Which of the following is a common material used for valley flashing?

Valley flashing is critical in roofing systems as it directs water runoff from the roof's slopes into the gutters, helping to prevent water damage and leaks. The use of metal flashing, such as galvanized steel, is common for valley flashing due to its durability, resistance to rust and corrosion, and ability to create a watertight seal. Metal can withstand the elements better than other materials, ensuring longevity and effectiveness in channeling water away.

Using wood shingles, plastic sheeting, or composite shingles as valley flashing would not provide the same level of protection. Wood shingles are susceptible to rot and may not hold up well against water exposure. Plastic sheeting lacks the structural integrity needed to effectively manage water runoff and can degrade over time when exposed to sunlight. Composite shingles, while useful for roofing, are not designed to function as valley flashing and would not provide the necessary protection against leaks. Therefore, metal flashing is the optimal choice for valley flashing, ensuring that roofs remain watertight and resilient.