Adhesive for steel solar brackets to aluminum roofs

I've used urethane adhesive (Sikaflex is one brand) with proper preparation of the bond surfaces for panels up to 200W and had no problems despite thousands of miles of washboarded roads.
Make sure the adhesive thickness is correct (thinner is rarely better) and the surface area of the bond is large enough and it will never come apart - if you ever need to get it off the roof, you will have to cut through the bond with a sharp knife.