What material is acceptable for a valley in asphalt shingle roof installation?

In asphalt shingle roof installation, using mineral surfaced asphalt for a valley is acceptable due to its compatibility with the shingle roofing system and its performance characteristics. Mineral surfaced asphalt is designed to provide a robust, water-resistant barrier that can effectively handle the flow of water through the valley area, which is crucial for preventing leaks and ensuring the longevity of the roof.

Valleys are critical areas on a roof where two sloping sections meet, and they require careful consideration during installation to ensure proper drainage. Mineral surfaced asphalt can provide a smooth surface that allows for seamless integration with asphalt shingles, creating a continuous water-shedding plane. Additionally, it adheres well to other roofing materials and is designed to withstand the elements, making it suitable for this purpose.

In contrast, while metal flashing is also commonly used and often considered for valleys due to its durability and water-shedding capabilities, it may not be the choice specified in the context of this particular question. Fiberglass and wood shingles don't provide the same level of protection or compatibility, so they are not suitable for use in valley applications within an asphalt roofing system.