Roof Valley Flashing Requirements Under IRC 2018

Quick Answer

Under IRC 2018 Section R905.2.8.2, roof valleys must be flashed using one of three acceptable methods: open metal valley flashing, closed-cut valley using shingles over a valley liner, or woven valley. Open metal valleys require a minimum 24-inch-wide corrosion-resistant metal flashing centered in the valley. The metal must be at least 0.019 inch thick (26-gauge galvanized steel, 0.019 aluminum, or heavier). Self-adhering polymer-modified bitumen membrane (ice and water shield) is required under the valley flashing as an additional moisture barrier at the valley. Valleys are the highest water volume location on a residential roof because they collect runoff from two adjoining roof planes — inadequate valley flashing is one of the most common locations for roof leaks in residential construction.

What R905.2.8.2 Actually Requires

Section R905.2.8.2 of IRC 2018 Chapter 9 permits three valley flashing methods for asphalt shingle roofing. The open valley method uses exposed metal flashing in the valley channel with shingles trimmed back to expose the metal on each side. The closed-cut valley method uses a continuous run of shingles from one roof plane extending through the valley and over the opposite side, with shingles from the adjoining plane cut along a line at the valley center. The woven valley method alternates overlapping courses from each adjoining plane, weaving them together through the valley without cutting either plane.

For open metal valleys, the metal flashing must be at least 24 inches wide and must be centered in the valley channel. The 24-inch width provides at least 12 inches of coverage on each side of the valley centerline. The metal must be at least 0.019 inch thick — equivalent to 26-gauge galvanized steel sheet or 0.019-inch aluminum sheet. Metal valleys thinner than the minimum are prone to cracking at bends under thermal cycling, which creates small open seams at the bends that allow water infiltration. The metal must be lapped at least 6 inches where sections join in a long valley.

In areas where ice barriers are required at eaves under R905.2.7.1, the ice and water shield protection must also be extended through the valley area. Self-adhering ice and water shield membrane must be applied over the full width of the valley before the valley metal or shingles are installed. The membrane provides a secondary waterproof layer under the valley flashing that prevents water infiltration if the valley flashing develops a small leak — particularly important in valleys where ice dams can force water uphill under the valley metal in cold climates.

For closed-cut valleys, the continuous shingles from one plane extend past the valley centerline by at least 12 inches before the other plane shingles are cut along the valley line. The shingles from both planes must be bedded in roofing cement at the cut edge for the first 6 inches from the cut to prevent wind-driven water from infiltrating at the cut edge. A valley lining of at least one layer of 36-inch-wide No. 15 felt or an equivalent self-adhering membrane must be installed in the valley before the shingles are applied.

For woven valleys, both planes of shingles are applied simultaneously, alternating courses from each plane through the valley zone. A 36-inch-wide valley lining must be installed in the valley before the woven shingle courses are applied. Each shingle course from each plane extends at least 12 inches past the valley centerline before the next course is applied. The woven valley does not expose any metal flashing — it relies on the overlapping shingle layers to shed valley water.

Why This Rule Exists

Valleys are the most hydrologically active locations on a residential roof. Two roof planes collect rainwater and channel all of it into the valley at the intersection, creating a concentrated water flow that can be orders of magnitude greater per unit length than the field area of either roof plane. During a heavy rainstorm, the valley runs full with fast-moving water that can splash and infiltrate under insufficiently overlapping shingle edges or metal valley laps. Inadequate valley flashing — too narrow, too thin, improperly lapped, or missing the secondary membrane layer — is the most common cause of roof leaks in residential buildings after improper flashing at chimneys and skylights.

The multiple acceptable methods — open, closed-cut, and woven — recognize that each provides adequate drainage when properly executed, while allowing roofers to use the technique appropriate for the shingle type, valley geometry, and local practice. The code does not mandate one method over another; it establishes minimum standards that each must meet to provide adequate weather protection.

What the Inspector Checks at Rough and Final

At the roofing inspection, the inspector verifies that the valley flashing method used matches one of the three permitted methods and that the specific requirements for the chosen method are met. For open metal valleys, the inspector checks the metal width, thickness, and lapping at section joints. The inspector verifies that ice and water shield is present under the valley metal where required by the climate zone. For closed-cut valleys, the inspector checks that the continuous plane extends at least 12 inches past the centerline and that the cut edge is sealed with roofing cement. For woven valleys, the inspector verifies that each course extends at least 12 inches past the centerline.

The inspector also checks the valley area for proper integration with the overlying shingles — open valley shingles must be trimmed and sealed correctly, closed-cut shingles must show the correct overlap distance, and woven valley courses must alternate correctly without gaps or misalignment at the valley centerline.

What Contractors Need to Know

Select the valley method appropriate for the shingle type and valley geometry before starting. Woven and closed-cut valleys work best on relatively straight, well-defined valleys. Complex valley geometry with tight angles or valley intersections at unusual configurations may work better with open metal valley flashing because the metal can be fitted to unusual shapes more easily than woven or cut shingle courses. For architectural laminated shingles, the closed-cut method is generally preferred because the thicker laminated shingle profile does not weave as cleanly as standard three-tab shingles, and some manufacturers recommend or require open valley installation for laminated products. Verify the manufacturer valley installation recommendation for the specific shingle product.

For open metal valleys, use a valley metal product with the correct width and material for the roofing conditions. In cold climates with ice dams, a minimum 24-inch width is the code minimum but 28 or 36-inch-wide valley metal provides additional protection for locations with high valley water volumes. Longer laps at metal section joints — 12 inches or more rather than the 6-inch minimum — improve watertightness at the most vulnerable joint location in the valley metal system.

Install ice and water shield in the valley before metal flashing in all climates, not only in areas where the code specifically requires it for the eave zone. Ice and water shield under valley metal is a construction best practice that provides a meaningful additional layer of protection against infiltration at the valley, one of the highest-risk leak locations on any residential roof.

What Homeowners Get Wrong

Homeowners re-roofing sometimes accept bids from contractors who propose woven valley construction for laminated architectural shingles without knowing that many architectural shingle manufacturers do not recommend or warrant woven valley installation for their thicker laminated products. The thick profile of laminated shingles does not create the flat, tight weave that effectively sheds valley water — instead it creates a series of ridges and hollows in the valley that can dam water and allow infiltration. Verify the manufacturer recommendation for the specific shingle product before accepting a woven valley bid.

Another common error is homeowners patching valley leaks with roof cement or sealant applied over an existing valley that has deteriorated. A deteriorated valley metal with corroded sections, uplifted edges, or open seams must be replaced, not patched. Roof cement applied over failing valley metal creates a temporary cosmetic fix that fails when the cement ages and cracks, and the underlying failing metal continues to deteriorate under the cement. A re-valley using new metal and new ice and water shield is the only durable repair for a failing open metal valley.

Homeowners also sometimes assume that valleys never need maintenance because they are installed as part of the roofing system. Valley metal accumulates leaf debris, pine needles, and sediment that slow drainage and hold moisture against the metal, accelerating corrosion. Valleys should be inspected and cleaned at least annually, and any debris accumulation that dams the valley should be removed before the next rain season.

State and Local Amendments

IRC 2018 states including TX, GA, VA, NC, SC, TN, AL, MS, KY, and MO adopted R905.2.8.2 valley flashing requirements. The three permitted valley methods and the 24-inch minimum metal width are uniform across all adopting jurisdictions. In cold climates with ice dam risk — VA mountains, NC mountains, KY, and MO — ice and water shield under valley flashing is required in the eave zone and is strongly recommended as a best practice throughout the valley length. Coastal states with high wind and rain exposure benefit from wider valley metal and more robust lapping at metal joints. IRC 2021 retained R905.2.8.2 with no substantive change to the valley flashing methods, minimum metal width, or material thickness requirements for asphalt shingle roofing.

Some jurisdictions require that valley flashings be shown on the permit drawings for complex roof configurations with multiple valleys. Verify with the local AHJ whether detailed valley flashing documentation is required for the permit application.

When to Hire a Licensed Contractor

Valley flashing is a technically demanding portion of the roofing installation that requires correct method selection, correct underlayment sequencing, and proper integration with the adjacent shingle field. A licensed roofing contractor experienced with the specific shingle product and valley method should perform the valley installation. For existing homes with valley leaks, a licensed roofing contractor should diagnose the specific failure mode and determine whether repair or full valley replacement is appropriate. Partial repairs over deteriorated valley metal typically do not provide long-term weather protection.

Common Violations Found at Inspection

• Open metal valley width less than 24 inches — metal narrower than the required minimum provides insufficient coverage at each side of the valley centerline.
• Metal valley thickness below 0.019 inch — thin metal cracks at bends under thermal cycling, creating open seams.
• Ice and water shield not installed under valley flashing in a climate zone where ice barrier protection is required at the eave zone.
• Metal valley laps less than 6 inches — section joints in the metal may separate under water pressure or thermal movement, creating infiltration paths at the laps.
• Closed-cut valley shingles not sealed with roofing cement at the cut edge — wind-driven water can infiltrate behind the unsealed cut edges at the valley line.
• Woven valley used for laminated architectural shingles where the manufacturer specifies open or closed-cut valley installation only.
• Valley metal installed without primary underlayment under the metal — metal sits directly on bare deck without the required underlayment layer.