This takes a bit of practice but if done correctly then you can look at the Allied Spawn and cover your flag really easy (great for sniping), so this is how you do it:

From the German Spawn run to the house on the right with the MG in the room, go up the stairs and go into the left hand room.
Go to the window on the right and drop down, you should be on a ledge you can't get to.
Then go to the little raised bit on this ledge, look at the corner, run jump and flick your right thumbstick to the right to where the roof is (roof is very visible).

If you need help message me. It does take some practice to get the hang of it!