open valley

An open valley is a roof construction detail where two sloped roof planes intersect, and the valley flashing (almost always metal, typically 24 gauge galvanized steel or painted aluminum) runs visibly down the centre between them. Unlike closed valleys, where roofing material covers the flashing, the metal in an open valley is left exposed to weather and view. Both roof slopes are cut back and trimmed clean along the valley edges, usually 4 to 6 inches back from the centre line on each side, leaving a clear channel through which water can flow freely downslope.

The function of this geometry is drainage. Water running down either slope converges into the valley and must be shed quickly without pooling or backing up under the shingles. The open design eliminates the risk of ice damming or debris blockage that can plague closed valleys where leaves, silt, or ice can trap water at the junction. In regions with heavy rain or snow melt, the unobstructed path through an open valley is genuinely superior hydraulically.

Variations and Installation

Open valleys are common in residential asphalt shingle work and in metal roofing systems. With shingles, the valley metal is fastened down first, then each sloped shingle course is cut along a chalk line and nailed just inboard of the valley edge. Metal roofing valleys use similar principle but often integrate the valley as part of the panel system itself. W-valleys and V-valleys refer to the cross-sectional profile of the flashing; W-profiles, with a subtle centre ridge, are preferred because they prevent water from running back under the edge shingles during heavy rain.

Installation quality depends heavily on fastening and sealant. Fasteners along the valley edge are typically driven within 1 inch of the edge and sealed with compatible roofing cement; exposed fasteners in the open channel corrode quickly if not sealed. The valley flashing must be at least 18 inches wide where it emerges from the roof opening, with 9 inches minimum coverage on each slope. Undersized flashing is a common fault and leads to water intrusion and rot.

Open valleys are often preferred by professional roofers and building code authorities in North America because their performance is predictable and inspectable. The weakness is aesthetic: the continuous line of metal can appear institutional or industrial to some homeowners, which is why some prefer the softer look of closed valleys despite their higher maintenance burden. Material costs are roughly equivalent, but labour for an open valley is simpler because the roofer need not weave shingles across the junction.