to Order together with every supporting figure in one pass — no value is carried over from a previous entry.
4. The supporting outputs — Strip Width, Strips Slit from One Roll Width, Joins along the Run, Strip Length incl. Joins, Roll Length Used incl. Allowance, Geotextile Laid, Drainage Stone and Drainage Stone by Mass — come from the same pass, so they always describe the same case as the headline figure.
5. Results are rounded for display only. The full-precision value is used throughout the chain, so reading a rounded intermediate figure back into the tool by hand can shift the last digit.

What each input means

Where to read each value on the floor, the unit it must be in, and the range the tool accepts.

What the tool returns

The headline figure and every supporting value it is built from.

Worked example

Substituting

A 40 cm trench takes a 2.3 m strip of fabric: almost six times its width.

How to use it

1. Work through the input groups in order — Trench and Geotextile Roll. The defaults are a realistic case, so you can change one value at a time and watch what moves.
2. There is no calculate button. Every figure recalculates as you type or drag, which is what makes this usable for a what-if sweep rather than a single answer.
3. Read Rolls to Order in the dark results panel — that is the headline figure, expressed in rolls.
4. Check the supporting rows underneath (Strip Width, Strips Slit from One Roll Width, Joins along the Run, Strip Length incl. Joins, Roll Length Used incl. Allowance, Geotextile Laid, Drainage Stone and Drainage Stone by Mass) before acting on the headline — they are where an implausible input usually shows itself first.
5. Reset to defaults returns every field to the reference case, which is the quickest way to check whether a surprising result came from the tool or from an input you had changed earlier.

Where this is used

• Process planning — establishing Rolls to Order before a trial is booked, so machine time and material in Geosynthetics & Civil Engineering are committed against a calculated figure rather than an estimate.
• Costing and quotation — Rolls to Order is an input to the cost sheet, and quoting from a worked number rather than a remembered one is what keeps a margin intact.
• Troubleshooting — when the floor result drifts from plan, entering the measured values (starting with Drain Length) shows how much of the gap in Rolls to Order each variable explains.
• Teaching and study — the accepted ranges bracket normal Geosynthetics & Civil Engineering practice, so moving one variable at a time shows the shape of the relationship rather than a single answer.

Reading the result

Typical bands and what each one is telling you.

Assumptions and limits

• 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.**
• Every input is bounded to the range normal practice occupies (Drain Length 1 to 100000 m, Trench Width 10 to 300 cm and Depth of Stone to Wrap 10 to 500 cm, and so on for the rest). Those bounds are guard rails against typing errors, not a claim that the formula fails one unit outside them.
• The calculation is deterministic: the same inputs always give the same result. It carries no allowance for machine condition, operator skill, ambient conditions or lot-to-lot material variation unless an input above explicitly represents one.
• Nothing is sent anywhere. The maths runs in your browser, so the numbers you type never leave the page.

Standards and further reading

• EN 13252, Geotextiles and geotextile-related products: characteristics required for use in drainage systems.
• AASHTO M 288, Geosynthetic Specification for Highway Applications, which includes subsurface drainage geotextiles.
• The drain detail on the drawings, which sets trench size, stone, overlap and laps.

Questions people ask