Wherever two roof planes lean into each other — an L-shaped house, a dormer joining the main roof — they form a valley. Everything both planes catch funnels into it, so a valley moves several times the water of any ordinary section of roof. Builders treat it accordingly: valleys get lined with ice and water shield and finished either "open" (an exposed metal channel) or "closed-cut" (shingles woven or lapped across).
Why valleys matter in Douglas County
Two local realities gang up on valleys. First, volume: roughly 130 rain days pushing concentrated flow down that channel. Second, debris: fir needles and oak leaves slide down the planes and wedge exactly there, building little dams that pond water and feed the moss that loves a damp, shaded channel. A valley that stays buried under needles through a Roseburg winter is a leak in training — one big reason seasonal roof cleaning and moss removal earns its keep here (more in our moss guide).
When it fails, what it costs you
When a dammed or worn-through valley backs up, water goes sideways under the shingles of both planes at once — and because valleys often run above living space, the damage bill is usually interior: insulation, drywall, sometimes flooring. Valley leaks are among the most expensive "small" roof failures we see, and among the most preventable: keep the channel clear, and re-line it properly at reroof time.
Why do roof valleys leak first in Oregon?
Concentrated flow plus debris dams. The valley already carries the most water on the roof; add a wedge of wet fir needles and the water ponds, creeps under the shingle edges, and finds the nail holes. Old felt-only valleys with no membrane fail fastest.
Related terms
Ice & water shield · Dormer · Cricket · Eave