Felt vs Synthetic Roof Underlayment: Differences Compared

Researched & Written by Craig

Planning a roofing project can be intimidating.

It’s not a common project, and there’s some real decisions to be made.

While some intense research may be done on laying metal or shingles, some forget to address whether a felt vs synthetic roof underlayment is the right choice.

The underlayment is a critical component of your roof and deserves attention.

The two common types of underlayment used on homes are asphalt-saturated felt and non-bitumen synthetic underlayment. The first, also known as tarpaper, is older, cheaper, and water-resistant. Synthetic underlayment is more expensive and adds increased stability plus better resistance to tearing.

Let’s dig in and talk about the differences between felt vs synthetic roof underlayment. First, I’ll quickly introduce the subject – click the contents table if you want to skip ahead.

What Is Roofing Underlayment?

When people think about roofing, many just consider the final outer layer.

Whether they should go with asphalt shingles or metal roofing. They may not think about underlayment at all.

Roofing underlayment is sandwiched between the roof deck, which is often plywood or OSB (Oriented Strand Board), and the shingles.

The roof deck is installed, and then the underlayment goes down right on top of it. Its purpose is to provide another layer of protection from all the elements your roof is exposed to. Wind, rain, snow, sleet. Whatever the sky can throw at you.

Types of Roofing Underlayment

You have two main options to choose from:

  • Felt
  • Synthetic

Regardless of which you choose, each comes with its own pros and cons. Other factors to consider when researching is where you live because that may limit your choice.  The following factors could come into play as well.

  • The materials the roof was constructed with
  • The design of the roof
  • Budget
  • Your roofing contractor

And of the above could impact your choice, regardless of your individual preference.

Felt Roofing Underlayment

This is the traditional choice. Felt underlayment is one of the oldest types of roofing underlayment. It’s manufactured by taking sheets of paper or fiberglass mats and saturating them in asphalt.

You again have two options to choose from here.

  • Felt #15
  • Felt #30

What’s the Difference Between #15 and #30 Roofing Felt?

The main difference is the weight. Historically, weight was calculated by taking 100 square feet of the felt underlayment and weighing it. #15 weighed in at 15 pounds and #30 weighed in at 30 pounds.

However, times have changed. Although the naming system of #15 and #30 is still used, those weights are no longer accurate. Technological advances have brought about significant changes in felt underlayment, and the weight of it has been reduced. It’s lighter, but it’s also stronger than it used to be.

But of course, #30 is still the stronger of the two. It’s also thicker than its counterpart, and there is less chance of it ripping or tearing off due to either the installation process itself or the effects of bad weather.

Felt Roofing Pros

The price. That is the largest pro of going with a felt underlayment. It’s cheaper than the synthetic alternative, so if lower final costs are a large part of your bottom line, this is your best option.

It’s breathable. Felt underlayment, while water-resistant, is still breathable. This is a significant plus for your roofing materials, especially asphalt shingles. Since the plywood roof deck can breathe, it adds to the longevity of your shingles.

You can use common tools. Felt can be installed with common tools but synthetic underlayment can’t.

Felt Roofing Cons

Limited exposure. Felt can’t be left exposed to the elements for any length of time, so the recommendation is to install roofing materials as quickly as possible after the felt goes down. Failure to do this can result in a leaky roof.

Tears easily. If it’s windy while being installed, the chances of it tearing are high.

It can wrinkle. If it’s exposed to any moisture during the install, it can wrinkle and harden, making it nearly impossible for shingles to lay flat.

It’s very slippery. This can pose a challenge for installers.

Felt underlayment can be a real hazard to work on in the cold.

Synthetic Roofing Underlayment

Like felt, synthetic underlayment is water-resistant, but it does offer greater protection from the elements. Made from long-lasting polymers, this type of underlayment brings added strength and a longer life.

Synthetic – or ‘man-made’ roofing is more expensive, but lighter and easier to install.

One thing to watch for when choosing a synthetic underlayment is who manufactures it and is your contractor trusted and respectable. Unfortunately, the synthetic underlayment industry isn’t standardized. Meaning there are no guaranteed minimums in terms of quality and performance.

So be careful when you find that super cheap deal on underlayment. You really do get what you pay for sometimes.

Synthetic Roofing Pros

Durability. This stuff is tough. It takes a lot to get it to tear. This is great since it means there is less chance of your installed destroying while walking on it. It will also last longer in situations where it’s exposed to UV or moisture for extended periods.

Lightweight. In some cases, it’s up to 4 times lighter than felt.

Fast and easy to install. Since it’s lighter, there is more on a roll. This means an installer can make fewer trips between the ground and the roof for new rolls.

Safer to install. Most synthetic underlayment has a slip-resistant surface. Your installer can walk the roof with a little more confidence, not so worried about sliding off.

Synthetic Roofing Cons

Price. Synthetic underlayment is going to cost you more when compared to felt. So if your upfront costs are an issue, this could be a significant con. However, if you’re more interested in long-term costs—and you plan to be in your home for many more years—then the initial investment can pay off over that time.

Only you can decide what’s better in your situation. Pay more now… or pay again later.

Is Felt Roof Underlayment Waterproof? Is Synthetic?

Neither asphalt-saturated felt or synthetic underlayment is waterproof. Instead, they are water-resistant.

Where waterproof underlayment is necessary, you need to use a rubberized asphalt underlayment. As the name suggests, it has a higher ratio of rubber, which makes it waterproof.

Rubberized asphalt underlayment is recommended in places where severe weather is the norm and is typically used around the eaves and roof protrusions, as well as in roof valleys.

Waterproof means that there is absolutely no chance of water infiltration. Water-resistant underlayment, while able to protect your roof deck from moisture, does not guarantee water infiltration.

Conclusion

If you are about to reroof your home—or even if it’s a new build—you knew you have some choices to make about metal or asphalt shingles. Now you know that choosing your underlayment is just as important.

Choose felt if:

  • You are looking for cheaper upfront costs
  • Breathability
  • You are doing the job yourself and don’t want to buy special tools

Choose synthetic if:

  • You’re okay with the upfront cost. You want something that can last
  • More durable against the elements
  • Installs faster

Whether you are thinking of redoing your roof yourself or hiring a contractor, this info should help you make the right decision.

Thanks for taking the time to read this article.

Why not check below to see if there are any other questions we can answer for you?