On the far corner of the map there should be a platform (make sure you have a class with M9, marathon and lightweight). Switch to your sidearm then run towards the platform

Jump just before the end of the floor then use the side of the platform for a ladder and climb up (this might take a couple of times).

Then you will see a wooden ledge. Walk onto that and carry on till you reach another platform dirctly on the other side of the wall infornt of you.

Then press A, then walk up the platform bar, face the wall, then press A again, then walk left and you should be on the roof.