Most roofing experts would agree that it is a best practice to apply underlayments on steep-slope shingle applications. Underlayments provide benefits to the roof system at the deck and shingle components. These benefits add to the long-term weatherproofing success of the roof system. Another primary reason for their use is that most building codes require the application of underlayments on steep-slope roofs.
Underlayments serve as a weatherproofing barrier protecting the deck from moisture prior to shingle application and can be applied to dry in the roof in advance of the shingle installation. This is of particular importance on new construction projects, where there can be a lengthy lapse between deck installation and shingle application. Moisture absorption in the deck could lead to delamination or warping of the deck surface, providing an unsuitable long-term substrate.