One way to do it is to use abaseiling/rope access harness and throw the ropes over the roof anchoring them the other side, but you would have to know your knots. This is a rope access technique (IRATA) which would only be effective up to the apex of the opposite side to where the ropes (2 attachments- main and chunt rope) are anchored. It does require some training ideally, it cost me nearly £600 for a weeks course as I was abseiling on offshore platforms blasting etc. But its the best and when done correctly, safest form of access.