What the pattern can mean
Valleys concentrate runoff. Debris, ice, damaged underlayment or an undersized drainage path can produce leakage that appears well below the valley line.
Why timing matters
Typical trigger: Heavy rain, leaf fall, snowmelt or freeze-thaw cycles.
Concentrated flow raises water depth and can push water beneath laps or over hidden barriers.
Evidence that increases the fit
- Only intense storms reproduce the mark
- Debris accumulates in the valley
- The interior path follows rafters downslope
Evidence that weakens it
- Light rain causes the same response
- The valley remains clean and drains freely
- A nearby plumbing event is a stronger trigger
Safe first checks
- Observe drainage from the ground during rain
- Record storm intensity and delay
- Inspect accessible attic framing for a directional track
Important boundary
Do not enter a wet or icy roof valley.
Professional handoff: A roofer should inspect the valley covering, underlayment and adjacent flashing.
Primary references for this topic family
These references ground the general building-science or safety context. They do not diagnose a specific house, replace local requirements or certify a repair.
US Environmental Protection Agency · government guidanceMoisture Control Guidance for Building Design, Construction and MaintenanceMoisture sources, transport, design, construction and maintenance controls.Verified 2026-07-20 · review due 2027-01-20Open source ↗US Department of Energy · government research libraryBuilding America Publications and Solution CenterBuilding enclosure, HVAC, windows, indoor air and high-performance housing research.Verified 2026-07-20 · review due 2027-01-20Open source ↗
Observe→Compare triggers→Repair source→Verify outcome
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