Synthetic underlayment is a roofing accessory created by weaving/spinning together polypropylene or polyethylene and a polymer to form an all-over protective barrier to put between the roofing material and the roof deck. 

Synthetic underlayment is a newer innovation that is taking over the metal roofing industry as a better, more advanced replacement for felt underlayment. Most synthetic underlayments are mechanically attached with roofing caps and staples, while others have a glue backing.