What is the purpose of building paper in the context of a roof?

Building paper serves a critical function in roofing systems primarily as a moisture barrier. Its primary purpose is to help prevent water from penetrating into the roofing structure, which could lead to problems such as wood rot, mold growth, and degradation of insulation. The building paper acts as an additional layer of protection beneath the roofing materials, ensuring that any water that might infiltrate the roofing materials is diverted away from the underlying structures.

While insulating properties may be important in roofing systems, building paper is not primarily designed to enhance insulation. Rather, its key role is to create a barrier against moisture. Supporting structural integrity is typically provided by the framing and roofing system itself, not by building paper. Additionally, a finishing layer is usually associated with aesthetic coverings, such as shingles or tiles, rather than the protective functions of building paper. This clarity helps one understand why the moisture barrier role of building paper is essential in maintaining the integrity and longevity of roofing systems.