How a French Drain Works
A French drain is not a brand, but a type of drainage system. It consists of a perforated pipe buried in a gravel-filled trench. The key principle is that water always flows downhill and seeks the path of least resistance.
Groundwater enters the trench, flows easily through the gravel, and enters the perforated pipe. The pipe then carries the water away to a safe discharge point, such as a dry well, a storm drain, or a low-lying area of the property away from buildings.
Key Installation Tips
| Component | Recommendation |
|---|---|
| Slope | The trench must have a consistent downward slope of at least 1% (about 1 inch of fall for every 8 feet of length). |
| Landscape Fabric | Line the entire trench with water-permeable landscape fabric before adding gravel. This prevents soil and silt from clogging the gravel and pipe. |
| Gravel | Use clean, washed drainage gravel, typically 1/2" to 1.5" in diameter. This allows for maximum water flow. |
| Pipe Placement | Lay the perforated pipe with the holes facing DOWN. This allows rising water to enter the pipe from below. |
How to Use the Calculator
- Measure the total length of the trench you plan to dig.
- Enter the planned width and depth of your trench in inches.
- Enter the cost per ton for drainage gravel from your local supplier.
- Click "Calculate Materials" to estimate the volume of the trench and the amount of gravel and pipe you need.
Frequently Asked Questions
Do I fill the entire trench with gravel?
Typically, yes. You lay a base of gravel, place the pipe on top, and then fill the trench with gravel to within a few inches of the surface. You can then cover the top with soil and sod.
What kind of pipe should I use?
Use a rigid, perforated PVC pipe or a flexible, corrugated pipe specifically designed for drainage. Flexible pipes are easier to work with on curved trenches, but rigid PVC is more durable.