GSM is the weight of the fabric in grams per square metre. In woven coir geotextile, a heavier grade means more yarn in every square metre: a tighter weave, a smaller open area, more strength and a longer-lasting cover. The right grade is the lightest one that will still hold your soil until plants take over.
What changes as the grade goes up
The table brings together figures from California’s transport department and minimum values published in Indian coir geotextile specifications. Individual products vary, so treat these as reference points, and ask for a test report for the lot you buy.
| Grade | Open area | Minimum tensile strength (machine direction) | Opening size | Steepest slope (H:V) |
|---|---|---|---|---|
| 400 GSM | 65% | 7 kN/m | about 16 × 21 mm | 3:1 |
| 700 GSM | 50% | 8 kN/m | about 7.5 × 7.3 mm | 1:1 |
| 900 GSM | 39% | 15 kN/m | about 4.2 × 5.1 mm | 1:1 |
Open area and slope limits: Caltrans. Tensile strength and opening size: Indian specification minimums as published in Technical Textiles. Slopes are given as horizontal to vertical, so 3:1 is gentler than 1:1.
Four questions that decide the grade
1. How steep is the slope?
Caltrans limits 400 GSM netting to slopes of 3:1 or gentler and allows 700 and 900 GSM up to 1:1. If your slope is steeper than 3:1, start at 700.
2. Will water run along it?
Run-off across a slope is gentle; water flowing along a channel, ditch or riverbank is not. Where water runs along the fabric, the heavier, tighter grades are the usual choice.
3. How much light do the plants need?
Open area is where seedlings grow through. A 400 GSM net leaves about two-thirds of the ground open, which suits fast germination. A 900 GSM net covers more than half the ground. Caltrans advises seeding before laying 900 GSM, because it is too dense to seed through afterwards.
4. How long must the cover last?
Caltrans rates all three grades at about three years of functional life on the surface. Heavier grades hold their strength longer, which matters where vegetation is slow to establish.
A simple starting point
- 400 GSM for gentle slopes, landscaping and seedbed cover, with little water flow.
- 600 GSM as a step between 400 and 700. Published reference figures for this weight are less standardised, so rely on the lot test report.
- 700 GSM for most embankments, cuttings and moderate banks.
- 900 GSM for channels, riverbanks, swales and steep or exposed ground.
These are starting points, not a specification. Soil type, rainfall, design flow and the planting plan should confirm the final choice.
See the full details on our pages for 400, 600, 700 and 900 GSM, or send us your site details for a recommendation.