The middle of an undisturbed roof plane rarely leaks. Details do: the places where the roof meets something else. This is where most Seattle-area roof repair work actually happens.
Skylights
Look for cracked or slipped flashing, failed seals between frame and glazing, and debris damming water above the curb. Interior condensation on the skylight well can mimic a leak.
Chimneys
Chimneys need step flashing, counter-flashing and often a cricket above them. Staining that tracks down a chimney chase after wind-driven rain points here; so does failing masonry that sheds water behind the flashing.
Roof-to-Wall Transitions
Additions and dormers create wall intersections where step flashing must integrate with the siding and weather barrier. When these details fail, water travels along framing before appearing indoors — which is why leak investigation traces the probable path instead of patching the stain’s location.