MB4 215b: Flimsy Walls (360:3); Roof Adjacent to a Yard and a Reshus Harabim (361:1)

If three people in a שיירא enclose their own adjacent areas with flimsy walls can all three carry throughout the whole area?

If a rooftop is adjacent to a reshut harabim (within ten tefachim) and to a chatzer (more than ten tefachim) what is it's status?