Roof Valley Flashing Code in Georgia

IRC R905.2.8.2 Valley Flashing Overview

Georgia regulates roof valley flashing through the International Residential Code (IRC), adopted with state-specific amendments by the Georgia Department of Community Affairs (DCA). Section R905.2.8.2 is the controlling code section for valley treatment on asphalt shingle roofs. It defines three permitted valley methods, sets material standards for each, and establishes the minimum dimensions that protect the intersection where two roof planes converge.

A valley collects water from two roof surfaces and channels it toward the eave. This concentration creates the highest water flow rate on the entire roof. During a metro Atlanta thunderstorm, a single valley on a 2,500-square-foot roof may carry 50 to 100 gallons of water per minute to the gutter. That volume demands a treatment method engineered to handle sustained high flow without allowing water to migrate sideways under shingle edges.

The three valley methods permitted under R905.2.8.2 are open valley (metal-lined), closed-cut valley (shingle-over with trimmed course), and woven valley (interlaced shingles from both planes). Each method has distinct material requirements, installation sequences, and performance characteristics. The code does not express a preference among them. All three pass inspection when installed to the specifications in R905.2.8.2.

Manufacturer preferences, however, narrow the field. GAF recommends open valleys for its premium warranty programs. CertainTeed permits both open and closed methods but publishes specific requirements for each. A contractor who follows the IRC but ignores the manufacturer's valley specifications may void the system warranty. Both code compliance and manufacturer compliance must align on the same project.

Local jurisdictions in Alpharetta, Buckhead, Sandy Springs, Marietta, Roswell, and Johns Creek enforce R905.2.8.2 through their building departments. Inspectors check valley treatment during the final roof inspection. Incorrect valley method, missing underlayment, insufficient metal width, or improper shingle trimming will result in a failed inspection and a correction notice.

For the broader context of Georgia's roofing code framework, including the permit process and inspection requirements, see our Georgia residential roofing code guide.

Open Valley: Metal-Lined Valley Construction

The open valley method uses a sheet metal channel that remains exposed between trimmed shingle courses from each roof plane. Water flows down the visible metal channel and exits at the eave. This method provides the highest water-carrying capacity and the longest service life of the three permitted valley treatments.

Metal Specifications

IRC R905.2.8.2 requires open valley metal to be at least 24 inches wide, measured as the flat sheet width before forming into the W-valley profile. The W-profile creates a center crimp (typically 1 inch high) that runs the length of the valley. This crimp prevents water flowing down one roof plane from crossing to the opposite side, a failure mode called "cross-wash" that damages the shingle system on the receiving side.

Acceptable valley metal materials under the IRC include:

• Galvanized steel: Minimum 26-gauge (0.0179-inch base metal thickness). The zinc coating must meet ASTM A653 G90 (0.90 oz/sq ft per side). This is the most common valley metal in metro Atlanta residential construction.
• Copper: Minimum 16 oz per square foot (approximately 0.0216-inch thickness). Copper valleys cost more but last the life of the structure without corrosion.
• Aluminum: Minimum 0.024-inch thickness. Aluminum resists corrosion but dents more than steel or copper.

Valley Metal Material	Minimum Thickness	ASTM Standard	Typical Service Life	Relative Cost
Galvanized steel	26-gauge (0.0179")	ASTM A653 G90	20 to 30 years	$
Copper	16 oz/sq ft (0.0216")	ASTM B370	50+ years	$$$$
Aluminum	0.024"	ASTM B209	25 to 40 years	$$
Painted steel	26-gauge	ASTM A653 + paint system	25 to 35 years	$$

Shingle Trimming Requirements

Shingles on each side of an open valley must be trimmed to maintain a minimum exposed metal width. The IRC does not specify a minimum exposed width, but manufacturer standards and industry practice require at least 4 inches of exposed metal at the top of the valley, widening by 1/8 inch per foot toward the eave. On a 20-foot valley, the exposed metal width at the bottom reaches approximately 6.5 inches. This taper accounts for increasing water volume as the valley descends.

The trimmed shingle edge must be sealed with roofing cement applied in a 3-inch band along the cut line. This sealant prevents wind-driven rain from migrating under the trimmed edge and reaching the valley metal surface. The sealant must not extend onto the exposed metal surface, where it would trap debris and create a dam.

Underlayment in Open Valleys

Before the valley metal goes down, the code requires underlayment in the valley zone. Best practice and manufacturer warranty requirements call for a 36-inch-wide self-adhering ice and water shield membrane (ASTM D1970) centered on the valley line. This membrane creates a waterproof secondary barrier beneath the metal. If the metal corrodes, shifts, or gets damaged by debris, the membrane prevents water from reaching the roof deck.

Standard felt or synthetic underlayment from each roof plane overlaps the edges of the valley membrane by at least 6 inches. The metal then lays over the membrane, with roofing nails placed along the outer edges of the metal only. Nails must never penetrate the center channel of the valley metal, as this creates a water entry point at the highest-flow zone.

Closed-Cut and Woven Valley Methods

Closed-Cut Valley

The closed-cut valley method extends shingles from one roof plane across the valley line and onto the adjacent plane by at least 12 inches. The opposing roof plane's shingles are then trimmed along a chalk line 2 inches from the valley centerline. This creates a clean, visible cut line with one layer of shingles crossing the valley and a trimmed layer on top.

The advantage of the closed-cut method is appearance. The valley presents a clean line without exposed metal, which some homeowners prefer on the front elevation of their home. The disadvantage is reduced water-carrying capacity compared to the open method. The shingle layer crossing the valley creates friction that slows water flow and can trap debris.

The code requires the same underlayment treatment for closed-cut valleys as for open valleys. A 36-inch-wide layer of underlayment (ice and water shield recommended) must run the full length of the valley before any shingles are applied. The crossing shingles from the first plane lay over this underlayment, and the trimmed shingles from the second plane lay over the crossing shingles. Roofing cement seals the trimmed edge in a 3-inch band.

Woven Valley

The woven valley method interleaves shingles from both roof planes across the valley line. Each course from Plane A extends across the valley and onto Plane B by at least 12 inches. The next course from Plane B extends across and onto Plane A by at least 12 inches. This alternating pattern creates a woven appearance with no cut lines or exposed metal.

Woven valleys have limitations. The interlaced shingles create a thick, uneven surface at the valley that can trap debris and slow water flow. The multiple layers at the crossover point make it difficult to identify and repair leaks. Woven valleys also create a bulky appearance that detracts from the roof line on steep-slope homes in premium neighborhoods.

Neither GAF nor CertainTeed recommends woven valleys for their premium warranty programs. GAF's installation guidelines state that woven valleys are the least preferred method for Timberline series products. CertainTeed's technical bulletin for Landmark shingles recommends open or closed-cut methods. A contractor who installs a woven valley may compromise the manufacturer warranty.

Valleys carry more water per square foot than any other roof surface. The treatment method determines whether that water exits the roof or enters the home. Open valleys with W-metal handle the heaviest Georgia thunderstorms.

Method Comparison

Characteristic	Open Valley	Closed-Cut Valley	Woven Valley
Water capacity	Highest	Moderate	Lowest
Debris shedding	Best (metal channel)	Moderate	Poor (traps debris)
Leak repair access	Easy	Moderate	Difficult
Manufacturer preference	Recommended	Permitted	Least preferred
Material cost	Higher (metal + membrane)	Lower (membrane only)	Lowest
Service life	30+ years	20 to 25 years	15 to 20 years

Why Valleys Are the Most Common Roof Failure Point

Insurance adjusters and roofing contractors agree: valleys produce more leak callbacks than any other roof location. The concentrated water flow, complex geometry, and multi-component construction create more opportunities for failure than a simple field area. Understanding the common failure modes helps homeowners recognize early warning signs and helps contractors avoid the defects that cause them.

Top Valley Failure Modes

• Missing ice and water shield: A valley without a self-adhering membrane beneath the metal or shingles relies on the top layer alone to prevent water entry. When shingles age and lose sealant bond, or when metal shifts from thermal expansion, water reaches the deck through nail holes in the underlayment. This is the most common valley failure on roofs installed before ice and water shield became standard practice.
• Insufficient metal width: Valley metal narrower than 24 inches does not provide adequate channel width for high-volume water flow during heavy rain. Water overflows the metal edges and enters under shingles on both sides. This failure is common on budget installations that use 18-inch or 20-inch metal to reduce material cost.
• Nails in the metal channel: A nail placed in the center of the valley metal creates a direct water entry point at the highest-flow zone. Every nail in the channel will leak. Nails must be placed along the outer edges of the metal only, at least 1 inch from the edge.
• Missing shingle sealant: The trimmed shingle edge in an open or closed-cut valley must be sealed with roofing cement. Without sealant, wind-driven rain pushes under the cut edge and pools on the metal or membrane surface. Over time, this persistent moisture accelerates metal corrosion and undermines the shingle bond.

Storm Damage and Valleys

After a storm damage event, valleys should be the first area a contractor inspects. Wind-driven debris collects in valleys because the converging geometry funnels branches, leaves, and granules toward the center channel. Hail concentrates in valleys for the same reason. A valley that sustained hail damage may show fractured shingles, displaced metal, and compromised sealant lines that are invisible from the ground.

When you file an insurance claim for storm damage, the adjuster inspects valleys as a primary damage indicator. A valley with pre-existing defects (missing sealant, inadequate metal, no ice and water shield) complicates the claim because the adjuster must distinguish between storm damage and pre-existing conditions. A valley installed to code with proper materials provides a clean baseline that supports your claim.

Valley Flashing and Related Code Connections

Valley flashing does not exist in isolation. It connects to several other code sections that govern the materials and methods used in the valley zone. A compliant valley installation must satisfy all of these related requirements.

Underlayment (IRC R905.1.1)

The underlayment code requires weather protection across the entire roof deck, including valleys. The valley underlayment (ice and water shield membrane) must integrate with the field underlayment through proper overlapping sequences. Field underlayment courses must overlap the valley membrane edges by at least 6 inches. The valley membrane must extend continuously from ridge to eave without end laps in the valley center zone.

Flashing General Requirements (IRC R903.2)

The general flashing code requires flashing at all intersections of the roof surface with vertical surfaces, penetrations, and direction changes. Valleys are direction changes. The general flashing requirement in R903.2 establishes the baseline that R905.2.8.2 refines with specific valley requirements. A valley that meets R905.2.8.2 also satisfies R903.2.

Drip Edge Integration (IRC R905.2.8.5)

Where a valley terminates at the eave, the valley metal must integrate with the drip edge. The valley metal overlaps the drip edge at the eave, directing water from the valley channel onto the drip edge and into the gutter. A gap between the valley metal and the drip edge creates a water entry point at the fascia board.

Fastener Requirements (IRC R905.2.5)

The fastener code applies to shingles in the valley zone. Shingles adjacent to the valley cut line must be fastened according to the same nail pattern and nail specification as field shingles. The nailing zone must be maintained. Nails placed too close to the valley edge can miss the roof deck or penetrate the valley metal.

Wind Speed Requirements

The wind speed code affects valley installations because wind uplift pressures differ at valleys compared to field areas. Valleys experience complex aerodynamic forces where wind flowing over two converging planes creates turbulence. Enhanced fastener patterns near valley edges help resist these concentrated uplift forces.

A valley connects underlayment, flashing, drip edge, fasteners, and shingle trim into a single assembly. Each component must meet its own code section. Miss one, and the valley leaks.

How 1 Source Installs Valleys in Metro Atlanta

Every roof replacement that 1 Source Roofing performs uses the open valley method with W-valley metal and ice and water shield membrane. This approach exceeds the code minimum, satisfies manufacturer warranty requirements, and provides the longest service life for metro Atlanta's storm-heavy climate.

Our Valley Installation Sequence

Step 1: After deck inspection and repair, we install a 36-inch-wide course of GAF WeatherWatch (ASTM D1970) ice and water shield centered on the valley line, running from eave to ridge. Step 2: We install 24-inch minimum W-valley metal in galvanized steel or painted steel, nailing along the outer edges only with roofing nails spaced 12 inches apart. Step 3: Field underlayment from each roof plane overlaps the valley membrane edges by at least 6 inches. Step 4: Shingles are installed from each plane, trimmed along a chalk line that maintains the 4-inch minimum exposed metal width at the top, widening toward the eave. Step 5: We apply roofing cement along the trimmed edges in a 3-inch band, keeping sealant off the exposed metal surface.

Quality Verification

Our project manager inspects every valley before the crew proceeds to ridge cap installation. We verify metal width, shingle trim lines, sealant application, nail placement, and membrane coverage. We photograph each valley for the project documentation file. This documentation supports manufacturer warranty claims and insurance claims if damage occurs during the warranty period.

For homeowners in Buckhead, Sandy Springs, Johns Creek, and other premium neighborhoods with complex roof lines, valley count matters. A home with 8 to 12 valleys has 8 to 12 potential failure points that must be constructed to code. The difference between a roof that leaks in five years and a roof that lasts 30 years often comes down to how the valleys were treated.

Related Building Code and Technical Guides

These pages cover related code requirements and technical installation standards for Georgia residential roofing:

• Georgia Residential Roofing Code Guide — Complete overview of IRC Chapter 9, permits, and inspections
• Roof Flashing Code Requirements — IRC R903.2 flashing materials, methods, and penetration details
• Ice Barrier and Ice Dam Code — IRC R905.1.2 ice barrier requirements and ASTM D1970 membrane specs
• Chimney Flashing Code Requirements — Base flashing, counter flashing, and cricket requirements
• Drip Edge Code Requirements — IRC R905.2.8.5 material specs, installation sequence, and inspection standards
• Roof Underlayment Code Requirements — IRC R905.1 underlayment standards, synthetic vs. felt, attachment methods
• Asphalt Shingle Code Requirements — IRC R905.2 wind ratings, fastener patterns, slope minimums
• Roof Fastener and Nailing Code — Nail specifications, patterns, and pneumatic nailer requirements
• Roof Flashing Installation Atlanta — Professional flashing installation for metro Atlanta homes
• Chimney Flashing Installation — Step flashing, counter flashing, and cricket construction

Questions about valley flashing on your roof? Call 1 Source Roofing at (404) 277-1377 for a free roof inspection and valley assessment.