Usually when u place an object directly ontop of a building it goes onto the roof (if u use the flats on nogova) not sure about other buildings. if not put this in the init field on the unite u want on roof-
this setpos [(getpos this select 0), (getpos this select1), 7]
The 7 at the end is the height in feet, play around until you have the desired height.
Hope that helps.