Underlayment

Underlayment is defined as a water-resistant or waterproof barrier material installed directly on the roof deck beneath the primary roofing material to provide secondary protection against moisture intrusion. The three main types of underlayment are asphalt-saturated felt, synthetic (polypropylene), and self-adhering rubberized asphalt (ice and water shield). Building codes require underlayment on all asphalt shingle roofs, and the type and coverage area are specified based on roof slope, climate zone, and local amendments.