Just what is a “French Drain?” Many times people incorrectly use the phrase, “French Drain,” to apply to many different types of Drains that could be used in a Drainage System. The term is widely used and many times incorrectly connected to “Surface Drains,” or “Channel Drains.”
A Surface Drain has a grate that sits on top of a basin. The basin is underground. A Drain Pipe is connected underground to an outlet on the Drain Basin. Storm-Water Drains through the grate into the Drain Basin. Once in the Drain Basin, water travels out of the basin through a Drain Pipe. The Storm-Water continues downhill in an underground Drain Pipe to an exit point.
THIS IS NOT A FRENCH DRAIN!
A French Drain is completely different from a Surface Drain. A basic French Drain consists of a Perforated Drain Pipe in the bottom of a trench. The Drain Pipe should have a neoprene sock around the Perforated Drain Pipe. This is to ensure that the Drain Pipe does not become clogged.
The dirt that was taken out to make the trench is hauled away. It is replaced by some type of small stone or gravel depending on what is desired or available. I prefer crushed 1 inch lime stone. It is the most economical option in my area. Pea Gravel or some other type of small stone can work just as well. The lime-stone or gravel is placed in the trench on top of the perforated Drain Pipe and filled all the way to the surface (ground level). In some cases where the French Drain needs to be deep or is being placed in sandy soil, a special trench liner must be placed in the trench before the perforated Drain Pipe or the Gravel are installed. This helps maintain the integrity of the trench over time. It also increases the cost of the French Drain and the amount of time to install it. I install a trench liner in a French Drain about 20% of the time. Most of the time a liner in not needed.
- French Drain March 2011