Underlayment product selection is influenced by many factors. Primary roof covering, substrate type, roof design criteria, building codes, material cost, labor cost savings, life cycle expectations, walkability, exposure rating, warranties, climate and customer preference, to name a few, can all factor into the choice of underlayment.
To place these various factors into perspective, a good place to begin is a glance backwards at the history of traditional asphalt-saturated organic felt. That will help us to appreciate from a marketing perspective the entries of alternatives into the residential roofing industry. The new entries include premium self-adhering and synthetic underlayment products. The successful introduction of these new product categories into the residential roofing market is a remarkable story.
Traditional asphalt-saturated organic felt remains important for many roofing applications. “Tarco Roofing” gets its name from this traditional type of underlayment. In fact, the development and use of new types of underlayment can be better appreciated by comparison with traditional roofing. Knowing the advantages and disadvantages of specific products and their features and benefits in each of the major categories will lead to the right choice of underlayment for any given application.
The earliest type of underlayment is asphalt-saturated organic felt, also known as felt paper or tar paper. As asphalt shingles were developed for roofing, felt paper was used as an underlayment, serving a dual purpose: It prevented wood sap from being soaked into shingles and dissolving the asphalt; and it also shed water from under shingles, thereby protecting the deck from damage from moisture.
Industry specifications for organic felt as well as bitumen were needed primarily because sources of the asphalt and other materials as well as the manufacturing methods vary widely and affect the quality of the asphalt saturated organic felts. Standardization evolved over several decades such that today the quality of these types of felts is tightly controlled by industry standards. Examples of standards include the American Society for Testing and Materials (ASTM) “Standard Specification for Asphalt-Saturated Organic Felt Used in Roofing and Waterproofing” (ASTM D226 / D226M); and “Standard Specification for Asphalt-Saturated Organic Felt Underlayment Used in Steep Slope Roofing” (ASTM D4869 / D4869M); as well as test standards relating to asphalt physical and chemical properties characterization and properties of dry felts.
While it is reassuring to know that the “tear strength” of a synthetic material far exceeds the minimum requirement for asphalt felts, it is not meaningful to claim a synthetic felt “meets the standard” for one particular property mentioned in a standard that applies to asphalt felts. The standards developed for asphalt saturated felts were not intended for other types of underlayment, which have their own separate set of standards.
Another type of asphalt-saturated roll roofing used in commercial roof applications is BUR, or built-up roofing. BUR uses multiple layers of felt that are mopped in place between the layers using hot asphalt. These felts differ from those used in residential roofing in that they are typically reinforced with a fiberglass mat and hence are better able to withstand building stress to a higher degree. They are covered by their own set of standards as well.