Should You Pay a Bit More for Synthetic Underlayment?


When you think about a roof, what comes to mind? The shingles and the gutter, right? Well, most people don’t know there is another layer of protection that is under the shingles and plays a pivotal role in the performance of your roof.

What is underlayment?

It is the second layer of protection under the shingles. It is installed directly to the deck with cap nails and it protects your home from water intrusion!

Types of Underlayment

  1. Synthetic
  2. Felt

These two products each have pros and cons. However, most of the time it is going to make more sense to install synthetic underlayment. There are many factors that can determine what is right for your roofing system. These include geographical area, roofing materials used, roof design, and budget.

Felt Underlayment

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

Felt roofing underlayment is available in two types: No.15 felt and No. 30 felt. Compared to No. 15 felt, No. 30 felt is typically thicker, and 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:

  • Prone to tearing in high winds and during the strain of installation. 
  • If exposed to moisture, the mat can absorb water and wrinkle the felt, making it harder for the shingles to lay flat. Therefore, shingles should be installed immediately after felt roofing underlayment is installed if possible to ensure optimal protection.
  • Felt underlayment also weighs more, which can make it harder for roofing contractors to drag rolls of it up a ladder and onto a roof.
  • It also has a slippery surface, which can sometimes make it more difficult to install. 
  • The weight also leads to less material per roll. This means more potential seams instead of a single course with no laps.

Felt Roofing Underlayment and Warranties

If felt underlayment is installed it may also prevent you from being protected under the manufacturer’s warranty, which may require synthetic underlayment.

Synthetic Underlayment

For enhanced water resistance and protection from the elements, we are choosing to go the route of synthetic underlayment. These products are usually made from long-lasting polymers, which provide added strength and longevity. This type of underlayment is typically moisture-resistant, and when it’s installed correctly, it offers better protection from the weather compared to felt.


There are four main advantages to installing synthetic roof underlayment rather than felt. 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 also stands up to boot traffic, which is important when your roofing contractor is walking around on its surface as it’s being installed.

Synthetic roofing underlayment also tends to be:

  • Lighter – Up to four times lighter in some cases
  • Fast to install – Because there is more material per roll compared to felt (synthetic roofing underlayment comes in wider and longer rolls), it results in fewer trips up the ladder for your roofers, saving them time and perhaps helping the job move along faster. For instance, a typical 2700-square-foot home might require three rolls of synthetic underlayment compared to 14 rolls of No.30 felt to cover the same area.
  • Safe – Synthetic underlayment is also advantageous for worker safety — the surface of many synthetic roofing underlayments, including those offered by Owens Corning, features a variety of slip-resistant surfaces for enhanced walkability. It’s also usually well-marked with overlap guides and indicators of where fasteners should be placed, helping to improve consistency and accuracy during installation.
  • Moisture-resistant – Where felt products tend to absorb water, synthetic roofing underlayments are built to repel water. This is important for homeowners concerned about moisture infiltration, especially if they plan to leave the underlayment exposed for a prolonged period of time.

Because it’s made of plastic, synthetic underlayment is typically resistant to mold growth, a definite advantage over felt.


Many synthetics are competitively priced, but when compared to felt, the main drawback of synthetic roofing underlayment is the cost. The upfront investment in higher quality roofing materials, however, could save you money down the road. You can’t put a price on the peace of mind knowing that your roof is sufficiently protected from moisture.

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    By Big Time Roof Team | May Wed, 2022| Roofing Advice