Valley linear footage drives several material orders: metal valley flashing, ice-and-water shield wrap, and shingle waste for cut-valley installations.
Valleys waste shingles because shingles get cut at angles where two planes meet. For closed-cut valleys (a single shingle course covering the seam), waste is moderate. For woven valleys (shingles interlocking through the seam), waste is higher. Metal valleys with cut-back shingles waste the most.
Building codes typically require ice-and-water shield under valleys for the full valley length plus a setback on each side.