how deep a french drain should be
Comments. Submit. French drains should be dug to a depth of 8 to 12 inches, for a 4-inch pipe, with a minimum of 2 inches of gravel on all sides.omments. Submit. French drains should be dug to a depth of 8 to 12 inches, for a 4-inch pipe, with a minimum of 2 inches of gravel on all sides.