French Drains

Water that is migrating under the surface is best intercepted and controlled by use of a French drain. People refer to many schemes as French drains. When we use the term French drain we mean a trench, preferably with the bottom below the level of the sub-floor, or better yet below the bottom of the house footings, and:

• the bottom smoothly graded and declining at the rate of one percent,
• 4 inch perforated pipe laid on the bottom of the trench,
• backfilled with gravel either all the way to the surface, or to within 8 inches of the surface
• discharging to an appropriate drainage course or structure.

Sometimes the trench is wrapped with filter fabric to reduce root invasion, though that has the disadvantage that it tends to fill up with silt which can stop the drain from functioning.

These French drains typically run all the way across one or two sides of a house, and occasionally three or four sides. The critical consideration is depth. Almost any water intrusion problem can be solved with a deep enough French drain. However, a French drain is expensive to construct, and the deeper it is the costs go up more than proportionately. We have adopted the guideline of preferring to start the drain at an elevation 6 inches below the bottom of the foundation footing whenever practical. This is not always practical due to structural considerations, cost, or outlet requirements.

French drains are susceptible to silt intrusion and root invasion, since they are not sealed. Tall shrubs and trees should not be near the drain when possible to prolong drain life. Shallow drains (12 to 18 inches) have been known to clog up within 2 years when installed next to ivy. Other drains have been in service for fifteen or twenty years and appear to be functioning well.