rosin paper
A heavy-duty felt paper used in construction as a layer under flooring and siding.
rosin paper: kraft backing that stops moisture creep
Rosin paper is a saturated kraft paper sheet, typically 35 to 40 pounds per 3000 square feet, used as a moisture barrier and slip layer in construction assemblies. It sits between structural substrates and finished materials, allowing vapor to move in one direction while blocking liquid water and reducing friction during installation. The paper itself is kraft (made from wood pulp), and the rosin coating, derived from pine resin or synthetic equivalents, gives it water resistance and a slippery surface.
In residential flooring, rosin paper is laid over concrete slabs or wooden subfloors before hardwood, engineered wood, or resilient flooring goes down. The slip property lets flooring materials expand and contract seasonally without binding to the substrate. Under siding, it serves as a weather barrier behind cladding, allowing the wall cavity to breathe while shedding bulk water. Some builders also run it beneath roof sheathing as a secondary water block, though modern synthetic underlayments have displaced it in many high-end applications.
Rosin paper tears and punctures easily. Sharp subfloor fasteners, aggregate on concrete, or foot traffic on unprotected rolls will compromise it before flooring is installed. Once breached, moisture wicks through the break, potentially reaching wood framing or causing floor cupping. The material also degrades under extended UV exposure; it becomes brittle and loses its water resistance, so it should not be left exposed for more than a few weeks during construction.
The name reflects its composition: rosin, the solid resin byproduct of pine oil distillation, was historically the standard water-repellent additive. Synthetic versions now often replace natural rosin, but the term persists in the trade. Builders sometimes confuse it with asphalt felt (tar paper), which is heavier and less slippery but less vapor-permeable. Rosin paper is the lighter, breathable choice; asphalt felt is the vapor blocker. Neither should be substituted for the other without considering the wall or floor assembly design.