Roof Flashing Requirements Under IRC 2018

Quick Answer

Under IRC 2018 Section R903.2, flashing must be installed at all roof and wall intersections, at the perimeters of skylights and roof openings, at penetrations through the roof covering, and at any change in roof slope or direction. Flashing must be corrosion-resistant metal with a minimum thickness of 0.019 inch (26-gauge galvanized steel or equivalent), or an approved non-metallic material. The purpose of flashing is to direct water away from the roof surface and prevent infiltration at the transitions and penetrations where the roof covering cannot provide continuous weather protection. Flashing at walls, chimneys, skylights, vents, and valleys must be integrated with the roof covering and underlayment to form a watertight assembly at each location.

What R903.2 Actually Requires

Section R903.2 of IRC 2018 Chapter 9 requires flashing at all roof and wall intersections, valleys, and penetrations through the roof surface. The code identifies specific locations where flashing is required: where the roof surface meets a vertical wall (such as a dormer wall, chimney, parapet, or exterior wall above the roof), at penetrations of roofing by chimneys, vents, pipes, skylights, and mechanical equipment, at roof valleys, and at any change in roof plane direction. At each of these locations, the roof covering material alone cannot provide continuous weather protection without supplemental flashing that bridges the transition between the roof plane and the adjacent vertical or horizontal surface.

Flashing materials must be corrosion-resistant. IRC R903.2.1 permits galvanized steel, stainless steel, aluminum, copper, lead-coated copper, and approved plastic or rubber materials. The minimum thickness for metal flashing is 0.019 inch for galvanized steel (approximately 26-gauge sheet metal) or equivalent. Thinner materials are prone to distortion, cracking at bends, and failure at fastener holes under thermal cycling and wind loading. Lead flashing, while historically used, is limited in many applications due to environmental concerns — galvanized steel and aluminum are the most common materials in modern residential construction.

The specific installation requirements for flashing at different locations are provided in R905 by roofing material type. For asphalt shingle roofing, step flashing at walls is required per R905.2.8. Step flashing consists of individual L-shaped metal pieces, one per shingle course, interlaced with the shingles and turned up the vertical wall surface. A separate counterflashing or cap flashing covers the turned-up step flashing leg and is embedded in mortar or fastened to the wall surface. Continuous piece flashing folded to cover both the roof and wall surfaces in one piece is a common substitution that is less waterproof than properly interwoven step flashing because it relies on sealant rather than mechanical overlap and drainage.

At chimney flashings, the code requires base flashing integrated with the shingles at the front and sides, step flashing at the sides, and saddle or cricket flashing at the rear of chimneys wider than 30 inches. The cricket is a ridge structure behind the chimney that diverts water around the chimney and prevents ponding against the rear face of the chimney. Missing crickets on wide chimneys are a common source of chimney flashing leaks.

Why This Rule Exists

Water infiltration at roof transitions and penetrations is the leading cause of water damage in residential buildings. A well-installed roof covering on the field area will shed water effectively, but the complex geometry at walls, chimneys, skylights, and valleys creates locations where water must be actively redirected rather than simply flowing off the roof surface. Improperly installed or missing flashing at these locations allows water to infiltrate behind the roof covering where it contacts wood framing, insulation, and interior finishes, causing structural rot, mold growth, and damage to interior materials. Proper flashing installed at all required locations prevents these infiltration paths and maintains the weather-tight integrity of the building envelope at its most vulnerable transition zones.

The corrosion-resistance requirement ensures that flashings remain functional for the expected service life of the roofing system. Flashing that corrodes, cracks, or pulls away from its attachment in the first few years after installation is worse than no flashing because it creates hidden pockets where water can pool and infiltrate without being visible from the exterior.

What the Inspector Checks at Rough and Final

At the roofing inspection, the inspector verifies that flashing is present at all required locations — wall intersections, chimneys, skylights, pipe penetrations, and valleys. The inspector checks that metal flashing is the correct minimum thickness and that it is properly integrated with the roof covering and underlayment. For step flashing at walls, the inspector verifies that individual step pieces are interwoven with shingle courses rather than folded continuous flashing relying solely on sealant. At chimney locations wider than 30 inches, the inspector checks for the required cricket or saddle flashing at the rear upslope face. At pipe penetrations, the inspector checks for listed pipe boot flashing products or lead-flashed boots at each vent pipe.

At final inspection, the inspector may probe suspicious flashing locations to check for secure attachment and watertight integration with the roofing materials. Flashings that are not embedded in the shingle layers, not caulked at critical joints, or not secured against wind uplift are deficiencies that must be corrected before the final inspection passes.

What Contractors Need to Know

Install step flashing at every wall-roof intersection using individual step pieces for each shingle course, not a continuous bent piece. Step flashing properly installed creates a mechanical water diversion that does not rely on sealant to be watertight — the overlapping geometry of shingles and flashing pieces directs water off the roof under all weather conditions. Continuous bent flashing with sealant at the wall-roof intersection may appear similar at installation but fails when the sealant ages, cracks, or separates, creating a hidden leak path that is difficult to diagnose and repair.

Install ice and water shield membrane under flashing at all valley, chimney, and skylight locations in addition to the required metal flashing. The membrane provides redundant waterproofing that functions even if the metal flashing develops a small leak. Ice and water shield at chimney bases is especially important in northern climates where ice damming can back water up under the chimney base flashing.

Crickets behind chimneys wider than 30 inches must be properly flashed as a waterproof assembly — the cricket flashing must integrate with the rear base flashing and the main roof shingles. A cricket that is framed and sheathed but not properly flashed and shingled provides no water protection. Include cricket flashing details in the roofing scope for every chimney wider than 30 inches in the completed installation.

What Homeowners Get Wrong

Homeowners frequently apply roof sealant or caulk over failing flashing as a temporary fix without repairing or replacing the underlying flashing. Roof sealant applied over corroded, cracked, or lifted flashing creates a temporary cosmetic fix that masks the failing flashing without addressing the underlying failure. Sealant degrades in UV exposure and temperature cycling, and when it fails, the water infiltration resumes at the same location. Replacing failing flashing completely is the only durable solution.

Another common error is homeowners sealing roof pipe penetrations with foam sealant or spray foam rather than using listed pipe boot flashing products. Foam sealant around pipe penetrations is not a listed roofing flashing material and will crack and separate with the differential movement between the rigid foam and the pipe and roofing materials. Listed pipe boot flashings — lead, neoprene, or aluminum boot products designed for roofing applications — provide durable watertight pipe penetration seals that accommodate pipe movement and maintain weather resistance over decades.

Homeowners also sometimes remove or cut back flashing at chimney or wall locations when performing interior renovation, not realizing the flashing is providing critical weather protection from the exterior. Any modification to flashing at the exterior envelope must be evaluated by a roofer before the interior work is performed, and the flashing must be properly reinstated as part of the renovation work.

State and Local Amendments

IRC 2018 states including TX, GA, VA, NC, SC, TN, AL, MS, KY, and MO adopted R903.2 flashing requirements. The 0.019-inch minimum thickness for metal flashing and the step flashing requirements at walls are uniform across all adopting jurisdictions. High-humidity and coastal states with aggressive corrosion environments — SC, GA, AL, coastal TX, and MS — benefit from higher corrosion-resistance flashing materials such as aluminum or stainless steel at coastal locations where galvanized steel corrodes faster due to salt air exposure. IRC 2021 retained R903.2 flashing requirements with no substantive change to the flashing material requirements or the required flashing locations. Some provisions for self-adhering membrane flashing were clarified.

Some jurisdictions have adopted storm-resistant construction requirements for coastal areas that specify enhanced flashing details at wall-roof intersections and chimney locations beyond the base IRC minimum. Verify local coastal construction requirements for the project site.

When to Hire a Licensed Contractor

Roof flashing installation is part of the roofing contractor scope and must be performed by an experienced roofer who understands the integration of flashings with the specific roofing material installed. Flashing details for asphalt shingles, tile, metal roofing, and other systems differ substantially — a roofer experienced with the specific material can install correct flashings that function correctly and meet the code requirements. For existing homes with active roof leaks at flashings, a licensed roofing contractor should be engaged to diagnose and repair the failing flashing rather than attempting a sealant repair over the existing failed flashing.

Common Violations Found at Inspection

• Step flashing at wall-roof intersections replaced with continuous bent flashing relying on sealant — not equivalent to properly interwoven step flashing and fails when sealant ages.
• Chimney wider than 30 inches without a cricket or saddle flashing at the rear upslope face of the chimney.
• Metal flashing below the 0.019-inch minimum thickness — thin aluminum scraps or tar paper at flashing locations do not meet the material requirement.
• Pipe penetrations through the roof surface not flashed with listed pipe boot products — foam sealant or caulk applied around the pipe is not an acceptable flashing.
• Flashing not integrated with the underlayment — metal flashing sitting on top of shingles rather than integrated under shingles and over the underlayment in the correct weather-lapping order.
• Valley flashing installed without proper integration into the shingle field — open metal valley not lapped under the adjacent shingle courses as required.
• Skylight flashings incomplete — missing end caps at upper and lower ends of the skylight frame or step flashings at the sides not properly integrated with shingle courses.