Different Types Of Roof Underlayment Explained

Over the past several years, we have seen significant changes in the types of roof underlayment that’s being used. As you already know, felt is the most commonly known roofing underlayment that has been widely used for a long time.

However, there have been products that have come out that are more suited for longer lifespans and higher temperatures when it comes to roofing. In most modern residential roofing systems, underlayment is a crucial component yet often unseen.

That’s why it’s understandable that not every homeowner is aware of this component. In this article, we will talk about different types of roofing underlayment, their uses, and compare them to see which type will be best for you.

In some places, people refer to underlayment as Ice and Water shield or Felt paper. While they are also amongst the various types of underlayment, they differ in material and design.

We will talk more about them later on. For now, if you don’t know what a roof underlayment is, the following section is for you.

What Is A Roofing Underlayment?

The job of your roof is to protect your home, right? That’s basic stuff, but a lot more goes into the roof than just sheer protection.

In your roofing system, there are multiple components that work together to provide you the necessary protection, and underlayment is one of them.

Consider this underlayment as a waterproof/water-resistant material that acts as a barrier. This gets installed directly on the roof deck and serves as an added layer of protection. They are usually applied under other roofing materials in a roofing system.

How Does It Work?

An underlayment provides protection during and after installation of your roof and also acts as a moisture barrier.

This extra layer of protection fills the gap or the weakness that shingles have and can even add protection to them.

Different regions require different types of underlayment installation by law. For example, if you live in a region that is prone to ice damning or high-winds, you will need to install a waterproof underlayment.

Why Do You Need Roofing Underlayment?

If you have extreme weather with hurricanes and whatnot, and god forbid if something happens like your roof system leaves or gets blown away, you will still have an adhered underlayment underneath it to protect your house. This product will protect water from infiltrating into your system. Make sense?

Types Of Roof Underlayment

Now that we have covered the basics, it’s time to look at the main types of roofing underlayment.

There are three main types:

  • Asphalt-saturated felt aka Felt Paper or Tar Paper
  • Non-Bitumen Synthetics aka Synthetic Underlayment
  • Rubberized Asphalt aka Peel & Stick underlayment

Let’s have a look at each of these types, as well as their uses.

1. Asphalt-Saturated Felt

More than 15 years ago, Asphalt-saturated Felt used to the go-to roof underlayment material for most residential, steep-slope applications. The term “Felt Paper” or “Roofing Felt” actually came from this underlayment material.

Before synthetic products gained popularity, every household used felt paper in their roofing system. It has a backing material that is the same as tar paper, but instead of a tar backing, it is saturated with asphalt and hence the name Asphalt-Saturated Felt.

Felt underlayment was either made from fiberglass substrate or organic. The organic underlayment was much more common back then because of the cellulose base.

Saturated Felt paper comes in two main sizes.

  • 15-pound
  • 30-pound

The size depends on how much oil is in the asphalt felt itself. 30-pound felt is obviously a better choice here as it is stiffer and much thicker. It will also do a better job when you are installing roof covering material.

Because of the thickness, it will last longer than the 15-pound unit even if the underlayment somehow gets exposed to bad weather. This material has been in use like forever until synthetic underlayment came and took all the glory.

If you have to be careful when choosing a corresponding size because many manufacturers do not saturate their asphalt to comply with regulation standards. One common problem that this type of underlayment face is water absorption.

We will talk more about the drawbacks of this type of underlayment later on.

Installation

Synthetic underlayment is mechanically attached. Depending on what the manufacturers recommend, it is usually done with cap nails. They can also be fastened to the surface using screws.

Walkability is also a major concern. Some underlayments are textured to help improve walkability on steeper slopes.

  • Fastening

They are usually installed using staples, but every product will have its own preferred method of installation. In high-wind areas, it’s recommended to use plastic wind strips alongside with staples along the edge. This will prevent the saturated felt from tearing.

Plastic caps are also used in high-wind areas to attach felts. It’s better than using staples and offers better wind resistance. It also helps prevent water leakage through the holes that are created by the fasteners.

  • Slope Limitations

Felt papers should be overlapped in low-slope roofs by a minimum of 19-inches. This includes roofs from 2:12 and up to 4:12. The point is to add two layers of underlayment across the entire roof instead of one for better protection.

  • Edge Metal Laps

Unlike synthetics, rubberized asphalt underlayment should overlap the edge metal at the eaves. The edge metal situated on the rakes should overlap the underlayment surface.

Benefits

  • Tried and true underlayment type
  • Similar characteristics of a felt paper
  • Adds stiffness to your roof deck
  • Provides protection from large tree branches and airborne debris
  • While this type does have water-resistant properties, it isn’t completely waterproof

Drawbacks

Asphalt contains volatile compounds that will dissipate over time. This will make the underlayment moisture-absorbent and more fragile, and this process can speed up if the material gets exposed to heat.

The heat from a particular type of roof covering material, harsh sunlight, warm climate, or bad roof-structure ventilation can easily destroy felt.

There is also a chance that the product can get damaged during installation either via footfall or other materials when roof-covering material is being installed.

The natural enemy of felt paper is UV radiation. If it gets exposed to direct sunlight, the deterioration process will accelerate, thus weakening the structure and its protective features.

Another major drawback of this type of underlayment is the lack of not following proper ASTM standards. You will only see the size labeled and not any ASTM rating.

This means, there is no way of telling the quality asphalt-saturated paper. You will only see them labeled as 15-pound or 30-pound, but most of them do not even comply with any standards.

2. Non-Bitumen Synthetics

Synthetic is equivalent to felt but lighter and lasts longer and built to withstand higher temperatures. It also has a longer lifespan compared to saturated felt papers. This underlayment type is made from polyethylene or polypropylene and can also be used to make a wide range of roofing products.

When it comes to the roofing system, whether it’s tiles, shingles, or metal, synthetics are by far the most popular option and for good reasons. The products in this category are made using asphalt-saturated base-mat and later fortified using a mixture of fiberglass.

Different manufacturers will have their own version of this product with enhanced features. The reason why this type is very popular is because of their anti-slip properties, which makes installation a breeze.

It is much stronger than other underlayment types and also extremely water-resistant. They are also more resistant to wear and tears. This is why most modern roofs have non-bitumen synthetic underlayment installed.

These synthetic polymers are often used in clothing, ropes as well as in food storage containers. Just like its felt counterpart, this material, too, has its own pros and cons.

Installation

The product itself is lightweight, and the surface is non-skid, which makes installation much easier compared to a heavy felt counterpart. This type of underlayment is safer to walk on during installation.

Some can get tacky when wet, and because of the improved walkability, a lot of roofing contractors are now using synthetics. Synthetics are also considered as moisture barriers because they don’t absorb moisture.

This means you will have more leeway when installing them and don’t have to worry about poor roof ventilation.

  • Fastening

Fastening synthetic underlayment is usually done with roofing nails or plastic caps. Since the material is not self-sealing, it is not recommended to use staples. Check the product label for information regarding installation.

  • Roof Edges

To protect the edges of the roof sheathing as well as to avoid problems from moisture-wicking, a lot of these synthetic underlayment’s come with a wrap-around design.

You wrap around the edges and install edge metal over the underlayment over the rakes and eaves. As for the slope limitations, it will vary from manufacturer to manufacturer.

Benefits 

  • It has an inherent resistant to fungal growth
  • Exceptionally strong and longer lasting
  • Good waterproofing and elastic properties
  • Doesn’t take very long to install
  • They are resistant to fungal growth
  • Improved walkability and gets tacky when wet
  • Highly resistant to UV damage

Drawbacks

When it comes to synthetics, the pros outweigh the cons. You don’t have to worry about any fungal growth or wrinkles. One common concern regarding synthetics was that manufacturers weren’t willing to follow ASTM standards.

However, most modern products now meet those standards, and they even provide strong after-sale support. It’s very important to follow the manufacture’s installation guide to avoid any problems in the future.

Otherwise, it might create moisture problems in the future. Whatever you do, make sure to talk with your roofing contractors, and see what their recommendations are before you decide on your own.

Not everything has to do with product quality. The ease of installation also plays an important role, and the people who will install it for you will know better.

3. Rubberized Asphalt/Peel & Stick

This is probably the most expensive option when it comes to roofing underlayment. This type has an adhesive on the back that sticks down to the deck surface of the roof shield.

For underlayment, it is quite common to see different types of rubber being used and hence the name rubberized asphalt. They have adhesive on one side that is protected by a peel-off membrane.

The synthetic and peel & stick are usually made out same material and are also available from the same manufacturer. They only differ in grade. The main difference would be how these underlayments are installed.

One unique characteristic of this type of product is the thickness of it. There is this idea process that thicker is better. However, that is not necessarily the case.

You should look at how it performs, installation, fastening, and how well the product sticks. There are some underlayment’s that will stick to a roof deck a lot better than others.

You should also consider the adhesive factor as well as the temperature. In snow country, it’s important to consider the lowest temperature, and how low of temperature you can actually install a peel and stick product.

Where and when should you use this material?

The answer would be anywhere you have extreme weather like snow countries, middle America, Florida, etc. You can also get away using a synthetic and peel and stick combo. 

The fun fact is, it has little to none asphalt content even though it has the word asphalt in its name. These self-adhering papers are fiberglass-reinforced that further improves the protection.

Modified Bitumen 

Mod-bit, which is the shortened version of Modified Bitumen, refers to the asphaltic roofing materials used as an underlayment. Bitumen is a generic term here, and it can mean a lot of things.

For example, there may be different mixtures of hydrocarbons, and you can call them all bitumen. In the roofing industry, this is a common term to refer to asphalt and other asphalt materials used in built-up roofing, asphalt shingles, and underlayment.

As for the term “modified,” polymers are used to modify bitumen to improve certain characteristics such as elasticity and strength. This gives this particular type of underlayment rubber or plastic-like properties, although it will depend on the modification process being used.

Polymers can give underlayment material-specific properties to strengthen certain aspects. The molecules of polymers are custom designed and are used with different types of roofing products to increase their resistance to deterioration and damage and increasing lifespan.

There are some underlayment products that have the term “cross-linked polymer” labeled. The molecules of polymers in this type of product are bonded at the atomic level that can greatly increase the strength.

Selvage Edge

Another common characteristic you will often see is the Selvedge Edge on one side of the rubberized asphalt underlayment rolls. It’s a strong edge that creates a water-tight seal along the edges. This eliminates the gap where the rolls of underlayment overlap.

If a particular product is being installed in courses across the roof, the selvage edge is kept along the top to create a water-tight seal.

Installation

Installation is simple and is much easier than felt because of the adhesive. This type of underlayment has them on the back for ease of installation. They also have a non-skid upper surface, and because the surface contains material like polyethylene or polyester, this type of product has great weather-resistant qualities.

Benefits

There are a couple of benefits if you choose to install peel and stick underlayment.

  • Anytime you have a low slope condition, this is the right type for you.
  • It makes your roof weatherproof.
  • Suitable for high-risk areas on the roof.
  • In extreme weather environments, it’s a good idea to cover the entire roof with adhesive-backed underlayment to provide maximum weather-tightness.
  • They are self-sealing and do a great job of sealing well around the fasteners.
  • Improved moisture resistance thanks to the polymer film that is bonded to the weather-surface.

Drawbacks

There are no shortcomings of this type of underlayment except for the price. However, it’s worth spending the extra money if you want the absolute best protection.

Underlayment Attributes

When it comes to the attributes of different types of underlayment, warranties play a big part. When you are installing a metal roofing system, you are probably getting a 50-60 years+ roof system.

This is the minimum expected lifespan of the roof. In that case, you would want to use a product, in my opinion, that will have a warranty period somewhat close to the lifespan of the roofing system that you are installing.

Don’t use a product that will only provide you will three or five years of warranty under a roof that is probably going to last half a century.  

Final Thoughts

In short, there are multiple factors like fastening capability, thickness, size, price, warranty, temperature, and many more to consider when you are considering installing one.

There are basically three roof underlayment types, but amongst them are different product categories with different specs. There are other things that play into it, depending on which product you are using. So, take your time, consider the different types, and make your choice.

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