I can't help with the ladder question, but I also carry my porta-bote (12' model) on the roof. Drilled into roof supports, and also used a cedar spacer that I made to fit the curvature of the roof. Then used 3M 5200 adhesive/sealant to coat both sides of the cedar spacer and bottom of the porta-bote carrier bracket, then screwed in place. It's worked out good for me, and is easy to take up and down from the roof.
USAF 1965-1971; USCG Master
2002 Horizon 36LD - 2000 GMC Sierra 1500 4x4
Indian River, De