IRC 2024 Valley Flashing: Open vs Closed Valley Installation Methods

Quick Answer

IRC 2024 Section R905.2.8 requires that roof valleys be lined with either a minimum 24-inch-wide valley metal flashing or a self-adhering ice and water shield membrane. Metal flashing must be minimum 26-gauge galvanized steel, aluminum, or equivalent corrosion-resistant material. For asphalt shingle roofing, three installation methods are permitted: open valleys (metal exposed), closed-cut valleys (shingles cut diagonally over valley flashing), and woven valleys (alternating shingle courses woven across the valley).

Under IRC 2024, no exposed nails are permitted in the valley itself in any method, as nail holes in the valley centerline create direct water entry points. In cold climates, ice and water shield is additionally required at valleys regardless of which installation method is used.

What IRC 2024 Actually Requires

Section R905.2.8 addresses all flashing for asphalt shingle roofs. The valley provisions within this section establish minimum material, size, and installation requirements while permitting the three industry-standard valley methods.

Valley metal specifications. When metal is used as the valley material, it must be a minimum of 24 inches wide (12 inches on each side of the valley centerline), minimum 26-gauge galvanized steel, 0.019-inch-thick aluminum, copper, or equivalent corrosion-resistant metal. The metal must be free of holes, tears, or seams along the centerline. Sections of metal valley are lapped a minimum of 4 inches with the upper piece over the lower piece (like shingles) so water sheds over the lap rather than into it.

Ice and water shield at valleys. In climates where the average January temperature is 25°F or below, the ice and water shield requirement for eaves extends to valleys. The membrane must be a minimum of 36 inches wide (18 inches on each side of the centerline) and applied before the metal flashing or any shingles. In mild climates, a valley liner equivalent to the underlayment is required at minimum.

Open valley method. In an open valley, metal flashing is installed in the valley and left exposed — the shingles on each side are cut at an angle, leaving a visible strip of metal 3 to 5 inches wide at the top of the valley, widening to 5 to 8 inches at the bottom as the valley collects more water volume. The clip line (the edge where shingles are cut) must be chalk-lined to ensure straight edges. Nails must not penetrate the metal valley — shingles are nailed outside the valley centerline and the metal itself is secured only at its edges, away from the flow channel.

Closed-cut valley method. One roof plane’s shingles are run across the valley and extended at least 12 inches beyond the centerline onto the opposite slope. The second plane’s shingles are then run to the valley and cut diagonally along a chalk line 2 inches from the valley centerline. The cut edge is embedded in roofing cement or a bead of compatible sealant. No nails may be placed within 6 inches of the valley centerline. An ice and water shield membrane or metal liner must be installed before any shingles are applied in a closed-cut valley.

Woven valley method. Both roof planes’ shingles are extended across the valley and woven (alternated) with each course. Each shingle from both planes extends at least 12 inches beyond the centerline onto the opposite slope. No nails may be placed within 6 inches of the valley centerline. A valley liner must be installed before weaving begins. Woven valleys are the least common of the three methods in modern construction because they do not work with laminated (architectural) shingles — the thickness variation creates an uneven surface through the valley.

Why This Rule Exists

Valleys are the highest-risk area of any roof system. A valley collects runoff from two intersecting roof planes and channels a disproportionate volume of water through a relatively narrow area. During heavy rainfall, valley flow rates can exceed the capacity of improperly installed valley systems. The concentration of water, combined with the relatively stagnant flow at the valley centerline where the two slopes meet, creates ideal conditions for capillary infiltration under any shingle or lap that is not well above the waterline.

The prohibition on exposed nails in the valley is particularly important. A nail in the valley centerline creates a hole in the primary drainage channel. Even with sealant applied over the nail head, sealant degrades in 5 to 15 years and the hole eventually admits water. The valley metal itself — or the membrane — must remain free of fastener penetrations in the flow zone.

Ice dam formation is especially severe in valleys. Water backing up behind an eave ice dam travels upslope; in a valley it also travels horizontally into the valley throat from both sides simultaneously. Ice and water shield’s self-sealing membrane is the only underlayment product that provides adequate protection in these conditions.

The widening of open valleys toward the bottom (3-inch minimum clearance at top widening to 5-inch at bottom) is calibrated to flow volume. As the valley descends, it collects runoff from an increasing area of both roof planes. Keeping the shingle edges far enough from center prevents water from overtopping the valley edge under high-volume flow events.

What the Inspector Checks at Rough and Final

Inspectors check valleys at two stages: before shingles are applied (to verify the liner is present and correctly installed) and at the final roofing inspection. Inspectors evaluate:

• Valley liner material — metal width confirmed at 24 inches minimum; ice and water shield width confirmed at 36 inches minimum
• Metal gauge confirmed by product label or material specification
• No nails driven through the metal valley material in the flow zone
• Metal lap joints oriented with upper section overlapping lower, minimum 4-inch lap
• Shingle cut lines for open and closed valleys are straight and at the correct offset from centerline
• Closed-cut valley: extending shingles cross the valley centerline by at least 12 inches
• Closed-cut valley: cut edge sealant applied at the diagonal cut line
• No nails within 6 inches of centerline in closed-cut or woven valleys
• Ice and water shield at valley in cold climates, installed before metal liner if both are used

What Contractors Need to Know

The choice of valley method has practical consequences beyond code compliance. Woven valleys, while permitted by the IRC, cannot be executed with architectural (laminated) shingles because the dimensional thickness variation of laminates makes the woven surface irregular, creating voids that trap debris and moisture. Essentially all modern shingle manufacturers explicitly prohibit woven valleys with their laminated products. Since architectural shingles are the dominant product in the current market, woven valleys are rarely appropriate.

Closed-cut valleys require that the cross-run shingles (those extending past the centerline) be fully nailed before the cut shingles are installed. Nailing the cross-run shingles after installation of the cut course requires nailing through both layers — which places nails in or near the valley and violates the no-nail zone requirement.

Valley metal must be pre-bent to the valley angle before installation. Forcing flat metal into a valley without pre-bending creates stress concentrations at the centerline that cause the metal to crack over time, particularly in climates with significant thermal cycling. A V-crimp or W-crimp profile pre-bent to the valley angle is far more durable than flat sheet bent in the field.

Galvanic corrosion is a concern when dissimilar metals are in contact. Aluminum valley metal should not be in direct contact with copper flashings (such as those around some chimney flashing applications), and zinc-coated (galvanized) metal should not contact copper either. In these cases, isolation tape or a non-reactive sealant bead prevents galvanic deterioration.

What Homeowners Get Wrong

Homeowners often do not know which valley method is installed on their roof until a leak occurs. The distinction matters for maintenance and repair. Open valleys are easy to inspect visually — the metal is visible. Closed-cut and woven valleys require lifting shingles to access the valley liner, which should only be done by a roofing professional.

A common homeowner mistake is applying tube sealant over a leaking valley as a DIY repair. Caulk does not adhere reliably to weathered shingles and metal in wet conditions, and any caulk that bridges a gap in the valley becomes a debris trap. The correct repair for a leaking valley involves removing shingles to access the liner, identifying and correcting the failure point, and reinstalling new shingles. Caulk is not a substitute for a proper valley liner.

Homeowners shopping for a re-roof should ask specifically whether the valley liners will be replaced. Some contractors reinstall new shingles over old valley metal, particularly in closed-cut valleys where the metal is not visible and its condition cannot be assessed without demolition. The cost savings are minimal relative to the risk of leaving a corroded or perforated liner in place under new shingles.

State and Local Amendments

In California’s Wildland-Urban Interface (WUI) zones, open valleys that could trap burning debris are sometimes disfavored in favor of closed-cut or woven installations. Local fire officials may have additional requirements about valley design in high fire-risk areas.

Florida Building Code in High Velocity Hurricane Zones requires specific metal gauges and widths that exceed IRC minimums. Miami-Dade County product approval requirements for valley materials are among the most stringent in the country and govern product selection in addition to installation method.

Some mountain jurisdictions with high snow and ice loads extend the required coverage of ice and water shield beyond the standard eave-and-valley minimum, requiring it throughout the entire valley for the full roof height rather than only at the eave portion of the valley.

When to Hire a Professional

Valley flashing is not DIY-appropriate for most homeowners. The precision required in cutting open and closed-cut valleys, integrating the liner with adjacent underlayment, and sealing the system against wind-driven rain requires significant experience. Errors in valley installation are among the most common causes of chronic roof leaks — leaks that appear years after installation and require investigative work to trace back to the valley origin.

Any home with a history of valley leaks should be evaluated by a licensed roofing contractor who will inspect the liner condition from below (from the attic) before recommending whether a partial re-flash or complete re-roof of the valley area is appropriate.

Common Violations Found at Inspection

• Valley metal narrower than 24 inches, leaving insufficient coverage on one or both slopes
• Nails driven through valley metal in the flow zone, creating holes in the primary drainage channel
• Metal lap joints reversed — lower section overlapping upper — so water is directed into the lap seam
• Closed-cut valley shingles not extending 12 inches past the centerline onto the opposite slope
• No ice and water shield at valleys in cold-climate jurisdictions where it is required
• Woven valley installation with architectural (laminated) shingles, which manufacturers prohibit
• Open valley cut lines not straight — irregular edges trap debris and divert water out of the channel
• Nails within the 6-inch no-nail zone on either side of the centerline in closed-cut or woven installations