Roofing Underlayment Water Barrier

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

Most people think of shingles and gutters when they think of roofing. Many are unaware that there is another layer of protection just between the shingle layer and your roof deck that plays a key part in preventing moisture damage to your property. It’s called roofing underlayment and it is a very key component in your roofing system. Let’s learn more about this important part of your roof.

What Is Roofing Underlayment and How Does It Work?

Roofing underlayment is the layer of material that sits underneath the shingle layer of your roof. It’s a secondary layer built directly on the roof deck that protects against rain, snow, and wind, and is engineered to ensure no moisture reaches the wooden layer of your roof.

Types of Roofing Underlayment

There are two main types of roofing underlayment, Felt and Synthetic. There are advantages and disadvantages to each and the type you choose may depend on your geographical location, roofing materials, roof design, budget, and your roofing contractor.

Felt Roofing Underlayment

Felt roofing underlayment is one of the oldest types of roofing underlayment. It’s created by saturating paper or fiberglass mat with asphalt.

Felt roofing underlayment is typically available in two types: No.15 felt and No. 30 felt. Compared to No. 15 felt, No. 30 felt is typically thicker, stronger, and may be less prone to tearing or ripping off during installation or weather events.

The main advantage of using felt roofing underlayment is cost. Felt underlayment tends to cost less compared to synthetic underlayment, which is why it’s often the go-to for budget-conscious homeowners.

There are several disadvantages to using felt underlayment on a roof. One disadvantage of traditional felt roofing underlayment is it generally can’t be left exposed for more than a few hours. The material may dry out or leach oils in the heat. This would impact the felt’s ability to protect against moisture.

Other drawbacks of felt underlayment include:

  • In severe gusts and during the installation process, it is prone to tearing.
  • When the mat is exposed to moisture, it absorbs it and wrinkles the felt, making it difficult for the shingles to rest flat. To offer the best protection, shingles should be put in as soon as feasible after the felt roofing underlayment is installed.
  • Felt underlayment is also heavier, making it more difficult for roofing professionals to haul rolls up a ladder and onto a roof.
  • It also has a slick surface, which can make installation more challenging at times.
  • Because of the weight, there is less material per roll. Instead of a single circuit with no laps, this creates more potential seams.

Synthetic Roofing Underlayment

Today, people are opting for synthetic roofing underlayment for improved water resistance and protection from the weather. Synthetic underlayments are typically made of long-lasting polymers, which enhance strength and durability. This sort of underlayment is usually moisture-resistant, and when laid properly, it provides superior weather protection than felt.

Because synthetic roofing underlayment materials are not standardized, different producers may manufacture their products differently, resulting in varying levels of performance. Make sure you do your homework and consult with a reputable contractor who can assist you in choosing the best roofing materials for your property.


Installing synthetic roof underlayment rather than felt has four major advantages. Synthetic roofing underlayment has the following advantages over felt:

  • Tough
  • Fast to install
  • Safe
  • Repels water
  • Synthetic underlayment has a tough and durable construction with an extremely high tear strength compared to felt.
  • Synthetic roof underlayment is extremely durable. It typically doesn’t tear and is suitable for extended UV and moisture exposure in some cases, which is especially helpful if there’s a bit of lead time before your roof covering is installed.

Synthetic underlayment can also withstand boot activity, which is vital when your roofing contractor is walking around on it while installing it. 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 up to four times lighter than felt in some cases.

Fast to install – 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 time and maybe speeding up the job. For example, three rolls of synthetic underlayment may be required to cover a standard 2700 square-foot home, but 14 rolls of No.30 felt may be required to cover the same space.

Safe – The surface of many synthetic roofing underlayments, including those offered by Owens Corning, provides a range of slip-resistant surfaces for better walkability. It’s frequently well-marked with overlap guides and indicators of where fasteners should go, which helps with consistency and precision during installation.

Moisture Resistant — Synthetic roofing underlayments are designed to reject water, whereas felt goods absorb it. This is critical for homeowners concerned about moisture intrusion, particularly if the underlayment will be exposed for an extended length of time. Synthetic underlayment, unlike felt, is often resistant to mold growth because it is constructed of plastic.


Although many synthetics are reasonably priced, the biggest disadvantage of synthetic roofing underlayment is its expense as compared to felt. However, making an initial investment in higher-quality roofing materials may save you money in the long run. You can’t put a premium on the peace of mind that comes with knowing your roof is well-protected against moisture.

The Right Underlayment for Your Roof

There are numerous elements to consider when choosing an underlayment for a reroofing project or a new house construction project. Synthetic roofing underlayment has a number of advantages over felt and may be a valuable investment in protecting your roof and home from water and moisture intrusion.

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