• May 13, 2016

Spotlight on Underlayment: Synthetic vs Felt

Jessica Taylor

What’s under your customer's metal roof matters.

Roof underlayment is the first layer of material laid prior to the metal roofing panels. It separates the roof covering from the roof deck, thus protecting it from detrimental resins, unevenness and—in re-roofing applications—fasteners and other old building materials. This water-shedding layer also provides secondary protection from weather, shields the roof deck during construction and is often required for warranty adherence and for roofs that must meet Underwriter’s Laboratory (UL) fire ratings. Outside of its traditional use, synthetic underlayment can also offer a myriad of benefits for your customers.

Synthetic vs Felt

Felt underlayment, also known as “felt paper” has been the traditional underlayment of choice for decades. It was used to prevent damage to the roof covering caused by sap leakage from wooden building materials, however—as even most wood structures are now constructed using plywood or other, non-wood materials—this is no longer an issue. Felt underlayment is saturated with asphalt composed of volatile compounds that dissipate over time. This makes it more fragile and likely to absorb moisture, which further weakens the roof. This deterioration is accelerated when asphalt is exposed to UV rays, heat, poor ventilation and less energy-efficient roof coverings. Additionally, the crude oil refining techniques that previously resulted in the creation of asphalt as a byproduct, now extract the maximum amount of product from the crude resource. As a result of this advancement in the refining process, only a powder-like residue that is not conducive to the development of asphalt is generated. Consequently, more customers are turning to a synthetic alternative for a longer-lasting, economically-efficient solution. Compared to traditional organic felt, synthetic underlayment is significantly more lightweight, permitting many more squares per roll, which makes it an economical solution for your customers. While non-woven materials are generally moisture permeable, woven scrim synthetic roof underlayment greatly enhances tear resistance and ease of installation. While it can be designed to be moisture-permeable, synthetic underlayment is typically manufactured as a barrier that doesn’t absorb moisture, making it resistant to fungal growth and wrinkling.

Underlayment Requirements and Resources

Although synthetic underlayment may be the superior choice for your customers, it is imperative that its use does not violate building code requirements or manufacturer’s warranties for the intended roof covering and that it is installed according to the manufacturer’s provided instructions. The International Code Council (ICC) has developed a single set of comprehensive and coordinated national model codes and standards used in the design, build and compliance processes to ensure the safe, sustainable and affordable construction of structures. The ICC’s set of International Codes (I-Codes) are available for purchase, and include, among others: ·         International Building Code (IBC)
  • International Energy Conservation Code (IECC)
  • International Fire Code (IFC)
  • International Green Construction Code (IgCC)
  • International Residential Code (IRC)
Of particular importance to the roofing industry are the IRC and IBC I-Codes. These codes include specific underlayment requirements and installation/ attachment directions for simple enforcement by inspectors. Provided in Chapter 9 of the 2015 IRC and Chapter 15 of the IBC, they cover material types, application and attachment requirements, including specific information for metal panels with denotations for variances specific to high-wind areas. Because of the wide variety of underlayment and metal panel styles, the IRC and IBC require installation according to manufacturer’s instructions. In the absence of manufacturers’ instructions and/ or when using alternative materials, the specific requirements outlined in the IRC and IBC I-Codes should be used where applicable. Remember, a metal roof is designed to have a long service life, and the underlayment’s service life should at least match that of the metal roof. Don’t allow your customers to be shortsighted when designing for longevity by using an underlayment not guaranteed to last for the duration of their roof.

ABC Underlayment

Advantages of Underlayment

Woven, coated polypropylene synthetic underlayment fabric is not only designed to protect against weather damage, but also provides greater strength, a longer UV exposure window and more secure surface for installation safety. Some underlayment options are 100 percent recyclable and up to 25 degrees cooler than asphalt felt, which can result in decreasing energy costs for your customers. At one-fourth the weight of traditional felt, but up to 20 times stronger, it ensures longevity that can’t be matched using traditional felt, particularly in areas that experience extreme heat and weather events. To prevent downhill slide during installation, underlayment is manufactured with anti-skid coating on both sides, ensuring a more sure-footed feel is achieved once attached. In addition, underlayment is certified to withstand up to six months of exposure to the sun and other weather elements before the final roofing material must be laid.

Learn More

For more information on ABC’s metal roof and wall products and for more tips on underlayment, please contact your local ABC sales representative.    

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