How high off the ground should a roost be?

Excuse me, could you elaborate on the ideal height for a roost to be off the ground? I understand that it's a crucial factor in maintaining the health and safety of the birds, but I'm not quite sure what the specific height recommendation is. Is there a standard measurement that farmers and poultry enthusiasts typically adhere to? Or does it depend on the specific breed or size of the birds? I'd appreciate any insights you could provide on this matter.