How do French Drains Work?

The term “French” is often used to separate something special from others of its kind, in categories as diverse as hairstyles (French braids) and food (French bread).  Even though French drains get their name from their nineteenth-century inventor (Henry French) instead of the country, the idea that a French drain is a special type of drain is still correct.  But what makes it special?  What exactly is a French drain, and how does it work?

Like any other drain, a French drain is a pipe that moves water from one place to another.  Unlike the drain in your shower, however, French drains are designed to collect water from a large outdoor area.  They collect groundwater after a hard rain and carry it out of your yard and to an appropriate wastewater collection point (like a city storm drain).  A well placed and properly installed French drain can protect your foundation, basement, and retaining walls from damage as well as preventing soil erosion or pools of standing water in your yard.

The first difference between a normal drain and a French drain is the type of pipe used.  French drains are made from pipes that are perforated (meaning, they are full of holes) on one side.  While this might sound counterintuitive, these holes actually allow the pipe to absorb groundwater as it rises during heavy rainfall.  The pipes are also wide, usually between four and six inches in diameter.

In order to function properly, the pipes must be buried in a water-permeable material like gravel.  Water must be able to easily reach the drain pipe for it to be of any use, and the less dirt and small debris that gets into the pipe, the better it will work.  Remember that French drains absorb groundwater as it rises, so the pipes should be installed with the holes facing down.  Instead of saturating the ground and pooling on the surface, the water will enter the pipe and be directed away from your property.

The final piece of a French drain design is the slope of the pipe.  In order to drain the water effectively as the pipe fills with water, the pipe should slope approximately one inch for every eight feet of length.  Too little slope and the water won’t drain quickly enough; too much slope can cause erosion issues at the exit point of the drain.

In short, a French drain is a perforated pipe that is buried in ground that often becomes saturated during heavy rains.  As groundwater rises, the gravel or loose rocks surrounding the pipe make it easy for water to flow into the pipe. The pipe then guides the water to a more desirable collection point.

French drains operate on a fairly simple concept, but they must be installed with precision and the proper materials to be effective.  If you struggle with standing water on your property, call us to see if a professionally installed French drain could be the solution!