9.2 Step Flashing, Counterflashing, Chimney Crickets & Reglet Details
Key Takeaways
Base flashing against a sidewall must be continuous or step flashing at least 4 inches high and 4 inches wide (IRC R905.2.8.3), with step flashing woven into each shingle course.
Nail each step flashing piece to the roof deck only, typically with one nail near the top corner, so the roof and wall can move independently.
The IRC (R903.2.1) requires a flashing to divert water away where the eave of a sloped roof meets a vertical sidewall, which is the kick-out flashing.
A cricket or saddle is required on the ridge side of any chimney or penetration more than 30 inches wide, measured perpendicular to the slope (IBC 1503.5, IRC R903.2.2).
Counterflashing is set into a reglet or mortar joint and laps over the base and step flashing, so the two parts can move without opening the joint.
Step Flashing, Counterflashing, Chimney Crickets & Reglet Details
Where a sloping roof plane terminates against a vertical wall, parapet, or chimney, two independent structural elements meet at an angle. These intersections are subjected to severe hydraulic loads, as rainwater flowing down the roof gathers against the vertical barrier, creating concentrated runoff channels. Furthermore, because the vertical wall framing and the horizontal roof trusses expand, contract, and settle independently, these junctions undergo ongoing differential structural movement. Continuous L-flashings nailed to both the roof and the wall along a sloping sidewall can tear, buckle, or pull fasteners free as the two move. Mastering step flashing, counterflashing, diverter kick-outs, and chimney crickets is essential for constructing a durable, code-compliant roofing assembly.
1. Step Flashing Mechanics and Geometry
Step flashing consists of individual, prefabricated or field-bent L-shaped pieces of sheet metal interwoven with each successive course of roofing shingles along a sloping roof-to-wall intersection.
STEP FLASHING DIMENSIONS & GEOMETRY
<───── 4" Vertical Leg ─────>
┌───────────────────────────┐
│ │ <── Extends up wall behind siding
│ │ or behind counterflashing
│ │
│ │
Length: 7" to 8" │ │
(Exposure + 2" min) ├───────────────────────────┤ <── 90° Precision Brake Bend
│ │
│ │ <── Nailing Zone (Top corner only)
│ │
│ │ <── Extends onto roof deck
└───────────────────────────┘ beneath overlapping shingle
<──── 4" Horizontal Flange ──>
Dimensional Standards (IRC R905.2.8.3 and Manufacturer Instructions)
IRC R905.2.8.3 Sidewall Flashing: base flashing against a vertical sidewall must be continuous or step flashing, at least 4 inches high and 4 inches wide, and must direct water away from the sidewall onto the roof or into the gutter. Where siding is present, the vertical leg must run continuously under the siding. Masonry veneer and stucco walls also need counterflashing or weep details under the wall-covering sections of the code.
- Vertical Leg: at least 4 inches up the wall. Many contractors use 5-inch legs for extra protection against wind-driven water.
- Horizontal Flange: at least 4 inches onto the roof deck.
- Length: shingle manufacturers call for each piece to be at least 2 inches longer than the shingle exposure, so each piece laps the one below. With a 5-inch exposure, that is a 7-inch piece; with a 5-5/8-inch metric exposure, pieces are about 8 inches long.
- Material: corrosion-resistant metal at least 0.019 inch (No. 26 galvanized), such as G90 galvanized steel, aluminum, or 16-ounce copper (IBC 1507.2.8.1, IRC R903.2.1).
Course-by-Course Weaving Sequence
- Step 1: The roofing underlayment is turned up the vertical wall a minimum of 3 to 4 inches (or an ASTM D1970 membrane strip is applied in the angle).
- Step 2: The starter shingle course is placed at the eave. The first piece of step flashing is placed over the starter shingle, tight against the wall framing.
- Step 3: The first regular course shingle is installed over the horizontal flange of the first step flashing piece.
- Step 4: The second piece of step flashing is installed over the end of the first course shingle, overlapping the lower step flashing piece by a minimum of 2 inches.
- Step 5: The sequence repeats with every shingle course up the wall. Each piece of step flashing is covered by the shingle above it.
The Golden Fastening Rule: Fasten to Deck ONLY
Do not nail step flashing to both the wall and the roof deck:
- Thermal Movement and Structural Racking: The wall framing and the roof rafters expand, contract, and deflect at different rates. If a flashing piece is rigidly nailed to both the wall stud and the roof sheathing, building movement will sheer the fasteners, buckle the metal, or tear nail holes in the flashing.
- Correct Nailing: Each step flashing piece is secured with one single roofing nail driven into the roof deck only, positioned high up in the top corner of the horizontal flange (approximately 1 inch down from the top edge and 1 inch in from the outer edge). The overlying shingle covers the fastener, and the weight of subsequent courses holds the flashing flat.
2. Kick-Out Flashing (Diverter Flashing) at Eave Terminations
A common cause of hidden wall rot in houses is the omission of kick-out flashing (also known as diverter flashing) at the lower termination of a sidewall where the eave projects past the wall line.
KICK-OUT FLASHING MECHANICS
Wall Cladding / Siding
│ ▲
▼ │ Diverts water outward
┌─────────────┐ │ away from wall face
│ │ │
│ Step Flash │ │
│ │ │
└──────┬──────┘ │
Roof Plane │ │
═══════════════════════════╪═══════════╪═════════════════════════════
│ │
┌─────┴───────────▼─────────────┐
│ KICK-OUT DIVERTER │ <── One-piece, leak-free unit
│ Angled Outward to Gutter │ Seamless Welded or
└───────────────┬───────────────┘ Factory-Molded Metal/Polymer
│
▼ Discharges Directly into Gutter
[ GUTTER TROUGH ]
The Problem: Concealed Wall Cavity Rot
When water flows down a roof-to-wall intersection, the step flashing channels a heavy, concentrated stream of runoff down the angle. At the eave, the roof slope ends, but the vertical exterior wall continues downward. Without a kick-out diverter, this concentrated stream strikes the corner fascia or siding, wicks behind the exterior cladding (stucco, fiber cement, vinyl, or brick veneer), and flows inside the wall cavity. Over time, this hidden leak can rot the wall sheathing, studs, and framing before any sign appears inside the home.
Code Requirement (IRC R903.2.1)
- IRC R903.2.1: "A flashing shall be installed to divert the water away from where the eave of a sloped roof intersects a vertical sidewall." This is the kick-out (diverter) flashing. The IBC's general flashing rules (1503.2 and 1503.2.1) require flashing at wall and roof intersections for the same reason.
- Geometry: A kick-out is a one-piece (seamless, welded, or molded) diverter whose outward-angled leg carries the water from the step flashing past the wall face and into the gutter. A folded piece with an open seam can leak into the wall it is meant to protect.
- Integration: The kick-out is installed at the bottom of the step-flashing run, before the wall covering. The water-resistive barrier laps over its vertical leg. Siding manufacturers typically require their siding to be held about 1 to 2 inches above the roof surface.
3. Chimney Flashing Assemblies: The Four Core Components
A masonry or framed chimney penetrating a steep-slope roof represents a multi-sided penetration that must shed water from all four directions. A complete chimney flashing assembly consists of four distinct, coordinated flashing elements:
COMPLETE CHIMNEY FLASHING ANATOMY
RIDGE (Upslope)
▲
│
┌───────────────────┐
│ CHIMNEY CRICKET │ <── Required if Chimney > 30" Wide
│ (Or Back-Pan) │ (IBC 1503.5 / IRC R903.2.2)
└─────────┬─────────┘
│
┌──────────────────────┼──────────────────────┐
│ │ │
▼ ▼ ▼
┌───────────────┐ ┌───────────────┐ ┌───────────────┐
│ STEP FLASHING │ │ MASONRY │ │ STEP FLASHING │
│ (Left Side) │ │ CHIMNEY CORE │ │ (Right Side) │
│ & Counter │ │ (Penetration)│ │ & Counter │
└───────┬───────┘ └───────┬───────┘ └───────┬───────┘
│ │ │
└──────────────────────┼──────────────────────┘
│
┌─────────┴─────────┐
│ APRON FLASHING │ <── Installed First at Chimney Base
│ (Downslope Face)│ (Laps Over Shingles Below)
└───────────────────┘
│
▼
EAVES (Downslope)
1. Apron Flashing (Base Flashing)
- Location: The downslope (bottom) face of the chimney.
- Geometry: A continuous L-shaped metal piece extending a minimum of 4 inches up the vertical masonry face and 4 to 6 inches out across the top surface of the asphalt shingles installed directly below the chimney.
- Sequence: Installed first in the assembly. Corner tabs are folded around the left and right corners to receive the first pieces of side step flashing.
2. Side Step Flashing
- Location: Both sides of the chimney running parallel to the roof pitch.
- Installation: Individual step flashing pieces are woven with each shingle course up the chimney flanks, exactly matching sidewall step flashing details. The vertical legs extend up the chimney face, and horizontal flanges are covered by shingles.
3. Upslope Back-Pan vs. Chimney Cricket (Saddle)
- Location: The upslope (ridge-facing) backside of the chimney.
- Small Chimneys (Width 30 Inches or Less): A cricket is not required, and a continuous metal back-pan (back flashing) is common. The back-pan extends a minimum of 4 to 6 inches up the chimney, bends over the roof deck, extends at least 12 to 18 inches up the slope beneath shingles, and may include a raised rib or crimp to steer water around the chimney.
- Wide Chimneys (More Than 30 Inches: The 30-Inch Rule): Under IBC 1503.5 and IRC R903.2.2, any chimney or penetration more than 30 inches wide, measured perpendicular to the slope (across the roof), must have a cricket or saddle on the ridge side.
4. Counterflashing (Cap Flashing)
- Function: Step flashing and apron flashing protect the roof deck, but their upright vertical legs leave an open vertical joint against the chimney wall. Counterflashing covers these vertical legs, preventing water running down the masonry from getting behind the metal.
- Two-Piece Principle: The counterflashing is mechanically anchored to the masonry wall, hanging down to overlap the base apron, step flashing, and cricket upright legs, typically by about 4 inches. Because the base flashing is attached to the roof and the counterflashing is attached to the chimney, the two pieces can slide past each other during building settlement without breaking the waterproof seal.
4. Chimney Cricket Sizing and Code Rules: IBC 1503.5 / IRC R903.2.2
A chimney cricket (also called a saddle) is a small peaked roof built on the upslope side of a chimney. It splits the water flowing down from the ridge and sends it around both sides of the chimney.
IBC 1503.5 Crickets and Saddles: "A cricket or saddle shall be installed on the ridge side of any chimney or penetration greater than 30 inches (762 mm) wide as measured perpendicular to the slope. Cricket or saddle coverings shall be sheet metal or of the same material as the roof covering." Exception: unit skylights installed per IBC 2405.5 and flashed per the manufacturer's instructions may be installed without a cricket or saddle. IRC R903.2.2 says the same for dwellings, with the exception for unit skylights installed per R308.6.
Framing and Covering
- Framing Construction: The cricket is framed on the roof deck with lumber (commonly a small ridge board and rafters) and sheathed with wood structural panels. It forms two small valleys that run to the left and right sides of the chimney.
- Cricket Slope: The code sets no special cricket slope, but the cricket's covering must meet its own minimum slope. For example, asphalt shingles need at least 2:12 (IBC 1507.2.2), with double underlayment below 4:12. A cricket too flat for shingles gets a sheet metal covering, which the code allows. Many contractors build crickets as steep as the main roof so water clears quickly.
- Cricket Waterproofing: Line the cricket and its valleys with self-adhered membrane (ASTM D1970) carried up the chimney. An open metal valley lining on a shingle roof must be at least 24 inches wide (IBC 1507.2.8.2 / IRC R905.2.8.2), in metals such as 0.0179-inch (26-gauge G90) galvanized steel, 0.024-inch aluminum, or 16-ounce copper. The cricket's upturned flashing runs up the chimney and is counterflashed.
5. Masonry Reglet Cutting and Counterflashing Detailing
Counterflashing must be anchored into masonry chimneys or brick walls in a manner that creates a permanent, waterproof seal capable of withstanding severe coastal wind and thermal cycling.
Surface-Mounted vs. Inset Saw-Cut Reglet Counterflashing
| Feature / Metric | Surface-Mounted Counterflashing | Inset Saw-Cut Reglet Counterflashing (Preferred) |
|---|---|---|
| Attachment Method | Fastened flat against the brick face with masonry fasteners | Flashing lip inserted into a saw-cut groove (reglet) in a mortar joint |
| Sealant Dependence | Depends on the exposed sealant bead, which weathers | Metal is locked into the joint; sealant is a secondary seal |
| Maintenance | Sealant must be inspected and renewed regularly | Longer-lasting; sealant still needs periodic checks |
| Trade Practice | Used where a reglet cannot be cut; weaker detail | The detail NRCA and SMACNA guidance generally favor for masonry |
The Step-by-Step Reglet Installation Protocol
INSET SAW-CUT REGLET DETAIL
Brick Masonry Mortar Joint
┌────────────────────────┐
│ │
1" Deep Diamond │ ┌──────────────────┤ <── Lead Drive Wedge
Saw-Cut Reglet │ │ ================╡ (Folded Lead Sheet)
│ │ ║ │
└─────┴──╫───────────────┘
Polyurethane Sealant ──────> █║ <── S-Fold Locking Lip
Flush with Brick Face █║
█║
║ <── 26-Ga Counterflashing
║
║ Overlaps Step Flashing min. 4"
║
▼
- Step 1: Precision Diamond Cutting: Using an angle grinder equipped with a diamond tuckpointing blade, the roofer cuts a clean, continuous groove (reglet), commonly about 3/4 to 1 inch deep, into a mortar joint (or stepped along mortar joints following the roof angle). The groove is blown clean of all dust and mortar debris with compressed air.
- Step 2: Flashing Fabrication: The sheet metal counterflashing is bent with an S-fold or hook bend along its top edge, creating a spring-loaded lip that matches the cut groove depth.
- Step 3: Mechanical Lead Wedging: The folded metal lip is inserted fully into the cut joint. To permanently lock the metal in place, the installer drives lead wedges (folded strips of sheet lead) into the joint above the metal at 12 to 18 inches on center. The soft lead expands under hammer taps, binding tightly against the rough brick and preventing the flashing from pulling out under extreme wind vibration.
- Step 4: High-Performance Sealant Application: The joint is sealed flush with the brick face using an elastomeric sealant compatible with both masonry and metal, such as an ASTM C920 polyurethane.
Under IBC 1503.5 and IRC R903.2.2, when must a cricket (saddle) be built on the upslope side of a chimney or penetration?
When the chimney is wider than 24 inches, measured along the slope.
When the chimney extends more than 48 inches above the ridge.
When the chimney is within 10 feet of a gable rake.
When the chimney or penetration is more than 30 inches wide, measured perpendicular to the slope.
How should individual step flashing pieces be fastened along a roof-to-sidewall intersection?
With one nail into the roof deck near the top corner of each piece, and none into the wall.
With two nails into the wall studs and two into the roof deck.
With silicone caulk only, and no nails.
With lag screws through the fascia board.
What does a kick-out (diverter) flashing do where a sloped roof's eave meets a vertical sidewall?
It holds the end of the gutter against the fascia in high winds.
It provides intake air for attic ventilation.
It diverts the runoff from the step flashing away from the wall covering and into the gutter, as IRC R903.2.1 requires.
It grounds the drip edge against lightning.
Sections you finish are checked off in the contents.