Synthetic Roof Underlayment Vs Felt: The Ultimate Comparison

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There are several questions you may constantly ask yourself, whether you are a property owner or a contractor when it comes to metal roofing and you both somehow are engrossed in roofing underlayments, I guess so! 

In a world full of options between synthetic roof underlayment and felt, it is not easy to figure out which one outweighs the other. It is also challenging to get home a product that actually works as per your desire. You don’t wanna make a mistake even if not having been knowledgeable? It is possible if you make time to read this thoroughly. 

What Is Roof Underlayment?

Roof underlayment is the intersection between the roof sheathing and the shingles, or sometimes roof deck as well, which is commonly OSB (Oriented strand board). 

It’s normally installed on the deck of the roof and is famous for providing an extra layer of good protection from some factors inclusive of rain, snow, and from time to time, wind as well.

synthetic roof underlayment vs felt

How Many Types Of Roofing Underlayment Are There?

There are 2 primary types that you may have ever heard of: Felt and Synthetic.

Each type contains its benefits and drawbacks, and the kind you choose may be dependent on a range of factors such as your area, or roofing materials made, or roof design, sometimes related to budget and even what your contractor normally recommends.

Read more abour Gable Roof Vs Hip Roof Comparison: What Are The Differences?

Felt Roofing Underlayment

It is one of the most long-lasting types that humans have ever known. The process of saturating paper or sometimes fiberglass mat is known to be the primary way how it is produced, along with asphalt.

Typically, felt roofing underlayment comes in two different types: No.15 and No. 30. In comparison to No. 15, No. 30 is often far thicker, stronger, and might be less vulnerable to external factors during the installation process or some kinds of weather events also.

a. Advantages

The primary benefit of using felt is its price. Felt underlayment is inclined to cost just a fraction of the price than the other, which is why it’s usually the first option for money-conscious homeowners.

b. Disadvantages

There are some downsides to applying felt on a roof. One of those is that it often can’t be exposed heavily. The material may actually wear out or at times oils may be leached. This will then influence the felt’s capability to protect from moisture. 

Synthetic Roofing Underlayment

To enhance water resistance as well as safeguard from external factors, many people are leaning towards synthetic roofing underlayment. These are usually made of durable polymers, providing added resistance and durability as well. 

This type is commonly moisture-proof, and as it’s installed in the right way, it provides users with better weather protection than that of felt

Distinguished manufacturers may individually produce their products because materials are not uniformed and hence may have some levels of performance unrelated. Make sure to do conduct research first yourself and then consult a professional contractor to seek guides in picking the right materials to keep your home from being adversely impacted.`

a. Advantages

There are four primary advantages to applying synthetic type instead of felt roofing underlayment:

– Tough:

This type is long-lasting. It generally doesn’t wear out soon and is compatible with extended UV as well as the exposure to moisture in several cases, which is extremely helpful if you need more time before any roof covering is placed.

Synthetic underlayment is also resistant to enhance traffic, which is crucial as a contractor is checking on its surface by walking around since it is being installed. 

Faster to install than the other:

As there is more material added per roll rather than felt (synthetic underlayment normally features rolls with wider and longer features), it leads to the decrease of trips up the ladder, which roofers them time and probably helps the job move along in a flash. 

For example, a 2700 square-foot home might need at least three rolls of synthetic type in comparison to 14 rolls of No.30 felt to fully keep the same hidden area as such.

– Safe:

Synthetic type is also very beneficial for the safety of workers – the surface of them has various slip-proof surfaces for better walkability. It’s also well-marked with overlap guides as well as indicators of where fasteners should be placed during the installation process as meant.


Where felt products are likely to benefit you further in terms of water absorption, synthetic roofing types are basically meant to force back water as a replacement. This is essential for homeowners worried about the infiltration of moisture, particularly if they intend to let the underlayment be completely exposed for a long period.

b. Disadvantages

Loads of synthetics are priced, but as you see it together with felt, the main downside of synthetic underlayment is obviously the price. The investment in better materials, nevertheless, could help you save a great deal of money in the future. 


Based on the comparison between synthetic roof underlayment and felt, you can clearly see the difference between these two. Of course, with the benefits outweighing the drawbacks, the latter one is still pale in comparison with the former. However, choosing the best one for your house still depends on what you prefer!

If you need a video to be sure about the process, here you are:


1. Are felt products waterproof?

Felt products are not considered waterproof, they are actually water-resistant, and cannot be exposed to UV rays and rain for more than 48 hours based on manufacturer specifications because the oil embedded in the felt will certainly break down and dry out and result in it losing its waterproofing properties too. 

This can be a big problem when installing a shingle product that is already meant to last 30 years if the felt has broken down and is not watertight after 5-10 years due to improper care during installation.

Read more: How to make your Roof ready for Raining Season

2. How to choose the best product regardless of their types?

There are a couple of different factors at play, but one of the most crucial is what type of top layer your roof applies. With conventional shingles, roofing felt is typically a not-too-bad idea – 15-pound roofing felt provides adequate protection while still giving air room to move around comfortably. 

However, in the case that you have a metal roof then you may wanna use a synthetic layer with a bit more heat resistance, as metal roofing may trap heat more quickly than any other material.

Climate is also a vital factor to choose from. Hotter climates or even some very wet areas may be better if considered synthetic underlayments, but if you live in a location receiving loads of snow and windstorms, durable felt may be better. 

3. How long can synthetic roof underlayment be exposed to weather?

Generally, most synthetic underlayments can be left exposed for at least a period of 6 months and some can even last for 12 months before showing some signs of wearing out. That is time you should think of a new replacement.

synthetic roof underlayment vs felt