I'm installing a ceiling-mounted extractor fan above a kitchen island. The problem is with the placement of ventilation ducting in the ceiling: the joists are running in the opposite direction to the only viable wall where the ducting needs to travel towards.
However; in the ceiling there is about a 120mm (4.5 inch) void above the joists before the the floor above. This gap is easily big enough to fit flat channel ducting so that I can rest the ducting on the joists in a perpendicular fashion. (see diagram below)
My question is: Is this OK to do? I haven't seen this done elsewhere.
Addition: Heres a quick photo: