Who this guide is for
- Homeowners planning a roof with valleys
- People with a valley that leaks in heavy rain
- Renovators adding a dormer or wing that creates a valley
- Anyone briefing a roofer on valley detailing
Why valleys concentrate water
A valley collects runoff from two slopes and funnels it down a single line, so it sees the highest flow on the roof, especially in a downpour. That concentration is exactly why valleys leak when the lining or width is inadequate. Recognising the valley as the roof's hardest-working line drives the planning.
Open versus closed valleys
Valleys are commonly detailed as open, with a visible metal lining, or closed, where the covering runs across or laps over the valley. Each handles water and looks different, and the choice suits the covering and the flow. Decide the style as a deliberate, flow-aware decision.
- Open valley: a visible lined channel handling high flow
- Closed valley: covering runs across or laps the line
- The covering type influences which style suits
- Style affects both performance and appearance
Lining and width
The valley lining and its width determine how much water it can carry without overshooting or backing up. A valley too narrow for the roof area above it can flood in heavy rain. Plan the lining and width to the catchment the valley serves.
Debris and where valleys feed
Valleys collect leaves and debris that can dam the flow, and they usually feed into a gutter, so the valley and gutter have to cope together. A blocked valley sends water under the covering. Plan for debris and the valley's discharge point.
Roof work and professionals
Valley lining and detailing are weather-critical, handle the heaviest flow and are at height, so they belong to qualified roofers. A leaking valley should be assessed rather than patched blindly. Requirements vary by location and project, and this is not DIY territory.
Roof valley planning checklist
- 1Recognise the valley carries the roof's heaviest flow
- 2Choose open or closed valley to suit the covering
- 3Plan lining and width to the catchment above
- 4Detail how the covering meets the valley line
- 5Plan for debris that can dam the valley
- 6Coordinate where the valley discharges into a gutter
- 7Have a leaking valley assessed, not just patched
- 8Use a qualified roofer for the valley detailing
Common mistakes to avoid
- Underestimating the concentrated flow a valley carries
- Sizing a valley too narrow for the roof area above
- Choosing a valley style that doesn't suit the covering
- Ignoring debris that dams the valley and backs water up
- Forgetting the valley has to work with the gutter below
- Patching a valley leak without finding the cause
When to involve a professional
- Have a qualified roofer detail the valley lining and width
- Treat valley work as weather-critical, high-flow and at height
- Have a leaking valley assessed rather than blindly patched
- Coordinate the valley discharge with the gutter
- Requirements vary by location and project; verify with your professionals
Frequently asked questions
Questions readers ask about this topic
Why do roof valleys leak so often?
A valley collects runoff from two slopes and funnels it down one line, so it carries the heaviest, fastest flow on the roof. That concentration means an under-sized or under-detailed valley leaks, especially in a downpour.
What is the difference between an open and closed valley?
An open valley has a visible lined channel that handles high flow, while a closed valley has the roof covering run across or lap over the line. Each handles water and looks different, and the covering type influences which style suits.
Can a valley be too narrow?
Yes. The lining and width determine how much water a valley can carry, and one too narrow for the roof area above can flood or back up in heavy rain. The width should be planned to the catchment the valley serves.
Should I patch a leaking valley myself?
No. Valley detailing is weather-critical, handles the roof's heaviest flow and is at height, so a leaking valley should be assessed by a qualified roofer rather than patched blindly. Requirements vary by location and project.
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