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Have you ever approached a roofing project and wondered: is roofing felt necessary?
For more than 25 years organizations such as the National Roofing Contractors Association (NRCA) and the Canadian Asphalt Shingle Manufacturer’s Association (CASMA) have included roofing felt in their roofing and waterproofing recommendations. It is also a requirement under many building codes.
Read on to understand the purpose of felt paper on roofs, and how this product will add an integral layer of protection to your roof – and your residence.
(Working on a commercial roofing project? Visit IKO’s Commercial Roofing section to learn more about our commercial roofing membranes, and related products and accessories.)
Sometimes called roofing felt underlayment, roofing tar paper, or roll roofing, this is a layer of protection installed between the roof deck and the roofing shingles.
Roofing felt is comprised of a base, made from natural materials (such as wood cellulose) or synthetic ones (such as fiberglass or polyester), and then coated or saturated with a protective coating such as bitumen (asphalt), which repels water but still allows the product to breathe.
Roofing felt is an underlayment. At one point, all roofing underlayments were made of felt. However, there are now synthetic underlayment options.
All roofs have underlayment. At a minimum, a roof with asphalt shingles should be completely covered with one layer of underlayment. Roofers may consider adding layers of underlayment for superior protection, which is especially useful on low-pitch roofs.
Roofing felt can shed water. This property helps it provide additional water protection, which is one reason it is an important part of a roofing system. However, roofing felt does not offer the lowered moisture permeability characteristics of synthetic underlayment.
Roofing experts have discovered that adding a layer of protection between your structural roof decking (the layer of wood that covers your rafters) and your roof shingles helps create a better-looking and longer-lasting roof. In fact, there are a number of important ways roofing felt not only protects your roof, but also your residence. For example:
No matter what type of asphalt shingle you are using, whether traditional or more robust styles (such as under premium designer, architectural and performance shingles) it is recommended to use a underlayment.
For many years, roofers used asphalt saturated felt under roofing shingles, and it was historically available in two standard weights: 15 pounds per hundred square feet and 30 pounds per hundred square feet. The 15 pound was for lighter weight projects, while the 30 offered more heavy duty protection. Today, the actual weights have changed, but the categories have stuck.
|No. 15||No. 30|
|Use a single layer of this thinner product for lighter duty projects, and steeper pitched roofs (which allow water to run off easier).||Use one or two layers of this thicker product for superior protection.|
|In some cases, a synthetic underlayment can be used as a suitable alternative.||Use especially under slate or tile shingles.|
|This is the minimum weight many professional associations recommend, and many building codes require.||Consider for slopes between 2:12 and 4:12 which are more prone to wind-driven rain.|
Roofing felt is sold in rolls and the product label will give information on the area contained. Divide the area of your roof by the area of the felt rolls you plan to use, and allow an approximate 10% extra for waste.
No matter what residential roofing project you’re undertaking, in the words of the Canadian Asphalt Shingle Manufacturer’s Association, using a felt or synthetic underlayment just makes “good roofing sense.”
In general, you may use nails or staples to secure the roofing felt to the decking. However, you should check with your product’s application instructions to see if the manufacturer recommends or requires a certain kind of fastener. In addition, the manufacturer’s instructions should indicate which side of the roofing felt goes down if that matters for the specific product.
Roofing felt products should not be glued to the decking in lieu of using nails or screws. However, on low-slope roof applications from 2:12 to 4:12, roofers may use adhesive on felt. If you have a low-slope roof, your roofer may need to use roofing felt adhesive to bond together multiple layers of roofing felt or to secure sections that overlap. The specific manufacturer’s instructions are the best source of knowledge on whether any individual product should be glued down and in which circumstances.
No, you should not install shingles over roofing felt that has collected frost or snow. Roofing underlayment may be left uncovered for short periods, but it still needs to be completely dry before you can add shingles on top.
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