Our house is only 80 or so years old, but that should give some reference. The back 20 feet or so of the roof are flat, covered with tar and gravel.
We bought the house about 7 years ago, and had building inspectors look at it. Their comment was that the tar and gravel was in reasonable shape, but looked like it had been there for quite a while, and they suggested we plan on replacing it in a few years. Well, finally this summer we are doing so.
This is what I know so far:
-tar and gravel only has a lifespan, they say, of around a dozen years.
- the newer way to cover a flat roof is with a water-proof membrane. There are various systems, and they have lives of more like 20 years.
You can get it re-done with tar and gravel, which is the cheapest, but not so many contractors will do that anymore. Many will do a membrane.
- membranes are much lighter, so put less load on old roof boards.
For telling if the roof is in good shape I'm far from an expert, but know a small bit:
- the gravel is there to protect the tar from drying up and cracking. So if the gravel is worn away in places, look at the state of the tar (one bit of maintenance is to add gravel to places missing it, if the tar is in good shape).
- is the underlying roof sound? Water shouldn't be pooling anywhere, so if you can look at if not long after a rain storm, see how it is. Also when you walk on the roof, does it feel solid?
- Of course the biggest test is does the roof leak? The most obvious sign is discoloration of the cieling on the top story. If you can get into the attic and crawl around (flat roofs generally have low clearance attics, so crawling around is a literal term here....), look for signs of water staining in the attic.
One additional thing, many flat roofs have a roof drain. The 80 year one on ours is cast iron, and just rusted through below the roof. If you do have a roof drain in an old roof, check the state of it, and maybe start thinking about whether you can replace it....not so easy I think! (I'll be posting a question on this topic in a minute....)
I hope this helped a bit.