Sure it's possible to build helipads on roofs. But those usually have to be designed in from the start and very few buildings have them. It's tough to add one to an existing building due to weight limits and obstructions from antennas and HVAC machinery. Real estate developers won't take on that expense until VTOL aircraft become more popular, so it's a "chicken or the egg" problem.
I looked up some weights. Helicopters (other than ultra-light) are measured in tons, and that's indeed something you don't want on a roof that's not designed for it. But this kind of VTOLs will most likely be equivalent to ultra light helicopters, so I can imagine building a metal platform on top of the HVAC machinery.
No these eVTOL air taxis will have to be roughly the same weight as a regular turbine powered helicopter in order to be able to carry a useful load. Batteries are heavy.
Ultralight helicopters are limited to 254 pounds vehicle weight. While that does make it easier to build suitable rooftop helipads without major structural renovations, such aircraft can't be used for air taxi service. It isn't legally allowed, and even if the law was changed they wouldn't have enough load capacity or range to do anything useful.
