Hi Shane, Thanks. I think, and I maybe wrong, that your answer assumes that the roof is flat at 2.0m above the ceiling and thus the plenum is consistent height., however in the case of a pitched roof the plenum height will vary from 0m on the roof edge…

Thanks Shane. That's brilliant and it has worked for me! One point that i would mention is that the floor height should be set to the apex height of the building.