First, you have to get to the room where you can come up a ladder or up the stairs.

So, when you are there go to the edge of the window, press the "A" button and get on the window ledge, just go left a tiny bit on you will be on a ledge, walk along until you get to a gap, jump across and you will get onto a kinda legdge thing.

Walk along then jump and you will get to the roof.

Advice: Have a sniper and a Gewer it works so well!