A French drain is a method of removing surface and ground water away from a building foundation. This is a preventative measure often used to avoid damage to the foundation. The French drain is a trench filled with gravel or rock or a perforated pipe encased in crushed stone. It is a method used to help keep a basement dry.
French drains operate on the gravity system and that means the system must slope from beginning to end. The high-end of the drain is where the excess water enters the system and the low-end, or exit point is where the excess water leaves the drain system. The optimum slope for a French drain system is 1 percent. A good rule of thumb for calculating the slope is for every 8 feet of length the drain must drop one inch.
When establishing the location for the french drains installation, select a location for the exit point. The exit point should always be away from the foundation or the wet part of the yard. Ideal locations to consider are grassy slopes and areas that have exposure to the sun for a good part of the day. The sun will help with evaporation and the grass will absorb some of the water. If the local municipality allows, locate the exit point where it will drain into the street and the municipal drainage system. Locating the exit close to the problem area will keep the system as short as possible.
It is never a good idea to locate the exit point directed at a neighbor’s yard or where the water could run into a sidewalk or driveway. Freezing conditions will result in icing conditions and this might be in violation of building codes. It is never a good idea to exit French drain installations where the water could cause erosion.
If there is no acceptable exit point, the French drains installation will have to exit into a dry well. A dry well is a vertical hole at least 12 inches in diameter and 4 feet in-depth and filled with gravel. The dry well will collect the excess runoff and the water will be absorbed into the surrounding soils.