I need a reality check: is this a bright idea or wishful thinking?

I have a townhouse with a flat roof which has a lot of mechanical equipment on it. In 22 years the roof has leaked many times and it always enters the 4th floor ceiling in the same 4×8 foot area. Now that the roof has been repaired again, I’m fixing to repair the ceiling section for at least the third time.

My bright idea is to use concrete backer board with a skim coat of joint compound this time instead of sheetrock. Then when the roof leaks again I can just repair the skim coat.

Sane or insane? Joint compound or is there a better choice? Thanks for any comments.