The purpose of a French Drainage System is to carry unwanted free standing water away from a building, such as your home. French drains are commonly installed near the perimeter of a building at the lowest point or anywhere where standing water is found. The system may terminate at any point where the water will not drain back toward the house. The best approach is to run the water out to the street or to a dry well.
French drains are generally shallow drainage systems. However, as with your plumbing waste drainage, French drains operate on the principle of gravity. The drain must slope downward. The minimum recommended slope is 1/4 inch per foot. If the landscape grade runs upward along the French drain path you will have to dig deeper to maintain a downward slope.