Building a Tighter Roof with Roofing Underlayments

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The underlayment is applied directly over an ice-and-water barrier to seal the roof sheathing and protect against the elements. Synthetic underlayments are cooler, breathable, and less slippery than traditional builder’s felt. Roofing underlayment was originally used for temporary protection against the elements but is now an integral part of a home's overall roof system. Underlayment provides a vital second layer of protection on top of the sheathing to help keep moisture out. It is necessary for roofing manufacturers' warranties and is now considered a necessary piece of the roofing puzzle. State and municipal codes require roofing underlayment for the system to meet standards like fire resistance, wind uplift resistance, puncture resistance, and resistance to wind-driven rain. The choice of one underlayment over another depends on a number of conditions, including its application, whether it will be used on a steep or low-slope roof, the fire code requirements in place, and in what region the structure is located. Fortunately for consumers and roofing contractors, there are many underlayment manufacturers on the market. Underlayments fall under three basic categories: felts, synthetics, and self-adhering ice-and-water barriers. Asphalt-Saturated Felt—The Grandaddy of Underlayments Also called builder's felt or felt paper, asphalt-saturated felt has been covering roofs for more than 50 years. The felt is made of a paper base that is impregnated or saturated with asphalt to make it more resistant to the elements. Some papers are actually coated in asphalt, while others are truly saturated. Roofing paper or felt is rolled across a roof deck and stapled or nailed in place. Builder's felt differs by weight and may be No. 30 or No. 15 felt, a reference to the original weights of each underlayment. "The name changed because today's 15 lb. felt is actually lighter than it used to be, because of modern technology and production," says Joan Crowe, technical manager of the National Roofing Contractors Association in Illinois. Although inexpensive and fairly easy to install, felt paper can be susceptible to tearing, especially in hot temperatures, and makes for a slippery surface to walk on while installing the roof covering. Asphalt felt also tends to trap moisture within the roofing system since it does not breathe. Synthetic Underlayment—A Growing Trend Much newer to the market are the polypropylene underlayments that are making waves in the roofing world. Made of synthetic polymers, the exact composition of these underlayments varies from manufacturer to manufacturer and remains strictly proprietary. These synthetic underlayments are marketed as wrinkle-free, tear resistant, and safer for workers to walk on during installation. "Synthetic underlayments are generally more durable than asphalt-saturated felt," explains Crowe. They are more expensive than felt, but it may be that you get what you pay for.

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