A French drain system is a trench laid with gravel designed to reroute water away from homes, gardens or other areas susceptible to moisture damage. Typically a 1 percent grade is optimal, the trench depth increasing by 1 inch for every 100 feet of the distance towards the proposed destination.
The drain field is the high end of a French drain where water enters the system, and the exit is the lower end where water spews out. The first decision necessary in building a French drain is where to locate the exit. Inappropriate choices include diverting water to other properties, running water into a street or spilling runoff into public drains. A better option is redirecting water to a point on the affected property that is not susceptible to damage, usually into a water feature, a rain garden, a sunny grassy slope or a dry well. A dry well is simply a hole approximately 4 feet deep, filled with gravel.
Taking proper measurements and checking depths while digging the trench are critical to successful drainage. The plan can be as simple as stakes joined by a leveled string at the drain field and exit. Obstacles such as trees, large rocks, garden beds and utilities necessitate a less direct path. Leveraging a natural slope lessens the amount of digging necessary.
Landscape fabric laid before adding gravel prevents weed growth from disrupting a drain system. Laying roofing felt or drainage tiles over the gravel before sodding stabilizes the system.