What Is Roof Underlayment?

By Amanda Lutz Updated March 28, 2024

When you think of your roof, you may picture shingles, metal roofing, or tiles. This outer layer protects your home from elements such as rain, snow, UV rays, and wind. Roof underlayment is a thin layer of protective material placed just below your roof’s outermost layer and covering your roof deck. This secondary layer of protection prevents moisture from entering your home if your roof’s outer materials become damaged.

Whether you need a roof replacement or are building a home, read our comprehensive guide to roof underlayment to help you make the right choice for your home.


Types of Roof Underlayment

The primary purpose of roof underlayment is to repel moisture. Different materials can be used to achieve this goal. There are three common types of roof underlayment used in modern roof installations. Each material has distinct properties that affect its performance. 

Asphalt Saturated Felt

Sometimes called felt paper or tar paper, felt is one of the oldest types of underlayment. It’s composed of organic materials saturated with asphalt to improve water resistance. 

Felt underlayment is available in two thicknesses or weights: 15 pound and 30 pound (15 pounds per 100 square feet or 30 pounds per 100 square feet). The 30-pound felt underlayment is generally recommended as it’s thicker and more durable than the 15-pound felt. 

Benefits

Felt underlayment is used for many roofing projects due to its affordable price tag and local building codes.

Drawbacks

Despite its heavy material, felt is prone to tearing during installation. Other drawbacks also make it difficult to install. 

Synthetic Roof Underlayment

Synthetic roof underlayment is newer than felt, and it’s becoming increasingly popular. Non-bitumen synthetics are made from synthetic polymers (usually polypropylene or polyethylene). Synthetic roof underlay is lightweight, durable, and mold- and water-resistant. 

Synthetic roof underlayment was developed in the early 2000s to address the shortcomings of felt. Its durability and easy installation make it one of the most popular choices. Since synthetics are water-resistant and unaffected by UV rays, they can safely be used when the roof’s outer layer isn’t installed immediately. 

Benefits

A synthetic roof underlayment’s durability and resistance to the elements make it easy and safe to install. 

Drawbacks

Because synthetic underlay is a newer material option, it may not meet warranty requirements or building codes. 

Rubberized Asphalt

Designed for optimal waterproofing, rubberized asphalt is composed of rubber polymers and asphalt, and it typically has an adhesive backing. The adhesive sticks directly to the roof deck, creating a waterproof seal. 

Rubberized asphalt underlayment can be used as a primary underlayment material or used in areas where water leaks are most likely to occur. When not used for a full underlay, it may be applied around vents, valleys, eaves, skylights, and chimneys. It can be useful in locations with extreme temperature changes or under metal roofing in harsh environments.

Benefits

Rubberized asphalt is a versatile material suited for a variety of weather conditions. 

Drawbacks

Rubberized asphalt’s benefits come with a high cost. It’s the most expensive type of underlayment.


Choosing the Right Roof Underlayment

Similar to choosing the best type of roof for your home, determining the right roof underlayment requires considering your environment, roofing materials, budget, and needs. Consider these factors when choosing the right roof underlayment for your home. 

Installation Method

No matter what type of underlayment you choose, roofing professionals must cover the entire roof deck before installing the outer roofing materials. Depending on the complexity of your roof replacement, getting the job completed in a single day can be challenging. 

While the installation process for each type is similar, some materials are better suited for specific circumstances. For example, felt underlay can wrinkle when exposed to moisture, making it a poor choice for a multiday installation. Synthetic roof underlayment is lighter than felt and requires fewer rolls, which can speed up the installation process. Since it’s resistant to UV rays and moisture, it can also be used when outer roofing materials won’t be applied immediately. Although rubberized asphalt requires the biggest upfront investment, its adhesive backing helps roofers install it quickly.

Climate Considerations 

The life span of your underlayment impacts the life span of your roof. While underlayment should last for decades, your climate can impact how quickly it degrades. 

Over time, heat causes volatile compounds in asphalt to dissipate, making felt roof underlayment more fragile. The deterioration is accelerated when areas are exposed to sunlight. 

Synthetic underlayment is resistant to moisture and UV rays, making it a longer-lasting solution. However, extended exposure to UV rays can cause it to become brittle. It can also be damaged in extreme weather if it becomes exposed. 

Rubberized adhesive underlayment offers more protection in colder, wetter climates thanks to its superior waterproofing. However, some types of adhesive can deteriorate when exposed to extreme heat. When installed with metal roofing or in hotter climates, choose a product formulated for use in high-temperature conditions. 

Roof Type 

Your roof slope and roofing materials are the most important factors to consider when choosing a roof underlayment. Low-slope roofs have reduced water-shedding capabilities, requiring greater overlap to avoid trapping moisture in your roof deck. Felt roof underlayment requires full overlapping on low-slope roofs, increasing the materials required. 

Your roofing materials will also affect how your underlayment performs. Synthetic underlayment is often recommended for use with asphalt shingles because of its durability. If shingles become damaged, synthetic underlay is less likely to be immediately affected by the elements. 

While all types of underlayment can be used under metal roofing, heat tolerance is essential. Certain types of rubberized asphalt adhesive can degrade in high heat, destroying its water-tight seal. Felt underlayment can also degrade faster when exposed to extreme heat. 

Durability 

When properly installed, all types of roof underlayment can be depended on for durability and to extend the life of outer roof materials. Roof underlay is designed to be covered by outer roofing materials. It can be affected by extreme conditions or damaged during installation.

Under ideal conditions, roof underlayment can last 15 to 40 years. Felt underlayment is the most susceptible to damage without a protective roof covering. It can be damaged during installation. It’s also susceptible to moisture and UV rays, which means it must be protected from the elements if outer roofing materials aren’t installed immediately. 

Once installed, ensure your roof underlay stays in good shape. Regular inspections can help you understand how your roofing materials are holding up and enable you to conduct repairs before your home sustains damage. 

Cost 

Like other roofing materials, there is no fixed cost for roof underlayment. The cost of your roof underlayment will be affected by several factors, including the size of your roof, the type of underlayment you choose, labor costs, and the complexity of your roof. 

The average cost to install roof underlayment on a 2,000-square-foot roof ranges from $800 to $1,200. However, this is a basic estimate. A complex roof install that requires the reuse of outer roofing materials such as tiles can increase the cost substantially. 

Roof underlay is priced by the square foot and based on the material, as follows:

The cost of each type of underlay varies by quality and brand. For example, expect a 30-pound felt underlayment to be more expensive than the 15-pound option. While the upfront cost of synthetic underlayment and rubberized asphalt is more expensive, its longevity can result in lower costs over time. 

Cost data in this section was sourced via Fixr.


Our Recommendation

Roof underlayment is a critical part of your roof system, and the type you use can impact the installation process and the product’s life span. While all types of roof underlayment can protect your home for decades, synthetic and rubberized asphalt roof underlays typically provide more durability and a longer life span. No matter which type of roof underlayment you choose, do your research and ensure it’s permitted by the building codes in your location. 


Roof Underlayment FAQ

Is roof underlayment necessary?

Roof underlayment is necessary to add a secondary layer of protection to a home and is typically required to meet the warranty requirements for your roof’s outer materials. However, it may not be required by building codes in all areas. Roof underlayment should be used with all types of roofing materials for best results.

What is the most common underlayment for roofing?

Felt underlayment is the most common underlayment for roofing, likely because it’s the oldest. Today, many roofing professionals choose synthetic underlayment because of its improved durability and easy installation properties.

What is the purpose of roof underlay?

The main purpose of roof underlay is to add a secondary layer of protection against moisture. It can also act as an insulation to keep out harsh weather and deaden sound.

What are three options for roofing underlayments?

The three most common options for roofing underlayments are felt, rubberized asphalt, and synthetic underlayment. Each type has different properties that should be considered when choosing the right one for your home. 

How long does roof underlayment last?

While roof underlayment is designed to last decades, roof underlayment materials have different levels of durability. Depending on the type of underlayment you choose, it lasts an average of 15 to 40 years.