Truck top with kayak rack - load capacity?

I'm unsure of the strength differences between aluminum and fiberglass canopies but imagine the load bearing limits are similar. The fiberglass look better but the aluminum have better access from the sides. We've hauled 2 kayaks on the pictured style canopy/rack arraignment for several years. First on a Tacoma and now a Ram1500. No problems. We swing the rear of the kayaks diagonally and load/unload one end at a time. We're not spring chicks anymore and have lousy shoulders so watching's probably a funniest home video moment for anyone watching, but it all works. The 2x4's are so we can easily raise and lower them both on and off the truck. We store them hanging.