The component of sloped roofs that is most vulnerable to early deterioration is the area around the flashings (chimneys, plumbing stacks, valleys, etc., and particularly skylights). It is not uncommon for these areas to develop a leak before the rest of the roof material has aged significantly. Also, because these areas are frequently made of metal they can be more susceptible than the rest of the roof coverings to wind and other mechanical damage. So while the flashings may appear fine today, and your roof may be reliably young, the flashings should be monitored on a regular basis (semi-annually) to ensure they will function properly. Often minor roof leaks are usually more than a nuisance than a major problem. Usually the amount of damage is to a small area of the interior finishes, and if the leak is repaired immediately, the damage can be minimized.
Flat roof technology has been improving and some of the newer methods are predicted to last longer with fewer problems. However, as a generalization, flat roofs are more vulnerable to leakage than sloped roofs, all other things being equal. The traditional method of covering flat roofs with built up material does work, but it has a shorter life expectancy and there is a higher likelihood of premature leakage. It seems one of the primary factors in the life of a built up roof is the workmanship of the installers. Built up roofs with excellent workmanship will last up to 15 years, while the work of less skilled tradesmen may develop problems in 5 years. Additionally a flat roof that is past mid-life may develop leaks reliably quickly. Because of this behavior characteristic regular monitoring of flat roofs is recommended, to ensure they continue to provide protection.
Typically water will penetrate flat roofs through small cracks in the tar. If you notice cracks, they can be repaired with roofing tar and the roof may be made to last another season or two with regular monitoring and spot repairs, depending on the degree of deterioration.
THE BOTTOM LINE: The roof is to your house, as tires are to your car – and just as one tire maker's advertisement suggests, "You've got a lot riding on your tires …"