Types Of Roof Underlayment: Comprehensive Guide To Essential Roofing Layers

Roof underlayment plays a crucial role in providing a secondary barrier against water, weather, and temperature changes. Choosing the right underlayment can enhance roof protection, extend lifespan, and improve energy efficiency for American homes. This guide explores each type, their advantages, drawbacks, and the best uses for residential and commercial applications.

Underlayment Type	Material	Main Benefits	Best For
Asphalt-Saturated Felt	Organic/Natural Fibers	Affordable, Traditional, Water-Resistant	Budget & DIY Projects
Synthetic	Polypropylene/Polyethylene	Lightweight, Durable, Mold and Tear-Resistant	New Construction & Longevity
Rubberized Asphalt (Peel-and-Stick)	Rubberized Asphalt with Adhesive	Superior Waterproofing, Self-Sealing	Ice-Prone, Valued Areas

What Is Roof Underlayment?

Roof underlayment is a protective layer installed directly onto the roof deck, beneath the shingles or primary outer roofing material. Its main function is to act as a secondary weather barrier, shielding the structure from wind-driven rain, snow, and moisture infiltration in case the outer roofing is damaged or fails.

There are several types, each tailored to specific climates, budgets, and roofing materials. Selecting the right underlayment significantly impacts overall roof performance, durability, and energy efficiency.

Asphalt-Saturated Felt (Felt Paper)

Historical Importance

Asphalt-saturated felt—commonly called tar paper or felt paper—has been the traditional standard in American roofing for decades. It is composed of a mat made from organic or fiberglass materials, saturated with asphalt for waterproofing. Typically black in color, it comes in rolls of varying thickness.

Types Of Felt Underlayment

• 15-Pound Felt: Lightweight, economical, commonly used for steep-pitch residential roofs.
• 30-Pound Felt: Heavier, thicker, more durable, preferred for lower pitches or regions with harsher climates.

Advantages Of Asphalt Felt

• Affordability: Cost-effective, widely available at any home improvement store.
• Easy Installation: Suitable for DIY or professional application, flexible and cuttable.
• Good Breathability: Allows moisture in the attic to escape, reducing condensation risk.

Drawbacks Of Asphalt-Saturated Felt

• Lower Water Resistance: Prone to breaking down if exposed for extended periods before roof installation.
• Shorter Lifespan: Less UV and tear resistance compared to synthetics.
• Heavier Weights: 30-pound felt is difficult to handle, especially on steep or large roofs.

Synthetic Roof Underlayment

Modern Roofing Technology

Synthetic underlayment has surged in popularity in recent years. Made from woven or spun polypropylene or polyethylene fabrics, synthetic underlayments offer superior strength, weather resistance, and ease of installation.

Features And Performance

• Lightweight: Rolls cover more area and are easier to lift than traditional felt.
• Slip Resistance: Advanced coating provides safer footing for installers.
• Excellent Tear Strength: Withstands high winds and foot traffic without ripping.
• Longer Exposure Tolerance: Can remain uncovered for weeks if roof installation is delayed.
• Moisture and Mold Resistant: Unaffected by humidity and does not absorb water.

Popular Synthetic Brands And Types

• GAF Tiger Paw: A tough, breathable synthetic layer.
• Grace Triflex: Known for its durability and UV resistance.
• Titanium UDL: Lightweight and highly slip-resistant.

Limitations Of Synthetic Underlayment

• Higher Cost: Premium materials lead to increased upfront investment.
• UV Degradation: Prolonged sun exposure (months) without primary roofing can still damage some products.

Rubberized Asphalt (Peel-And-Stick) Underlayment

High-Performance Waterproofing

Rubberized asphalt—sometimes called “peel and stick” or “ice and water shield”—is engineered for superior waterproofing in the most vulnerable roof areas.

Main Characteristics

• Self-Adhering: Rubberized backing with a peel-away film adheres directly to the roof deck.
• Self-Sealing: Forms watertight seals around nails and fasteners penetrating the material.
• Flexible Application: Easily shapes around valleys, dormers, chimneys, and skylights.

Best Use Cases

• Ice Dams: Essential in northern climates with heavy snowfall and ice buildup risk.
• Penetrations: Highly recommended around roof features prone to leaks.
• Low-Slope Roofs: Adds vital water resistance where shingles alone may not suffice.

Pros And Cons Of Rubberized Asphalt

• Superior Leak Protection: Offers unmatched waterproofing and peace of mind.
• Cold Flexibility: Maintains performance even in freezing temperatures.
• Cost: More expensive than both felt and most synthetic varieties.
• Installation: Proper surface prep and application required for best results—generally not a DIY solution.

Self-Adhered Roof Underlayment

Although rubberized asphalt is the most well-known, self-adhered underlayment options increasingly use flexible polymers and advanced adhesives for new roof assemblies and repairs. These sheets bond directly to the roof deck, eliminating the need for mechanical fasteners.

Self-Adhered Benefits

• Zero Nails: Reduces potential water entry points.
• Customized Layers: Available in different thicknesses for specific climates and roof slopes.
• UV Stability: Some premium versions are designed for extended exposure before coverage is applied.

Comparison Table: Roof Underlayment Options

Underlayment Type	Material	Average Cost (per sq ft)	Water Resistance	Durability	Ease Of Installation
Felt Paper (15 lb)	Organic Asphalt	$0.15 – $0.25	Basic	Good (if covered quickly)	Easy
Felt Paper (30 lb)	Organic Asphalt	$0.25 – $0.40	Better	Moderate	Fair
Synthetic	Polypropylene/Polyethylene	$0.20 – $0.50	Excellent	Excellent	Easy/Moderate
Rubberized Asphalt	Rubber, Asphalt, Polymers	$0.65 – $1.00+	Superior	High	Advanced

Specific Applications Of Roof Underlayment

Steep-Slope Roofs

Traditional asphalt shingles, wood shakes, and slate or tile roofs, typically installed on slopes greater than 4/12, require underlayment to meet building code and shingle manufacturer specifications.

• Felt Paper: Economical for steep-pitched houses in mild climates.
• Synthetic: Adds robustness in areas prone to high winds and sun exposure.

Low-Slope Roofs

Roofs with a low pitch—often below 4/12—demand more effective waterproofing. Rubberized asphalt or multiple synthetic layers are preferred due to their superior water resistance and durability.

Ice And Water Shield Areas

Areas of a roof most susceptible to leaks—such as valleys, eaves, around chimneys, and skylights—are typically protected by specialty ice and water shield underlayments.

Climate Considerations When Selecting Underlayment

Cold, Snowy Regions

The northern U.S. faces snow and ice dams. Building codes often require ice and water shields for the first three feet from the eaves.

• Rubberized asphalt underlayment is standard for these climates.

Hot, Humid States

In southern or coastal states, heat reflection, mold resistance, and UV stability become crucial selection criteria.

• Synthetic underlayments with mold inhibitors and longer exposure ratings work best.

Wildfire-Prone Areas

Some synthetic underlayments are manufactured with enhanced fire resistance, a valuable option for Western states experiencing wildfires.

Building Code Requirements For Roof Underlayment

Local and state building codes across the U.S. specify minimum roof underlayment standards. Most codes reference the International Residential Code (IRC), which dictates:

• At least one layer (or more, depending on roof slope and material).
• Ice and water membrane coverage in ice-prone areas.
• Proper overlap and fastening of underlayment sheets.

Manufacturers’ installation instructions will typically meet or exceed these requirements to maintain roofing warranties.

Energy Efficiency And Roof Underlayment

Breathability And Thermal Performance

The right roofs can decrease attic temperatures in summer and prevent heat loss in winter. Synthetics, especially, can offer improved vapor transmission rates, mitigating attic condensation.

Radiant Barriers

Some high-tech synthetic and self-adhered underlayments feature reflective surfaces to block solar heat gain. This can reduce energy bills and improve comfort, particularly in southern climates.

Compatibility With Roofing Materials

Roofing Material	Recommended Underlayment	Notes
Asphalt Shingles	15 or 30 lb felt, synthetic, or rubberized asphalt (at penetrations)	Building code compliant, rubberized at eaves and valleys
Metal Roofing	Synthetic or self-adhered	Protection against heat and condensation
Tile or Slate	Synthetic or multiple layers of felt	Support heavy tiles, enhance waterproofing
Wood Shakes	Felt or synthetic, breather-type may be needed	Must allow air circulation below shakes
Flat Roofs	Rubberized asphalt, specialized membranes	Key for water and UV protection

Installation Best Practices

• Surface Preparation: Roof deck should be dry, clean, and smooth to prevent punctures and ensure adhesion.
• Proper Overlap: Each row of underlayment must overlap the lower by several inches to channel water downward, away from the deck.
• Secure Fastening: Use nails, staples, or adhesives as specified for the product and climate.
• Managing Penetrations: All pipes, vents, and valleys should be carefully flashed and sealed with compatible materials.

Common Installation Mistakes To Avoid

• Insufficient Overlap: Can allow water infiltration between sheets.
• Loose Fastening: High winds can lift poorly secured underlayment.
• Ignoring Manufacturer Specs: Noncompliance may void material warranties.
• Extended Exposure: Leaving underlayment uncovered too long risks UV and weather damage, especially with felt paper.

Longevity And Maintenance Of Roof Underlayment

Life Expectancy By Type

• Felt Paper: Typically designed to last 10–20 years under shingles; exposure shortens lifespan significantly.
• Synthetic: Can last 20–30 years when kept covered, some products last as long as the outer roof.
• Rubberized Asphalt & Self-Adhered: Often rated for 30 years or more, especially in protected zones.

Inspection And Replacement

• Inspect after severe storms, leaks, or prior to reroofing.
• Replace underlayment whenever installing new shingles or roofing material, unless specifically labeled “re-usable.”

Environmental Impact And Sustainable Options

Green Roofing Initiatives

Some synthetic brands are now made from recycled materials and can be further recycled after removal.

• Asphalt Felt: Biodegradable, but short lifespan can contribute more landfill waste.
• Synthetics: Often last longer, reducing material turnover and jobsite waste.

Emerging Technology

Innovations include plant-based waterproofing, cool-roof underlayments, and recyclable polymer blends. Always check product documentation for environmental certifications.

Choosing The Right Underlayment For Your Roof

There is no “one size fits all” answer. Consider:

• Local climate
• Roof pitch and shape
• Primary roofing material
• Budget and long-term durability
• Building code requirements

The right choice balances initial cost with desired performance, longevity, and environmental impact.

Frequently Asked Questions About Roof Underlayment

How Many Layers Of Underlayment Are Required?

Most building codes require a single layer for standard pitch roofs, but two layers or specialized membranes are called for on low-slope roofs and in ice-prone areas.

Can You Mix Underlayment Types?

Yes. Using peel-and-stick in valleys and eaves, with synthetic or felt across the main deck, is a common and recommended combination for added protection.

Is Roof Underlayment Waterproof?

Most underlayment is water-resistant, not technically waterproof. Rubberized asphalt and some high-performance synthetics offer the best waterproof properties, especially at high-risk locations.

Does Underlayment Affect Roof Warranties?

Yes. Not using the manufacturer-specified or code-approved underlayment may void your shingle or roofing warranty.

How Much Does Roof Underlayment Cost?

Material costs range from $15 per square (100 square feet) for basic felt, up to $100+ per square for premium self-adhered membranes. Installation adds additional labor expenses.

How Long Can Underlayment Be Left Exposed?

Felt paper typically must be covered within days; synthetics may remain exposed for 1–6 months based on manufacturer instructions.

Is DIY Underlayment Installation Recommended?

Felt and some synthetics are feasible for advanced DIYers; rubberized asphalt and self-adhered systems usually require professional installation for optimal performance.