I used adhesive and skipped the screws. Holding fine now for 3 years running. I inspect regularly and I'm sure I'll notice if one or two brackets start coming loose before a complete failure.
However, if you are going to use screws, attach everything first, then take it apart and apply the adhesive so the stuff will go down into the hole with the screw, thereby sealing any leak at that point. There isn't a real need to encapsulate everything in adhesive if the point of entry, the screw hole, is filled with adhesive as the screw goes in as that's where any moisture can possibly do any damage.
