For decades, a roofing underlayment was just an asphalt-saturated paper, or "felt." Today, contractors have a broader selection of options, as the underlayment category has evolved to include synthetic and hybrid products that are enticing them with performance and installation benefits.

Though many contractors are still sticking with traditional paper-based, asphalt-saturated roofing underlayments, some have switched to newer technologies. Synthetic underlayments, such as Grace's Tri-Flex Xtreme (above), offer greater tear resistance, longer exposure times, and more coverage per roll of product. Made from various types of poly-based films, synthetics are engineered to overcome the shortcomings of traditional 15- and 30-pound asphaltic felts, such as susceptibility to tearing, UV degradation, and water absorption. Synthetic underlayments–such as Grace Weather Barriers' Tri-Flex Xtreme, DuPont's RoofLiner with Elvaloy, Carlisle Coatings & Waterproofing's EZ-Roof Base, Valeron's Synthetic Roof Underlayment, Fortifiber's PlyDry 30, and GAF's Deck-Armor–are said to offer greater tear strength, resistance to UV degradation and the elements, and greater water hold-out than traditional felts.

"You're seeing high-end builders go to these other underlayments, because even though the materials cost more up front, they more than pay for themselves" by preventing some of the common issues with traditional roofing felts, says Steve Ratcliff, president of Tarco, manufacturer of several types of roofing underlayments.

And though synthetic underlayments may cost 40% to 50% more per roll than asphaltic underlayments, they provide more roof coverage per roll: 10 squares compared to the two squares of coverage provided by one roll of standard 30-pound asphaltic product. For installers, this means fewer trips up and down the ladder, says Wes Settlemyre, eastern sales manager for Grace. Plus, because the material is thinner, a 10-square roll of synthetic product weighs about the same as the 30-pound asphaltic roll.

Their durability and resistance to tearing mean synthetics also allow longer exposure periods (up to six months), giving builders substantial leeway in scheduling trades.

Still, synthetic roofing underlayments aren't perfect. Most have slick surfaces, posing a safety issue. Some manufacturers have reduced this hazard by creating textured surfaces, such as on Grace's Tri-Flex Xtreme, that provide better walkability. Synthetics also do not self-seal around fasteners.

Some underlayment manufacturers maintain that synthetic underlayments, because they act as vapor barriers as well as waterproofing layers, can lead to moisture damage because they don't allow interior moisture to escape. To address this, GAF's Deck-Armor and Fortifiber's Rooftex 30B synthetic underlayments allow moisture vapor to perspire through the roofing system. But some are skeptical about the value of a permeable roofing underlayment. "Our general view is that you seal a house properly and you let the HVAC system be what lets it breathe; you don't try to breathe through the walls or roof," says Bob Stout, vice president and general manager for Carlisle.

Products that combine synthetic material with asphalt or SBS-modified asphalt could provide the best of both worlds. These hybrid underlayments–such as Protecto Wrap Co.'s Rainproof 60 Plus 180-day (circle 109), Tarco's LeakBarrier EasyLay, Tamko's TW underlayment (circle 110), Elk's Quantum 30 (circle 111), and Owens Corning's Weatherlock Metal (circle 112)–incorporate either a layer of poly material coated or saturated with asphalt or a top layer of poly material over an asphaltic base. Hybrid underlayments offer tear resistance and longer exposure times similar to 100% synthetics plus the nail-sealability and walkability of standard asphaltic felts.

–Stephani L. Miller