[Coleman]

The easiest way to do it is to type these commands in console (these keys are free if you have default setup):

bind b sv_gravity 750
bind n sv_gravity 150
bind m sv_gravity -8000
bind , sv_gravity 0

So whenever you hit those keys in the game, the gravity will change (SP and only if you are hosting the server). If you want to be "hoisted up", then just press the M key on your keyboard and move around. That will suck you up to the ceiling of the map. Since you don't want to be pushed up there the whole time constantly, press the "," key so you will just stay at the same height in the air. Then if you want to drop onto a building roof-top, hit the N key and then press foward to stay at the same level in the air. Let go of the foward key and then you will drop. Once you are on the top of the roof, you'd probably want the gravity to go back to normal so you can jump. Then just press the B button. Hope this helps