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.
Underlayments themselves are required by most shingle manufacturers and recommended by ALL of them. Synthetic underlayment is the better choice, offering better performance and more advantages.
Many of the advantages of synthetic underlayment are the quality and performance but it also has some hidden benefits that help you get a better quality roofing installation. It lays flatter and resists wrinkling to reduce labor costs, won’t tear and sacrifice performance as work crews walk on it, holds up during exposure even in high wind conditions, and even increases safety for workers by reducing the likelihood of slipping on it.