About French Drain Calculation
A **French Drain** is an effective drainage system that uses a sloped trench, gravel, and perforated pipe to divert water away from a specific area, preventing pooling and foundation damage. The calculator provides volume estimates for the trench fill and area for the filter fabric.
Material Formulas Used
$$\text{Volume (yd}^3) = \frac{\text{Length (ft)} \times \text{Width (ft)} \times \text{Depth (ft)}}{27} \times (1 + \frac{\text{Waste}\%}{100})$$ $$\text{Fabric Area (sq yd)} = \frac{\text{Length (ft)} \times (\text{Width} + 2 \times \text{Depth} + 1 \text{ ft})}{9} \times (1 + \frac{\text{Waste}\%}{100})$$ We divide by 27 (to convert ft³ to yd³) and by 9 (to convert ft² to yd²). The Fabric Girth includes 1 foot for top overlap.
Drain Components & Best Practices
Trench Construction
- **Slope:** The trench must slope downward 1% (1 inch per 10 feet) towards the outlet to ensure gravity drainage.
- **Depth:** For shallow water, 1.5–2 ft deep is sufficient. For hydrostatic pressure control near a foundation, deeper (3–4 ft) may be required.
- **Outlet:** Water must be directed to a safe location (e.g., street, dry well, low area) at least 10 feet from the structure.
Material Selection
- **Gravel:** Use clean, washed rock (like #57 stone) with no fines or dirt, as these clog the system.
- **Pipe:** Use 4-inch corrugated pipe with a filter sock, or rigid PVC pipe. Holes must face down to allow water to enter the pipe base.
- **Fabric:** Non-woven filter fabric is critical to prevent surrounding soil and dirt from migrating into the gravel and clogging the drainage system.
Installation Steps & Costs
DIY vs. Professional Cost (Approx.)
- **DIY Material Cost:** ₹1,000 – ₹2,500 per linear foot.
- **Professional Installation:** ₹3,500 – ₹7,000 per linear foot (includes labor, machinery, and disposal).
- **Major Costs:** Trenching labor (or machinery rental) and gravel delivery are the largest cost factors.
Installation Time (DIY)
- **Trenching (Manual):** Slow and difficult, approx. 1-2 hours per 10 linear feet.
- **Trenching (Machine):** Fast, 1 hour per 50-100 linear feet.
- **Laying Pipe/Fabric:** Moderate speed, 1-2 hours per 50 feet.
- **Filling/Finishing:** Most time-consuming stage due to material handling.
Typical Project Duration
Short Drain (20 ft)
1 Full Day
(Manual digging)
Medium Drain (50 ft)
1-2 Weekends
(Manual digging)
Large Drain (100 ft+)
2-3 Full Days
(Requires machinery)