A French drain, also called a land drain, is made up of a ditch filled with stones or gravel. The ditch directs water away from an undesirable area, such as the base of the house. These drains can be a simple and useful way to redirect water on a property, as long as they are angled from high to low ground. However, French drains also have a number of disadvantages.
Water seeping through the soil to reach a French drain often carries sediment into the drain. The water then deposits this sediment on the gravel in the drain as it flows through the ditch. Over time, this sediment build-up can slow the flow of water through the drain or block it up entirely. Once this occurs, the water no longer passes through the drain correctly, and can flood homes or other areas of the property. Repairing this problem often requires digging up the entire drain and replacing the gravel. Some French drain installations attempt to avoid this problem by using a filter at the beginning of the drain. When it clogs, only the filter must be replaced.
Difficulty of Installation
Some French drains are built along with the house, bust most are installed after home construction, usually in response to a wet basement or other problem. This requires digging down to the level of the footer outside the home, and may require the homeowner to remove shrubs, decks, porches and other structures. This digging may encounter gas and water lines, electrical wires and sewer pipes, all of which pose hazards. It's also important not to dig too deep – if the drain is dug below the level of the footer, it may undermine the foundation, causing the home to settle.
French drains installed inside the home to reduce flooding require a sump pump to operate correctly. This pump moves the water outside and into the drain. Unfortunately, a pump adds to the cost of installation, and many lower-end pumps have a short lifespan. Pumps installed without a battery backup will not function during a power outage.
In areas with poorly-draining soils, draining water from a French drain into the traditional dry well may cause problems. When the soil becomes saturated, the well fills with water, sending flow back up the French drain and into the home or original problem area. In these cases, it may be necessary to direct the drain into street storm drains or another area. Many homeowners choose to tie their French drains into a downspout, but as the house settles over the years, this method may cease to work, causing water to flow back into the home.