Understanding Roof Underlayment

What To Consider When Choosing Between Traditional Felt And Synthetic

Oliver Olinger, Writer
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Roofing underlayment is required by the International Residential Code (IRC) and local building codes and must meet certain ASTM standards. Roof underlayment provides an extra layer of protection between roofing shingles and the roof deck. This added barrier further protects the roof and attic from the elements and encourages proper draining of any water that manages to seep through the asphalt shingles.

The two primary options are traditional felt and synthetic, polymer-based roof underlayment. When deciding which one to use, roofing contractors should take several important factors into consideration, including cost, climate, local code requirements, degree of slope and individual preference.

Felt Roof Underlayment: Low Cost And Popular

The use of felt roof underlayment, also known as tarpaper, goes back to the early 20th century. Roofing felt is composed of organic materials with a cellulose base (such as recycled paper or sawdust) or inorganic fiberglass materials. Typically, the organic or inorganic felt paper is saturated with asphalt or bitumen (similar to asphalt) to reduce the vapor permeability of the felt underlayment.

Felt underlayment is available as #15 basic or #30 heavy felt. Due to the low cost of felt underlayment products and their long-standing presence in the roofing industry, many roofers continue to prefer felt to synthetic materials. Felt, however, does allow some moisture to permeate over time, which can lead to problems with the roof deck.

According to the International Association of Certified Home Inspectors (InterNACHI), problems associated with felt underlayment include wrinkling caused by expansion (as water permeates the felt) and deteriorating or weakening caused by volatile compounds or exposure to UV light. Felt underlayment can’t be left on the roof for long before it needs to be covered by a course of asphalt shingles.

Atlas Roofing offers both #15 and #30 asphalt-saturated felt underlayment products designed to meet both ASTM D4869 and ASTM D226 standards. Felt rolls are tightly wound and include alignment lines printed for ease of installation.

The Durability Of Synthetic Roof Underlayment

Slightly more expensive than felt products, synthetic roof underlayment is composed of woven polypropylene fabric. This polymer base is both thinner and stronger than traditional felt. Atlas Roofing’s Summit® synthetic roofing underlayment, for example, is six times lighter and three times stronger than #30 felt paper.

According to Green Building Advisor, synthetic polypropylene fabric is not permeable by moisture or vapor, can be exposed to UV light and the elements for up to six months before asphalt shingles are installed (useful in delivery-delay scenarios), and offers outstanding slip resistance and traction for workers’ shoes.

Synthetic underlayment products are more effective than felt at providing a temporary weather barrier during roof installation. After the roof is installed, the non-permeable synthetic fibers prevent shingle tar and other volatile components from seeping down into the roof deck over time. Synthetic underlayment also exceeds felt at preventing rain or snow from passing down through the joints in the overlying shingles and damaging the roof.

The Atlas Roofing Underlayment Solution

Atlas Summit® underlayment meets ASTM D6757 standards for inorganic shingle underlayment and exceeds ASTM D4869 and D226 standards. Summit underlayment is an eligible component of the Atlas Signature Select® Roofing System, which also includes WeatherMaster® Ice and Water peel-and-stick roofing underlayment, Atlas starter shingles, Atlas roof shingles and Atlas hip and ridge shingles.

For more information about Atlas Roofing’s organic felt and synthetic roof underlayment products, contact Atlas today.