Roof Underlayment

Roof underlayment is a waterproof or water-resistant layer installed beneath the primary roofing material (tiles, metal panels, or shingles) and on top of the roof deck or battens. Its primary function is to serve as a secondary water barrier, catching any moisture that penetrates past the roofing material due to wind-driven rain, ice dams, or damaged tiles.

In addition to water protection, underlayment helps manage condensation that can form on the underside of metal roofing materials. By providing a drainage path to the eaves, the underlayment prevents moisture from accumulating on the roof structure, protecting the insulation, rafters, and sheathing from rot and mold. In Finnish construction, breathable underlayments are preferred because they allow moisture vapor to escape upward while blocking liquid water.

There are several types of roof underlayment: traditional bitumen-felt, modern synthetic (polyethylene or polypropylene) membranes, and rubberized asphalt peel-and-stick products. The choice depends on the roof type, slope, and local building code requirements. Low-slope roofs require higher-performance underlayments with better waterproofing properties.

The condition of the underlayment should be inspected whenever roofing work is performed, and especially if leaks are detected. A deteriorated underlayment compromises the entire roof system, as it is the last line of defense against water reaching the attic structure. If your roof is being re-covered, replacing the underlayment at the same time is strongly recommended.