In addition it is important to avoid nailing the shingles through the valley metal.
Roof valleys metal or shingle.
Roofing shingles are installed then cut and sealed where they overlap the metal channel.
A chalk line is then used to strike a line from the top of the valley to the bottom of the valley.
The metal is usually pre painted to best complement the shingles color blend.
For an open valley a sheet of metal usually steel sometimes copper is fastened to the roof deck.
The shingles are then cut out of the valley area opening the surface of the valley lining to water run off and the environment.
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.
That flashing is left exposed so the roof has visible lines of metal running down the valley.