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Why Do I Need Roofing Felt?

Why Do I Need Roofing Felt?

Why do I need roofing felt?There is a lot of confusion with home and building owners alike when it comes to the felt layer of a sloped roof. Also known as tar paper or a felt underlayment, this material offers some benefits for roof protection. But once the roof is complete, felt fails to make your home more waterproof.  This is especially important for those of us living in rainy climates, such as Portland, OR, and the greater Pacific Northwest. So, why exactly does your roof need felt?

Why is roofing felt used?

Variations of roofing felt have been around for more than a century. Before roofing contractors were so common, this blanket of sorts was designed to protect a roof’s inner structure during lengthy DIY roofing projects. Replacing a roof back then would easily take more than a few days, so working homeowners would remove existing shingles one weekend, and install new ones the next. In the meantime, the felt was used to temporarily protect their home from rain. This is important in rainy climates, especially here in the Pacific Northwest. Eventually, shingle manufacturers began producing their own felt underlayments, making improvements in the design, and marketing them as a necessary roofing component. Since then, additional benefits of using felt have emerged.

Benefits of roofing feltroofer

  • Protects the roof deck while shingles are removed and the inner structure is exposed.
  • Gives workers better traction, helping to keep them from slipping and providing a safer working environment.
  • Has been shown to increase the fire rating of some roofing systems.

No one can argue the advantages of using a felt underlayment. But, in an attempt to upsell the customer on a “felt upgrade,” some contractors have been making claims that simply don’t hold water. Be wary of salesman-type tactics that suggest felt can protect your home from leaks and water damage. It certainly cannot repel the amount of rain and moisture that Oregon and Washington experience every year.

Every home and building owner should understand that once the shingles are nailed in place, the underlayment is essentially ruined as a waterproofing agent. No matter how thick it is, those nail holes just defeated any waterproofing benefits the paper offered during construction. If the roof should develop a leak or lose a shingle, the underlayment will not prevent water from reaching the decking and framing. Yet, most roofing contractors use it and many area building codes require it. When using felt or tar paper as the product was intended, installing it does make sense.

Different roofing felt types, thickness

First, while the terms “felt” and “tar paper” are often used synonymously, they are different. Tar paper is less commonly used by far. It is made of tear-resistant paper or fiberglass matting and soaked in tar. Roofing felt is generally made of recycled paper products and then impregnated with asphalt. Some felt paper is only coated with asphalt on the exterior layers while others are saturated all the way through. Historically, these underlayment products came in two different weights. Fifteen-pound felt was referred to as such because it weighed fifteen pounds per square (100 square feet). Thirty-pound felt also weighed as much per square. But, as the products were improved upon, they became lighter. Fifteen-pound felt now generally weighs between 7 and 12 lbs, and 30lb. felt weighs between 16 and 27 lbs. Because of this, these options are now referred to as #15 and #30.

cedar shake roofThere are circumstances that justify the thicker #30  felt, the most common being a steep pitch. On a more dangerous roof, the heavier felt will resist tearing a bit better, making conditions safer for our workers. The most advantageous improvement made to underlayment materials has been the introduction of synthetic products. These are made of polyethylene or polypropylene polymers rather than paper or fiberglass. It is far more tear-resistant and provides a tight seal around fasteners, which does improve its waterproofing abilities after the roof is installed, but still it isn’t waterproof. A synthetic underlayment will actually allow your roof to “breathe,” unlike standard materials which can sometimes create a vapor barrier and actually trap moisture between the shingles and sheathing.

Find roofer in Portland, OR

Have more questions about your roof and what kind of underlayment you need? Contact Pacific West Roofing and speak to a qualified Oregon roofer about your needs, or click here to visit our contact page.

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Showing 2 comments
  • Brad Caldwell

    Great article! Only thing I might ask is, you said, “A synthetic underlayment will actually allow your roof to “breathe,” unlike standard materials which can sometimes create a vapor barrier and actually trap moisture between the shingles and sheathing” – 15- or 30-lb felt is going to be rated at about 5 or 6 perms (a little “breatheable”), whereas synthetics are going to range from less than 1 perm (almost a” vapor barrier”) to 16 perms (“breatheable”) to even 300+ perms (very breatheable) in some cases; however, if your attic is properly ventilated, then the moisture gets out via ventilation, not via breatheable synthetic, right? Also, Ph.D.s have argued that even if synthetic is breatheable, what good does it do when there’s a layer of shingles (0.15 – 0.30 perms) on top to stop whatever small amount of water vapor got through? Albeit, breatheable synthetics could be useful for conditioned, non-related attics, or for use under metal roofs, tile roofs, etc.

    • Pacific West Roofing

      I agree with what you say about permeability and it’s also true about the ventilation doing the work as long as you have a balanced in to out flow not just Sq. Ft. but spaciously speaking to allow air to move from every corner. Synthetics are only easier to install due to weight but create the same “slip sheet” that allows the shingle to NOT stick to the decking then shear during expansion and contraction from heat and moisture.