Underlayment — felt (15# / 30#) vs synthetic, IRC R905.1 baseline

Felt underlayment is the legacy product — paper saturated with asphalt to repel water — sold in two weights (15-pound and 30-pound, named for the historical weight per 100 square feet). It is breathable, inexpensive, and broadly familiar to installers, but it absorbs water rather than shedding it, tears under foot traffic and wind, and degrades quickly under UV when left exposed between tear-off and shingle-on. Synthetic underlayment is woven or extruded polypropylene or polyethylene, sometimes laminated, with a higher tear strength, water-shedding rather than water-absorbing surface, lighter weight per roll, and significantly higher UV tolerance — most synthetic products list 30 to 180 days of permitted UV exposure during installation, where felt is typically rated only days. Synthetic also walks safer in light rain and lays flat without the wrinkling felt develops in humidity. The IRC's underlayment requirement (R905.1.1) is product-neutral — both felt and synthetic underlayments meeting ASTM standards satisfy code — but most major manufacturers' enhanced warranties (cross-reference D9-004) require installation of the manufacturer's own underlayment as part of the qualifying full-system package, and that product is typically synthetic. Some installers still prefer felt for cost reasons or familiarity, and felt remains a code-compliant choice. The install detail that matters most regardless of material is the lap pattern — minimum head laps and side laps are specified by manufacturer install guides and IRC R905.1.1, and a deficient lap pattern is a failure path independent of which material is used. [Source: IRC Section R905.1.1 underlayment; ASTM D226 asphalt-saturated felt standard; ASTM D4869 underlayment for asphalt shingles; NRCA Roofing Manual underlayment chapter; GAF Tiger Paw synthetic underlayment install guide; Owens Corning RhinoRoof synthetic underlayment install guide]