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Home » Synthetic vs. Felt Roofing Underlayment: What’s the Difference?

Synthetic vs. Felt Roofing Underlayment: What’s the Difference?

    Most people think of shingles or metal roofing when they think of roofs. However, some people may need to realize that there is another layer of protection just on top of the roof deck and beneath the roof covering that plays an integral part in protecting your property from moisture damage. It’s known as roofing underlayment.

    Learn more about this essential structural component of your roof.

    What Is Roofing Underlayment?

    Roofing underlayment is the material that sits between the shingles and the roof sheathing, or roof deck, which is usually plywood or OSB. It is erected directly on the roof deck and provides an additional layer of protection from the elements, such as rain, snow, and wind.

    Types of Roofing Underlayment

    Roofing underlayment is classified into two types:

    • Felt
    • Synthetic

    Each product has advantages and disadvantages, and the type you choose may be determined by your geographical location, roofing materials used, roof design, price, and what your roofing contractor recommends.

    Felt Roofing Underlayment

    One of the oldest types of roofing underlayment is felt roofing underlayment. It is made by soaking paper or fiberglass mats in asphalt.

    There are two varieties of felt roofing underlayment: No.15 felt, and No.30 felt. No. 30 felt is often thicker, stronger, and less prone to tearing or ripping off during installation or weather events than No. 15 felt.


    The primary benefit of employing felt roofing underlayment is affordability. Felt underlayment is less expensive than synthetic underlayment, making it a popular choice among cost-conscious homeowners.


    Using felt underlayment on a roof has various drawbacks. Traditional felt roofing underlayment has the problem of not being able to be exposed for more than a few hours. In the heat, the material may dry out or leach oils. This would affect the felt’s capacity to repel moisture.

    Felt underlayment also has the following disadvantages:

    • Prone to tearing in high winds and during installation stress.
    • If the mat is exposed to moisture, it might absorb water and wrinkle the felt, making it difficult for the shingles to lay flat. Therefore, shingles should be put in to offer the best protection as soon as the felt roofing underlayment is installed.
    • Felt underlayment is also heavier, making it more difficult for roofing professionals to bring rolls of it up a ladder and onto a roof.
    • It also has a slick surface, which might sometimes make installation more challenging.
    • The weight also means that there is less material per roll. Instead of a single track with no laps, this creates more potential seams.

    Felt Roofing Underlayment and Warranties

    If you use felt underlayment, you may not be covered by the manufacturer’s guarantee, which may need synthetic underlayment.

    Synthetic Roofing Underlayment

    Many roofers opt for synthetic roofing underlayment for increased water resistance and protection from the weather. These items are typically made of durable polymers, which enhance strength and lifespan. In addition, this underlayment is often moisture-resistant, and when properly put, it provides superior weather protection than felt.

    Because synthetic roofing underlayment materials are not specified, different producers may make their products differently, resulting in varying performance levels. Therefore, do your homework and consult a reputable contractor who can assist you in picking the best roofing materials to protect your home.


    There are four significant advantages of using synthetic roof underlayment instead of felt. First, synthetic roofing underlayment is superior to felt in the following ways:

    • Tough
    • Fast to install
    • Safe
    • Repels water

    Unlike felt, synthetic underlayment has a robust and resilient construction with an incredibly high tear strength.
    Synthetic roof underlayment is highly long-lasting. It usually does not tear and, in certain situations, is acceptable for long UV and moisture exposure, which is very useful if there is some lead time before your roof covering is put in.

    Synthetic underlayment is also resistant to boot activity, which is vital when your roofing contractor walks around on its surface during installation. We term this “usage after abuse” at Owens Corning Roofing because the product may still work as designed even after the trauma it receives during installation.

    Synthetic roofing underlayment also tends to be:

    • Lighter* – In some situations, up to four times lighter.
    • Fast installation: Because there is more material per roll than felt (synthetic roofing underlayment comes in broader and longer rolls), your roofers will make fewer trips up the ladder, saving them time and speeding up the work. For example, a 2700-square-foot home may require three rolls of synthetic underlayment to cover the same area as 14 rolls of No.30 felt.
    • Safe – The surface of many synthetic roofing underlayments, including those produced by Owens Corning, provides a range of slip-resistant surfaces for better walkability. It’s also frequently well-marked with overlap guides and fastener placement indicators, which helps to increase uniformity and precision during installation.
    • Moisture-resistant – Unlike felt goods, synthetic roofing underlayments are designed to repel water. This is critical for homeowners concerned about moisture intrusion, especially if the underlayment will be exposed for an extended time.
    • Because it is constructed of plastic, synthetic underlayment is usually resistant to mold growth, which gives it a significant benefit over felt.


    Many synthetics are economically priced; however, the expense is the most significant disadvantage of synthetic roofing underlayment compared to felt. However, the initial investment in higher-quality roofing materials may save you money in the long run. You can’t put a premium on the piece of mind that comes from knowing your roof is adequately protected from moisture.

    The Right Underlayment for Your Roof

    When choosing an underlayment for a reroofing job or new home building, there are numerous aspects to consider. The advantages of synthetic roofing underlayment over felt are multiple, and it may be a wise investment to protect your roof and home from water and moisture infiltration risks.

    Learn more: When Do You Need Immediate Roof Repair?