Hi FSX345,
You may want to start by deciding how you will place a rooftop pad.
There are various ways to place objects into flight simulator, unfortunately rooftops, mountaintops, and cliffsides can be problematic for most of those options.
Whether you use an existing rooftop pad library object, or make your own in Gmax, the best way I know of getting it onto a rooftop or similar high altitude location is with the payware utility "Instant Scenery" which is a "what you see is what you get" format where the object is displayed in the sim, and you move it into position with your mouse, and then adjust altitude, heading, etc within the user interface.
However, placing objects on top of tall buildings and mountaintops is a bit of a challenge since you must ascertain the height at which the object is to be placed in order to enter that altitude into the Instant Scenery user interface to get it to the general altitude of the rooftop, at which point you can make smaller adjustments in height.
Generally you would fly or slew into position above the building and pause the flight, open Instant Scenery, select the pad from a list and locate it on the ground below by moving your mouse. Then it is a matter of entering altitude adjustments to raise it to the desired altitude at which point it can be moved into position and placed.
You could certainly make some pads in Gmax, or perhaps even figure out how to take advantage of the object placement tool in FSX to place FSX default pads or towers, and perhaps some other readers who have successfully used other methods will have some advice how they would place a rooftop pad in FSX.
There are several collections of rooftop pads for FS9, but most of them do not work for FSX because FSX handles the hardened platforms which make the pads landable differently.
As a workaround so that I could use our FS9 FinneyAir rooftop pads in FSX, I made a set of semi-invisible landable platforms for Instant Scenery which can be used in FSX to harden the landing surfaces of the pads and towers.
If you want to play around with Instant Scenery to see what it is all about, you can download a trial version from the Abacus website which can be used within a fifty mile radius of Seattle. In my humble opinion, it is well worth the money it costs.
To find some helicopter related pads and towers, you can use the FinneyAir ChopperWorld download which is at AVSIM (finneyair_chopperworld.zip), and to harden the landing surfaces use the FinneyAir Landable Platforms collection at AVSIM (fa_isp_x.zip).
I am currently working on FinneyAir ChopperWorld for FSX, but it will also require Instant Scenery or ??? to place the pads and towers on a rooftop.
Hope this helps
Gman