Abstract: A gravity sub-drainage system for drainage of level-graded natural grass athletic fields. A plurality of parallel trenches are formed in a level-graded subgrade beneath the surface of the field. A main drainage system is provided at the lowest point of the subgrade. A collector pipe is placed into a trench at one end of the field sloping downward into the main drainage system. Perforated drain pipes are placed in the parallel trenches to slope downward toward the collector pipe and to slope downward toward the main drainage system to ensure that water collected from the surface will flow down into the collector pipe and into the main drainage system by the forces of gravity without the need for a mechanical pumping system.A layer of pea gravel capable of draining at a preferred rate of eighteen inches per hour is placed around and above the perforated drain pipe up to a height of four inches above the top of the subgrade.