Purpose of Roofing Felt: Function, Types, and Installation Tips

Roofing felt is a traditional underlayment used beneath shingles and other roofing materials to add protection against water, wind, and weather. This article explains the purpose of roofing felt, compares common types, outlines installation best practices, and reviews pros and cons so homeowners and contractors can make informed choices.

Type	Weight/Thickness	Common Uses
15-pound Felt	Lightweight	Standard underlayment for asphalt shingles
30-pound Felt	Heavier, More Durable	Steeper slopes, higher durability needs
Synthetic Underlayment	Varies, Lightweight	High-performance alternative to felt

What Roofing Felt Is And Why It Matters

Roofing felt, also called tar paper, is a sheet material made of a base (organic or fiberglass) saturated with asphalt to create a water-resistant barrier. Its primary purpose is to act as a secondary water-shedding layer beneath primary roofing materials, preventing moisture intrusion if shingles fail or during wind-driven rain.

Beyond waterproofing, roofing felt protects the roof deck during installation from rain and UV exposure and provides a temporary weatherproof barrier while work is in progress.

Key Functions Of Roofing Felt

Water Resistance: The most critical role is reducing the chance of water reaching the roof deck when shingles are damaged, aged, or temporarily absent.

Slip Sheet And Fire Resistance: Felt creates a smoother substrate for shingles to lie on and some felt products provide modest fire resistance, improving overall roof assembly performance.

Ice And Wind Protection: In colder climates, felt helps limit ice dam backup into the roof deck, and it provides a secondary barrier against wind-driven rain and snow.

Types Of Roofing Felt And Alternatives

Organic Felt

Organic felt is made from recycled paper or cellulosic fibers saturated with asphalt. It is less expensive and commonly used with asphalt shingles. However, it can be heavier, more prone to tearing, and more susceptible to mold if it remains wet for long periods.

Fiberglass Felt

Fiberglass felt uses a glass-fiber mat coated with asphalt. It resists tearing and shrinks less than organic felt, improving dimensional stability. Fiberglass felt tends to be lighter and more durable under prolonged exposure.

Synthetic Underlayment

Synthetic underlayments are made from polypropylene or polyester blends and are designed to outperform asphalt-saturated felts. They offer superior UV resistance, tear strength, and lighter weight, but usually cost more than traditional felt.

Self-Adhered Membranes

Self-adhered waterproof membranes (e.g., peel-and-stick) are often used at eaves, valleys, and around penetrations. They create a highly reliable waterproof seal where water exposure risk is highest. These membranes are commonly paired with felt or synthetic underlayments across the rest of the roof.

How Roofing Felt Is Installed

Proper installation matters as much as material choice. Felt is typically rolled horizontally across the roof deck from the eave to the ridge, overlapping each course per manufacturer or local code. Common overlaps are 2–4 inches on horizontal laps and 4–6 inches at end laps or as specified by product instructions.

Fastening methods include roofing nails or staples spaced according to the product and roof slope. Careful attention around valleys, vents, chimneys, and flashings ensures water is directed away from vulnerable areas.

Underlayment Placement Best Practices

• Start At The Eave: Lay the first course with a slight overhang to guide water into gutters and away from the fascia.
• Stagger Seams: Avoid continuous vertical seams; stagger end laps to reduce leak paths.
• Seal Critical Areas: Use self-adhered membrane or additional layers at eaves, rakes, valleys, and roof penetrations.
• Follow Slope Guidelines: Some underlayments recommend full coverage only above certain slopes; low-slope roofs may require more robust waterproofing.

When To Use 15-Pound Vs 30-Pound Felt

15-pound felt is lighter and easier to handle, often used on standard residential roofs with asphalt shingles. It provides basic protection but is less durable under prolonged exposure or heavy foot traffic.

30-pound felt is thicker and more tear-resistant, preferred in harsher environments or where shingles may take longer to install. 30-pound felt is recommended when added robustness is needed, though many contractors now choose synthetic underlayment for enhanced performance.

Advantages And Limitations

Advantages: Felt is cost-effective, familiar to roofing professionals, and provides reliable secondary water resistance and temporary protection during construction.

Limitations: Traditional felt can be heavy, tear under wind exposure, and suffer degradation from prolonged UV exposure or wet conditions. It also adds weight to the roof, which may matter for some structures.

Comparing Felt To Modern Alternatives

Synthetic underlayments and advanced self-adhered membranes offer improved tear strength, UV stability, and lighter weight compared to asphalt-saturated felt. They often extend the window of exposure before roof covering installation and reduce installation risks.

Cost considerations remain important: synthetic products typically cost more up front but can reduce replacement and warranty claims, potentially saving money over a roof’s life cycle.

Code, Warranty, And Insurance Considerations

Building codes and shingle manufacturer warranties often specify acceptable underlayment materials and installation practices. Using improper underlayment can void roofing warranties and affect insurance claims after leaks or storm damage.

Contractors should follow local building code requirements and shingle manufacturer instructions to ensure compliance, warranty coverage, and optimal roof performance.

Maintenance And Inspection Tips

Roofing felt is mostly concealed, but its condition can be inferred from roof performance. After storms or high winds, inspect for missing shingles, sagging roof deck, or interior stains that indicate underlayment failure. Promptly address visible damage and maintain gutters to prevent water backup that can stress the underlayment.

During re-roofing, assess underlayment condition and replace if brittle, torn, or waterlogged. Upgrading to synthetic underlayment is often recommended when replacing shingles.

Cost Considerations And Value

Material cost for 15- and 30-pound felt is lower than most synthetic underlayments, making it attractive for budget-sensitive projects. However, long-term value depends on durability, installation speed, and risk of water intrusion.

When evaluating bids, owners should weigh material cost versus potential benefits such as reduced tear risk, longer exposure windows, manufacturer warranties, and insurance implications.

Common Misconceptions

One misconception is that felt alone waterproofs a roof permanently. Felt is a secondary barrier; proper shingles, flashing, and drainage are essential for a waterproof roof system.

Another misconception is that all felt products perform the same. Differences in fiber base, asphalt saturation, and product weight affect tear resistance, UV tolerance, and handling characteristics, which matter for roof longevity and installation quality.

Decision Guide For Homeowners And Contractors

1. Evaluate Climate And Exposure: Harsh climates or long installation windows favor synthetic underlayments or additional self-adhered membranes.
2. Check Manufacturer Requirements: Follow shingle manufacturer recommendations to maintain warranty eligibility.
3. Consider Budget Versus Long-Term Value: Lower-cost felt may be acceptable on short projects, but higher-value underlayments reduce installation risk and potential future repairs.
4. Inspect Existing Roof: On reroofs, assess felt condition and replace damaged or waterlogged underlayment.

Resources For Further Information

For the most reliable guidance, consult local building codes, shingle manufacturer technical documentation, and product data sheets for underlayments. Professional roofers and building inspectors can provide site-specific recommendations based on slope, ventilation, and local weather risks.

Industry organizations such as the Asphalt Roofing Manufacturers Association (ARMA) and National Roofing Contractors Association (NRCA) offer technical resources that clarify underlayment performance and installation standards.

Summary: Roofing felt remains a cost-effective, familiar underlayment whose primary purpose is to provide a secondary water-resistant barrier, temporary protection during installation, and improved roof performance in many conditions. Choosing between 15-pound, 30-pound, synthetic, or self-adhered products should be based on climate, exposure, code requirements, and long-term value considerations.