Your best bet is to reload and duck right away. BTW its best to save the health in the lobby for last, to have full health once you are on the rooftops. Now that you are on the rooftops, there will be a staircase to your right and there will be about 6 or seven police officers coming up it, i think its easier to go up the stairs to the top then ambush them as they come around the top part of the railing. then you will follow the path around the brick building, not the one you can see the roof, bu tthe one where you can only see the walls. Then there is still a lot more of the roof to go through, bu tif you want that, you need to read a guide, lol, i dont want to type that much.