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What is a roof valley?

A roof valley is the joint where two sloping roof surfaces meet and channel water together. An open valley exposes flashing; a closed-cut valley hides it beneath shingles. Because runoff concentrates at this joint, ask the roofer to specify the valley detail and flashing for the roofing material being used.

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By InstaBids editorial team with AI research agents. Updated

What this means for your home project

  1. Read the roofing manufacturer’s recommendations for the proposed valley application; use that document when discussing the detail with the roofer. Source: Asphalt Roofing Manufacturers Association, read 2026-11-02.
  2. For reroofing, request a written inspection scope covering missing, damaged or poorly installed valley flashing, with the proposed repair or replacement identified. Source: Pacific Northwest National Laboratory, read 2026-11-02.
  3. Ask the written sealing scope to identify the membrane’s integration with the adjoining roofing materials, so the proposed water-control detail is explicit. Source: Pacific Northwest National Laboratory, read 2026-11-02.
  4. Ask how the installer will leave the membrane flat; request confirmation that the finished layer has no gaps, creases or folds. Source: Pacific Northwest National Laboratory, read 2026-11-02.
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Why do roof valleys leak?

Water from both roof slopes concentrates at the valley joint. In snowy conditions, PNNL also identifies a second mechanism: poor valley ventilation can encourage snow melt, ice dams and water leakage. A leak therefore calls for attention to both the drainage joint and, where relevant, ice-dam conditions. “Because roof valleys are difficult to ventilate, they are more prone to heat accumulation and, therefore, snow melt, which can contribute to ice dam formation and subsequent water leakage.” Source: Pacific Northwest National Laboratory, read 2026-11-02.

Open or closed roof valley: what is the difference?

An open valley leaves its flashing visible between shingle edges; a closed-cut valley conceals it. In ARMA’s 2014 asphalt-roofing manual, the closed-cut detail is described for intersecting roofs of the same slope. Ask which detail the proposed shingles support, rather than choosing by appearance alone. “Shingles do not extend across the valley.” Source: Asphalt Roofing Manufacturers Association, read 2026-11-02. “Valley flashing is exposed.” Source: Asphalt Roofing Manufacturers Association, read 2026-11-02. “The valley flashing is not exposed.” Source: Asphalt Roofing Manufacturers Association, read 2026-11-02.

How do you seal roof valleys?

PNNL describes sealing beneath the roof covering with a self-sealing membrane: a waterproof sheet, often supplied with peel-and-stick backing. Its valley sequence puts the membrane directly on the roof sheathing and overlaps successive pieces by 6 inches from the bottom upward. Request this layering detail in the written scope; this is roof work for the installer. “Apply from the lowest point to the highest, overlapping membrane sections by 6 inches in a shingle fashion to allow water to flow unobstructed down the length of the valley.” Source: Pacific Northwest National Laboratory, read 2026-11-02.

How do you replace roof valley flashing?

Start with an inspection that identifies missing, damaged or poorly installed flashing and says what will be repaired or replaced. PNNL specifically recommends that inspection at valleys when an older home is reroofed. Ask for the findings and proposed flashing work in writing; replacement should respond to the condition found. “When the roofing on an older home is replaced, take the opportunity to inspect all of the flashing and repair or replace missing, damaged, or poorly installed flashing at all roof penetrations as well as at roof-wall joints and at valleys.” Source: Pacific Northwest National Laboratory, read 2026-11-02.

What roof valley flashing sizes are used?

For its open metal valley detail, ARMA’s 2014 manual gives a minimum total flashing width of 24 inches; local code or the shingle manufacturer may require more. That is the sheet width, rather than a promised visible opening. The same manual specifies 12-inch cemented laps in its valley-underlayment instructions. PNNL notes that code requirements vary with roofing material. “Laps should be 12" and cemented.” Source: Asphalt Roofing Manufacturers Association, read 2026-11-02. “Building code requirements for roof valley flashing vary by roofing material type.” Source: Pacific Northwest National Laboratory, read 2026-11-02.

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