For more than a century, roofers have interpreted that a separate layer the water protection need to be added between a roof’s deck, or substrate, and the overlying shingles. Much more recently, both International and Residential structure codes call for the usage of an authorized underlayment in between the deck and shingles.

You are watching: What is the difference between 15 and 30 roofing felt

A widely provided roof underlayment ~ above the market today is traditional felt or tar paper.

Felt document is make of organic materials with a cellulose basic permeated v a waterproofing certified dealer such as asphalt or bitumen, a byproduct of the oil-refining process. Likewise known together tar paper, this beforehand underlayment confirmed to be an effective and inexpensive roofing solution for decades, providing roofing decks a secondary layer of water protection versus the damaging impacts of the elements.

However, since of recent advancements in technology, fabricated underlayment is obtaining popularity and offers additional benefits over traditional felt.

As technical enhancements have increased, manufacturers emerged synthetic underlayment written of a woven polypropylene fabric to replace traditional felt paper. Synthetic underlayment provides many benefits to the contractor and also homeowner over timeless saturated feel paper.

Synthetics, since of your woven polypropylene fabric, are lot stronger and also won’t tear contrasted to feeling paper. Synthetics can also remain exposed to UV light because that much much longer than feel product and also are less permeable come moisture. In addition, man-made underlayment is easily accessible in lighter 10-square rolls (1,000 square feet), contrasted to the much heavier and smaller 4-square roll of timeless felt. Synthetic underlayment costs only slightly much more than felt, and many contractors consider the benefits to be well worth the extra cost.

The services of each form of underlayment room generally famed throughout the roofing industry, yet how is each kind weighed and measured because that roofing applications? and how execute the weights and measurements that felt and also synthetic underlayment compare to each other?

Weighing #15 vs. #30 feeling Underlayment

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

The #15 and also #30 tar record designators supplied to mean that a 100-foot roll of 36-inch large tar paper would weigh either 15 or 30 pounds, respectively, depending upon the thickness the the paper. Today, follow to the needs laid out by ASTM D4869, #15 felt should weigh in at 34.6 pounds every 4-square roll, and also #30 felt must weigh in at 43.2 pounds per 2-square roll.

Felt underlayment is water-resistant thanks to the asphalt or bitumen provided to make it, however is no waterproof. And the measurement of their permeability, or perms, go not vary as lot as one might guess. Number 15 feel is rated in ~ 6 perms and #30 feel is rated in ~ 5 perms. To placed those measurements right into perspective: a perm rating the 0.1 is thought about impervious come vapor. A rating in between .01 and also 1 is a class II semi-permeable vapor retarder, and also anything in between 1.0 and also 10 is a class III permeable vapor retarder.

How Does man-made Underlayment measure up Up?

Synthetic underlayment assets are considerably lighter and also stronger than felt paper. Lighter and also thinner rolls mean more square coverage per roll, bring about fewer trips increase a ladder because that roofers.

Instead of weighing the rolls like classic felt, synthetics are designated by grams every square meter (GSM). To carry out a side-by-side comparison and also illustrate the diminished product weight, 86.5 pounds of felt will cover the same area as around a 22-pound solitary roll of fabricated underlayment.

Synthetic underlayment manufacturers architecture their polypropylene weave to meet and exceed the strength demands of the code-approved ASTM D4869 and also ASTM D226 requirements for agree #15 and also #30 felt underlayment products. Synthetics additionally adhere to ASTM D6757, i m sorry covers inorganic fiber reinforcements in underlayment products.

Finally, the permeability ratings of fabricated underlayment can be reduced than 0.1, do synthetics essentially waterproof vapor barriers. Some synthetics, however, space engineered to administer a tiny degree of vapor permeability to enable the roofing substrate or deck come breath. One of two people way, perm ratings ~ above synthetics are far lower than those of feeling products.

Low-Slope vs. Steep-Slope Installation

Building codes, both residential and international, call for that a roof through a short slope (2:12 approximately 4:12), which has actually a greater chance of humidity infiltration than a steep-slope roof, obtain a twin layer of underlayment. Standard and also steep-slope roofs (greater than 4:12) require only a solitary layer the underlayment. Follow the printed manufacturer’s installation accuse for an ext detailed guidelines.

See more: How Many Entrances Did The Cave Have, In Last Of The Mohicans

The Atlas Underlayment Advantage

Atlas Roofing® uses asphalt saturation felt underlayment and also its Summit® 60 and also Summit 180 progressed synthetic underlayment assets for every roofers’ needs. Additionally, Atlas additionally provides peer and also stick water and ice underlayment and also fire-retardant slip-sheets. Visit our website today to learn much more about Atlas Roofing’s underlayment product offerings.