There are arguments for and against hanging ducting. We don’t hang ducting because over the life of your ducted air conditioning system there will be stress on the point where the ducting is hung. We believe this is more of a risk than the ducting getting damaged because it is lying on the ceiling joists. When the duct starts to sag (which it will) it will start to cause issues with the quantity of air and the air flow. Also, it is a lot harder for other trades to move about in the roof with ducting hung in the air.
