Difference Between Synthetic And Felt Roofing Underlayment

Typically, most people associate roofs with metal roofing or shingles. However, many don’t know there is another layer of protection on top of your roof deck and underneath the roof covering. This detail protects your home from moisture damage. It’s called roofing underlayment.

Learn more about this aspect of your roof’s structure.

What’s a Roofing Underlayment?

The roofing underlayment (known as a roof deck or roof sheathing) is the layer between the roof deck, roof deck, or shingles. It is typically plywood or OSB. It is usually plywood or OSB and is attached directly to the roof deck. This aspect protects against the elements, including rain, snow, wind, and freezing temperatures.

Types and applications of roofing underlayment

There are two types:

  • Felt
  • Synthetic

Each product has its pros and cons. The type of underlayment you choose will depend upon where you live, what roofing materials you use, your budget, and any suggestions you receive from your contractor.

Felt Roofing Underlayment

Felt roofing is one of the oldest types of roofing underlayment. It is made from saturating papers with asphalt or fiberglass mat.

Two types of felt roofing underlayments are available: No.15 or No. 30 felt. Compared to No. 15 felt, No. 30 felt is thicker than 15 feet. It is also less likely to be ripped during installation.


The main advantage of using felt underlayment is its price. Felt underlayment can be cheaper than synthetic and is a popular choice among budget-conscious homeowners.


There are several disadvantages to using felt roofing underlayment. Traditional felt roofing underlayment could not be left out for more than a few hours. Heat can dry the material and cause it to leach oils. This could affect the felt’s ability to protect against moisture.

Another drawback to felt underlayment is:

  • Prone to being torn by high winds or during the strain of installation.
  • When exposed to moisture, the mat may absorb water and wrinkle felt, making it difficult for shingles to lay flat. To ensure maximum protection, You should always install shingles as soon as possible after installing the felt roofing underlayment.
  • Felt underlayment is heavier, making it more difficult for roofing contractors to drag the rolls up a ladder to a roof.
  • The surface is slippery, making it more challenging to install.
  • It has less material per roll, resulting in more seams than a single course with no laps.

Felt Roof Underlayment and Warranties

You may install felt underlayment to prevent you from receiving the manufacturer’s warranty. This point may lead to your need for synthetic underlayment.

Synthetic Roofing Sublayment

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 commonly water-resistant and, when installed correctly, offers more excellent protection against damage than felt.

Various manufacturers make synthetic roofing underlayment materials differently. This aspect means that they may have multiple performance levels. Contact your dependable contractor to help choose the best roofing material for your house.


Synthetic roof underlayment is better than felt. But, first, synthetic roofing underlayment has the following advantages over felt:

  • Tough
  • Installation is quick
  • Secure
  • Repels water

Synthetic roof underlayment is very durable. This attribute is handy if you have little time before installing your roof covering.

The synthetic underlayment is also resistant to boot traffic. This feature is crucial for roofing contractors walking on the surface while installing the underlayment.

Roofing calls this “use after abuse.” This underlayment can still perform as intended even after being subject to a lot of abuse during installation.

Additionally, synthetic roofing underlayment tends to be:

  • Lighter – Some cases are up to four times lighter.
  • Easy to install. Synthetic roofing underlayment is faster to install than felt. Because it has more material per roll, your roofers will need to make fewer trips up the ladder. This feature 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 favorable for worker safety. Many synthetic roofing underlayments, including Framingham Roofing MA, have slip-resistant surfaces that allow for better walking, which 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 underlayments are designed to repel water. This feature is especially essential for homeowners who are worried about moisture infiltration.

Synthetic underlayment is made from plastic and has a higher aversion 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, investing in better-quality roofing materials could help you save money later. In addition, it’s hard to beat the comfort of knowing your roof is adequately protected from water.

The proper underlayment for your roof

When choosing the proper type of underlayment for your roofing project, there are many things to consider. Synthetic roofing underlayment has many advantages over felt and can be an excellent investment to protect your roof from moisture and water infiltration.

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