Exterior Wall Flashing Requirements Under the IRC

The exterior wall flashing requirements under the International Residential Code sit in Section R703.4, which calls for corrosion-resistant flashing at seven specific locations on the outside of a home. Flashing intercepts water that gets past the siding and redirects it away from the framing before it can cause rot or mold. The rules below reflect the 2024 IRC, which forms the baseline most local codes build on. Nearly every state has adopted some version of the IRC, but local jurisdictions often amend it, so confirm the edition and any local changes with your building department before you start.¹

Approved Flashing Materials

Section R703.4 requires that all flashing be corrosion-resistant. The code accepts copper, lead, aluminum, and stainless steel, along with self-adhered membranes that comply with AAMA 711 and fluid-applied membranes that comply with AAMA 714.¹ Table R703.4 sets a minimum for each metal:

Copper: at least 16 ounces per square foot
Lead: at least 2.5 pounds per square foot
Stainless steel: at least 0.015 inches thick
Aluminum: at least 0.019 inches thick

Aluminum carries one important restriction: it cannot be placed in direct contact with masonry or concrete, because the alkaline compounds in those materials corrode it over time. Where flashing meets masonry, use one of the other metals or a membrane product. Flashing can be factory-fabricated or shaped on site, and either satisfies the code as long as the material meets the specifications above.

The Seven Locations Where Flashing Is Required

Section R703.4 lists every place on the exterior wall where flashing must be installed:¹

Around exterior window and door openings, following the detailed provisions in Section R703.4.1
At the intersection of chimneys or other masonry with frame or stucco walls, with projecting lips on both sides under stucco copings
Under and at the ends of masonry, wood, or metal copings and sills
Continuously above all projecting wood trim
Where porches, decks, and stairs attach to a wall or floor assembly of wood-frame construction
Wherever a vertical wall meets a roof plane
At built-in gutters

At every one of these locations, flashing must be applied in a shingle-lapped pattern so each upper piece overlaps the one below it, and it must extend to the surface of the exterior wall finish so water actually discharges instead of getting trapped behind the cladding.¹

Window and Door Openings

Section R703.4.1 gives windows and doors their own detailed rules because they are the largest intentional gaps in the wall envelope. Flashing must extend to the surface of the exterior wall finish or connect to the water-resistive barrier so water can drain. Air sealing is also required around every window and door opening on the interior side of the rough framing gap.²

The code gives you a hierarchy for the installation method. First choice is the window or door manufacturer’s flashing instructions. If those don’t cover your situation, follow the flashing or water-resistive barrier manufacturer’s instructions. If no manufacturer instructions exist, the code’s default applies: pan flashing at the sill, sloped toward the exterior or sealed so water drains outward, with flashing or protection also at the head and sides.² A registered design professional can specify a custom method instead.

Self-adhering flashing tapes are common at these openings. When mechanically attached flexible flashings are used, they must meet AAMA 712. Sequence matters. The sill piece goes on first, then the jambs overlapping the sill, then the head overlapping the jambs. That order creates the shingle-lap that carries water down and out. Reversing it builds a dam against the framing and can also void the window manufacturer’s warranty.

Wall-to-Roof Intersections and Kick-Out Flashing

The junction where a wall meets a roof is where inspectors find the most failures. Step flashing runs along the slope where the roof deck meets the wall, and each piece must extend at least four inches up the wall and four inches out along the roof deck. Metal step flashing must be corrosion-resistant and at least 0.019 inches thick, which corresponds to 26-gauge galvanized sheet.³

The spot that causes the most hidden damage is where the bottom edge of a sloped roof terminates into a sidewall. Without a kick-out diverter, water running down the roof-wall seam has nowhere to go except behind the siding. A kick-out angles outward at the bottom of the step flashing run and pushes water into the gutter or away from the wall. The IRC has required kick-out diverters since the 2009 edition, and they must be sized to handle expected stormwater flow at the termination point.³

Deck Ledger and Attached Structures

Porches, decks, and exterior stairs that bolt to the house create one of the highest-risk penetrations on the entire wall. The 2024 IRC addresses this from two directions. Section R703.4 requires flashing wherever a porch, deck, or stair attaches to a wood-frame wall or floor assembly.¹ Section R507.9.1.5 specifically requires flashing above the deck ledger, and that flashing must prevent water from contacting the house band joist. Deck-specific flashing must be corrosion-resistant metal at least 0.019 inches thick, or an approved nonmetallic material compatible with both the house structure and the decking materials.

The 2024 edition also updated R703.2 to explicitly require the water-resistive barrier to be continuous behind deck ledgers, not just behind the wall veneer.¹ Building officials frequently require photos or a mid-construction inspection here, because once the decking is installed the flashing cannot be verified without disassembly.

Weep Screeds for Stucco Walls

Stucco and plaster trap moisture differently than lap siding, and Section R703.7.2.1 addresses that with a dedicated weep screed requirement. A weep screed is a flashing component at the base of a stucco wall that lets trapped moisture drain to the exterior rather than pooling behind the plaster.⁴

The weep screed must be corrosion-resistant metal or plastic, at least 0.019 inches thick, with a vertical attachment flange of at least 3½ inches. It goes at or below the foundation plate line on exterior stud walls and must sit no less than four inches above bare earth or two inches above paved surfaces. The weather-resistive barrier laps over the attachment flange, and the exterior lath covers and terminates on it.⁴ Burying the weep screed below grade or leaving it out is one of the most common stucco failures.

How Flashing Integrates With the Water-Resistive Barrier

Flashing only works if it connects properly to the water-resistive barrier behind your siding. Section R703.2 requires at least one layer of water-resistive barrier over the studs or sheathing of every exterior wall. Acceptable materials include No. 15 asphalt felt meeting ASTM D226, materials meeting ASTM E2556, foam plastic insulating sheathing systems, and other approved products installed per the manufacturer’s instructions.¹

The barrier must be applied horizontally, with the upper layer lapping over the lower layer by at least two inches. Where horizontal joints occur, the overlap must be at least six inches. The barrier must run continuously to the top of the wall and be terminated at penetrations and building appendages in a way that maintains the exterior envelope.¹

One principle governs the relationship: water hitting any layer must have a downhill path to the outside. Flashing tucks behind the barrier at the top so water flowing down behind the siding meets the barrier, runs down to the flashing, and exits. If the flashing sits on top of the barrier at the upper edge, water runs behind the flashing instead of over it. That layering mistake is invisible once the siding is on but causes concealed rot.

Wall Penetrations

Plumbing vents, dryer exhausts, electrical conduits, and HVAC lines all pass through the exterior wall, and each is a potential entry point for water. The IRC does not list these penetrations as standalone items in the R703.4 flashing location list. Instead, R703.1 requires the exterior wall envelope to provide a weather-resistant barrier, and R703.2 requires the water-resistive barrier to be terminated at penetrations in a way that maintains that envelope.¹ In practice, that means sealing around each penetration with flashing tape, caulk, or a boot so water cannot follow the pipe or conduit into the cavity. Many inspectors treat an unsealed penetration as a code violation under the general envelope requirements even without a line-item listing.

Inspection and What Failure Looks Like

Local building departments enforce flashing requirements through permitting and inspection. An inspector who finds missing or improperly installed flashing will issue a correction notice, and the work cannot proceed until the deficiency is resolved. On existing homes, violations discovered during a sale inspection or renovation permit can delay closing or require remediation before the project moves forward.

The stakes are high because flashing failures hide inside the wall for years. By the time staining appears on interior drywall or a deck feels spongy underfoot, the sheathing, framing, and insulation behind the cladding may need full replacement. Getting each of the seven locations right during construction, and confirming that any local amendments to the IRC don’t add more, is the reliable way to keep water out of the wall.

1
International Code Council. 2024 International Residential Code Chapter 7 Wall Covering
2
UpCodes. R703.4.1 Flashing Installation at Exterior Window and Door Openings
3
Building America Solution Center. Step and Kick-Out Flashing at Roof-Wall Intersections
4
UpCodes. R703.7 Exterior Plaster (Stucco)