Guardrail height requirements vary depending on the specific regulations and application. Here's a summary:

• General Height: Guardrails should typically have a horizontal top member between 920 mm and 1070 mm above the working surface [1]. Some regulations allow for a height between 36 inches (0.9 m) and 45 inches (1.2 m) 4, 7.
• Older Guardrails: Guardrails installed on or before October 30, 1988, can be between 900 mm and 1 meter above the working surface [1].
• Scaffolds: For supported scaffolds manufactured or placed in service after January 1, 2000, the top edge of the guardrail should be between 38 inches (0.97 m) and 45 inches (1.2 m) above the platform surface 4, 7.
• Underground Mines: Handrails in underground mines should have a top rail between 900 mm and 1.07 m above the floor level [2].

It's important to consult the specific regulations applicable to your location and type of work to ensure compliance.