Roof warning line systems create a defined perimeter warning workers of the roof edge. Warning lines consist of ropes, cables, or chains marking the boundary. Lines are typically 10 feet from the roof edge or as close as practical. Height of the warning line is typically 34 to 39 inches above the walking surface. Warning line posts are spaced no more than 6 feet apart. The line must be strong enough to withstand impact from a person. A safety monitor is stationed to observe work and ensure compliance with fall protection requirements. The monitor must be trained and have a clear view of the work area. The monitor focuses on safety, not other job responsibilities. The monitor can enforce safety procedures and halt unsafe work. Warning line systems may be used with or without fall arrest systems. Systems with fall arrest systems provide maximum protection. Systems with warning lines alone rely on the worker staying behind the warning line. Continuous safety monitoring is required when using only warning lines. Work must stop if personnel approach the warning line. OSHA considers fall arrest systems more protective than warning line systems alone. Warning line systems are typically used only where fall arrest systems are not feasible. Temporary warning line systems are common during construction. Permanent warning line systems protect workers during roof maintenance.