Valley construction is one of those choices homeowners never make, because nobody presents it as a choice. It is worth presenting, because under Seattle's tree canopy the difference in service life is real.
A valley is where two roof planes meet at an internal angle, and it carries the concentrated runoff of both. The water volume moving through a valley during heavy rain is far greater than anywhere else on the roof, moving faster, with more energy.
It is also, in this region, where organic debris ends up. Everything that lands on either slope migrates toward the valley and accumulates there.
Open metal. A metal channel runs the length of the valley and is left exposed, with shingles from both planes trimmed back to leave a visible strip. Water runs on metal.
Closed-cut. Shingles from one plane run through the valley and continue onto the other side; shingles from the second plane are trimmed to a line a few inches off the centre. Water runs on shingle.
Woven. Courses from both planes alternate through the valley, interlacing. Water runs on shingle, over a lumpier surface.
This is the part that is specific to where we live.
Seattle valleys collect Douglas fir and cedar needles, bigleaf maple seed and leaves, and moss fragments — continuously, not seasonally, because conifers shed year-round. What happens to that debris depends entirely on the surface it lands on.
A smooth metal valley lets debris wash through with normal runoff, and what remains can be swept clear without difficulty. A shingled valley presents a granular surface with the texture of coarse sandpaper, and needles catch on it and stay. Over a season, that produces a felted mat that water cannot penetrate. The mat dams flow, spreads water sideways under the shingle courses on both sides, and holds constant moisture against the assembly through the entire wet season.
This is why we favour open metal valleys in this market. It is not a general claim that one method is superior everywhere — closed-cut valleys perform perfectly well in regions without a conifer canopy. It is a claim about the specific interaction between shingle texture and needle debris in the Puget Sound lowlands.
For open metal: faster water movement, easier cleaning, easier inspection since the channel is visible, no shingle edges in the highest-flow area, and considerably longer service life under debris load.
For closed-cut: a cleaner uninterrupted look that many homeowners prefer, marginally lower cost, and no visible metal strip if that matters aesthetically.
Against woven: it is the most debris-retentive of the three and the hardest to repair, since fixing one plane means disturbing both. Rarely the right answer here.
Self-adhered membrane underneath, full length, on every valley regardless of type. This is the actual waterproofing layer. Everything above it manages flow; the membrane is what stops water if flow management is overwhelmed. A valley without membrane beneath is relying entirely on the surface working perfectly.
W-profile metal rather than plain V. A W-valley has a raised rib running down the centre. That rib prevents water sheeting across from the heavier-flowing plane and driving up under the shingles on the opposite side during intense runoff. On roofs where two planes of very different size meet, this matters.
Metal type and gauge. Galvanised is standard and adequate; aluminium does not rust; copper lasts indefinitely and sheds trace ions that suppress moss below it, which in a shaded valley is a genuine functional benefit rather than a luxury.
The termination at the eave. Frequently botched. The valley metal must extend past the fascia and lap correctly over the drip edge so the concentrated flow discharges into the gutter rather than behind it. A valley that ends short delivers its entire volume to the fascia board.
Fastening at the edges only. Valley metal should be secured at its outer edges, never fastened through the centre channel, and it needs room to move with temperature.
Whatever the construction, valleys need clearing. Twice a year under canopy is a realistic minimum, and it can generally be done from a ladder at the eave with a long-handled brush rather than by walking the roof.
Look for: accumulated needles and leaf litter, moss establishing along the edges where the shingles meet the channel, granule accumulation indicating the shingle edges are wearing, and any lifting or separation of the metal.
A clear valley moves water in seconds. A packed one holds it for a season. That is the entire difference, and it is within a homeowner's control.
This post is part of our in-depth coverage of roof repair & leak detection in Seattle.
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