Mark Swinkels wrote:

The pump requirements are not that extreme. Since the piping up to the roof
and back is a closed system the pump doesn't need enough head to get to the
roof. The water on its way up to the roof is balanced by the water on its
way back down. The pump only has to overcome the dynamic head caused by
friction losses in the pipe. The system would have to be primed by
something like a garden hose.

Good point about being a closed "balenced" system. I had not thought
about that.