How does a roof stay waterproof, especially in the Seattle/Redmond area where there's a lot of rain?
The trick to keeping a home waterproof and protected from the elements, at least in terms of the role your roof plays, it by creating multiple redundancies. That is, when one ‘layer’ (figuratively and literally) fails, the next layer can be expected to do its job. In short, there is the outer roofing material (shake, composite, synthetics, etc.), a waterproofing layer usually made of a plastic sheeting specifically design for roofs, and a plywood layer under that.