What Is Roof Underlayment? (Material, Function, & More)

When replacing or installing a new roofing system, understanding what roof underlayment is and why it matters can help you make better decisions for your home. Roof underlayment is a protective barrier that’s installed between the roof deck and the visible roofing material—like shingles or shakes—and it plays a crucial role in shielding your home from water damage.

Moisture protection: Underlayment adds an extra layer of defense against rain, ice, and snow.
Structural support: It reinforces your roof deck and helps prevent leaks.
System longevity: Quality underlayment extends the life of your entire roofing system.

In this guide, we’ll explain the different types of roof underlayment, what they’re made of, how they function, and when replacement is necessary.

👉 What Roof Underlayment Does

Acts as a Moisture Barrier

Even the best roofing materials can allow small amounts of moisture to seep through—especially in heavy rain or wind-driven storms. Underlayment provides a waterproof or water-resistant layer that catches any infiltration before it reaches your roof deck.

Protects wood sheathing: Prevents water from soaking into the structural base of your roof.
Stops mold and rot: Keeps moisture from accumulating where it can cause decay.
Improves leak prevention: Acts as backup protection if shingles or shakes are damaged.

Adds Durability to the Roofing System

Underlayment provides temporary protection during roof installation and extra security once the roof is complete. It’s a key component of a well-functioning roofing system.

Holds up during construction: Shields the deck from rain between tear-off and shingle installation.
Reduces uplift: Adds friction between the decking and shingles during wind events.
Supports even wear: Helps distribute the load evenly across the roof surface.

Helps With Fire Ratings and Building Codes

Certain types of underlayment improve the fire rating of a roofing system, especially when used with wood-based materials like cedar shakes. Many local building codes also require underlayment for new construction and roof replacements.

Code compliance: Most municipalities require underlayment to meet safety and performance standards.
Improved fire safety: Synthetic and fiberglass-reinforced options often have better fire resistance.
Supports insurance eligibility: Proper underlayment may be required for certain policies or claims.

📋 3 Main Types of Roof Underlayment

This section uses a numbered list instead of H3s.

1. Felt Underlayment (Asphalt-Saturated Paper): Traditional felt, often called tar paper, is made from organic or fiberglass mat saturated with asphalt.
   Material: Paper or fiberglass base with asphalt coating
   Best for: Budget-friendly installs and basic protection
   Pros: Affordable, widely available
   Cons: Tears easily, absorbs water, heavy, shorter lifespan
2. Synthetic Underlayment: Made from woven or spun polyethylene or polypropylene, synthetic underlayment has quickly become the go-to option for modern roofing systems.
   Material: Engineered plastic compounds
   Best for: Premium roofing systems like cedar shakes or metal
   Pros: Lightweight, durable, water-resistant, tear-proof
   Cons: Slightly higher cost than felt, can be slippery when wet
3. Rubberized Asphalt Underlayment (Peel-and-Stick): This high-performance material features a self-adhesive backing that seals tightly to the roof deck.
   Material: Rubberized asphalt or butyl with adhesive backing
   Best for: Ice dam protection and high-risk leak areas
   Pros: Waterproof, self-sealing, flexible
   Cons: Expensive, difficult to reposition once applied

🤔 Where Underlayment Is Installed

Across the Entire Roof Deck

Most roofing systems include underlayment beneath all shingled areas to ensure complete protection. It acts as a moisture barrier from the eaves up to the ridge.

Full coverage: Shields every square foot of your roof deck from potential leaks.
Base for roofing: Creates a smooth surface for installing shingles or shakes.

In Valleys and Around Penetrations

Roof valleys, chimneys, vents, and skylights are all prone to water intrusion. Extra underlayment—often peel-and-stick—is used in these areas to provide a stronger seal.

High-risk zones: Protects where water is most likely to collect or infiltrate.
Double-layer defense: Often paired with flashing or specialized tape for added security.

Under Starter Courses and Along Eaves

Specialized underlayment is often installed along the lower edges of the roof to protect against ice dams and wind-driven rain.

Ice and water shield: Rubberized options are used to stop melting snow from backing up under shingles.
Prevents edge leaks: Reinforces areas most exposed to water flow.

⚠️ Signs Your Roof Underlayment Needs Replacement

Aged or Brittle Underlayment

Over time, exposure to heat, humidity, and poor attic ventilation can cause underlayment to break down—even if the shingles still look intact.

Cracking and tearing: Aged felt and even some synthetics can become brittle.
Shortened lifespan: Underlayment often wears out before the roofing material above it.

Roof Leaks or Staining

If you’re experiencing water leaks but your shingles look fine, the problem may be a failed underlayment layer underneath.

Hidden moisture paths: Leaks can bypass shingles and rot the decking below.
Stains on ceilings or walls: Water damage inside the home is a red flag.

Recent Storm Damage

High winds or flying debris may not only tear shingles but also damage or displace the underlayment below them.

Uplifted materials: Storms can expose the underlayment or peel it away entirely.
Insurance considerations: Replacing underlayment may be covered under storm-related policies.

✅ Benefits of Professional Underlayment Installation

Correct Material Selection

Tailored to your roof: Experts match the underlayment to your home’s slope, climate, and roofing type.
Durability and compatibility: Choosing the right product prevents failures and extends lifespan.

Clean and Secure Application

Proper alignment: Professionals avoid wrinkles, folds, or gaps that compromise protection.
Fasteners and sealing: Roofers use the correct staples, cap nails, or adhesives for lasting results.

Warranty Protection

Manufacturer requirements: Roofing warranties often mandate specific underlayment brands or types.
Code-compliant work: Licensed roofers ensure all materials meet local building standards.

🪵 Underlayment and Cedar Shake Roofing

Cedar shake roofs have unique ventilation and moisture management needs. The right underlayment helps regulate airflow and drainage beneath the shakes.

Breathable systems: Specialized underlayments allow air movement and reduce trapped moisture.
Stain prevention: Helps minimize moisture-related streaks or discoloration on cedar.
Shingle life extension: Aids in protecting both the shakes and the decking underneath.

💪 Protect Your Home From the Inside Out With Shake Guys

Roof underlayment may be hidden beneath your shingles, but it plays one of the most important roles in protecting your home. From resisting leaks to supporting your entire roof structure, underlayment is the unsung hero of a healthy, long-lasting roofing system.

At Shake Guys, we understand that the strength of your roof comes from the layers you can’t see. That’s why we use the highest-quality underlayment materials and install them with care and precision—especially on premium cedar shake and specialty roofing systems.

Request your free inspection today and let Shake Guys deliver total roof protection, from the decking up.