A French drain only works as long as the gravel around the pipe stays clean. Once soil washes into the stone, the drain slowly clogs and water backs up again. The fabric you wrap the trench with is what keeps that from happening. Here is how to pick the right one.
Use non woven geotextile, not woven or weed barrier
For French drains you want non woven geotextile. It is a double needle punched felt-like fabric that lets water pass through quickly while holding back soil fines. That combination of flow and filtration is exactly what a drain needs.
- Woven geotextile is built for strength and soil separation under gravel driveways. It passes water much more slowly, so it is the wrong choice for wrapping a drain.
- Weed barrier is made to block weeds in planting beds. It is not designed to filter soil around a buried pipe.
4 oz, 6 oz or 8 oz?
The weight (ounces per square yard) tells you how thick and tough the fabric is. Heavier fabric resists punctures from sharp stone better, while lighter fabric moves water faster. Here is how Super Geotextile's three non woven weights compare (MARV values from our spec sheets):
| Spec | 4 oz | 6 oz | 8 oz |
|---|---|---|---|
| Grab tensile | 100 lb | 160 lb | 205 lb |
| CBR puncture | 300 lb | 410 lb | 535 lb |
| Water flow rate | 140 gpm/ft² | 110 gpm/ft² | 90 gpm/ft² |
| AOS (sieve) | 70 | 70 | 80 |
- 4 oz non woven is the go-to for most residential French drains, downspout drains and dry wells with typical drain rock. It has the highest flow rate of the three.
- 6 oz non woven is a step up for larger or angular stone, longer runs, or when you just want more puncture resistance during installation.
- 8 oz non woven is for heavy duty work: retaining wall drainage, rip rap, and commercial jobs where the fabric will take a beating.
How wide a roll do you need?
Wrap the whole trench: bottom, both sides, and enough to fold over the top of the gravel with some overlap. A quick way to estimate width is:
Roll width = trench width + (2 x trench depth) + 12 inches of overlap
Example: a 12 inch wide by 18 inch deep trench needs about 12 + 36 + 12 = 60 inches, so a 6 ft wide roll works well. For roll length, use the length of the trench plus a little extra for any seams.
Basic installation steps
- Dig the trench with a slight slope toward the outlet.
- Line the trench with fabric, leaving the extra width hanging over both sides.
- Add a layer of clean drain rock, set the perforated pipe, and fill with more rock.
- Fold the fabric over the top of the stone, overlapping the edges.
- Hold the fabric in place with landscape staples while you work, then backfill with soil or top with decorative stone.
Where two pieces meet, overlap them by at least 12 inches so soil can't slip through the seam.
Not sure which weight to get?
Order a 1 ft x 1 ft sample to feel the difference, or reach out to us and tell us about your project. Every order ships free.
