Have you ever walked underneath a small waterfall, maybe from a stream somewhere? You could probably feel every drop on your head. Water can exert quite a force on a surface. That is why water will eat away at rock and other materials as long as it has enough time.
This should make you think about the way your roof drains. If you have a section of roof that is higher than another, does the water just drop down on it? If so, you should do something to remedy the situation.
The proper way to drain a roof is to have a gutter that collects the water and then funnels it, not to the roof section below it, but instead directly to the gutter at the bottom of the roof. While this might take some extra work and cost a little more, you are making sure that your roof lasts longer.