A French drain, or a drain tile, is made when a trench is filled with gravel, and many times a perforated pipe at the bottom of the trench is used as part of the drainage solution. A textile fabric wraps the pipe and gravel to act as a sieve where fine sediments will not pass into the gravel area. A French drain, when properly designed, will reduce the pressure exerted upon retaining walls and subgrade walls that, with time, can induce moisture inside basements and crawl spaces.
This drainage system is an indispensable component of moisture control.

The drainage system should not be more than 12 inches wide; between 6 and 8 inches will be ideal because it will last longer, and water will be collected and transported better with fewer obstructions. As a good practice, you can also have at least 2” of gravel around the pipe, and the Geotextile fabric must be installed so the pipe is completely covered. The pipe installed in the trench must also be installed with holes on the bottom.
A French drain must be installed correctly; otherwise, the cost will add up. In addition, the drainage system should be installed deep enough, so water will eventually run into the pipe, instead of moving near or into the structure.
The usual recommended slope of the perforated pipe for a French drain system is a one-inch drop per ten feet horizontal run.
How deep does the pipe need to be installed? Well, that depends on the application of the drain. For example, if a French drain is used to protect a living space, the bottom invert shall be placed at least 2 inches below the level of the finish floor.