General-Purpose Underlayments. These materials are typically recommended for use with one or more of the common roofing types. Generally they are the lightest weight, least expensive products, and carry shorter warranties.

The last time JLC treated the subject of synthetic roofing underlayments in depth (May 2006), author John Nicol acknowledged that synthetics are stronger, lighter, and faster to install than asphalt felts, but asked whether they’re worth the extra cost — estimated at about 30 percent at the time. Today, with even more synthetic underlayments on the market and their premium over felt a bit higher, the question is worth revisiting.My own acquaintance with felt underlayment began more than 40 years ago (at the time 15# felt actually weighed 15 pounds per square), when I went up on the roof of...