One of the main reasons a roof rots is that the shingles are installed incorrectly, are damaged or have ripped off of the roof, leaving a hole where the elements can effect the roof itself.
Another reason for rotted roofing materials is the flashing or gutters are not water-proof. This can also result from an incorrect installation, or age and wear.
Additionally, incorrect attic ventilation can cause rotting. Because of our moisture-rich climate a poorly ventilated attic can cause moisture to be trapped in a house and create a rot issue that way.
This is insidious as a piece of wood might look just fine from the outside, but may be rotted within. This results in something that looks structurally sound, but is just waiting to break and crumble.
The solution? Make sure you get your roof inspected if you so much as suspect that it is unsound. Also, you can do your own visual inspection from the ground.
Additionally, get any roof repairs done right away – especially during this time of year – when rain and freezing temperatures are a factor.