Roofing Underlayment and Ice Dam Protection Standards

What Underlayment Does and Why It Matters More Than Most Homeowners Realize

Underlayment is the layer between the roof deck and the shingles. It is the component that most homeowners never see, never think about, and never evaluate when comparing roofing proposals. It is also the component that determines whether your roof deck sustains water damage when — not if — wind-driven rain penetrates beneath the shingle surface during a severe storm. Shingles are the primary water barrier on a roof. Underlayment is the secondary barrier. And the secondary barrier is what saves you when the primary barrier is overwhelmed.

Every shingle roof in metro Atlanta will experience conditions where water gets beneath the shingle surface. Wind-driven rain during thunderstorms forces water upward beneath shingle edges. Hail impacts can crack shingles and expose the underlayment beneath. Ice accumulation during Georgia's winter weather events can dam water at the eaves and force it upward beneath the shingle courses. In each of these conditions, the underlayment is the only remaining barrier between the water and the roof deck. If the underlayment is inadequate — wrong type, wrong application, wrong overlap — the water reaches the deck, saturates the plywood or OSB, and begins the rot and mold progression that leads to structural damage and interior water stains.

Both GAF and CertainTeed specify underlayment requirements for their warranty-eligible installations. The type, the application method, and the locations where enhanced underlayment (ice and water shield) is required are all defined in their installation specifications. A contractor who skips underlayment, uses the wrong type, or fails to install ice and water shield at required locations has installed a roof system that does not meet manufacturer specifications — and the resulting installation may not be covered under the manufacturer warranty.

This page covers the underlayment specifications from both manufacturers, the difference between underlayment types, and the specific locations where ice and water shield is mandatory. For related information on flashing that integrates with underlayment at critical junctions, see our flashing standards page.

Synthetic Underlayment vs. Felt Underlayment — A Direct Comparison

The roofing industry has undergone a significant transition in underlayment materials over the past two decades. Asphalt-saturated felt (commonly called tar paper or felt paper) served as the standard underlayment for generations. Synthetic underlayment — manufactured from woven or spun polypropylene — has replaced felt as the preferred material for the majority of residential installations. Both products serve the same fundamental purpose. Their performance characteristics, however, differ substantially.

Asphalt-Saturated Felt (15-lb and 30-lb)

Felt underlayment has been used in roofing since the early twentieth century. It consists of a cellulose (paper) or fiberglass base saturated with asphalt to provide water resistance. The "15-lb" and "30-lb" designations originally referred to the weight of the material per 100 square feet (per square), though modern felt products are typically lighter than these historical designations suggest.

• Water resistance: Felt absorbs moisture. When exposed to sustained rain during installation (before shingles are applied), felt can absorb water, swell, and wrinkle. These wrinkles create an uneven surface beneath the shingles and can telegraph through the shingle surface as visible ridges after installation.
• Tear resistance: Felt tears relatively easily, particularly when wet. Crews walking on wet felt during installation can tear the material, creating gaps in coverage that are not always visible after shingles are installed over them.
• UV exposure tolerance: Felt degrades rapidly under direct UV exposure. If a roof is not shingled within 30 days of felt application, the felt can deteriorate to the point where it no longer provides adequate secondary protection. On projects where weather delays extend the exposure period, felt may need to be replaced before shingle installation begins.
• Cost: Felt remains less expensive per square than synthetic underlayment. For budget-constrained projects, 30-lb felt meets minimum code requirements in most jurisdictions.

Synthetic Underlayment

Synthetic underlayment is manufactured from polypropylene or polyethylene polymers, typically in a woven or spun-bonded configuration. The material is inherently water-resistant (it does not absorb moisture), significantly stronger than felt, and lighter weight per square foot.

• Water resistance: Synthetic underlayment does not absorb water. It can be exposed to rain during installation without swelling, wrinkling, or degrading. This is a meaningful advantage in Atlanta, where afternoon thunderstorms can interrupt installation unexpectedly.
• Tear resistance: Synthetic products are dramatically more tear-resistant than felt. Crews can walk on synthetic underlayment without tearing it, even on steep slopes where foot traction is aggressive.
• UV exposure tolerance: Most synthetic underlayment products are rated for 60 to 180 days of UV exposure, depending on the product line. This extended exposure window provides a margin of safety for projects where shingle installation is delayed.
• Traction: Many synthetic products include a textured surface that provides superior foot traction compared to felt — a meaningful safety consideration on steep-pitch roofs.
• Coverage: Synthetic underlayment rolls cover more square footage per roll than felt, reducing the number of seams across the roof. Fewer seams mean fewer potential water entry points.

Both GAF and CertainTeed offer their own branded synthetic underlayment products and recommend synthetic underlayment for their premium installation systems. GAF's FeltBuster and Deck-Armor are their synthetic options. CertainTeed offers DiamondDeck and RoofRunner. Using the manufacturer's own underlayment products is typically recommended (though not always required) for their enhanced warranty programs. 1 Source Roofing uses synthetic underlayment on every residential project in metro Atlanta. The performance advantages over felt are clear, and the marginal cost increase is offset by installation efficiency gains.

Self-Adhered vs. Mechanically Attached — Where Each Type Is Required

The distinction between self-adhered and mechanically attached underlayment is one of the most important installation details on any roof. These are not interchangeable products. Each serves a specific function at specific locations on the roof, and using the wrong type in the wrong location creates a waterproofing deficiency that the manufacturer specifications are designed to prevent.

Mechanically Attached Underlayment (Field Application)

Mechanically attached underlayment — whether synthetic or felt — is fastened to the roof deck using cap nails, button-cap staples, or roofing nails with plastic caps. The fasteners hold the underlayment in position on the deck, but each fastener point is a penetration through the underlayment material. Water that reaches the underlayment surface can pass through these fastener holes and reach the roof deck beneath. This is acceptable in the general roof field because the shingle system above provides the primary water barrier — the underlayment only needs to shed water temporarily during installation and provide a secondary barrier during the limited conditions when water penetrates beneath the shingles.

Both GAF and CertainTeed specify mechanically attached underlayment across the general roof field. The overlap requirement between courses is a minimum of 4 inches for synthetic products and 2 inches for felt. On slopes steeper than 4:12, the overlap requirements increase. Fasteners must be driven flush — a protruding fastener creates a bump beneath the shingle that can telegraph through the shingle surface and prevent proper shingle sealing.

Self-Adhered Underlayment (Ice and Water Shield)

Self-adhered underlayment — commonly called ice and water shield or leak barrier — has a rubberized asphalt adhesive backing that bonds directly to the roof deck when the release liner is removed. This adhesive bond creates a waterproof membrane that seals around every nail that subsequently penetrates it during shingle installation. Unlike mechanically attached products, self-adhered underlayment does not leak at nail penetrations. The rubberized asphalt deforms around the nail shank, creating a watertight seal at every fastener point.

This self-sealing property is why ice and water shield is required at every location where water concentration, ice damming, or wind-driven rain is most likely to force water to the underlayment level. These locations include eaves, valleys, penetrations, sidewalls, headwalls, and around chimneys. At these critical points, the underlayment must be fully waterproof — not just water-resistant — because these are the locations where water volume and pressure are highest.

Ice Dam Protection in Georgia — Why It Matters Even in the South

Many homeowners and some contractors in metro Atlanta dismiss ice dam protection as a northern climate concern. This is a mistake that costs Georgia homeowners thousands of dollars in preventable water damage every time a winter weather event moves through the metro area. Georgia does not experience the sustained freeze conditions of Minnesota or Massachusetts. But Georgia does experience freeze-thaw cycles, ice accumulation at eaves, and the specific conditions that cause ice dam formation — and those conditions occur often enough that both GAF and CertainTeed require ice and water shield at eaves for warranty-eligible installations in this region.

An ice dam forms when heat escaping through the roof deck melts snow or ice on the upper portions of the roof. The meltwater flows downward toward the eaves, where the roof extends beyond the heated building envelope and the deck temperature drops below freezing. The water refreezes at the eave, forming a ridge of ice. Subsequent meltwater backs up behind this ice ridge, pooling on the roof surface above the eave. This pooled water is under hydrostatic pressure — it pushes downward and laterally, seeking any gap in the shingle and underlayment system to enter the building. Standard mechanically attached underlayment at the eaves is not waterproof at its fastener penetrations. Ice and water shield is.

Georgia Building Code Requirements

The Georgia State Amendments to the International Building Code require ice barrier (ice and water shield) at eaves in areas where the average January temperature is 25 degrees Fahrenheit or below. While metro Atlanta's average January temperature typically sits above this threshold, the north Georgia counties and higher-elevation areas within the metro fall within or near the requirement zone. Regardless of the code minimum, both GAF and CertainTeed require ice and water shield at eaves as part of their warranty-eligible installation specifications for the entire state of Georgia.

Where Ice and Water Shield Is Required

Both manufacturers specify ice and water shield at these locations for warranty-eligible installations:

• Eaves: Ice and water shield must extend from the eave edge upslope to a point at least 24 inches past the interior wall line. On most residential construction, this means the ice and water shield extends 36 to 48 inches up the roof slope from the eave, depending on the overhang depth.
• Valleys: A 36-inch-wide application centered on the valley centerline — 18 inches on each side. See our valley installation page for complete valley specifications.
• Penetrations: Ice and water shield must surround all roof penetrations — pipe boots, vent stacks, exhaust vents, and electrical mast boots — extending a minimum of 12 inches in all directions from the penetration.
• Sidewalls: GAF requires a minimum 5-inch application of ice and water shield up the sidewall at all roof-to-wall transitions. See our flashing standards page for the complete sidewall specification.
• Headwalls: Ice and water shield at headwall junctions extending a minimum of 18-24 inches down the roof slope from the wall base.
• Chimneys: Ice and water shield must extend 24 inches from the chimney base in all directions.
• Skylights: Ice and water shield around the full perimeter of the skylight frame, extending a minimum of 12 inches beyond the frame on all sides.

The 5-Inch Sidewall Ice and Water Shield Requirement

GAF's updated installation specifications mandate a minimum 5-inch application of self-adhered ice and water shield extending up the sidewall at every roof-to-wall transition. This specification, covered in detail on our flashing standards page, represents a meaningful increase from the previous 4-inch standard.

The technical justification is capillary action at the roof-to-wall junction. During wind-driven rain — an event that occurs dozens of times per year in metro Atlanta — water is forced between the step flashing and the wall surface. Surface tension causes this water to wick upward between the two closely spaced surfaces, potentially rising above the height of the ice and water shield application. At 4 inches, the capillary draw during severe events could exceed the shield coverage. At 5 inches, the safety margin accommodates the worst-case capillary conditions that field testing and failure analysis have documented.

This requirement integrates directly with the step flashing installation at sidewalls. The ice and water shield is applied first — adhered to both the roof deck and the wall sheathing, extending 5 inches up the wall from the deck surface. Step flashing is then installed over the ice and water shield at each shingle course. The ice and water shield provides the waterproof backup layer, while the step flashing provides the durable physical barrier and water shedding mechanism. Together, they create a two-layer defense at the most leak-prone junction on any residential roof.

1 Source Roofing adopted the 5-inch standard immediately upon GAF's publication of the updated specification. Our crews measure the sidewall application at every roof-to-wall junction and photograph the coverage before step flashing is installed. This documentation verifies compliance and protects the homeowner's warranty eligibility. For a full discussion of flashing specifications that integrate with this requirement, see our flashing standards page.

GAF and CertainTeed Underlayment Specifications Compared

GAF Underlayment Requirements

GAF's installation specifications for their Timberline shingle systems require:

• Synthetic underlayment (GAF FeltBuster or Deck-Armor recommended) across the full roof deck field, installed over properly spaced and conditioned decking
• GAF WeatherWatch or StormGuard self-adhered leak barrier at all eaves, extending 24 inches past the interior wall line
• Leak barrier in all valleys — 36-inch minimum width centered on the valley centerline
• Leak barrier at all penetrations, extending 12 inches beyond the penetration in all directions
• Leak barrier at sidewalls — 5-inch minimum application up the wall surface
• Leak barrier at headwalls — extending 24 inches down the roof slope from the wall base
• Felt overlap specification: 2-inch side lap minimum for 15-lb felt, 4-inch for 30-lb felt. GAF recommends 6-inch overlap at all hips and ridges regardless of underlayment type

CertainTeed Underlayment Requirements

CertainTeed's installation specifications for their Landmark shingle systems require:

• CertainTeed DiamondDeck or RoofRunner synthetic underlayment recommended across the full roof deck field
• CertainTeed WinterGuard self-adhered underlayment at all eaves, extending a minimum of 24 inches past the interior wall line
• Self-adhered underlayment in all valleys — 36-inch minimum width
• Self-adhered underlayment around all penetrations and roof-to-wall transitions
• CertainTeed specifies that self-adhered underlayment must be applied over the field underlayment at valleys, not directly to the bare deck with field underlayment over it — ensuring the self-adhered product is the topmost underlayment layer at the valley
• Minimum side lap of 4 inches for synthetic underlayment. End laps (where one roll ends and the next begins on the same course) must be a minimum of 6 inches

For a comprehensive view of how underlayment integrates with the broader installation system, see our technical standards hub and our roof deck requirements page.

Continue Your Research

This page is part of our Roofing Technical Standards resource library:

• Roofing Flashing Standards — step, wall, chimney flashing specs including the 5-inch sidewall requirement that integrates with underlayment
• Roof Valley Installation Methods — open vs. closed-cut valley methods with valley underlayment specifications
• Shingle Nailing and Installation Guide — nail placement, nail count, and temperature specifications
• Roof Deck Requirements — deck condition and preparation before underlayment application
• Roof Flashing Installation in Atlanta — our flashing service page
• Roof Replacement Services — our full replacement process
• GAF Roofing Resources — manufacturer technical bulletins

Technical Bulletins from GAF and CertainTeed

The information on this page is backed by official manufacturer technical bulletins. These documents provide the installation specifications, warranty requirements, and best practices that certified contractors like 1 Source Roofing follow on every project.

• GAF — Underlayment Requirements for Hips and Ridges (PDF)
• CertainTeed — Ice Dam Protection (PDF)
• CertainTeed — Self-Adhered Underlayment (PDF)
• CertainTeed — Underlayment Application in Valleys (PDF)