The king of large flat roofs for the last 100 years has to be Asphalt.
Since its discovery more than 5,000 years ago, asphalt has proved to be one of nature's most useful, and abundant materials.
Though you may never have considered it, asphalt (a petroleum by product), is a naturally occurring part of the environment. However in recent years the new Asphalt roofs that I have seen don't seem to last as long as the old ones. In my opinion this could be down to 2 factors:
1) The quality of the raw materials is starting to deteriorate 2) Tradesmen laying the roof are not as precise as before.
Unfortunately shifting large amounts of heavy asphalt up ladders to lay small roofs is not something for the faint-hearted and the thickness of the roofs that are now being laid has diminished dramatically. We find that we are replacing asphalt roofs that are only years old rather than scores of years old.