Underlagspapp

Underlagspapp is a term used in Swedish construction for a sheet material that serves as the base layer under roofing or siding to protect the structure from moisture. It is typically asphalt-impregnated paper or felt, sometimes reinforced with fiberglass or polyester. Traditionally it acted as a tar-based underlayment on pitched roofs, providing a water-repellent base for later layers such as roofing felt, shingles, or tiles, and as a barrier against dampness from the attic space or wind-driven rain during construction.

In practice, underlagspapp is rolled out in courses from the eaves toward the ridge with overlaps of about 75–100 mm, depending on the product. It is attached with nails or staples, and joints are sealed according to manufacturer instructions. On walls, similar sheets can function as weather barriers behind exterior cladding.

Material variations include traditional asphalt-saturated paper or felt and modern asphalt-impregnated synthetic sheets; newer products may be fiberglass- or polyester-backed, and some are designed to be UV resistant for short exposure times.

Although still used, especially in older buildings or certain regions, many builders now use synthetic underlayments or breathable membranes that offer greater tear resistance and moisture management. Underlagspapp has limited vapor barrier properties and may degrade with sun exposure, making timely installation and proper sealing important. Disposal of tar-impregnated materials follows local regulations due to the bitumen content.