I bought a 50 year old architecturally designed home in 1967 that had an older flat roof. I lived there for 10 years without a problem. Fifteen years later, in 1992 or so), I stopped and talked to the owner and he had no idea how old it was because he never could find a problem and never worried. I assume the roof was replaced, but you can never tell.
It was a unique home designed by a real architect in 1917 and obviously built under his direction, since that was the tradition at that time.
The home had many swing-out casement windows with screens and weather stripped storms on the inside (a room painting nightmare), 1-' ceilings. All ceiling trim (Greek dental pattern), casement and windows and other trim was solid birch that was painted and never stained. The roof was separated from the house ceilings by 1'-3' with all roof drains in the center and no troublesome parapets that only cause problems. Not one inch of stain or varnish! Every piece of millwork had a penciled note of the date and the house location on the back and the ugly wide wall paper (30-36") had the installers name and date on the back.
It even came complete with a built in safe (4" thick door) in the basement and a few bullet scars in the stucco over the clay tile from either the Capone or Dillinger era.
I would love the opportunity to buy it back now, but it is not available and the price would be too high.
A good flat roof if designed and built properly is very cheap in the end. If done wrong or cheaply, it could be a disaster.