Onshore, two elevated truss-like structures having triangular cross sections. The pipe rack supports drillpipe, drill collars or casing above the ground. These structures are used in pairs located about 20 ft (6 m) apart and keep the pipe above ground level and closer to the level of the catwalk. Pipe stored horizontally on the pipe racks can have its threads cleaned and inspected and the rig crew may roll the pipe from one end of the pipe racks to the other with relative ease. The pipe racks are usually topped with a wooden board so as to not damage pipe, especially casing, as it is rolled back and forth along the racks. When large amounts of pipe are stored, wooden sills are placed between the layers of pipe to prevent damage.