Sniper rifles used to work on their own independent system that had more too do with the way combat worked 5 years ago then it does now.
They have now been brought back into the fold, and incorporate the newer aspects of the combat system!
On top of that, at this moment, ALL guns are capable of shooting off of a rooftop!
We have given each gun in the game an effective range. Hand guns effective range are usually 1, meaning that they are only optimal in the same room.
Now, what this means for rooftop shooting is that the further away you are, the harder it is to hit someone, and if you do hit them, the less damage you will do.
Weapons mods can have positive or negative effects on the accuracy of your guns. A scope will help you aim in general and definitely at longer ranges, just as a barrel extension will make your shots more accurate as will a trusight enhancement. Barrel shortening has the opposite effect, making you less accurate.
Take these things into account! Go to your nearest weapons modifier and get them to hook you up!
Also, start thinking tactically. Widdle your enemies down before engaging them directly, or have tactical support on rooftops watching your back!