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Red rosin paper (red building paper, brown rosin paper, slip sheet paper, rosin-sized sheathing paper, and building paper) is a 100% recycled heavy duty felt paper used in construction such as underlayment under flooring and siding. The names comes from the rosin used in the paper, the process of sizing it to add the rosin, and its use by builders. "Alum-rosin size was invented by Moritz Friedrich Illig in Germany in 1807..." and is known to have been used as a building paper by 1850.
In building construction red rosin paper is used to reduce air and moisture flow through a wall or floor, create a "slip sheet" so different materials can slip by each other as they expand and contract, keep dust from working down through a floor, minimize squeaking, and sometimes as part of built-up roofs. Rosin paper is also temporarily used to protect a work site during construction. Rosin paper may have a polyurethane coating to improve moisture resistance and tearing. It is not acid free particularly containing abietic acid and comes in many size rolls up to 96 inches (2.4 m) wide.
Papers from the Middle Ages were sized with gelatine but the invention of the paper-making machine in the late 18th century demanded a better size resulting in the creation of the rosin size.
A modern equivalent would be Synthetic Resin Bonded Paper (SRBP).