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 name "rosin-sized sheathing paper", commonly used to describe the material, 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.