For more than a century, roofers have understood that a separate layer of water protection should be added between a roof’s deck, or substrate, and the overlying shingles. More recently, both International and Residential Building codes require the use of an approved underlayment between the deck and shingles.

A widely used roof underlayment on the market today is traditional felt or tar paper.

Felt paper is made of organic materials with a cellulose base permeated with a waterproofing agent such as asphalt or bitumen, a byproduct of the oil-refining process. Also known as tar paper, this early underlayment proved to be an effective and inexpensive roofing solution for decades, giving roofing decks a secondary layer of water protection against the damaging effects of the elements.

However, because of recent advances in technology, synthetic underlayment is gaining popularity and offers additional benefits over traditional felt.

As technical enhancements have increased, manufacturers developed synthetic underlayment composed of a woven polypropylene fabric to replace traditional felt paper. Synthetic underlayment offers many advantages to the contractor and homeowner over traditional saturated felt paper. 

Synthetics, because of their woven polypropylene fabric, are much stronger and won’t tear compared to felt paper. Synthetics can also remain exposed to UV light for much longer than felt product and are less permeable to moisture. In addition, synthetic underlayment is available in lighter 10-square rolls (1,000 square feet), compared to the much heavier and smaller 4-square rolls of traditional felt. Synthetic underlayment costs only slightly more than felt, and many contractors consider the benefits to be well worth the extra cost.

The benefits of each type of underlayment are generally well known throughout the roofing industry, but how is each type weighed and measured for roofing applications? And how do the weights and measurements of felt and synthetic underlayment compare to each other?

Weighing #15 vs. #30 Felt Underlayment

When the roofing industry began using asphalt-saturated felt underlayment, manufacturers offered two different levels of tar paper thickness: #15 and #30.

The #15 and #30 tar paper designators used to mean that a 100-foot roll of 36-inch wide tar paper would weigh either 15 or 30 pounds, respectively, depending on the thickness of the paper. Today, according to the requirements laid out by ASTM D4869, #15 felt should weigh in at 34.6 pounds per 4-square roll, and #30 felt should weigh in at 43.2 pounds per 2-square roll.

Felt underlayment is water-resistant thanks to the asphalt or bitumen used to make it, but is not waterproof. And the measurement of their permeability, or perms, does not vary as much as one might guess. Number 15 felt is rated at 6 perms and #30 felt is rated at 5 perms. To put those measurements into perspective: a perm rating of 0.1 is considered impervious to vapor. A rating between .01 and 1 is a Class II semi-permeable vapor retarder, and anything between 1.0 and 10 is a Class III permeable vapor retarder.

How Does Synthetic Underlayment Measure Up?

Synthetic underlayment products are considerably lighter and stronger than felt paper. Lighter and thinner rolls mean more square coverage per roll, resulting in fewer trips up a ladder for roofers.

Instead of weighing the rolls like traditional felt, synthetics are designated by grams per square meter (GSM). To provide a side-by-side comparison and illustrate the reduced product weight, 86.5 pounds of felt will cover the same area as roughly a 22-pound single roll of synthetic underlayment.

Synthetic underlayment manufacturers design their polypropylene weave to meet and exceed the strength requirements of the code-approved ASTM D4869 and ASTM D226 standards for acceptable #15 and #30 felt underlayment products. Synthetics also adhere to ASTM D6757, which covers inorganic fiber reinforcements in underlayment products.

Finally, the permeability ratings of synthetic underlayment can be lower than 0.1, making synthetics essentially waterproof vapor barriers. Some synthetics, however, are engineered to provide a small degree of vapor permeability to allow the roofing substrate or deck to breath. Either way, perm ratings on synthetics are far lower than those of felt products.

Low-Slope vs. Steep-Slope Installation

Building codes, both residential and international, require that a roof with a low slope (2:12 up to 4:12), which has a greater chance of moisture infiltration than a steep-slope roof, receives a double layer of underlayment. Standard and steep-slope roofs (greater than 4:12) require only a single layer of underlayment. Follow the printed manufacturer’s installation instructions for more detailed guidelines. 

The Atlas Underlayment Advantage

Atlas Roofing® offers asphalt saturated felt underlayment as well as its Summit® 60 and Summit 180 advanced synthetic underlayment products for all roofers’ needs. Additionally, Atlas also provides peer and stick water and ice underlayment and fire-retardant slip-sheets. Visit our website today to learn more about Atlas Roofing’s underlayment product offerings.