The answer to 'what causes underlayment' is that underlayment is a critical component of a roof that serves as a secondary barrier against water infiltration. Underlayment is installed directly on the roof deck, beneath the outer roofing material like shingles or tiles. It provides an additional layer of protection against leaks, wind, and other weather elements that can damage the roof and the home's interior.
Underlayment is typically made from a variety of materials, including asphalt-saturated felt paper, synthetic polymers, or rubberized asphalt. The specific type of underlayment used depends on the climate, the roof slope, and the primary roofing material. In areas with heavy rainfall or snowfall, a more robust underlayment may be required to ensure proper waterproofing.