But despite this extensive catalog of ways water can attack a building, the main problem presented by water is still from rain entering at the top. Flat roofs are the worst. They must have a perfect seal to work and perfection is impossible. Worst is that when there is a leak, it is impossible to tell where it is from. The pitched roof sheds water well and provides room for storage or services.
Simple roofs are easiest to maintain.
Chimneys, dormers, valleys, skylights, etc., invite problems with flashing and make reroofing difficult. Multiple layers, each known to be imperfect, offer protection that one layer striving for perfection cannot. Wood shingles last only ten to fifteen years. Composition shingles last about five years more. They are also fireproof and beautiful.