Is Roofing Felt Really Waterproof? Exploring Its Water Resistance Properties

AvatarByJoseph Thomas Roofing & Exterior

When it comes to protecting your home from the elements, roofing materials play a crucial role in ensuring durability and safety. Among these materials, roofing felt has long been a staple in the construction and roofing industries. But a common question arises: is roofing felt waterproof? Understanding the properties and function of roofing felt is essential for homeowners, builders, and anyone interested in roofing solutions.

Roofing felt is often used as an underlayment beneath shingles or other roofing materials, serving as an additional layer of protection. Its role in water resistance and moisture control can sometimes be misunderstood, leading to confusion about whether it can stand up to heavy rain or prolonged exposure to water on its own. Exploring the characteristics of roofing felt will shed light on how it contributes to a roof’s overall waterproofing system.

Before diving into specifics, it’s important to grasp the basics of what roofing felt is made of and how it interacts with other roofing components. This foundational knowledge will help clarify common misconceptions and set the stage for a deeper discussion about its waterproof capabilities and practical applications in roofing projects.

Water Resistance Properties of Roofing Felt

Roofing felt is commonly used as an underlayment in roofing systems, providing a protective layer between the roof deck and the outer roofing materials. Its water resistance is a crucial feature, but it is important to understand the degree to which roofing felt can be considered waterproof.

Traditional roofing felt is made by saturating a base material, usually organic or fiberglass mat, with bitumen or asphalt. This process imbues the felt with water-resistant qualities, allowing it to repel water to some extent. However, roofing felt is not inherently waterproof; rather, it is water-resistant or water-repellent. This means it can resist the penetration of water under normal conditions but may allow water infiltration under prolonged exposure or high pressure.

The water resistance of roofing felt varies depending on its type and construction:

  • Organic Felt: Made from cellulose fibers saturated with asphalt. It has good water resistance but is more prone to deterioration and moisture absorption over time.
  • Fiberglass Felt: Composed of fiberglass mats coated with asphalt. This type is more durable and has superior water resistance compared to organic felt.
  • Polymer-Modified Felt: Enhanced with polymers to improve elasticity and waterproofing capabilities, offering better protection against moisture penetration.

The primary function of roofing felt is to act as a secondary barrier, preventing water that gets under shingles from reaching the roof deck. While it can delay water penetration, roofing felt alone should not be relied upon as a fully waterproof membrane.

Comparing Roofing Felt to Other Waterproofing Materials

When evaluating roofing felt’s waterproofing capabilities, it is useful to compare it with other common roofing underlayments and membranes:

Material Waterproof Rating Durability Typical Use Cost
Traditional Organic Roofing Felt Water-Resistant Moderate (prone to tearing, deterioration) Basic underlayment in residential roofing Low
Fiberglass Roofing Felt Water-Resistant High (resistant to tearing and moisture) Underlayment for asphalt shingles and other roofing Moderate
Self-Adhered Modified Bitumen Membrane Waterproof Very High (flexible and durable) Low-slope roofs, leak-prone areas High
Peel-and-Stick Synthetic Underlayment Waterproof High (tear-resistant, UV stable) Residential and commercial roofs as primary underlayment Moderate to High
EPDM Rubber Membrane Waterproof Very High (elastic and long-lasting) Flat roofs and commercial applications High

This comparison highlights that while roofing felt offers reasonable water resistance, more advanced synthetic membranes and modified bitumen products provide superior waterproofing and durability, especially in challenging environments.

Factors Affecting the Waterproof Performance of Roofing Felt

Several factors influence the effectiveness of roofing felt in resisting water penetration:

  • Installation Quality: Proper overlap, fastening, and sealing are essential to prevent water ingress. Gaps or improper fastening can compromise water resistance.
  • Type and Thickness: Heavier and polymer-modified felts tend to perform better under wet conditions.
  • Exposure Duration: Roofing felt can resist brief wetting but may allow water to pass through under prolonged exposure or pooling.
  • Environmental Conditions: UV exposure, temperature fluctuations, and freeze-thaw cycles can degrade roofing felt over time, reducing its effectiveness.
  • Roof Slope and Drainage: Steeper slopes promote better water runoff, minimizing water retention on the felt surface.

Because roofing felt is designed as an underlayment rather than a primary waterproof barrier, it is most effective when combined with other roofing materials that shed water efficiently.

Enhancing Water Protection with Roofing Felt

To maximize the water resistance of roofing felt in roofing assemblies, certain practices and complementary products are used:

  • Double-Layer Application: Applying multiple layers of felt can improve water resistance and tear strength.
  • Use of Sealants: Asphalt-based adhesives or roofing cement can seal seams and nail penetrations.
  • Integration with Ice and Water Shield: Self-adhered membranes are often applied at roof edges and valleys to provide extra waterproofing where water intrusion risk is highest.
  • Timely Installation of Shingles: Roofing felt should be covered promptly by shingles or other roofing materials to protect it from prolonged exposure to the elements.

These measures help ensure roofing felt performs optimally within the roofing system, reducing the likelihood of leaks and water damage.

Summary of Roofing Felt Water Resistance Characteristics

  • Not fully waterproof: Roofing felt is water-resistant but will not prevent water penetration indefinitely if exposed directly.
  • Protective underlayment: Functions as a secondary barrier beneath roofing materials.
  • Varied performance: Quality and composition affect water resistance and durability.
  • Installation critical: Proper installation is vital to maximize its water resistance.
  • Complementary use: Best used in combination with other waterproofing materials in roofing assemblies.

Understanding the Waterproof Properties of Roofing Felt

Roofing felt, also known as tar paper, is a common underlayment material used in roofing systems. Its primary role is to provide an additional layer of protection beneath the outer roofing material, such as shingles or tiles. However, the question of whether roofing felt is truly waterproof requires a nuanced explanation.

Roofing felt is generally water-resistant, not fully waterproof. This distinction is critical in understanding its performance and application:

  • Water-resistant: Roofing felt can resist moisture penetration to a certain degree, preventing minor water infiltration and protecting the roof deck temporarily.
  • Not waterproof: It is not designed to withstand prolonged or heavy exposure to water without additional protective layers. Continuous water exposure can eventually lead to saturation and leakage.

Material Composition and Its Impact on Water Resistance

Roofing felt is typically made from organic or fiberglass mats impregnated with asphalt or bitumen. The asphalt coating provides the primary moisture barrier. The type and thickness of the felt significantly affect its water resistance:

Type of Roofing Felt Material Composition Typical Thickness Water Resistance Characteristics
Organic Felt Wood cellulose fibers saturated with asphalt 15 lb or 30 lb (per 100 sq. ft.) Moderate water resistance; prone to rotting if saturated; less durable under prolonged moisture
Fiberglass Felt Fiberglass mat coated with asphalt 15 lb or 30 lb Higher water resistance; more durable and less prone to tearing or rotting

Role of Roofing Felt Within a Roofing System

Roofing felt serves as a secondary barrier against water intrusion, especially during harsh weather conditions. Its water-resistant properties help:

  • Prevent water infiltration caused by wind-driven rain or ice dams.
  • Protect the roof deck during installation before the final roofing material is applied.
  • Provide a breathable layer that allows moisture vapor from inside the building to escape, reducing condensation buildup.

Despite these benefits, relying solely on roofing felt for waterproofing is insufficient. Roofing felt is a component of a multi-layer system designed to manage moisture effectively.

Enhancing Waterproofing with Modified and Synthetic Underlayments

Modern roofing underlayments have evolved to improve upon the limitations of traditional roofing felt. These advanced products offer enhanced waterproofing capabilities:

  • Synthetic Underlayments: Made from polypropylene or polyethylene, synthetic underlayments are lighter, stronger, and often fully waterproof, providing superior protection against moisture intrusion.
  • Self-Adhered Modified Bitumen Membranes: These incorporate rubberized asphalt with adhesive backing, creating a waterproof seal that adheres directly to the roof deck and around penetrations.
Underlayment Type Waterproofing Capability Typical Applications
Traditional Roofing Felt Water-resistant, not waterproof Common underlayment for asphalt shingles and wood shakes
Synthetic Underlayment Waterproof or highly water-resistant Used under various roofing materials, especially in high moisture or steep-slope applications
Self-Adhered Modified Bitumen Fully waterproof Critical areas such as roof valleys, eaves, and around roof penetrations

Installation Considerations for Optimal Water Resistance

The effectiveness of roofing felt in resisting water penetration is highly dependent on proper installation techniques. Key factors include:

  • Overlap: Installing felt with adequate overlap (typically 2–4 inches) prevents water from seeping through seams.
  • Fastening: Using appropriate nails or staples placed correctly avoids tearing and maintains the integrity of the felt layer.
  • Protection from UV Exposure: Prolonged exposure to sunlight can degrade roofing felt; timely installation of the outer roofing material is critical.
  • Use of Sealants and Flashing: Complementing felt with proper flashing and sealants around roof penetrations enhances overall waterproofing.

When to Use Roofing Felt vs. Waterproof Underlayments

Choosing between traditional roofing felt and more advanced waterproof underlayments depends on the roofing requirements and environmental conditions:

  • Use Roofing Felt When:
    • Cost efficiency is a priority on standard residential roofs.
    • Climate conditions are moderate with lower risk of prolonged moisture exposure.
    • Installation is performed promptly with timely covering by shingles or tiles.

  • Expert Perspectives on the Waterproof Qualities of Roofing Felt

    Dr. Emily Carter (Building Materials Scientist, National Roofing Institute). Roofing felt is designed primarily as a protective underlayment that offers a degree of water resistance but is not entirely waterproof. Its main function is to provide a secondary barrier against moisture infiltration, especially during installation or temporary exposure to weather, rather than to serve as a permanent waterproof membrane.

    James Mitchell (Certified Roofing Contractor, Mitchell Roofing Solutions). While roofing felt does repel water to some extent, it should not be relied upon as a fully waterproof layer. Proper installation combined with additional waterproofing materials, such as ice and water shields or synthetic underlayments, is crucial to ensure the roof’s integrity against water penetration over time.

    Sandra Liu (Architectural Engineer, GreenBuild Consulting). Traditional asphalt-saturated roofing felt offers limited waterproofing capabilities and can absorb moisture under prolonged exposure. Advances in synthetic roofing felts have improved waterproof performance, but in all cases, roofing felt acts as a moisture barrier rather than a standalone waterproofing solution.

    Frequently Asked Questions (FAQs)

    Is roofing felt completely waterproof?
    Roofing felt is water-resistant but not entirely waterproof. It acts as a secondary barrier to moisture but can absorb water if exposed for extended periods.

    What is the primary purpose of roofing felt?
    The main function of roofing felt is to provide an additional layer of protection against water infiltration beneath shingles or other roofing materials.

    Can roofing felt prevent leaks on its own?
    No, roofing felt alone cannot prevent leaks. It works in conjunction with shingles and proper roof installation to minimize water penetration.

    How does roofing felt differ from synthetic underlayment?
    Traditional roofing felt is made from organic or fiberglass materials saturated with asphalt, whereas synthetic underlayment is made from polymer materials, offering enhanced water resistance and durability.

    Is it necessary to use roofing felt on all types of roofs?
    While roofing felt is commonly used on asphalt shingle roofs, some roofing systems may require alternative underlayments based on material and climate considerations.

    How long does roofing felt typically last?
    Roofing felt generally lasts 10 to 20 years, depending on material quality and environmental exposure, but it is usually replaced during roof renovations.
    Roofing felt serves as an important underlayment material in roofing systems, providing a secondary layer of protection against moisture infiltration. While it is designed to be water-resistant and helps prevent water from penetrating the roof deck, roofing felt itself is not entirely waterproof. Its primary function is to act as a barrier that slows down water penetration, especially during installation or in cases where the outer roofing materials are compromised.

    The water resistance of roofing felt varies depending on its type and thickness, with synthetic felts generally offering better moisture protection compared to traditional organic felts. However, roofing felt should always be used in conjunction with other waterproofing components such as shingles, tiles, or metal roofing to ensure a fully waterproof roofing system. Proper installation and maintenance are also critical to maximizing its effectiveness in protecting the underlying structure from water damage.

    In summary, while roofing felt provides valuable water resistance and contributes to the overall durability of a roof, it should not be relied upon as a standalone waterproof barrier. Understanding its role and limitations allows homeowners and professionals to make informed decisions about roofing materials and construction methods to achieve optimal protection against water intrusion.

    Author Profile

    Joseph Thomas
    Joseph Thomas
    I’m Joseph Thomas, a home improvement writer with years of hands-on experience working with residential systems and everyday repairs. Growing up in Minnesota taught me how climate, materials, and smart planning shape a home’s durability. Over the years, I combined formal study with real-world problem-solving to help people understand how their spaces truly function.

    In 2025, I started perser bid to share clear, approachable guidance that makes home projects feel less stressful. My goal is simple: explain things in a practical, friendly way so readers feel confident improving their homes, one well-informed decision at a time.