I just spoke to another roofing contractor, and he suggested yet
another solution: install "ice aprons" (essentially flashing) along the
edge of the roof. His claim is that the ice cannot hold on the apron,
so ice building is not a problem.

Does this make sense? I noticed one of the churches in our
neighbourhood has the same solution, except that they applied ice
aprons *and* ice railings.

Thanks

Dave I