The easiest way to get to the rooftop of the building is to go around the back of the construction site. Be sure to bring a silenced sniper rifle to take out the 3 enemies
between you and the elevator:

First of three guards you have to take out.

Second of three guards you have to take out with a silenced sniper rifle.

Third of three guards you have to take out with a sniper rifle.

Then take the elevator to the rooftop:

Take this elevator to the rooftop.