We need to transport a full size mattress and boxspring. It won't fit inside our Element so we were thinking about just putting it on top of the roof and tying it down. Is this okay to do? I was worried that it might be too heavy for the roof to support.
Yes, weight should be okay. Put mattress first, then box-spring on top. Open the doors, and strap to the four hand-holds inside along headliner edge. Then shutting the doors tightens the straps even further, nice and snug. ;-)
I pulled this off just yesterday. Wish I would've taken some pictures for ya. Here's the setup I used:
Rear seats & Passenger seat all layed flat. Managed to fit the mattress in the cab. You'll need to fold it a bit, lower right to upper left. The mattress will end up laying on the driver's seat/headrest. Be wary not to drag the mattress across any latches on the rear hatch. Box spring went up top. As its much lighter, I wasn't too worried about the weight. I used two wratchet straps, one through the front doors, one through the rear doors. The rear antenna helped prevent any backward movement, but I'd think the friction will keep the box spring from shifting at all.
It was a 4 mile deliver at 50 mph max. I was constantly checking its position while driving. Didn't notice anything but a little lift (maybe an inch) at 45 mph. Mattress and box spring delivered unscathed.