Roof underlayment is a critical part of any roof and serves as a secondary layer of protection against the elements. This roofing material also plays a vital role in keeping moisture out of your home. In the guide below, we’ll discuss why synthetic and felt are two of the most popular types of roof underlayment. We’ll also explain how underlayment protects your house, compare synthetic and felt varieties, and help you decide which roofing underlayment is right for you.

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What Is Roof Underlayment?

Roof underlayment is the layer of your roof system that’s directly beneath its outermost layer and on top of the roof deck. You can’t see it, but this secondary moisture barrier is critical to keeping your home dry. With this double layer of protection, you can preserve your roof’s life span and improve its performance.

There are two main types of roof underlayment: felt and synthetic. They’re very different as underlayment options, but both perform the following functions:

• Assist your heating and cooling efforts by acting as added insulation
• Comply with your outer roofing warranty
• Smooth the surface of your roof deck for uniform shingle or tile installation
• Soften the sound of rain by acting as a noise barrier
• Temporarily protect your home if outer roofing materials receive damage

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Synthetic Underlayment

Synthetic underlayment is newer among underlayment materials and has become popular in the roofing industry since it addresses some of the shortcomings of felt underlayment. It’s typically made from long-lasting polymers such as polypropylene or polyethylene.

Read more about the pros and cons of synthetic underlayment below.

Benefits of Synthetic Underlayment

• Durable: Synthetic underlayment is difficult to tear and can withstand exposure to ultraviolet (UV) rays and moisture.
• Easy to install: Synthetic underlayment is thinner than felt and easier to work with, saving time during the installation process.
• Lightweight: Synthetic underlayment is substantially lighter than felt, making it easier to carry.
• Moisture-resistant: Synthetic underlay is temporarily safe to use without a roof covering because of its water resistance.
• Safe to install: Synthetic underlay isn’t slippery, increasing walkability and making installation safer.

Drawbacks of Synthetic Underlayment

• May not comply with building codes: Some building codes require the use of felt underlayment, so using synthetic may not be an option.
• Not vapor permeable: Synthetic underlayment acts as a vapor barrier, so it may not be suitable for rooftops lacking ventilation.
• Pricier than felt: Synthetic underlay is more expensive than felt, which can be a deterrent for homeowners on a budget.

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Felt Underlayment

Felt underlayment is one of the oldest types of roofing underlayment and is made from a compound of either organic materials or fiberglass and asphalt. It’s a durable material that lasts up to 30 years with proper installation. It resists water infiltration but isn’t completely waterproof.

Traditional felt underlayment is available in 15- and 30-pound thickness. If your roof doesn’t have a steep pitch, opt for 15-pound felt. If your roof has a steep pitch, consider 30-pound felt as it’s thicker and more resistant to damage during installation. You can either use staples to fasten felt to your roof or use plastic caps, which offer better wind resistance and protect against leakage.

Read more about the pros and cons of felt underlayment below.

Benefits of Felt Underlayment

• Affordability: Felt underlayment tends to cost less than synthetic underlayment, so it could be a good option for budget-conscious homeowners.
• Meets building code requirements: Felt underlay has been a common roofing material for decades and is consistent with most building codes.

Drawbacks of Felt Underlayment

• Can be dangerous to work on: Because felt underlayment is smooth, roofers may slip on it during installation, especially if your roof has a steep slope.
• Easy to rip or tear: High winds or missteps during installation could rip or tear felt underlay.
• Heavy to carry: Felt is a heavy material, creating more work for roofers. The additional work might increase the price of installation.
• Wrinkles when exposed to moisture: Felt paper is prone to wrinkling when it absorbs moisture. If this occurs during installation, it can be a challenge to get shingles to lay flat.

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Cost Comparison: Synthetic vs. Felt

The cost of roof replacement is significant for any homeowner, and the choice between felt and synthetic underlayment warrants serious consideration. The average cost for roof underlayment replacement on a 2,000-square-foot roof ranges from $800–$1,200.* Labor, materials, the condition of your roof, and roof size will determine the final price of underlayment replacement.

Here’s how the prices of felt underlayment and synthetic underlayment compare:

• Felt roof underlay costs: The price of felt underlayment ranges from five to 80 cents per square foot for materials only.
• Synthetic roof underlay costs: The price of synthetic underlayment ranges from 10 to 55 cents per square foot for materials only.

Labor costs will vary due to the complexity and scope of your roofing project. For example, an underlayment installation that requires the removal and reuse of tiles will be more expensive than a project that includes asphalt shingles. Roof size, labor costs, material type and brand, roof repairs, permits, and product warranties will also influence the price of a project.

*Price data sourced from Fixr and HomeGuide.

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Durability: Synthetic vs. Felt

Synthetic and felt roof underlayment are both durable and can protect your roof from moisture damage for years. Synthetic underlayment was specifically designed with durability in mind and is stronger and longer-lasting than felt.

Asphalt-saturated felt comes reinforced with asphalt and provides a barrier that insulates against harsh weather. It can last between 15 and 30 years with proper care but could be susceptible to damage during installation and severe weather. Felt underlay also wrinkles when it encounters moisture, can degrade with UV exposure, and tears easily.

Synthetic underlayment is designed to be moisture-resistant and provides a secondary barrier against noise, UV rays, and harsh weather. It can last for up to 40 years with proper care and is tear-resistant. These protective properties make it safe for a multiday roof installation.

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Installation: Synthetic vs. Felt

Installing felt underlayment is similar to installing synthetic underlayment, but there are some critical differences.

Installing Felt Roof Underlayment

Roofers install felt roof underlayment in strips, the edges of which overlap. They fasten underlayment to your roof deck with staples or plastic caps and might use plastic wind strips to prevent tearing in areas with high winds.

Your roofing contractor may recommend 30-pound felt over 15-pound felt for installation since it’s more resistant to tearing, but the heaviness of the material can make installation difficult. Felt underlayment is vulnerable to damage from UV rays or moisture, so it’s best for a one-day installation. If installation takes longer than a day, a roofer may need to place a special covering over the underlayment for better protection during installation.

Installing Synthetic Roof Underlayment

Roofing contractors install synthetic roof underlayment in horizontal strips with overlapping edges and fasten them with plastic caps or roofing nails. They’ll wrap synthetic underlayment around the roof to avoid any issues that may stem from wicking moisture.

Synthetic materials are thinner and lighter than felt, making installation easier. Synthetic is slip-resistant and more durable, so roofers don’t have to worry about tearing the underlayment during installation. Synthetic roof underlayment is also resistant to UV rays and moisture, so roofers can leave it exposed temporarily if they don’t finish roof replacement in a single day.

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Our Recommendation

The right underlayment is an important part of the roofing system and helps protect your home from moisture and harsh weather conditions. If you need a new roof or just want to replace your current underlayment, contact a roofing professional to help you decide between synthetic and felt underlayment.

Felt and synthetic roof underlayment both have their merits, but synthetic roof underlayment is easier to install and lasts longer. However, some buildings don’t accept synthetic underlayment, in which case felt underlayment would be a better option.

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Synthetic Underlayment vs. Felt FAQ