Why valleys are your roof's weak point
A valley concentrates the runoff from two slopes into one channel, so it moves more water than any other part of the roof. It's also where two planes meet, which makes the shingle work and flashing trickier than open field shingling. More water plus more complexity equals the place where problems start.
Healthy valleys shed water fast. The moment debris, moss, or sagging lets water linger there, you've got constant moisture working on the seams of your roof.
What causes water to stand in a valley?
In our area it's almost always one of these:
- Debris dams: needles and leaves pile up in the valley and slow the flow until water pools behind them. This is the big one under Oregon tree cover.
- Moss growth: a moss colony in or beside the valley acts like a speed bump and a sponge at the same time.
- Ice and snow (occasionally): our valley snow events are rare, but when they happen, melting snow refreezing in valleys backs water up under shingles.
- Structural sagging: if the decking under the valley has softened or sagged, water finds the low spot and sits in it. This one means the moisture has already been at work for a while.
What standing water does over time
Trapped water accelerates granule loss, breaks down the shingle mat, and probes every seam and nail hole in the valley for a way in. Asphalt shingles overlap to shed moving water; they're not a swimming pool liner. Give water enough standing time and it finds the path through.
Once it's through, you're into wet underlayment, then wet decking, and the repair bill grows with each layer.
The fix is usually simple (if you catch it early)
If it's debris or moss, a proper roof cleaning clears the valley and restores the flow, done in an afternoon. If the decking has started to sag or the shingles around the valley are damaged, that's a repair conversation, and earlier is always cheaper.
Can't see your valleys well from the ground? That's normal, and it's why they're a standard checkpoint on our free inspections. For keeping valleys clear year-round, see the Oregon roof care calendar.