What if the metal support bracket that spans between the joists is drilled and tapped with 10-32 srews and the fan is supported by that? Would that not qualify as being independently supported from the box?
What if the metal support bracket that spans between the joists is drilled and tapped with 10-32 screws and a fan rated box is attached to the support bar with these 10-32 screws with the fan attached to the fan rated box?
