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Top 3 Synthetic Roof Underlayment Problems
Lauren Risotto

By: Lauren Risotto on September 1st, 2021

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Top 3 Synthetic Roof Underlayment Problems


The success of a roofing system depends on quality parts being installed and functioning correctly. By skipping over a step or using lower quality material, you are compromising the roof’s longevity. 

Roofing underlayment is one of the key components to ensuring a long lifespan for your roof. While there are several different types to choose from, synthetic underlayment is currently the most popular and preferred choice for underlayment in the roofing industry for its superior performance and longevity. 

At Western States Metal Roofing, we have been in the roofing industry for over 20 years manufacturing metal panels. We generally prefer the use of synthetic underlayment, as it is more durable and will last longer. However, that does not mean it does not come without issues.

In this article, we discuss the top 3 problems with synthetic roof underlayment and why it may not be the right fit for a project to help you decide if it’s right for you. First, let’s discuss why roof underlayment is necessary in the first place.

Is Roof Underlayment Necessary?

Underlayment is an important component to the overall roof system as it serves as an extra layer of protection for your roof.

Roof underlayment will:

  • Protect against moisture penetrating through the roof, resulting in leaks.
  • Add extra insulation to your home to keep it from being drafty.
  • Absorb noise from rain or other elements on the roof.
  • Ensure that the roofing material laid on it looks uniform and even .
  • Keep your roofing warranty valid if applicable.

Sharkskin Underlayment Comparison Guide

What Is Synthetic Roof Underlayment?

Synthetic underlayment is a roofing accessory composed of laminated polypropylene or polyethylene plastic that goes over the roof deck and serves as an extra layer of protection from the elements for a roofing system. It has excellent durability as the material does not absorb moisture. This makes it the right material to choose when drying in a roof.

How Long Can Synthetic Roof Underlayment Be Exposed?

Within a certain timeframe, synthetic underlayment can be left exposed to the elements and not be damaged. The time frame depends on the specific manufacturer of the underlayment as each is different. We recommend, Sharkskin roofing underlayment which can be exposed for 6 months to 12 months. 

What Are The Problems With Synthetic Underlayment?

  1. Synthetic roofing underlayment is more expensive than felt underlayment.
  2. Mil thickness versus weight
  3. Some building codes do not allow synthetic underlayment to be used.


Synthetic Underlayment Cost vs. Felt Underlayment Cost

Synthetic is a higher quality product but also comes with a higher cost. 


While non adhesive synthetic underlayment costs more than twice as much, there is long term value with synthetic material that you do not get with felt underlayment. It just doesn’t make sense to install an expensive roofing system and a cheap underlayment that will not last for the lifetime of the roof. On an average size roof, the difference will only be a few hundred dollars. It’s not that much additional money when you look at the overall cost of the metal roof.

Felt vs. Synthetic Roof Underlayment


Synthetic Underlayment Weight Vs. Mil Thickness

Non adhesive synthetic roofing underlayment isn't created equally. There are lots of high quality synthetic roof underlayment brands , but there are also some low quality brands. They try to trick you with a lousy product and fancy marketing.  

The weight of the synthetic underlayment is a great indication of the quality of the underlayment. The membrane layer is where you get your water proofing and abrasion resistance. In an effort to save money, there are brands of synthetic underlayment that boast about their mil thickness. This can be misleading though as some of these underlayment have a fuzzy top layer for walkability that’s a false indicator of the materials thickness. 

We suggest that you look at the weight of the underlayment instead of the mil thickness to ensure that you have the best synthetic roof underlayment.


Certain Building Code Prohibits Synthetic Roof Underlayment

Before making any decision regarding roof underlayment, make sure you or your roofing contractor refer to the building code. Building code will specify not only if underlayment is required, but also what underlayment should be used. It’s a rarity, but there are some instances where synthetic roof underlayment will be prohibited and felt underlayment will have to be used.


Is Synthetic Roof Underlayment Better Than Felt Underlayment?

Synthetic roof underlayment is the most preferred form of underlayment material for its strength and tear resistance. Synthetic was actually created to be a product that solved the problems that felt underlayment, or roofing felt, has. 

For years, felt was the go-to material to use for roofing underlayment. Only within the last two decades has synthetic underlayment hit the market as an improved product that would not have the problems with felt.


Problems With Felt Underlayment

  • Felt underlayment does not last as long as synthetic underlayment and will have to be replaced sooner.
  • Roofing felt is up to 4 times heavier than synthetic and harder to install which makes installation costs higher.
  • Felt cannot be left exposed to the elements and will have to be covered once installed.
  • Felt is not as safe to walk on for a roofer compared to synthetic which makes for a more hazardous installation.
  • Felt underlayment tears easier

Choosing The Right Type Of Synthetic Underlayment For Your Metal Roof

When you need a reliable underlayment that will last longer for your roof installation, you want to choose synthetic.

Synthetic underlayment is the right choice when you’re looking for: 

  • Easy installation. This material is lightweight and safer to walk on than felt, making for an easier installation.
  • Smoother appearance from laying flatter on the roof decking.
  • Resistance to mold and mildew.
  • Durability. Synthetic underlayment can withstand exposure to the elements for weeks or even months.

If you’ve decided on synthetic, there are different options to choose from. Continue on to learn about the different types of synthetic underlayment.


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About Lauren Risotto

Lauren is the Content Manager at Western States Metal Roofing and provides insight on content creation and product promotions.