Hi!
I guess, I can come in handy.
I already have a working prototype with the same functionality you need (drawing the roof overlay and the area calculation). With a huge Google Maps API development experience (as well as in frontend and backend in general - see my profile for details), I quickly provide you a ready-to-use solution, built with a modern tech stack.
Please check my profile and let's discus you project in more details.
Regards,