When beginning a roof replacement, many homeowners focus on the shingles or outer surface. However, one of the most important parts of any roofing system is hidden underneath—the underlayment.
Roof underlayment plays a key role in protecting the roof structure. It acts as a barrier between the roof deck and the final roofing material, such as asphalt shingles or tile. Understanding what this layer is and how it works helps clarify how the full roofing system functions.
This article explains roof underlayment types, installation considerations, and how different materials perform based on roof design. The goal is to help property owners be better informed when evaluating options for a new roof.
Understanding What Is Roof Underlayment
Roof underlayment is a sheet material installed directly on top of the roof deck and underneath the outer roofing material. It’s part of all modern roofing systems.
The underlayment serves as a secondary layer of protection. If water gets past the outer layer through wind-driven rain, damaged shingles, or ice buildup, the underlayment helps prevent that moisture from reaching the wood deck.
Older underlayments were made from felt saturated with asphalt, also known as tar paper. Today, most roofing systems use more advanced materials, including synthetic or rubberized asphalt options.
Key functions of roof underlayment include:
- Weather Protection: Stops water from rain, wind, or ice from reaching the roof deck
- Deck Protection: Prevents rot or mold in the wood sheathing
- Added Insulation: Helps maintain consistent temperatures in your home
- Sound Dampening: Reduces noise from rainfall or hailstorms
What Goes Under Shingles: The Critical Layer Of Protection
The material placed directly beneath shingles is called roof underlayment. It sits on top of the roof deck and serves as a protective layer before the final roofing material is applied.
Roof underlayment isn’t optional. It plays a structural role in the roofing system by helping to prevent water from reaching the wooden components of the roof in case the outer shingles are damaged or displaced.
A complete roofing system has three main layers:
- Roof deck – the base structure, usually made of plywood or oriented strand board (OSB)
- Underlayment – a water-resistant or waterproof barrier that covers the deck
- Shingles – the top layer that provides the roof’s outermost protection
These layers work together to manage water, resist wind, and support the overall integrity of the roof system.
Three Main Types Of Underlayment For Roofing
There are three primary types of underlayment used in roof replacements. Each type has unique properties and best-use scenarios.
1. Traditional Felt Underlayment (Asphalt Paper Roof)
Felt underlayment consists of a base layer of organic or fiberglass material saturated with asphalt. This material has been used in roofing for many years.
- Basic Protection: Provides moderate water resistance but breaks down when exposed to sunlight or moisture for too long
- Budget-Friendly: Costs less than other options but doesn’t last as long
- Traditional Choice: Has been used for decades but is being replaced by newer materials in many applications
Felt underlayment comes in different weights, typically labeled as #15 or #30, with the higher number indicating a thicker, more durable material.
2. Synthetic Roof Underlayment
Synthetic underlayment is made from plastic-based materials like polypropylene or polyethylene. It offers stronger protection than traditional felt.
- Superior Strength: Resists tearing and punctures better than felt, making it more durable during installation and over time
- Weather Resistant: Stands up to rain exposure for longer periods without degrading
- Lightweight Design: Easier to carry and install, reducing labor time and effort
Many professional roofers now prefer synthetic underlayment because it provides better protection and is easier to work with than traditional felt.
3. Rubberized Asphalt (Peel And Stick) Underlayment
Rubberized asphalt underlayment uses a self-adhesive backing that bonds directly to the roof deck. It creates a waterproof seal.
- Complete Waterproofing: Forms a watertight barrier that prevents even standing water from penetrating
- Self-Sealing: Creates a seal around nails and fasteners, preventing leaks at these common weak points
- Premium Protection: Offers the highest level of protection, especially in vulnerable roof areas
This type of underlayment is often used in areas prone to ice dams or heavy rainfall, such as roof valleys, eaves, and around roof penetrations like chimneys and vents.
Comparing Underlayment Options: Which Is Best For Your Roof?
The following table compares the main underlayment types to help determine the optimal choice for your project:
| Underlayment Type | Cost | Durability | Water Resistance | Best Applications | Limitations |
|---|---|---|---|---|---|
| Felt | $ | ★★☆☆☆ | ★★☆☆☆ | Budget projects, mild climates | Tears easily, limited water resistance, short lifespan |
| Synthetic | $$ | ★★★★☆ | ★★★★☆ | Most residential roofs, all climates | Higher cost than felt, may require specific fasteners |
| Rubberized Asphalt | $$$ | ★★★★★ | ★★★★★ | Critical waterproofing areas, severe weather regions | Highest cost, not always necessary for entire roof |
Felt underlayment is the most affordable option and works well in areas with moderate weather. However, it’s less durable and has lower water resistance compared to the other types.
Synthetic underlayment offers good durability and water resistance. It’s becoming the standard in modern roofing systems and performs well in a variety of weather conditions.
Rubberized asphalt underlayment provides the highest level of waterproofing. It’s often used in areas of the roof that are more vulnerable to leaks or in regions with frequent storms.
For most residential roofing projects, synthetic underlayment offers the best balance of performance and value. However, the ideal choice depends on your local climate, roof design, and budget.
Selecting The Best Roof Underlayment For Asphalt Shingles
Asphalt shingles work with a properly selected underlayment to create a complete roofing system. The right underlayment helps prevent moisture damage and maintains the roof’s structural integrity.
Synthetic underlayments are commonly used with asphalt shingles because they resist tearing and water exposure. Some shingle manufacturers test specific combinations of shingles and underlayments to ensure compatibility.
Climate affects underlayment choice. In areas with frequent rainfall or potential for ice dams, materials with higher water resistance are often installed in vulnerable areas like valleys and eaves.
The roof’s pitch (steepness) also matters. Low-slope roofs between 2:12 and 4:12 are more prone to water intrusion and often need self-adhering underlayments. Steeper roofs may work fine with standard synthetic materials.
Cost is another factor. Felt reduces upfront expenses but has limited durability. Synthetic materials cost more but remain stable longer during installation and throughout the roof’s life.
Many roofing warranties specify which underlayment types are approved for use. Using a non-approved underlayment might void the warranty coverage, so it’s important to check the manufacturer’s guidelines.
Specialized Underlayment For Different Roof Pitches
Roof pitch refers to how steep or flat a roof is. The pitch affects how water flows across the roof and which underlayment works best.
1. Underlayment For Standard Pitch Roofs (4:12 to 9:12)
Standard pitch roofs are common in residential construction. On these roofs, water flows at a normal rate, allowing for effective drainage.
Synthetic underlayment works well on standard pitch roofs because it:
- Provides consistent coverage
- Installs efficiently across typical slopes
- Offers good protection against occasional water backup
2. Best Underlayment For Low Pitch Roof Applications (2:12 to 4:12)
Low-slope roofs don’t drain water as quickly. Water can stay on the surface longer, increasing the chance of leaks.
For these roofs, self-adhering underlayments like rubberized asphalt membranes are often used because they:
- Form a watertight seal across the deck
- Prevent standing water from seeping through
- Provide extra protection where water moves more slowly
3. Underlayment Requirements For Steep Roofs (9:12 and greater)
Steep-slope roofs cause water to run off quickly, but they face stronger wind uplift and gravity-driven slippage.
Underlayments for steep roofs need:
- High tear strength to resist wind forces
- Slip resistance for safer installation
- Reinforced construction to prevent movement over time
Synthetic materials with reinforced scrims (internal mesh) are commonly used on steep roofs to maintain their position and reduce movement.
How To Choose The Best Underlayment For Flat Roof Projects
Flat roofs, with a slope of less than 2:12, need different underlayment materials than sloped roofs. Standard products like felt or basic synthetics aren’t designed for flat conditions.
Water drainage is slower on flat surfaces, so underlayment is exposed to standing water for longer periods. A waterproof barrier is essential to prevent moisture from seeping into the roof deck.
Flat roof underlayment needs to withstand:
- Standing Water: Completely waterproof materials prevent seepage when water pools
- UV Exposure: Sun-resistant formulations maintain integrity during installation delays
- Temperature Changes: Flexible materials that won’t crack or warp with expansion and contraction
- Foot Traffic: Durable construction that resists tears and punctures during installation
Specialized membranes such as rubberized asphalt, polymer-modified bitumen, or thermoplastic options are common choices for flat roof underlayment. These materials are specifically engineered for waterproofing and stability in low-slope applications.
Professional Installation Guidelines For Roof Underlayment
Proper installation ensures underlayment performs as designed and maintains warranty protection. While DIY installation is possible, professional installation often yields better results.
1. Proper Deck Preparation Techniques
Before applying underlayment, the roof deck must be properly prepared:
- Remove old nails, dust, and debris
- Replace any rotting or warped wood sections
- Ensure the surface is completely dry
A clean, solid deck creates the foundation for effective underlayment performance.
2. Correct Fastening Methods For Different Underlayments
Each underlayment type requires specific fastening methods:
- Felt underlayment typically uses roofing nails or cap staples
- Synthetic underlayments often need cap nails or plastic cap fasteners
- Rubberized asphalt underlayments are self-adhering and don’t require mechanical fasteners
Fasteners should be placed according to the manufacturer’s specific overlap and spacing instructions, especially in areas with high wind exposure.
3. Critical Areas Requiring Extra Protection
Some roof areas need additional underlayment protection:
- Valleys where water collects and flows
- Eaves where ice dams can form
- Around vents, chimneys, and other roof penetrations
These sections often receive an additional layer of underlayment or a self-sealing membrane applied before the primary underlayment to ensure continuous protection.
Partner With A Trusted Roofing Expert In Texas
Working with a knowledgeable roofing contractor helps ensure the correct underlayment is used for your specific project. Local weather patterns and building codes can affect which materials perform best.
Amstill Roofing has over 50 years of experience in Texas and holds certifications from major manufacturers like Owens Corning (Platinum) and GAF (Master Elite). These certifications indicate specialized training and consistent installation practices.
Underlayment selection varies depending on roof slope, material type, and local requirements. Professional installers can identify which underlayment type meets code, supports warranty compliance, and works with your chosen roofing material.
For a roof inspection or consultation regarding underlayment options, contact Amstill Roofing.
FAQs About Roof Underlayment
How long can roof underlayment be exposed before installing shingles?
Felt underlayment can typically remain exposed for up to 30 days, synthetic underlayment for 60 to 180 days, and rubberized asphalt underlayment for up to 90 days, depending on the specific product and manufacturer’s guidelines.
Can you install new shingles over existing underlayment?
Installing new shingles over existing underlayment is generally not recommended because the old underlayment may have tears, moisture damage, or reduced effectiveness that can lead to leaks and potential warranty issues.
What underlayment works best for metal roofing systems?
Synthetic underlayment is typically preferred for metal roofing systems because it resists heat transfer, prevents condensation, and withstands the expansion and contraction of metal panels better than other underlayment types.
How does underlayment affect the overall cost of a roof replacement?
Underlayment typically accounts for 5% to 10% of the total roofing project cost, with premium options like synthetic and rubberized asphalt materials adding more to the bottom line but potentially offering better long-term protection.
Do building codes require specific types of underlayment?
Many local building codes specify minimum requirements for underlayment types and installation methods, particularly in areas prone to high winds, heavy rainfall, or hurricane exposure, so checking local regulations is important.
