Understanding Underlayment: Separating Fact From Fiction About Roofing Felt

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Roofing felt is a simple blend of fibers and tar, but this paper-like material is also the most common underlayment installed under shingles to protect the wood decking below. This product is not well understood by the average homeowner seeking out roofing repair because there are more myths about this product than the Loch Ness monster. Cut through the hype about roofing felt and find out the hard facts to decide if you actually need it replaced during your next maintenance visit.

Waterproof vs. Water Resistant

There are a few underlayment products for roofing that are waterproof. However, roofing felt is not one of them. The material is only water resistant, and even if it was waterproof, it wouldn't stay that way with the hundreds of nails pushed through it when attaching asphalt shingles.

This means that roofing felt only slows down leaks when the shingles above get damaged or go missing. While many manufacturers advertise these products as protective, they only offer a limited amount of help in real world situations involving damaged roofs.

Necessary vs. Optional

Many homeowners jump to the conclusion that if roofing felt isn't waterproof, it's strictly optional. This is the case in some areas, but many local building codes require the application of an underlayment like felt before shingles can go on. Local roofing crews will know what your area requires so your roof repairs don't land you in hot water with the code enforcement department.

Aside from building codes, most asphalt shingle manufacturers also require something between the decking and the shingles. Instructing the crew to skip the felt or a similar product when repairing your roof could result in a voided warranty. Be sure to read the warranty information completely before assuming you can do away with an underlayment completely.

Benefits vs. Cost

When faced with an unexpected major repair, it's tempting to skip felt to trim down the costs a little. Making this decision causes you to lose out on a range of felt benefits like:

  • Protection for the decking during the repairs, which keeps the wood dry and prevents serious moisture issues from shortening the lifespan of the entire roof.
  • A barrier effect to keep pine plywood from releasing resins that shorten the lifespan of the shingles above them.
  • Smoothing properties to help hide unevenness between sheets of decking, a problem ruining the look of many new or recently repaired roofs.
  • Improved fire resistance, leading to a higher total fire rating for the roof.

Roofing felt is the most affordable underlayment option offering these benefits. Even when you're working with a tight budget, it's usually worth the extra cost because it also makes the roof's surface a lot safer for the contractors to work on.

15 Pound Felt vs. 30 Pound Felt

Once named this way for the weight of a piece covering 100 square feet, modern felts vary in weight but continue the tradition of the 30 pound felt being thicker and strong than the 15 pound products. The 15 pound felt is standard, but an upgrade to 30 pound felt provides better resistance against water when the shingles become compromised. It also tends to resist the wrinkling effect better from wind damage due to the heavier weight and stiffer composition.

Your roof may work fine without a layer of roofing felt, but only a professional can tell you if it's safe to skip. They may also offer you other underlayment products that do a better job of resisting water or slowing the spread of a fire. Discuss and consider all your options when investing in repairs or replacement to help your roof last as long as possible.


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