Patrick,
By using a Cowtail to clip onto the triples, you'll maintain a three point contact. We used a procedure based on Sky's. It was robust when correctly implemented, however, on no single occasion did I witness it correctly implemented.
From memory, the procedure involved:
Ladder against wall, fit ladder stay, fit ladder spurs, put on harness, clip on Cowtail, drill eyebolt into customer's wall, ratchet strap ladder to wall, place Cat ladder against wall, climb ladder, clip Cowtail onto ladder, pull Cat ladder up, turn over and position on roof, strap ladder to Cat ladder, Clip Cowtail onto Cat ladder, climb onto roof!
Forgive me, it was a few years back, however, you get the point. Of course, dependent on the situation, a safety line was also required and a rescue plan.
Hope this is of some help!
Simon