For an open valley a sheet of metal usually steel sometimes copper is fastened to the roof deck.
Roof valleys metal or shingle.
Open valley shingle installation an open valley relies on metal flashing for protection.
That flashing is left exposed so the roof has visible lines of metal running down the valley.
A chalk line is then used to strike a line from the top of the valley to the bottom of the valley.
Roofing shingles are installed then cut and sealed where they overlap the metal channel.
With an open valley a strip of metal will be visible on the roof wherever two opposing roof planes meet.
In addition it is important to avoid nailing the shingles through the valley metal.
These items can be fabricated in many different profiles.
For example in metal roofing systems valleys are commonly fabricated with hems but can also be fabricated without them in which case they could still be used for metal roofing or any other type of roofing.
Whether for slate shingle or metal roofing valley flashing adds detail and helps to direct water.