roof edges..you are reffering to the gutters correct?
I have seen these sorta gutter grabbing tie downs before. They are probably sufficient...but as a canoe owner I would feel much more secure if my canoe was strapped to a proper rack Yakima/Tuele ect. If you go with the pad/strap option just be very carefull and double, triple check.
Check every time you stop for fuel or whatnot so you familiarize yourself with the hold down system.