The underlayment is very important as a second layer of defense against the elements. The most commonly-used 15# tarpaper felt rolls out with large bubbles and wrinkles. It also tears easily and doesn’t stand up to adverse weather conditions. See the example below. We use high performance synthetic underlayment with every project. It does not tear, blow away, or wrinkle. It stands up to UV rays for up to 6 months by itself, shelters the roof from rain storms and heavy wind.