Exterior Wall Flashing Requirements — IRC 2018

Quick Answer

IRC 2018 R703.4 requires flashing at every location on an exterior wall where water could enter or accumulate: window and door heads and sills, wall penetrations, horizontal surfaces intersecting the wall, wall-to-roof intersections, and the top of masonry and concrete foundation walls at grade. Flashing must direct water back to the exterior and integrate with the water-resistive barrier behind the siding.

What R703.4 Actually Requires

Section R703.4 of the IRC 2018 requires that flashing be installed to prevent moisture from entering the wall assembly at vulnerable locations. The code lists specific required flashing locations:

• At the top of all exterior wall openings — a head flashing diverts water coming down the face of the wall away from the top of the window or door frame.
• At all exterior wall penetrations — pipes, wires, vents, and other penetrations require a flashing boot or weatherproof seal that integrates with the WRB.
• At the intersection of the exterior wall and roof, horizontal surface, or lower roof — continuous step flashing or counter-flashing must direct water away from the wall junction.
• At the top of masonry chimneys — flashing and counterflashing at the chimney-to-roof junction.
• At the tops of foundation and retaining walls — cap flashing directs water away from the top of masonry or concrete where it contacts wood framing.

Flashing materials permitted under R703.4 include: sheet metal (corrosion-resistant), self-adhering polymer-modified bitumen membrane (rubberized asphalt), self-adhering flashing tape, EPDM, or other materials specifically listed for this use. The material must be compatible with any adjacent materials — for example, flashing in contact with ACQ pressure-treated wood must be corrosion-resistant to copper-based preservatives.

The code does not specify a universal flashing width, but it requires the flashing to be wide enough to direct water to the exterior face of the siding, not into the wall assembly. Generally, head flashings must extend under the WRB above and lap over the top of the window or door frame a minimum of 3 inches. Sill flashings (pan flashings) must slope slightly to drain to the exterior.

R703.4 also addresses a critical integration requirement: flashing must be installed in proper sequence with the WRB so that the two systems work together as a continuous drainage plane. Water that drains to the WRB from behind the siding must flow to the flashing, which directs it out the face of the wall.

Why This Rule Exists

Water does not travel only straight down — it follows capillary action, wind pressure differentials, and gravity into every gap and joint in a wall assembly. Openings, penetrations, and horizontal surfaces are the highest-risk locations because they create gaps in the continuous WRB and provide horizontal surfaces where water can collect before finding a path inward. Flashing provides a physical barrier and drainage pathway at each of these high-risk locations, redirecting water to the exterior before it can reach the structural framing or sheathing.

What the Inspector Checks at Rough and Final

Flashing is inspected during the WRB and siding installation phase, before siding covers the flashing:

• Head flashing at each window and door — material type, integration with WRB, and overhang past the jamb on each side.
• Pan flashing at sill of each window and door — positive slope to the exterior, end dams at each side, integration with WRB.
• Wall penetrations — flashing boot or tape at each pipe, conduit, and vent penetration sealed to the WRB.
• Wall-to-roof intersections — step flashing or counter-flashing present and properly lapped with roofing and WRB.
• Foundation cap flashing where wood framing bears on masonry or concrete.
• Compatibility of flashing material with adjacent materials (no aluminum flashing against untreated masonry; no bare steel flashing in contact with copper-treated wood).

What Contractors Need to Know

Flashing installation must follow the window and door manufacturer's installation instructions, not just the IRC. Most manufacturers publish specific step-by-step flashing sequences that must be followed for the product warranty to be valid and for code compliance to be achieved. Document the flashing installation with photographs before covering it with siding — the inspector may not be on-site at precisely the right time, and photographic documentation avoids questions later.

Use self-adhering flashing tape (e.g., DuPont FlexWrap, Henry Blueskin, 3M Window and Door Flashing Tape) at window sills and anywhere the flashing must bend around corners. Rigid metal flashing cannot conform to the rough opening geometry, while flexible tape can seal the sill-to-jamb junction completely.

The sequence of flashing installation is as important as the flashing material itself. Flashing must be installed in the correct waterfall sequence so that each piece laps over the piece below, directing water outward and downward at every layer. A commonly reversed sequence is the window sill pan and side flashing: the side flashing must lap over the sill pan, not under it, so water entering at the window side is redirected onto the sill pan rather than behind it. Review the flashing sequence with framing and window installation crews before work starts, as sequence errors are difficult to correct once windows are installed and the surrounding frame is enclosed.

Dissimilar metals in contact create galvanic corrosion that degrades the less noble metal. Aluminum flashing in contact with copper piping or copper-treated wood will corrode within a few years in wet conditions. In coastal environments, even galvanized steel flashing corrodes faster than expected due to salt air exposure. Use compatible materials, aluminum with aluminum and copper with copper, and verify compatibility when dissimilar materials must be adjacent. Galvanic isolation tape or sealant can separate incompatible metals where adjacency cannot be avoided.

The sequence of flashing installation is as important as the flashing material itself. Flashing must be installed in the correct waterfall sequence so that each piece laps over the piece below, directing water outward and downward at every layer. A commonly reversed sequence is the window sill pan and side flashing: the side flashing must lap over the sill pan, not under it, so water entering at the window side is redirected onto the sill pan rather than behind it. Review the flashing sequence with framing and window installation crews before work starts, as sequence errors are difficult to correct once windows are installed and the surrounding frame is enclosed.

Dissimilar metals in contact create galvanic corrosion that degrades the less noble metal. Aluminum flashing in contact with copper piping or copper-treated wood will corrode within a few years in wet conditions. In coastal environments, even galvanized steel flashing corrodes faster than expected due to salt air exposure. Use compatible materials, aluminum with aluminum and copper with copper, and verify compatibility when dissimilar materials must be adjacent. Galvanic isolation tape or sealant can separate incompatible metals where adjacency cannot be avoided.

The sequence of flashing installation is as important as the flashing material itself. Flashing must be installed in the correct waterfall sequence so that each piece laps over the piece below, directing water outward and downward at every layer. A commonly reversed sequence is the window sill pan and side flashing: the side flashing must lap over the sill pan, not under it, so water entering at the window side is redirected onto the sill pan rather than behind it. Review the flashing sequence with framing and window installation crews before work starts, as sequence errors are difficult to correct once windows are installed and the surrounding frame is enclosed.

Dissimilar metals in contact create galvanic corrosion that degrades the less noble metal. Aluminum flashing in contact with copper piping or copper-treated wood will corrode within a few years in wet conditions. In coastal environments, even galvanized steel flashing corrodes faster than expected due to salt air exposure. Use compatible materials, aluminum with aluminum and copper with copper, and verify compatibility when dissimilar materials must be adjacent. Galvanic isolation tape or sealant can separate incompatible metals where adjacency cannot be avoided.

What Homeowners Get Wrong

Homeowners who install windows themselves frequently miss the pan flashing at the sill, which is the most important piece of flashing at any window opening. If a window leaks, it almost always leaks at the sill — and the water runs into the wall cavity and wets the rough sill framing before any visible sign of leaking appears at the interior. Once rot sets in at the sill framing, the repair is extensive.

Another misconception: caulking can substitute for flashing. Caulk is a maintenance material with a service life of 5 to 15 years. Flashing is a permanent installation. Replacing caulk with flashing is always the right approach at structural transitions.

In renovation work, existing window and door flashings that are covered by new siding must be evaluated before the new siding is installed. If the existing flashing is a single layer of 30-pound felt paper applied in the wrong sequence — head flashing under side flashing, for example — it is not providing adequate protection regardless of how well it has performed in the past. The renovation is an opportunity to correct the flashing system. Installing new siding over an improperly flashed opening perpetuates the problem and does not satisfy the current code requirements for weather protection at penetrations.

State and Local Amendments

IRC 2018 R703.4 flashing requirements are adopted across TX, GA, VA, NC, SC, TN, AL, MS, KY, and MO. In high-rainfall and high-wind areas of the Southeast, inspectors are particularly thorough about flashing compliance because water infiltration at walls is one of the most common and costly building defects in the region. Some coastal jurisdictions require self-adhering membrane head flashings (rather than rigid metal) for all windows and doors in the wind-driven rain zone.

IRC 2021 expanded the flashing requirement language in R703.4 to make pan flashing (sill flashing with end dams) explicit for window and door openings, where the 2018 edition implied but did not fully spell out the pan flashing requirement for sills. Jurisdictions on IRC 2018 that have not adopted this 2021 clarification should still install pan flashing as best practice, and many inspectors require it under the implied requirements of R703.4.

When to Hire a Licensed Contractor

Flashing installation is a skilled trade task that requires knowledge of sequencing, material compatibility, and integration with both the WRB and the window/door product. A licensed siding or waterproofing contractor should install all exterior wall flashing. Window and door installation should be performed by licensed contractors who follow the product's installation instructions, including all flashing requirements.

Common Violations Found at Inspection

• No head flashing at window or door tops — single most common exterior flashing violation.
• No pan flashing at window or door sills — water leaks into the rough sill framing without any visible exterior sign until rot develops.
• Flashing lapped in the wrong direction — head flashing installed under WRB rather than over it, directing water into the wall.
• Wall penetrations not flashed — pipes and vents pierce the WRB with no boot or tape seal, creating direct water infiltration pathways.
• Flashing at wall-to-roof intersection absent or improperly lapped — step flashing not integrated with roofing courses.
• Incompatible materials — aluminum flashing against Portland cement stucco (electrolytic corrosion), or standard galvanized flashing in contact with ACQ pressure-treated wood (accelerated corrosion).
• Flashing too narrow — head or sill flashing does not extend far enough to lap over adjacent materials and direct water to the exterior face.