Go to the blue or red spawn point in Treasury. It is where you take on all of the enemies on co-op on line.
Once you go to that first spawn point in the back of the building, there will be a door and behind that, all the way behind it, there will be a truck and a light post.
Stand by the truck and the light post and run at the glass door and as soon as you are getting ready to hit it dive your guy into it. If you do it right you will be on top of the building, i.e. on the roof.