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Underlayment for Metal Roofing: Synthetic vs. Felt Paper – Continuing Education Course

Credit: 0.25 LU/HSW (AIA) or 0.25 CEH (IIBEC)

Learning Objective: Upon successful completion of this learning program, you will come to appreciate the differences between synthetic and felt paper underlayment by learning of the advantages and disadvantages of each building material.

Please Note: After watching the video and reading the text, click the button at the bottom of this page to take the quiz.

For metal roofing, there are two main options for underlayment: Synthetic or felt paper.

Both are options to use for metal roofing, but there are some distinct differences to keep in mind. By the end of this 15-minute course, you’ll learn:

  • What synthetic underlayment is
  • The advantages and disadvantages of synthetic
  • What felt underlayment is
  • The advantages and disadvantages of felt

Synthetic Underlayment 

What is Synthetic Underlayment?

Synthetic underlayment is a roofing accessory created by weaving or spinning together polypropylene or polyethylene and a polymer to form a protective barrier to put between the roofing material and the roof deck. Synthetic underlayment is a newer innovation that is taking over the metal roofing industry as a better, more advanced replacement for felt underlayment. Most synthetic underlayments are mechanically attached with roofing caps, while others have a glue backing.

Synthetic Underlayment Advantages

LONGEVITY – Synthetic underlayment is going to last longer when compared to felt paper underlayment. In fact, most manufacturers guarantee that it’ll last as long as a metal roofing system (40+ years), if not longer. Longevity also applies to how long the bare underlayment can be exposed to the sun during the installation. The number of days or months varies by supplier, but some synthetics can last up to an entire year of UV exposure without drying out. This can be very helpful for projects where the installation is temporarily halted or where there are harsher weathering elements. The longer exposure ratings are often critical after catastrophic events like hurricanes and enable roofs to be dried-in and weathertight while waiting to have the main roof covering installed.

MORE DURABLE – One of the biggest complaints regarding felt underlayment is that it tears with relative ease. Plus, felt paper has been known to fly off the roof and into neighboring properties during the installation before it’s attached to the roof deck. Synthetic underlayment is engineered with thermoplastic polymers that are significantly stronger to resist tearing, moving, or other damage that could occur during installation.

HIGH-TEMPERATURE TOLERANCE – Synthetic underlayment makes a great choice in regions with higher temperatures. For example, some products can withstand temperatures of up to 260 degrees (Fahrenheit), which makes it a good choice for buildings in desert, tropical, or other high-heat climates.

RETAIN LESS HEAT – Most synthetic underlayments come in a mid-to-lighter gray color, which means the material retains less heat from the sun and doesn’t get as hot to the touch. This is especially helpful for the contractors handling and installing the underlayment.

COMES IN LARGER ROLLS – Synthetic underlayment rolls tend to have more material and square foot coverage per roll because it’s often longer; meaning it’s a better “bang for your buck.” For reference:

  • Peel and stick synthetic underlayment tends to come in rolls of 2 to 5 roofing squares (or 200 to 500 square feet per roll)
  • Mechanically attached synthetic underlayment tends to come in rolls of 10 roofing squares (or 1,000 square feet per roll)
  • Felt paper tends to come in rolls of 2 to 4 roofing squares (or 200 to 400 square feet per roll)

LIGHT-WEIGHT – Synthetic underlayment averages around 2 to 4 pounds per roofing square, compared to asphalt felt paper of about 15 to 30 pounds per square. Not only does this reduce the stress inflicted on the building’s frame/structure, but it also makes it easier to haul up onto the roof.

WARRANTIED – Most reputable synthetic underlayment manufacturers offer warranties on their products based on testing and industry standards. Some superior synthetic products even come with as high as a 50-year limited warranty.

WALKABILITY – One of the biggest concerns is having a good, slip-resistant grip while walking and working up on a roof. Luckily, many synthetic products have a slip-resistant surface built into the material.

OFTEN RECYCLABLE – One of the major perks to synthetic underlayments is that polyethylene and propylene are both recyclable materials. So, when it comes time for the underlayment to be disposed of or if there are unusable leftover scraps, it can be recycled appropriately.

ADHESIVE BACKING – Most synthetic underlayments offer their product in a peal and stick / ice and water shield version. This is a great option or roofs with lower slopes or ones in harsh environments, and is the most common choice for specified commercial projects.

FIRE RESISTANCE – Many synthetics have a Class-A fire rating, meaning that when used in conjunction with a Class-A fire rated roof assembly, it offers the highest resistance to combustion or catching on fire.

WRINKLE FREE – It’s crucial for the underlayment to lay flat on the roof deck so the metal panels aren’t exposed to any stresses that could cause the development of oil canning. Most synthetic underlayments do not wrinkle when exposed to moisture, sunlight, or other elements.

Synthetic Underlayment Disadvantages

MORE EXPENSIVE – The most significant disadvantage to synthetic underlayment is that it usually comes at a higher price point. Depending on the manufacturer or where it’s purchased, synthetics can be anywhere from two to five times more expensive than felt paper. Since it’s one of the best performing materials to use as underlayment and the fact that it’s a newer invention, consumers get what they pay for. However, synthetic underlayment comes in larger rolls, so you might have to purchase significantly more rolls of felt paper to cover the roofing structure, which may end up evening out the price difference in some cases.

LOW PERMEABILITY RATING – While it may be a good thing for a roof underlayment to have a low permeability rating, meaning that less water and air can pass through it, it also can cause issues if not installed correctly. With synthetic underlayment, moisture from within the building cannot escape from the inside outwards, which means that proper attic and building ventilation is imperative when installing synthetics. Most peel and stick underlayments have less than a 0.5 permeability rating, which makes it a better barrier product than mechanically attached synthetic underlayment (which has a higher perm rating). Be sure to check your products specific technical information.

LOWER SLOPES – For low-slope standing seam metal roof applications, strictly peel and stick underlayment is a better choice.

INSTALLATION TEMPERATURE – Peel and stick underlayments have a minimum temperature that the surface can be for the product to be properly installed. Most products require you to use a primer when installing the product in temperatures below 40 degrees. Be sure to check the specific product information. 

ROOF SUBSTRATE – Not all peel and stick underlayments can be adhered to every roof substrate. Again, check with the recommendations on the product you’re using.

Asphalt Felt/Tar Paper Underlayment

What is Asphalt Felt/Tar Paper Underlayment?

Asphalt felt underlayment is known by many different names, including felt paper, roofing tar paper, felt underlayment, asphalt-soaked felt, and so on. Felt underlayment is an all-over roofing accessory made of a natural (wood cellulose, natural fibers, etc.) or synthetic (polyester or fiberglass) base that is then saturated in asphalt to make it waterproof. It is then laid down and attached to the roof deck below the roofing material.

Felt paper was widely used in the mid-to-late 1900s as the preferred underlayment choice but is slowly phasing out due to new and improved product developments (synthetics). However, it is still used today in steep slope and shingle applications.

Felt Paper Underlayment Advantages

CHEAPER – The most notable advantage to installing felt paper is that one roll of felt paper is quite a bit less expensive than one roll of synthetic underlayment. If you’re operating on a strict budget, felt underlayment is the best choice. However, you might have to purchase numerous rolls of felt paper for one project, as opposed to just one roll of synthetic for the same project. Also, it will not offer the same warranty that a synthetic underlayment does.

WATER RESISTANT – Asphalt is a water-resistant substance, which does help keep water from penetrating the underlayment’s surface if water leaks through the roofing structure. Felt paper does have a higher permeability than synthetics, which means air and water are able to pass through the material easier.

MOST COMMONLY USED – Since felt paper was used in most roofing projects in the past, it’s cheaper than synthetic, and works well with shingle roofing, there is still an existing need for it. That’s why most home improvement stores sell felt paper, which makes it available nearly anywhere.

Felt Paper Underlayment Disadvantages

DOESN’T LAST AS LONG – The biggest issue with felt paper underlayment is that it doesn’t last as long as a synthetic underlayment. Since this material is most popular with shingle roofing, most felt paper is warranted for the lifetime of a shingle roof (usually 15 to 20 years). By nature, felt underlayment will break down and weaken over time, especially if exposed to high heat, sun exposure, or frequent temperate fluctuations.

HIGH-TEMPERATURE PROBLEMS – Another problem that could arise with felt underlayment is that it doesn’t respond well in high heat environments. Asphalt is a semi-solid form of petroleum and an oil-based product, so it is more susceptible to high temperatures breaking down the material, drying out the oil, cracking, and ultimately degrading prematurely.

LESS EXPOSURE TIME – Felt underlayment should not be exposed to sunlight for an extended period. How long it can be is dependent on how much asphalt/oil is in it and how quickly the material will dry out and not perform properly. Some roofing felt can bubble, buckle, and wrinkle within just 24 hours of UV exposure.

MIGHT REQUIRE A SLIP SHEET – A slip sheet is a layer of building paper that is installed beneath the roofing material to further protect the felt underlayment from damage or sticking that can occur when in contact with metal panels. The addition of a slip sheet will increase the cost.

FIRE CONCERNS – Asphalt-soaked felt paper comes with its own set of precautions regarding fires. Asphalt is made from petroleum, which is a highly flammable substance by itself. During the production of felt paper, UV degraded mineral granules are added on top of the felt to make it more fire-resistant, but if the felt is damaged and a fire source encounters the asphalt, it could potentially combust. Most felt underlayment is considered “slow-burning,” so if a fire does occur, it won’t catch on fire all at one time.

HEAVIER – Felt underlayment usually comes in two different sizes, 15-pound (#15) or 30-pound (#30). These numbers represent how much the underlayment will weigh per roofing square (100 square feet). One roll of felt underlayment weighs similar to one roll of synthetic underlayment, but there’s at least twice as much synthetic underlayment in one roll, meaning the installers will have to haul up more felt paper rolls.

LESS RECYCLABLITY – Much like asphalt shingles, you technically can recycle felt paper underlayment, but many people don’t know that you can or don’t take the necessary steps to do so. Unfortunately, a lot of the leftover scraps or tear-off felt paper are just thrown away and end up in a landfill.

Final Thoughts

For help deciding between these two options, ask yourself these questions:

  • How long do I expect the roof underlayment to last? How about the actual roof?
  • Does my clients’ budget allow for a higher end underlayment?
  • Will my clients’ underlayment be exposed to long periods of sun or heat?
  • Is fire resistance or having a Class-A fire rating important in my region?
  • What are the engineering requirements I need to adhere to for my project?
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