As A. I. Breveleri commented, you can do this with one measurement:
1) Take the stairs down until you reach the height of of the opposing roof. (the correct position is easy to find if the roof is level, as pictured.)
2) Stick your head out the window and use the laser to measure the distance down.
The traditional solution which requires zero measurements:
Take the stairs down to your office, phone the relevant city officials / the architect who designed the opposing building / the manager of the opposing building and ask/bribe/cajole them into telling you the height.
Alternative way with three measurements, without moving off the roof and without annoying the doorman of the opposite building:
O B <- Me
| F C___________
| | | | \
| | | | }
| | | | } measure this
| | | | }
_| D_________E | /
Stand at the corner of your roof, and measure the distances from your feet and from the top of your head to the opposing corner. (i.e. AC and BC)
Knowing your own height (AB), you can get the height difference (AF) of the buildings from the two triangles.
Then measure the distance from your feet to the ground, i.e. height of your own building (AD), and get the opposing building height CE = FD = AD - AF