what is the minimum requirement for guardrail height?

The top edge height of toprails or equivalent member on supported scaffolds manufactured or placed in service after January 1, 2000, should be installed between 38 inches (0.97 m) and 45 inches (1.2 m) above the platform surface [1]. For supported scaffolds manufactured and placed in service before January 1, 2000, and on all suspended scaffolds where both a guardrail and a personal fall arrest system are required, the top edge height should be between 36 inches (0.9 m) and 45 inches (1.2 m) [1]. The height of the top edge may exceed 45 inches if the guardrail system meets all other requirements [1].

Additionally, OSHA defines "unprotected sides and edges" as any side or edge of a walking/working surface where there is no wall or guardrail system at least 39 inches (1.0 m) high [2].