In a closed valley, how far should shingles from one side run underneath?

In a closed valley, shingles should extend 12 inches or 300 mm underneath from one side. This specific measurement is important to ensure proper water shedding and to prevent leaks. The extended overlap helps redirect water flow away from the valley and into the gutter system. If water is able to penetrate or if there is insufficient overlapping, it can lead to water pooling or backing up, which dramatically increases the risk of leaks.

Using 12 inches provides a good balance between effective water drainage and the amount of materials used, ensuring both the longevity of the roofing system and the overall efficiency of the shed roof design. This measure helps to create a secure, watertight seal that offers durability against the elements, particularly in areas prone to heavy rainfall. Proper overlap also allows for thermal expansion and contraction of the materials, minimizing the risk of damage over time.

Other measurements may not provide enough coverage to adequately protect the valley from potential leaks or may result in using unnecessary materials, complicating installation without adding significant benefits.