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Synthetic Vs. Felt Roofing Underlayment: Pros & Cons

Aug 25

Most people associate roofs with metal roofing or shingles. Many people don't know that there is another layer of protection on top of your roof deck and underneath the roof covering. This protects your home from moisture damage. It's called roofing underneath. Learn more about this crucial component of your roof's structure.

What is Roofing Underlayment?

The roofing underlayment, also known as roof sheathing or roof deck, is the layer between the shingles, roof sheathing, or roof deck. It is usually plywood or OSB. It is installed directly on the roof deck, providing a second layer of protection against the elements, including rain, snow, and wind.

Types of roofing underlayment

There are two types of roofing underlayment.

  • Felt
  • Synthetic

Every product has its advantages and disadvantages. The type of product you choose will depend on where you live, the roofing materials used, your budget, and any suggestions from your contractor.

Felt Roofing Underlayment

Felt roofing is the oldest type of roofing underlayment. It is made of saturating paper with asphalt or fibreglass mat. There are two felt roofing underlayments: No.15 and No. 30. 30 feet. Comparable to No. 15 felt, No. 30 felt is thicker and more robust than 15 feet. It may also be less susceptible to being ripped off during installation.


The primary advantage of using felt roofing underneath payment is its cost. Felt roofing underlayment is often less expensive than synthetic, making it a popular choice for budget-conscious homeowners.


There are many disadvantages to using felt roofing underlayment. Traditional felt roofing underlayment couldn't be left out for more than a few hours. The heat can cause the material to dry out and leach oils. This could impact the felt's ability to protect against moisture.

Another drawback to felt underlayment is:

  • Prone to being torn in high winds or during the strain of installing.
  • When exposed to moisture, the felt can absorb water and create wrinkles, making it difficult for the shingles to lay flat. Shingles should permanently be installed as soon as possible after the felt roofing underlayment has been installed to ensure maximum protection.
  • Felt underneath-payment is heavier, making it more difficult for roofing contractors to drag rolls up a ladder to a roof.
  • The surface is slippery, making it sometimes more difficult to install.
  • Less material per roll is also a result of the weight. This results in more seams than a single course that has no laps.

Felt Roof Underlayment and Warranties

Felt underlayment may prevent you from receiving the manufacturer's warranty. This may lead to your need for synthetic underlayment.

Synthetic Roofing Underlayment

Many roofers use synthetic roofing underneath for enhanced water resistance and protection from the elements. These products are often made of durable polymers that provide strength and durability. This underlayment is usually water-resistant and, when installed correctly, offers more excellent protection against the elements than felt.

Different manufacturers make synthetic roofing underlayment materials differently. This means that they may have different performance levels. Talk to a trusted contractor to help you choose the best roofing material to protect your home.


Synthetic roof underlayment is better than felt for four reasons. Synthetic roofing underlayment has the following advantages over felt:

  • Tough
  • Installation is quick
  • Secure
  • Repels water

Synthetic underlayment is more rigid than felt and has exceptionally high tear strength.

The synthetic roof underneath is very durable. This is particularly useful if you have little time before installing your roof covering.

The synthetic underlayment is also resistant to boot traffic. This is especially important for roofing contractors walking on the surface while it is installed. Owens Corning Roofing calls this "use after abuse." 

The product can still perform as intended even after being subject to a lot of abuse during installation.

Also, synthetic roofing underlayment tends to be:

  • Lighter* - Some cases are up to four times lighter
  • Easy to install. Synthetic roofing underlayment is faster than felt. Because it has more material per roll (compared to felt), your roofers will need to make fewer trips up the ladder. 

This may save them time and help the job move faster. A typical 2700 sq. ft. home may require three rolls of synthetic underlayment to cover the same area as 14 rolls of No.30 felt.

  • Safe Synthetic underlayment can also be beneficial for worker safety. Many synthetic roofing underlayments, including Owens Corning's, have slip-resistant surfaces that allow for better walking. 

It is also marked with overlap guides and indicators indicating where fasteners should go, which helps ensure installation consistency and accuracy.

  • Moisture-resistant- Unlike felt roofing products that absorb water, synthetic roofing underneath lays are designed to repel water. This is especially important for homeowners who are concerned about moisture infiltration.

Synthetic underlayment is made from plastic and has a higher resistance to mold growth than felt.


Although synthetics can be competitively priced compared to felt, the main disadvantage of synthetic roofing underlayment over felt is its high cost. However, the upfront investment in better-quality roofing materials could help you save money later. 

It's hard to beat the comfort of knowing your roof is adequately protected from water.


When choosing the right type of underlayment for your roofing project, there are many things to consider. Woodbridge Roofing Company offers many benefits over felt and can be an excellent investment to protect your roof from moisture and water infiltration.