Valley gutter
A valley gutter runs down the intersection of two roof planes, collecting water from both. Pre-formed metal. Maintenance and detail explained for builders.
Ask Chalkline about this →A valley gutter is the pre-formed metal channel that runs down the intersection of two roof planes (the “valley” formed where two slopes meet). It collects water shed from both adjacent roof planes and discharges it into the eaves gutter at the lower end. On hipped or complex roofs, the valley is one of the highest-flow-rate elements; it sees several times the volume of water that flows down a flat sheet of the same projected area.
Construction and profile. Standard residential valley gutters are pre-formed from the same Colorbond / Zincalume coil as the roof sheeting, profiled with:
- A central trough to carry water (typical 100 to 150 mm wide between fold lines).
- Side wings that lap under the adjacent roof sheets (typically 200 to 300 mm each side).
- A stop-end at the head where the valley meets a roof feature.
- An overlap at any join (sheets are typically 6 m long; longer valleys require an end-lap join with sealant).
Pitch and capacity. AS 1562.1 sets the geometric requirements; AS/NZS 3500.3 covers the discharge capacity calculation. The key design rules:
- Minimum slope along the valley line is typically 5 degrees; flatter valleys hold water and leak under sustained rain.
- Trough width is sized for the catchment of both intersecting planes plus a safety factor.
- Outlet into the eaves gutter must allow the design flow without backing up.
Why valley gutters fail. Three modes, in order of frequency on residential builds:
- Debris blockage, especially from overhanging trees. Leaves and twigs accumulate in the trough, water backs up, and overflow tracks sideways under the adjacent roof sheets into the roof space. Most common on inland sites with established trees; least common on bare new-development sites.
- Lap join failure. An end-lap join in a valley that wasn’t sealed at install opens after years of thermal cycling. Capillary creep then admits water along the lap.
- Side-wing lift under wind. The roof sheets cover the valley side wings; if the side wings are fixed only by sheet weight and the sheet lifts in high wind, the valley wing lifts with it and water enters the cavity.
Maintenance. Recommend quarterly clearing of valleys on tree-affected sites; annual on open sites. The valley is generally clean enough to inspect from the eaves gutter with a torch; full ladder access is needed for deep cleaning. Note: valley gutter inspection at heights is High Risk Construction Work under WHS regs (over 2 m), needs SWMS and edge protection or harness.
For builders.
- Plan tree clearance at the lot landscaping stage. Branches over valleys cause permanent maintenance burden for the homeowner.
- Spec a valley with margin. Bigger trough widths are cheap at install and forgive blockage events for longer.
- Use a continuous valley where possible; minimise lap joins. Off-roll lengths of 6+ m mean most residential valleys come as a single piece.
- Communicate the clearing schedule to the client at handover. Include in the maintenance manual.
Also known as: valley, roof valley, valley flashing.
Category: Practical / metal roofing / drainage.
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Last updated: 2026-05-14. Verified: 2026-05-14.