Buy Synthetic Roofing Underlayment rolls and ship them directly to your roof project address. Synthetic roofing underlayment is a layer of slip resistant laminated polypropylene material that is installed over the roof decking using cap nails. The synthetic underlayment purpose is to provide an added layer of protection to the roof system. One the eave, the synthetic underlayment is installed over the drip edge. On the rake, the synthetic underlayment is installed underneath the drip edge.