The French Drain: What is a French Drain?
The French drain, also called a sub-drain, is so called because the person who is supposed to have invented it, an American, was named French. First used for agricultural purposes in New England before the development of perforated plastic pipe, the system invented by Mr. French was probably somewhat different from what we call a French drain today. Unlike a surface drain system which collects and removes surface moisture, this type drain system collects and removes subsurface moisture.
A sub-drain, by modern definition, is basically a trench filled with gravel with a perforated pipe at the bottom. Moisture accumulating in the trench percolates down and enters the pipe which transports the moisture to some point of discharge.
A French drain system can be of great benefit in solving a variety of foundation repair problems. When surface drainage control is not enough, I often recommend French drain systems to dry out wet crawl space areas under a house or building.
Some contractors may be tempted to pursue a cheap solution and install surface drains in the crawl space area, collecting water when it starts to pond in the lowest areas. These contractors do not understand the problem. The crawl space must be kept dry at all times. Waiting for water to pond in the crawl space and then drawing it down is like living in a house with no roof and then mopping the water off the floor after each rainstorm!
A Typical French Drain system consists of the following.
- Excavate problem area at lowes point creating a ditch.
- Installation of ½ perforated drain-tile allowing water to enter on top of the pipe and flow through the bottom of the pipe to release at the designated area.
- Installation of clean stone on top of pipe acting as a filter for the top half of the pipe.
- Installation of soil/mulch, sod, seed/straw Etc… on top of “Ditch” allowing water to slowly seep down into soil, stone and into the pipe.