What primary purpose does roofing felt serve in a roofing system?

Roofing felt primarily serves as a vapor barrier within a roofing system. Its main function is to protect the underlying roofing materials from moisture and prevent water infiltration, which can lead to significant issues like mold growth, structural damage, and wood rot. By acting as a barrier, it helps divert any incidental moisture that might penetrate the outer roofing layers, ensuring that the underlying materials remain dry.

While roofing felt may provide certain minor benefits in terms of insulation—keeping some ambient temperatures in check or adding a minimal layer between the roof deck and the actual roofing material—its primary role is as a moisture and vapor barrier. Aesthetics and structural support are not its main functions; roofing felt is not designed to be visible once the roofing material is applied, nor does it contribute to the structural integrity of the roof itself. Instead, it is intended to serve a protective role against the elements, maintaining the health and longevity of the entire roofing system.