Tile Roofing Underalyment

Irene writes:  Our house in Los Angeles is 25 years old. One roofer recommended removing the composite roofing tiles and replacing the old underlayment with 90-pound paper. Then they will replace the roofing tiles. Another roofer recommended the same approach, but with two layers of 30-pound felt as the underlayment. Please explain the differences between paper and felt and the differences between using the 90-pound paper and the 30-pound felt? Is one better than the other?

Steve Bliss, of BuildingAdvisor.com, responds: With a tile roof, the underlayment is critical as it is the primary water barrier. The tiles are mostly for aesthetics and to help protect the underlayment. Since the tiles, themselves, should last 50 years or more, the underlayment should be of similar quality.

The terms “paper” and “felt” are used interchangeably and refer to organic felt paper, which is impregnated with asphalt and fillers to make roofing shingles and underlayments. Some products use fiberglass mesh rather than felt as the substrate.

The term “90-pound paper” usually refers to gravel-coated roll roofing, which is a low-quality product that rarely lasts more than a few years when exposed to the elements. It weighs about 90 pounds per square (100 sq. ft.) because of the heavy gravel surface. I would not recommend it for anything other than a utility shed as the quality today is very poor.

Two layers of #30 asphalt-impregnated felt is the minimum you should use under tile and should last 20 to 30 years. For a little more money, you can upgrade to a double layer of 35 or 40-pound rubber-modified (SBS) felt, such as LayfastSBS from MB Technology, which can increase the lifespan of the underlayment by 50 to 100%. Torch-applied membranes of organic or modified felt are commonly used as the second layer (the “capstock”) in wet areas like Florida, but are probably overkill in an arid region like southern California.

ASTM-rated roofing felt must comply with D 4869 or the older D 226 standard.

ASTM No. 30 roofing felt is far better than unrated “30 pound” felt, which may weigh as little as 15-20 pounds. Courtesy of Tamko.

There is a distinction between standard “30-pound”  felt, which actually weighs 15 to 20 pounds per square and ASTM-rated #30  felt which weighs closer to 30 pounds and is of higher quality. ASTM-rated felt (under ASTM D4869 or the older D226) is manufactured to strict specifications, while standard 30-pound felt varies in quality from one manufacturer to another. Both are made with an organic felt mat impregnated with asphalt and various fillers. Some premium felt underlayments include fiberglass reinforcement.

Many roofers now prefer synthetic underlayments, which are lighter, more tear-resistant, and easier to work with than felt paper. There are wide variety of these with different warranties and projected lifespans. As with any new product, however, how long they will really last is unknown and manufacturing standards have not yet been established.

Whatever underlayment is used, the installation is as important as the specific material. Most important are proper sealing and flashing at all penetrations for chimneys, skylights, pipes and roof vents. Valleys are also a problem area that need special treatment. I prefer an open valley design using a self-sealing flashing membrane (such as Grace Ice & Water Shield) as the underlayment, covered with a heavy-duty metal flashing. Avoid aluminum flashing as it is will fail long before your tiles.

Q&A Index


  1. Clay vs. Concrete Tile, Synthetic Underlayment?

    I have a Mediterranean-style custom home and currently have authentic one-piece clay S-tiles. I had fumigation done because of termites and the roof was extensively damaged. I am not sure should if I should go with concrete tiles or the same clay tiles. Also what kind of double underlayment is recommended? I have been recommended Fontana G40 for underlayment or synthetic Sharkskin. I have been reading about concrete tiles that their color will fade over time, are heavy, and are 30% less expensive than clay tiles. The 30% extra cost is not an is an issue. I just need the right advise because overall it is an expensive project. I would appreciate your response at your earliest.

    • buildingadvisor says:

      You are smart to focus on the underlayment as that is the true waterproofing layer on a tile roof. The industry-standard minimum underlayment for a tile roof with a slope of 4:12 or greater is a single layer of ASTM-rated #30 felt paper. Cheaper felt paper without an ASTM rating, widely sold in home centers, is much thinner and should not be used. (The ASTM standards are D226 Type II or D4869 Type IV for organic felt; D6757 or D4601Type II for fiberglass products.)

      A double layer of underlayment provides twice the protection and longevity as a single layer and is, therefore, highly recommended. Many tile roofing systems have outlived their felt underlayments, requiring expensive repairs. No sense in putting a 10 year underlayment on a 50-year roof! The specific underlayment used should be suitable for the climate. Freezing weather, extreme heat, and windblown rain require appropriate materials and installation.

      Nowadays there are many more options than asphalt felt – primarily rubber-modified asphalt (SBS) products and synthetics, such as the Sharkskin and Titanium lines of products.

      Adding synthetic rubber (SBS) to asphalt generally improves its flexibility, its performance in cold and hot weather, and its overall durability and longevity. Modified asphalt has been widely used in self-adhesive flashing membranes such as Grace Ice & Water Shield and is used on many premium roofing underlayment such as Layfast SBS TU35/43. I am not familiar with Fontana G40, but according to their website, it is a heavy-weight modified asphalt underlayment that provides a 20-year warranty for a two-layer installation. It also meets ASTM Standard D6757 for roofing felt with a fiberglass mat or reinforcement.

      There are also a number of newer synthetics on the market made from a variety of materials. Some, like Sharkskin, carry very long warranties. But like most roofing warranties, they don’t really cover much. For example, Sharkskin’s 50-year-warranty and Titanium’s lifetime warranty cover only the original cost of the material, not its installation. This might cover 10% of the actual cost of repairs.

      Some of the synthetic underlayments provide a Class A fire rating, improved walkability on steep slopes, and easier installation due to their light weight and resistance to tearing. The easy installation makes them popular with many roofing contractors. How well they will last over the long haul is unknown, however, and manufacturing standards have not yet been established.

      Personally, I would choose two layers of G40 over Sharkskin as the material has been around longer and has a proven track record.

      Sounds like you have done your homework about clay vs. concrete tile. As you point out, concrete tiles are less expensive and lighter, but will fade somewhat over time. Also because they are more porous, they can get stained. Clay tiles will keep their color pretty much forever whether natural or with a ceramic finish. If you are in a freezing climate, however, concrete tiles are a better choice as they are less prone to cracking.

      Good quality concrete tiles should last 30 to 50 years, while clay tiles may still be going strong in 100 years, well after you and I are long gone. Best of luck with your new roof!

  2. Composite Roofing Tiles

    Are composite roofing tiles very popular in California? I love the look of slate tiles and I thought that these were common in California. How durable are they?

    • buildingadvisor says:

      As far as I know, traditional clay tiles are still the most common (and most expensive) roofing material on the West Coast, followed by concrete tiles, and then composite tiles.

      Good quality concrete tiles should last 30-50 years. Clay tiles may last much longer. Composite tiles vary in their composition and lifespan.

      Composite tiles are made from a variety of materials so it is hard to generalize about quality and longevity. They are lighter weight than concrete or clay and less costly. They use various combinations of fiber-cement, plastics, polymers, synthetic rubber, and various fillers.

      If you go with composite, I would look for a product and company with a good track record and products that have been in the field, in your climate, for 10 to 20 years. Check the product online for consumer complaints, lawsuits, etc. A lot of composite roofing and decking products have not lived up to expectations and ended up being pulled from the market.

      Look for a strong warranty, which shows some level of confidence in the product. But remember that most roofing warranties are for materials only, are prorated by years of use, and require that the product be installed exactly as the manufacturer specified. And the company has to still be around when you make the claim. So, in reality, roofing product warranties are usually not worth very much.

  3. SBS-modified 40 or 90 lb felt is much higher quality than 30 lb paper. Two layers of unmodified 30 is cheaper than 1 layer of modified 90. I was disappointed to read your answer. I disagree with many specifics you gave, including the lack of the term “modified” and the actual weights vs names. Please refer to Malarkey Building Products for accurate information.

    • buildingadvisor says:

      I agree that SBS-modified felt is a much better product than unmodified paper and stated that above. Different companies offer different formulations and different weights, so I’m not sure what you find incorrect. For example, MBTechnology offers 37-, 43- and 75-pound modified felt underlayment. Other companies offer 40 and 90 pound. Since the nominal weight with any of these products does not always match the actual weight, so it’s always a good idea to check the product specifications before purchasing.

      • Brad -- A Roofer in Findlay says:

        I thought you answered the question pretty accurately. I too have moved away from the 90 pound paper to go with a different type of material like you mentioned. Thanks. Brad, Findlay Roofers, Findlay, OH



Please enter correct number before posting – to prove you're a person. * Time limit is exhausted. Please reload CAPTCHA.

Home | About Us | Contact Us | Privacy Policy
© 2016 BuildingAdvisor ®; All rights reserved.
Website Designed By Multi Pitch Media