Underlayment is a layer of fabric-like material that is rolled out and stapled to the sheeting. It adds an important level of moisture protection between the shingles and the sheeting.
How to do it right: A high quality underlayment is just as important in preventing leaks as the roofing material itself. Municipal codes require roofing underlayment to meet standards like fire resistance, wind uplift resistance, puncture resistance, and resistance to wind-driven rain. The most widely used underlayment is asphalt impregnated felt. The weight and specifications of the material should be sufficient to provide protection for the life of the roof.