Synthetic underlayment is a high tech, high performance layer of material installed prior to the shingles on areas of the roof decking not covered by ice and water shield (the importance of ice and water shield covered in another post). The purpose of underlayment is to absorb asphalt from the shingle and provide an extra level of water resistance. Until more recent years, felt paper was the standard. However, advancements in roofing technology have brought about the synthetic underlayments which consistently outperform and out test traditional felt paper.