Blackwater Roof
Immediately move to the left behind the guard searching the window. Push him in and then continue on. Look up and to the right and you should see a crane that you can shoot to drop its cargo. This should clear a path for you up ahead. Move forward a little and find the stairs leading up on the right side. Wait for the guard to move away from them before climbing up.
Hit the checkpoint at the top and start making your way across the roof. On the right will be a guard you can take out for a disguise. When you have it, drop him over the edge. Get rid of the next guard and dispose of him just the same. On the left side of this roof will be a door with a keycard swipe. Remember that door. Now just blend across the roof and enter the building on the right side.
Inside this room is a keycard. If you grab the keycard, you can go back to the locked door on the roof and open it. If you do, you'll be able to get the Evidence inside. The evidence is on a box on the left side of the second room. Return here when you are ready.
Go straight forward and grab the sledgehammer before coming back and going right. You'll find a locked door with a chain. Smash the chain with the sledgehammer to get through the door. Turn left and climb up the ladder to the helipad. At the top, smash Dexter in the face to finish the mission.
