BuildMeter guide · Excavation & Drainage
French Drain Geotextile Fabric Guide
Measure a full trench wrap, plan roll strips and understand why drainage geotextile selection must match the soil and aggregate rather than generic landscape fabric.
Direct answer
What to know first
A full trench wrap uses the trench bottom width, both lined sides and the top closure overlap. Plan strip width and length seams from the actual roll, but select filtration properties for the site soil so water can pass without carrying fines into the drain stone.
Open French Drain Fabric CalculatorA full wrap is wider than the trench bottom
For a trench lined on the bottom and both sides, the required fabric strip width is bottom width plus twice the lined depth plus the top closure overlap. Compare this wrap width with the roll width to determine whether one or multiple trench strips can be cut from each roll.
| Component | Width |
|---|---|
| Bottom | 2.0 ft |
| Two sides | 5.0 ft |
| Top overlap | 1.0 ft |
| Total strip width | 8.0 ft |
Worked roll-layout example
A 120 ft drain needs an 8 ft wrap width. A 12.5 × 300 ft roll yields one full-width strip. With an 18 in. end overlap, one length segment still covers the trench, and a 10% allowance remains within one roll. A narrower roll would not fully wrap the trench without a longitudinal seam.
Filtration is not the same as weed control
Drainage geotextile must retain soil while allowing water to pass over time. The appropriate opening size, permittivity, strength and survivability depend on the soil, aggregate and installation conditions. Generic landscape fabric may clog, tear or have unsuitable hydraulic properties.
Common mistakes
- Measuring only the trench bottom.
- Omitting top closure or end overlaps.
- Placing fine soil directly into clean drainage stone.
- Using fabric to compensate for a drain with no outlet.
- Wrapping the pipe so tightly that required stone envelope is lost.
Sources and professional limits
- FHWA — Aggregate trench drains and filters
- FHWA — Geosynthetics guidance
- FHWA — Drain construction considerations
Final field verification
Before using the french drain geotextile fabric estimate as a purchase order, walk the project and compare the calculated scope with the installed conditions. Verify measured flow, run length, elevation change, pipe or duct size and available service access at the site. Photograph or sketch unusual details and keep the measurements with the estimate so another person can understand how the quantity was produced.
- Check equipment manuals and local plumbing or mechanical requirements before selecting spacing, insulation or operating assumptions.
- Plan cleanouts, valves, traps, discharge points and maintenance access before material quantities are finalized.
- Round only after all measured sections have been combined; rounding every small section independently can materially overstate the total.
- Recheck units, package coverage and the number of full stock lengths or containers required before checkout.