Are you sure you are going to like the look? I find HVAC equipment on top of a flat roofed house unsightly, IMO.

Hi, I ran this by my husband. He has 2 years of school for hvac We now live in arkansas but we are from the deer valley area of phoenix. He has done heating/ac work in both the phoenix metropolitan area and here in arkansas. Many years of experience. He said this could be done. But...... you would have to insulate the heck out of it and it would never really be energy efficient He said it would be better to run it under your house out of the direct sunlight. .

"Is this a bad idea?"
IMO,no it's an insane idea from the standpoint of effecience. The material required to insulate plus that to resist weather damage makes it super expensive. Tack on more expense for pitch pans and curbs that must be done by roofers. Then when time comes to maintain roof the ducts are in the way. Holes in roofs aka penitrations is the cause of most leaks.
Consider yourself lucky if enegyrater doesn't send ugly men over to peep in your windows simply for entertaining the idea.

Underground returns, roof duct. Before you go anywhere with this I'd suggest you have your duct system pressure tested. DOE says the average home looses between 18-42% of its energy in duct leakage. Your best bet would be to abandon the system and go with mini-split heat pumps. Fujitsu and Mitsubishi are the two lines I would look at.

Any HVAC contractor who would recommend running ducts on top of the roof should have his license suspended in my opinion. I think running duct work in the attic in Arizona is almost as bad.

Thank you all! I will leave everything as is. Considering the size of the house and the amt. of glass, my AC bills are low. I would hate to see them sky rocket just to clear out some dropped ceilings. I also just put on a new foam roof and would hate to disrupt that. You were so helpful!