In 2007 the Packers installed an entirely new playing surface, including a completely new drainage and heating system, bringing the latest technology in field management to the famous stadium. Chief to the new system is DD GrassMaster, a natural grass surface reinforced with man-made fibers. All existing levels of the field were removed and the new system began with a clay sub-grade level, compacted and graded (with a .6 percent slope), including drain tile, irrigation pipe and thermostat wiring for the heating system. The second level consists of 5.5 inches of pea gravel. On top of the gravel layer is 30-plus miles of 3/4-inch tubing for the heating system which can maintain a root-zone temperature of 55-plus degrees to keep the ground from freezing during the season's latter months. Level three consists of 12 inches of root-zone sand and Kentucky bluegrass turf. DD GrassMaster's synthetic fibers are stitched into the surface, providing strength and stability to the field. Fibers extend approximately seven inches below the surface and are exposed approximately one inch above the surface and are spaced every three-quarters of an inch. Approximately 20 million individual stitches make up the process. The slope equates to a crown of about 5.5 inches on the new surface.