Roof Underlayment 101: What It Is & Why It’s Important

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When most homeowners think about their roof, they picture the tiles, shingles, or foam that cover the top of their home. But while these outer layers provide essential protection from the elements, the real workhorse of your roof system sits just beneath the surface and largely out of sight.. It’s called underlayment, and it’s one of the most important—yet most overlooked—components of a healthy, long-lasting roof.

Underlayment serves as the final barrier between the elements and your home. It protects against moisture, heat, and wind, while helping maintain the integrity of your roof structure overall. Underlayment can also provide thermal protection, and that means in a climate as extreme as Arizona’s, choosing the right underlayment matters more than most people realize.

Whether you're replacing your roof, making repairs, or preparing for solar, evaluating the health of your underlayment is the great place to start, while understanding how various underlayment materials perform will help you make the best decision if a re-roof is in the plan. 

What Is Roof Underlayment?

Underlayment is a layer of material installed between your roof deck and the outer roofing material (such as tiles or shingles). This hidden protective layer is designed to act as a secondary moisture barrier, shielding your home from water intrusion, dust, and other environmental elements. Some higher-grade underlayment also have thermal benefits, helping to keep the house warm in the winter and cool in the summer. 

All roofing systems use underlayment, regardless of the type of roofing material installed. The right underlayment should be durable, weather-resistant, and compatible with your roof type. This is especially important in Arizona, where extreme temperatures, high UV exposure, and monsoon storms pose unique challenges.

An Essential Part of the Roof System

Underlayment is part of a larger, layered roof system that works together to protect your home. A typical Arizona roof structure includes three main layers. From bottom to top those layers include:

1. Roof Deck – Usually plywood or OSB sheathing that forms the structural base of your roof.
2. Underlayment – The waterproof or water-resistant layer that sometimes also provides insulative properties and protects the decking material from exposure to the elements .
3. Roofing Material – The outward-facing tile, shingle, or foam system that you can see.

While the outer material sheds water, underlayment is what truly stops it. Tiles and shingles are designed to move water off the roof, not to waterproof it. In high winds, heavy monsoon rain, or during freeze-thaw cycles in higher-elevation parts of Arizona, water can easily get beneath the outer layer. That’s where underlayment steps in as the critical final defense.

How Underlayment Protects Your Home

Underlayment performs several key protective roles:

• Moisture Barrier: Prevents water from reaching the roof deck or inside your living area.
• Secondary Weather Defense: Protects in case tiles crack, shingles lift, or wind pushes rain sideways.
• Heat & UV Protection: Helps reflect heat and protect the deck from thermal damage. This is especially important here in Arizona.
• Dust and Debris Shield: Prevents the fine Arizona dust from entering vulnerable areas.
• Structural Longevity: Reduces strain on the roofing materials, deck boards and generally helps to prevent structural wear and tear.

A properly installed, high-quality underlayment can extend the life of your roof significantly—especially in Arizona’s harsh climate.

Why Underlayment Matters in Arizona’s Climate

Arizona is one of the most challenging roofing environments in the country. Standard materials that work fine in cooler or milder states may degrade much more quickly here.

Extreme Heat & UV Exposure

Daytime attic temperatures can reach 150–170°F in and around Phoenix, causing lesser-quality underlayment to warp, crack, or deteriorate prematurely. UV exposure also breaks down felt products quickly if they are exposed during installation.

Monsoon Storms & Wind-Driven Rain

Underlayment must withstand sudden, heavy rainfall and high winds that can force water sideways – pushing it up beneath roof tiles or shingles. In addition, wind uplift can stress fasteners and seams, and inferior underlayment may tear or allow moisture penetration.

Dust Intrusion

Fine dust and sand carried by seasonal winds can work their way into tiny gaps, compromising sealants and adhesives. Higher-quality synthetic and SBS materials are better at resisting this intrusion.

Thermal Shock

Daily temperature swings of 40–50°F cause expansion and contraction across the roof system. Flexible, durable underlayment perform better under these stress cycles.

Because of these unique pressures, choosing an appropriate underlayment is one of the most important decisions a homeowner can make during a roof replacement.

The Different Types of Roof Underlayment

There are three primary types of underlayment: felt, synthetic, and SBS modified. Each offers varying levels of protection, durability, and cost.

1. Felt Underlayment

Felt, commonly called “tar paper,” is the oldest and most common type of underlayment. It's typically made from paper or fiberglass mat soaked in asphalt.

Pros

• Low cost
• Readily available
• Traditional option for many roofing projects

Cons

• Prone to tearing during installation
• Absorbs moisture
• Poor UV resistance
• Shorter lifespan, especially in hot climates
• Not ideal for tile roofs, which dominate the Arizona housing market

In Arizona, felt underlayment typically lasts just 10–15 years—sometimes more and sometimes, depending on how well it was cared for. The good news is that you don’t always need a full roof replacement. In many cases, the underlayment can be replaced on its own, since tile roofing can often be reused as long as the tiles are not cracked or broken. 

2. Synthetic Underlayment

Synthetic underlayment is made from polymers that provide superior strength and weather resistance. It has become a popular choice nationwide—especially in the Southwest—thanks to its excellent UV tolerance.

Pros

• More durable than felt
• Lightweight and easier to install
• Highly tear-resistant
• Excellent UV exposure tolerance
• Better water resistance

Cons

• Slightly higher cost than felt
• Performance varies by brand and thickness
  
  

Synthetic underlayment is an excellent mid-tier option for most homes and a significant improvement over felt.

3. SBS Modified Underlayment

SBS stands for Styrene-Butadiene-Styrene, a rubberized compound added to asphalt to improve flexibility and durability. This type of underlayment is engineered for extreme temperatures, making it one of the best choices for long-term performance in our harsh desert climate.

Pros

• Exceptional durability
• High heat tolerance
• Flexible and resistant to cracking
• Excellent waterproofing
• Long lifespan
• Ideal for tile roofs

Cons

• Higher cost (but also much longer-lasting)
• Requires experienced installers

In Arizona, SBS modified underlayment is widely considered the gold standard, especially under tile roofs where long-term performance is essential.

Low-E ThermaSheet: A Reflective Underlayment Option

Low-E ThermaSheet is a type of underlayment with reflective properties that help reduce radiant heat transfer into your attic. This can improve your home’s energy efficiency and comfort, especially during hot Arizona summers. If you’re AC is working less, your bills are smaller too. 

Key Benefits

• Reflects radiant heat
• Can reduce attic temperatures
• Helps lower cooling costs
• Adds an extra layer of protection
• Works well under tile and metal roofs

Low-E underlayment isn’t appropriate for every home or roof configuration, but when used correctly, it can complement other insulation and ventilation strategies for increased energy efficiency in attic spaces. Keeping the attic cool is a great way to help offset those sky-high summer electric bills.

Read more: What Is Low-E Insulation & Is It Good For My Roof? 

How to Tell if Your Underlayment Needs to Be Replaced

When underlayment reaches the end of its lifespan—or is damaged—your roof becomes vulnerable. Because the underlayment sits beneath the roofing material, problems often go unnoticed until a leak appears inside the home.

Common Signs of Failing Underlayment

• Cracked or displaced tiles or shingles
• Ceiling stains or discoloration
• Soft spots on the roof deck
• Musty odors from attic moisture
• Increased attic temperatures
• Visible sagging or warping along rooflines
• Tiles blowing off more frequently during storms

If you're seeing any of these signs, it’s time to schedule a professional inspection.

What Do I Need to Replace if My Underlayment Is Damaged?

When underlayment is damaged, you may need to replace more than just the underlayment itself. Depending on the severity, other components of your roof may also need attention.

In many cases, homeowners can spot early warning signs—such as cracked or slipped tiles, visible gaps along ridges or valleys, staining on interior ceilings, or unusual attic odors that suggest moisture. These issues often indicate that water may be getting past the outer roof layer and potentially compromising the underlayment below.

However, true underlayment damage is not always visible from the ground. A roofer will lift tiles or shingles to inspect the condition of the underlayment directly, as well as inspect other critical components of your roofing system at the same time. For the underlayment, they’ll look for signs such as brittleness, tears, dried-out seams, improper overlaps, saturation from past leaks, or wood rot beginning on the deck beneath. This professional inspection helps determine whether you need a localized repair or a full underlayment replacement to protect the integrity of your roof system.

Potential Replacement Items

• Roof deck sections: If moisture has penetrated, wood rot may be present.
• Metal flashing: Often rusted or damaged around penetrations.
• Tiles or shingles: Cracked, damaged, or loose pieces may need replacement.
• Ventilation ducts or attic insulation: If moisture has spread.

Why Tile Roofs Require Extra Care

In Arizona, many roofs are tile systems. While tiles themselves can last 50+ years, the underlayment beneath them rarely does. This is why most tile re-roofing projects focus on replacing underlayment and reusing the existing tiles when possible.

Read More: Replace Or Repair A Roof: Factors To Consider Before You Decide

Underlayment & Solar: Why Coordination Matters

As more Arizona homeowners add rooftop solar, the intersection of roofing and solar installation has become increasingly important. Your roof and underlayment must be able to support both the physical demands and the long-term lifespan of a solar array, which is typically designed to last 25 years or more.

Key Considerations

• Solar racking creates roof penetrations.
  Mounting hardware secures the solar system to your roof using attachments that penetrate the roofing surface. These penetrations put added stress on your roof. If the underlayment is already aging or compromised, these penetrations can make problems worse.
• Panels must be removed to repair or replace the roof beneath them.
  If your underlayment fails after solar is installed, your solar panels will need to be removed before any roofing work can be performed. This process is almost always an out-of-pocket cost for the homeowner and can run into several thousands of dollars depending on system size.
• Older underlayment may not outlast your new solar system.
  Many Arizona roofs still have original felt underlayment that is already halfway or more through its functional life. Installing solar on top of aging underlayment increases the risk you’ll need major roofing work long before your solar system reaches end of life, triggering that costly removal and reinstallation process.
• Solar creates additional thermal load on the roof.
  Solar panels shade the roof surface but also trap heat underneath them. This creates a pocket of elevated temperature at the roofline, increasing thermal stress on both the roofing material and the underlayment. High-quality underlayment is better equipped to handle this long-term heat exposure.
• High-quality underlayment protects your investment.
  Ensuring the underlayment is durable, heat-resistant, and properly installed helps protect the roof deck beneath the solar array for decades. This reduces the risk of leaks, premature wear, and unexpected repairs.

If you're considering solar in the next few years, it's wise to coordinate your roofing and solar plans to avoid duplicated labor later. Ensuring your roof—and especially your underlayment—has plenty of life left before installing solar helps you avoid unnecessary costs and ensures both systems work together effectively for the long term.

 Read more: Is My Roof Suitable For Solar Panels? 

Best Practices for Installing Underlayment

A long-lasting roof depends not just on choosing the right underlayment material, but also on how well it’s installed. Even the highest-quality products can fail prematurely if they are not applied correctly, especially in a climate as demanding as Arizona’s Sonoran Desert. Proper installation ensures the underlayment forms a continuous, watertight barrier that can stand up to wind, heat, and seasonal monsoons This makes workmanship just as critical as material choice.

What Good Installation Looks Like

• Clean, dry roof deck
• Proper overlap at seams (often 4–6 inches)
• Correct fastener spacing
• Secure sealing around penetrations
• Minimal wrinkles or bubbles
• Underlayment not left exposed beyond its UV rating
• Proper integration with flashing and drip edge

These best practices help create a smooth, secure surface that prevents moisture intrusion and maximizes the life of both the roofing system and the home beneath it.

Warning Signs of Poor Installation

• Uneven or buckled underlayment
• Over- or under-driven fasteners
• Exposed edges
• Excessive foot traffic scarring
• Underlayment left exposed for too long

These issues can shorten the lifespan of the underlayment, compromise it’s waterproofing ability, and lead to leaks or premature roof failure. Some problems are visible from the ground, but many can only be identified by a trained roofer during an inspection.

Hiring a reputable roofing contractor ensures the installation meets manufacturer specifications and Arizona code requirements. Just as importantly, it gives you confidence that your roof’s most important protective layer has been installed correctly and will perform for the long haul.

If you’re unsure about the condition of your current roof or want a second opinion before major work or solar installation, a professional assessment can provide clarity and peace of mind.

Maintenance Tips for Longer Underlayment Life

Regular maintenance helps prolong the life of your roof and underlayment.

Recommended Maintenance

• Annual or bi-annual roof inspections
• Inspections after monsoon storms
• Clearing debris and overhanging branches
  Checking attic ventilation
• Monitoring for cracked tiles or shingles
• Ensuring gutters and downspouts are clear (if present)

Catching issues early is the best way to prevent underlayment damage and preserve the health of your roof system.

Read More: Four Ways Regular Roof Maintenance Can Save You Money

Final Thoughts

Underlayment may not be visible, but it plays a central role in your roof’s durability, performance, and long-term protection—especially in Arizona’s intense climate. Choosing the right material and ensuring proper installation can help safeguard your home from heat, moisture, and storm damage for decades.

If you have questions about your roof or suspect your underlayment may be nearing the end of its life, schedule a professional evaluation. Azul Roofing Solutions is here to help you make informed decisions and ensure your roof is ready for the challenges of Arizona weather.