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Synthetic Roof Underlayment

Housewrap for the roof

A contractor installs synthetic roofing underlayment before the roof tiles go on.  The underlayment is 6 times lighter and 20 times stronger than Number 30 felt.

Until the twenty-first century, most residential sloped roofs received a layer of asphalt-saturated felt building paper underneath the roofing material. Mimicking the attributes of housewraps, synthetic roof underlayments are now available to serve the same function as a secondary weather barrier with better resistance to tearing, moisture, and ultraviolet rays than traditional roofing felt. Synthetic underlayments are typically made from polypropylene, polyester, or fiberglass fabric which weighs less than felt building paper, can be manufactured with an anti-slip surface, and can withstand exposure to the elements for six months.

Recent natural disasters and subsequent rebuilding efforts highlighted the versatility of synthetics as roof underlayment by providing a real-life test environment. After several hurricanes ravaged southern coastal areas of the United States, many people were forced out of their damaged homes. At the same time, large numbers of homes required quick roof repair and "drying in" to minimize further damage due to water intrusion. With limited resources, contractors triaged homes, repairing the critical components and installing synthetic underlayments as temporary roofing. The underlayments performed better than FEMA’s blue tarps and didn’t require removal and discard when a roofing crew eventually arrived to install shingles.

One manufacturer offers a Class I fire-rated synthetic underlayment for roofs that require resistance to fire, as well as, the durable attributes of synthetic fabrics while the building is under construction.


Quality and Durability

Moisture resistance and hardiness make synthetic underlayment a good choice as a secondary weather resistant barrier under roof cladding.

Safety and Disaster Mitigation

Polypropylene and similar synthetic materials resist moisture, tearing, and degradation from UV rays, making them a durable, relatively long-term covering that can be utilized for disaster response. Fire-rated synthetic underlayments can provide added protection against fire spreading through the roof in multi-family housing and/or regions of the country that are prone to wildfires.


Easy

Installation is very similar to that of building felt paper. Proprietary fasteners may be required, but roofers can easily use the product. The lighter weight of the material and different fastening requirements may make the product simpler to install than #30 felt.


Although material cost starts at 30 percent more than traditional felt building paper, labor cost should remain the same or decrease. Because roof underlayment cost represents a small portion of the total cost of roof cladding installation, specifying a synthetic product on a 3,200-square-foot colonial home with an 8/12 gabled roof would add about $100 to the overall cost.


Not Applicable


Underlayment products should have an ICC-ES Report, or similar, filed listing the testing that has been conducted on the product. Roof underlayment is required by the 2003 International Residential Code (IRC) to conform to specified water-resistant test methods.

Where roof assembly fire resistance is required, a Class (I, II, or III) listing resulting from testing to ASTM E108 or UL 790 standards will be required.


Not Applicable


Generally, underlayments are rolled out parallel to the overhang eave edge (perpendicular to the slope), starting at the bottom of the roof and working toward the ridge. Minimum end and edge laps from 3 to 6 inches are required and vary with roof style and product selected. Nails and caps are the preferred attachment fasteners, however, fastener specification and spacing varies with each manufacturer’s product. Each manufacturer provides guidance for specific layout, attachment, and detailing. Therefore, it is important to review the installation instructions provided with the product.

Some underlayment products are compatible with self-adhering bituminous eave flashing, or contain installation details to adhere eave areas with double layers of underlayment secured with asphalt roofing cement. Many synthetics are only recommended for high-slope installations.


Product limited warranties range from 7 to 25 years.


Synthetic roof underlayment performs as a secondary weather resistant barrier under roof cladding. In addition, the products are more durable than building felt paper when exposed to wind, rain, and sun for prolonged periods (up to 6 months). This gives builders some time to endure material or labor shortages or unforeseen weather events while protecting the building’s interior. The same protection against water intrusion can benefit victims of natural disasters, so synthetic roof underlayment can be equally useful in the repair and remodeling industries.

Synthetic roof underlayments, like housewrap, tend to have lower perm ratings than building felt paper. Therefore, if the sheathing is wetted, it may dry more slowly than it would with a felt underlayment. Properly installed underlayment may act as an air barrier, making attic ventilation details more important in certain climates.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.