What Is Roof Felt Used for and Why It Matters

Roof felt is a traditional underlayment material used beneath roofing shingles and other coverings to provide a secondary layer of protection against water, improve fire resistance, and assist with roof installation. This article explains the primary uses, types, benefits, limitations, installation tips, and modern alternatives so homeowners and professionals can make informed decisions about roof protection.

Use	Benefit	Where Applied
Water Barrier	Temporary and secondary leak protection	Under asphalt shingles, flashing areas
Vapor Control	Reduces moisture movement into decking	Cold climates and attics
Slip Surface	Safer surface for installers	Roof deck during installation
Fire Resistance	Improves rated assembly	Commercial and residential roofs

What Roof Felt Is And How It Works

Roof felt, often called tar paper, is a sheet material made from a base (organic felt or fiberglass) saturated or coated with asphalt or bitumen. It acts as a secondary water-resistant layer between the roof deck and the finished roofing material, shedding incidental moisture and protecting against wind-driven rain that penetrates shingles.

Common Types Of Roof Felt

There Are Several Types Of Roof Felt Available, And Choice Depends On Climate, Budget, And Local Code. Organic Felt Uses cellulose fibers saturated with asphalt and is more pliable but less resistant to rot. Fiberglass Felt Uses inorganic glass mat for better tear resistance and dimensional stability. Synthetic Underlayment Is polymer-based and increasingly used as a high-performance alternative.

Standard Weights And What They Mean

Roof felt is commonly sold in rolls described by weight, such as 15-pound and 30-pound felt. 15-pound felt is lighter and easier to handle, while 30-pound felt is thicker and more durable, offering greater temporary protection during installation.

Primary Uses Of Roof Felt

Roof Felt Serves Multiple Purposes On A Roof System. Its core functions include providing a secondary water barrier, protecting the roof deck during installation, reducing the risk of interior water damage, and improving fire resistance. It also creates a smoother surface for shingle adhesion and reduces wear on shingles from friction.

• Secondary Water Barrier: Protects against leaks if shingles are compromised by wind or damage.
• Installation Safety: Offers a slip-resistant surface for roofers.
• Deck Protection: Shields decking from short-term exposure to weather during construction.
• Vapor Control: Helps moderate moisture movement through the roof assembly in some climates.

Benefits Of Using Roof Felt

Roof Felt Delivers Several Practical Advantages For Both New Roofs And Re-roofs. It extends the roof’s tolerance to water entry, simplifies installation, and can contribute to code-compliant roof assemblies. Felt is also cost-effective and widely available, making it a popular choice for many roofing projects.

Limitations And Risks

Despite Benefits, Roof Felt Has Limitations. Organic felt can absorb moisture and deteriorate over time, and both organic and fiberglass felts have limited UV exposure tolerance. Roof felt is not a permanent waterproofing membrane and will fail if exposed for extended periods. Additionally, improper installation can reduce its effectiveness.

Installation Best Practices

Proper Installation Maximizes The Protection Roof Felt Provides. Felt should be applied to a clean, dry deck, with laps oriented upslope and fastened according to manufacturer recommendations. Underlayment should extend into valleys and overlap correctly at eaves and rakes to ensure continuous protection.

1. Start at the eave and roll upward, overlapping horizontal seams by the specified distance.
2. Use roofing nails or staples at recommended intervals; avoid excessive fastening that tears felt.
3. Seal end laps and transitions at chimneys and vents with compatible flashing and roofing cement.

Building Codes And When Felt Is Required

Local Building Codes Often Specify Underlayment Requirements Based On Roofing Material And Slope. Many U.S. codes call for an underlayment like 15-pound or 30-pound felt under asphalt shingles, while some jurisdictions accept synthetic underlayments. High-wind or cold-climate areas may require specific types or multiple layers for ice and water protection.

Ice And Water Shield Vs. Traditional Felt

Ice And Water Shield Is A Self-Adhering Membrane Designed For Eave And Valley Protection. Unlike traditional felt, ice and water shield bonds to the roof deck to provide a watertight seal where ice dams and wind-driven rain are common. It’s typically used in critical areas while felt remains common over the rest of the roof deck.

Alternatives To Roof Felt

Synthetic Underlayments And Peel-And-Stick Membranes Provide Modern Choices. Synthetic options offer greater tear resistance, UV endurance, lighter weight, and often better water resistance than traditional felt. Choice depends on budget, roof design, and installer preference.

Cost Considerations

Roof Felt Is Generally Economical Compared To Synthetic Underlayments. A roll of 15-pound felt is inexpensive and widely available, while 30-pound felt costs more. Synthetic membranes typically carry a higher material cost but may reduce labor time and offer longer service life. Evaluating long-term performance versus upfront cost is important for informed decisions.

Maintenance And Longevity

Under Normal Conditions, Felt Serves As A Temporary But Effective Layer That Lasts As Long As The Shingles Above It Remain Intact. Felt Is Not Designed For Long-Term UV Exposure, so it should be covered by roof material soon after installation and inspected during roof replacements.Leaks, rot, and visible deterioration indicate replacement is necessary.

Environmental And Safety Considerations

Some Traditional Felts Contain Petroleum-Based Components. Disposal And handling should follow local regulations. Synthetic underlayments can reduce waste over time due to longer lifespans, but they also have production footprints to consider.Installers should use proper safety gear and ventilation when working with adhesives and bituminous products.

Choosing The Right Underlayment For A Project

Selection Depends On Climate, Roof Slope, Shingle Type, Budget, And Local Code Requirements. For standard asphalt shingle roofs in moderate climates, 15-pound or 30-pound felt remains a viable option. In areas with severe weather, synthetic underlayment or a combination of felt and peel-and-stick membranes is often recommended.

Frequently Asked Questions

Does Roof Felt Make A Roof Waterproof?

No. Roof Felt Is A Secondary, Water-Resistant Layer That Helps Prevent Water Intrusion Temporarily. Permanent waterproofing requires proper flashing, membrane systems, and intact exterior roofing materials.

Can Roof Felt Be Left Exposed?

Felt Should Not Be Left Exposed To Sunlight For Extended Periods. UV Exposure Causes Deterioration, So it must be covered by shingles or other roofing materials as quickly as possible.

Is Synthetic Underlayment Better Than Felt?

Synthetic Underlayments Offer Improved Tear Resistance And UV Tolerance, But They Cost More. They Are Often Preferred For Long-Term Performance, Especially In Harsh Climates.

Can Existing Roof Felt Be Reused?

Reusing Existing Felt Is Rarely Recommended. If The Felt Shows Signs Of Damage, Moisture, Or Rot, replacement of the underlayment is best to ensure long-term roof integrity.

Practical Tips For Homeowners

• Verify Underlayment Type: Confirm with the contractor which underlayment is specified and why it’s appropriate.
• Inspect After Installation: Ensure felt is properly lapped and sealed around penetrations and edges.
• Address Ice Damming: Use ice and water shield at eaves in cold climates to reduce damage risk.
• Consider Upgrades: Evaluate synthetic underlayments when replacing an entire roof for improved performance.

Understanding what roof felt is used for and how it functions helps homeowners and professionals make smarter roofing choices. Selecting the right underlayment, installing it correctly, and combining it with proper flashing and ventilation improves roof durability and reduces the likelihood of leaks and interior damage.