This is a common spot for snow and ice buildup, especially on roofs with lower pitches.In a premium roofing system, use of roll roofing material is required over the length of the valley.
Ice and water-proofing membrane rolls such as Progard Plus or Gripgard provide the optimum protection as the base for any valley construction and are critical in preventing problems with ice dams.
Ice damming develops as snow on the surface of the roof melts. Water runs under the edge of the roof creating a dam. Additional snow melts forming a pool against the dam causing water to back up, often getting under the shingles.