IRC 2024 Chimney Flashing: Step Flashing, Counter Flashing, and Cricket Details

What IRC 2024 § R1003.17 requires

IRC 2024 Section R1003.17 requires chimney flashing to be constructed of approved corrosion-resistant metal — galvanized steel, aluminum, copper, or lead — and installed to prevent water infiltration at all four sides of the chimney-roof intersection. The system consists of step flashing at the sides (one piece per shingle course, woven under shingles and over the water-resistive barrier), counter flashing embedded in the chimney mortar joints or cut into the masonry a minimum of 1 inch, a front apron at the downhill side, and a rear pan or saddle flashing at the uphill side. A cricket (saddle) — a peaked diverter structure built behind the chimney — is required by IRC 2024 R903.2.2 when the chimney is 30 inches or wider measured perpendicular to the roof slope, to redirect water around the chimney rather than pooling behind it.

Chimney flashing requirements appear in both Chapter 9 (Roof Assemblies) and Chapter 10 (Chimneys and Fireplaces). R1003.17 mandates flashing at all masonry chimney-roof intersections and specifies material requirements. R903.2.2 mandates crickets for wide chimneys. The two sections work together:

R1003.17 — Material requirements:

• Flashing must be minimum 26-gauge corrosion-resistant sheet metal (galvanized steel), or aluminum, copper, or lead
• Aluminum flashing is not permitted in direct contact with concrete or masonry due to galvanic corrosion — a bituminous coating or separation membrane is required
• All flashing must be installed to direct water away from the chimney-roof intersection to the exterior

Step flashing at the sides: Each piece of step flashing is an L-shaped metal piece, one per shingle course, installed so that it overlaps the piece below by at least 2 inches. Each step flashing leg lies flat on the roof deck (and WRB) under the shingle, and the vertical leg turns up against the side of the chimney at least 4 inches. The shingle laps over the horizontal leg; the counter flashing laps over the vertical leg from above.

Counter flashing: Counter flashing covers the vertical legs of the step flashing, creating a two-part system that allows the roof to move independently of the chimney without breaking the water seal. Counter flashing must be embedded a minimum of 1 inch into a mortar joint (the traditional “reglet” method) or cut into the masonry with a grinder or saw and sealed with approved sealant. Counter flashing must extend at least 4 inches down over the step flashing legs.

Front apron flashing: At the downhill face of the chimney, a front apron flashing (sometimes called base flashing) is installed under the shingles above it and over the shingles below it, turned up at least 4 inches against the front face of the chimney. The apron sheds water away from the chimney face onto the roof surface below.

R903.2.2 — Cricket requirement: A cricket (saddle) must be installed behind (on the uphill side of) any chimney that is 30 inches or wider measured perpendicular to the slope. The cricket is a peaked diverter that splits water flow and routes it around each side of the chimney rather than allowing it to pond against the uphill chimney face. Cricket construction materials must match or be compatible with the main roof covering material. The cricket must be covered with sheet metal or the same roofing material as the main roof, and flashed at the chimney intersection with the same step and counter flashing system used on the sides.

Why This Rule Exists

The chimney-roof intersection is one of the most common sources of roof leaks in residential construction, and chronic chimney leaks cause far more structural damage than dramatic roof failures. The multi-component flashing system required by IRC reflects the unique challenges of this intersection.

Why a two-part (step + counter) system: A masonry chimney and a wood-framed roof are two structurally independent systems that move differently in response to temperature changes, moisture, and settlement. A single-piece flashing system — which would require bonding both to the masonry and to the roof deck simultaneously — would be stressed by this differential movement and eventually crack or pull loose. The step-and-counter system allows the step flashing (attached to the roof deck) and the counter flashing (attached to the masonry) to slide over each other as the two structures move independently.

Why crickets are required at 30 inches: Behind a wide chimney, the triangular area of roof surface uphill of the chimney collects and concentrates water from a large catchment area. On a standard residential roof with a 6:12 pitch, a 36-inch wide chimney can accumulate water from 6–10 square feet of roof surface. This pooled water sits against the uphill chimney face under ice and snow conditions and can drive water into the masonry even through properly installed flashing — especially if ice damming occurs. The cricket creates two mini-valleys that route water to the sides of the chimney and off the roof, eliminating the ponding zone entirely. The 30-inch threshold reflects research into chimney widths where the water accumulation behind the chimney becomes significant enough to routinely defeat flat rear-pan flashing in severe weather.

Why aluminum cannot touch masonry: Aluminum and the alkaline compounds in concrete and mortar undergo a galvanic reaction that progressively corrodes the aluminum. Aluminum flashing in direct contact with masonry mortar can develop through-holes within 5–10 years, defeating the flashing system. Copper and galvanized steel are more resistant to alkali attack; aluminum requires a separation layer.

What the Inspector Checks at Rough and Final

Chimney flashing is evaluated at different stages depending on roof assembly sequence.

During roof sheathing inspection (if done separately):

• Water-resistive barrier (WRB) installation at the chimney penetration — WRB must lap over the front apron and run under step flashing
• Ice and water shield installation around the chimney in required zones (IRC requires ice and water shield from the eave to at least 24 inches inside the wall line in climate zones with potential for ice damming)

During roofing inspection or final inspection:

• Step flashing visible at the top of each shingle course at the chimney sides (inspector may lift shingles at the top row to verify step flashing is present)
• Counter flashing embedded in mortar joints or cut into masonry — inspector checks that counter flashing is not simply caulked to the chimney face without mechanical anchorage
• Counter flashing extends at least 4 inches down over step flashing legs
• Front apron present and properly integrated with shingles above and below
• Cricket present for chimneys 30 inches or wider — inspector will measure chimney width
• Cricket flashing integrated with main chimney flashing at the chimney face
• No visible gaps or open seams in the flashing system

What Contractors Need to Know

Chimney flashing is a specialty skill within roofing, and the sequencing of installation is critical. Several practices separate flashing systems that perform well from those that fail within a few years.

Integration sequence: The WRB must be installed before step flashing; step flashing must be installed as roofing proceeds up the slope (not at the end after shingles are done); and counter flashing must be installed after step flashing is complete. If counter flashing is installed first and the roofer then tries to weave step flashing behind it, the result is a system where step flashing is not properly integrated and water will bypass it.

Counter flashing anchorage: The reglet cut method — sawcutting a horizontal kerf into the masonry approximately 1.5 inches deep, inserting the counter flashing leg into the kerf, and sealing with approved sealant — is the most durable approach for existing chimneys where cutting out mortar is difficult. On new chimneys, traditional mortar joint embedding during construction is preferred because the mortar provides full contact along the length of the flashing. Either method must achieve the 1-inch minimum embedment.

Cricket construction: A cricket can be framed with wood (2×4 or 2×6 framing on the roof deck behind the chimney) or fabricated in sheet metal by a sheet metal fabricator. Sheet metal crickets are preferred because they eliminate the wood framing that can rot if the cricket flashing ever fails. The cricket must be flashed at its intersection with the chimney face using a full counter flashing detail — this intersection is where most cricket leaks originate.

Aluminum and masonry separation: Where aluminum step or counter flashing is used (a common cost-saving choice), apply a continuous strip of bituminous flashing membrane or butyl tape to the masonry face before the aluminum contacts it. This prevents galvanic corrosion and extends the aluminum flashing lifespan significantly.

What Homeowners Get Wrong

Chimney flashing maintenance is widely neglected, and the most common homeowner mistakes accelerate flashing failure rather than addressing its root causes.

Caulking over failed counter flashing instead of replacing it: When counter flashing pulls out of a mortar joint or separates at a seam, the visible gap often prompts homeowners or handymen to apply a bead of roofing caulk or tar over the opening. Caulk and tar are not substitutes for properly anchored flashing. The caulk crack, shrink, and fail within a few years — often before the next professional roof inspection — and the underlying leak continues undetected until interior damage becomes visible. Proper counter flashing repair requires re-cutting the reglet, inserting new flashing, and sealing.

Painting over mortar joints on the chimney face without removing flashing: Homeowners who apply waterproofing sealers or paint to the chimney masonry sometimes cover the counter flashing-mortar joint interface, trapping moisture and accelerating mortar deterioration behind the sealed surface. Masonry waterproofing should not be applied over flashing unless the flashing has been inspected and confirmed to be properly seated.

Not clearing debris from behind the chimney: Leaves, pine needles, and debris that accumulate behind the chimney in the area a cricket is designed to drain can clog the drainage path and create the pooling conditions the cricket was designed to prevent. Homeowners in treed settings should clear chimney debris annually.

State and Local Amendments

Chimney flashing requirements are consistent across most IRC jurisdictions. The 30-inch cricket threshold and the step-and-counter flashing requirement are adopted with minimal amendment in nearly all states.

Climate-based amendments are the primary source of variation. Jurisdictions in the upper Midwest, New England, and mountain states that have adopted IRC Table R301.2(1) ice barrier requirements mandate ice and water shield membrane around the chimney penetration for a greater distance than the base IRC minimum. Minnesota, Wisconsin, Michigan, Maine, and Colorado have jurisdiction-specific ice barrier requirements that extend the shield area around chimney penetrations.

California’s amendment to flashing material requirements — included in the CRC — adds provisions for copper flashing in coastal salt-air environments, where galvanized steel has a reduced service life due to accelerated corrosion. The CRC also adds requirements for secondary drainage paths at chimney flashings in seismically active areas, where movement can cause counter flashing to pull out of mortar joints during earthquakes.

When to Hire a Professional

Chimney flashing work — whether new installation or repair — is one of the most skill-dependent tasks in residential roofing. Errors create chronic leaks that are often misdiagnosed for years. Professional involvement is warranted in virtually all chimney flashing situations:

• New chimney construction: The masonry contractor and roofing contractor must coordinate flashing installation. Flashing should be specified in both the masonry and roofing scopes, with clear responsibility for the counter flashing installation assigned to one party (typically the roofer).
• Chimney flashing replacement: When counter flashing is pulling out of mortar joints or when visible rust staining or water damage is found below a chimney, full flashing replacement — not caulk repair — is the correct response. A qualified roofer with chimney flashing experience should perform this work.
• Cricket installation on wide chimneys: If an existing chimney wider than 30 inches does not have a cricket, adding one is a major roofing project requiring removal and replacement of the surrounding roof covering. A licensed roofing contractor with sheet metal experience should design and install the cricket.
• Post-storm leak investigation: After severe weather or hail damage, a roofer should inspect the chimney flashing for dislodged counter flashing, torn step flashing, or compromised cricket flashing before interior repairs are made.

Common Violations Found at Inspection

• Counter flashing applied with only roofing caulk or tar against the masonry face, with no mechanical embedment in the mortar or masonry — the most common chimney flashing defect
• Step flashing absent entirely, with only a continuous strip of caulk or roofing cement at the chimney-shingle interface
• Cricket absent on chimney 30 inches or wider, often because the builder or roofer was unaware of the R903.2.2 requirement
• Aluminum flashing in direct contact with masonry without a separating membrane, leading to corrosion within 5–10 years
• Counter flashing embedded less than 1 inch into the masonry, causing it to pull out under thermal movement
• Step flashing pieces running continuous up the chimney side rather than individual pieces per shingle course, eliminating the flexibility needed for differential movement
• Ice and water shield not installed around chimney penetration in climate zones where it is required by IRC Table R905.1.1(2)
• Cricket flashing not properly integrated with the main chimney counter flashing, leaving a gap at the cricket-chimney junction that is the most common cricket leak point