On a new roof, asphalt shingles should be applied over what material?

Asphalt shingles are specifically designed to be installed over an underlayment, which serves several important functions. The underlayment acts as a secondary barrier against moisture, protecting the roof deck from water infiltration. It also provides a smoother surface for the shingles to adhere to and can help to mitigate the risk of ice damming in colder climates by allowing for better drainage.

While plywood is a common decking material for roofs, and strapping may be used in certain construction practices, these options are not the ideal direct surface for asphalt shingles. Installing shingles directly on rafters is also inappropriate, as it would lack the necessary moisture protection and structural integrity. The underlayment is therefore essential, as it optimizes the overall performance of the roofing system and enhances durability against weather elements.