My first and only experience with well nuts - on 1/4" wall panel - was rather discouraging. I was installing a 5-pound controller and didn't want to use 4 tapping screws on this thin plywood. After I removed the screw (and controller) to see how the first well nut was doing, it pulled itself into the wall with the lip and all, and disappeared there. Had to put another one, with huge washer to prevent this from happening again.
For roof brackets I used 4" aluminum angle, 6" long. Where I couldn't hit the rafters, I used 5 screws #10 per bracket. After many calculations and estimates I've decided that 5 screws per 4"x6" plate was more than enough for a flat 40"x65" panel. With plenty of Dicor under the bracket and over the screw heads.
