5.2 Flashing Details: Valleys, Drip Edges, Step Flashing, Aprons, and Crickets

5.2 Flashing Details: Valleys, Drip Edges, Step Flashing, Aprons, and Crickets

Quick Answer: Sheet metal flashings are the primary defense against water intrusion at roof perimeters, intersections, valleys, and penetrations. Drip edge metal must be installed under underlayment at eaves and over underlayment at rakes (CRC § R905.2.8.5). Open valleys require minimum 24-gauge galvanized steel or 16 oz copper with a center W-crimp, fastened outside the water flow line. Wall transitions require 4" x 4" x 7" step flashing pieces nailed only to the roof deck, overlapped 2 inches, and integrated with siding or counterflashing. A chimney cricket is legally required on the ridge side of any chimney or penetration more than 30 inches wide perpendicular to the slope (CBC § 1503.2 / CRC § R903.2.2).

Water naturally tracks along surface tension lines, angles of intersection, and vertical structural walls. Over 90% of steep-slope roof leaks occur not in the field of shingles or tiles, but at transitions and penetrations where flashing assemblies fail. For a California C-39 roofing contractor, mastering sheet metal fabrication, expansion allowances, galvanic compatibility, and code-mandated flashing sequences is vital for constructing durable, leak-free assemblies that comply with California Building Code (CBC Chapter 15) and California Residential Code (CRC Chapter 9).

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1. Perimeter Drip Edge Metal Architecture

Drip edge flashing shields exposed wood sheathing and fascia boards from water wicking, rot, and wind-driven rain along roof perimeters.

Material and Dimensional Mandates (CRC § R905.2.8.5)

• Metal Gauge: Minimum 26-gauge galvanized sheet steel (or corrosion-resistant aluminum/copper).
• Flange Width: Must extend a minimum of 2 inches onto the roof decking.
• Drip Leg & Lip: Extends downward past the fascia with a bent 1/4-inch to 3/8-inch kick-out drip lip at the bottom to throw water clear of the wood trim.
• Attachment: Fastened with corrosion-resistant roofing nails spaced 12 inches on-center in a staggered pattern along the deck flange.

The Critical Eave vs. Rake Installation Sequence

A fundamental rule of water management governs drip edge installation:

• Eaves (Horizontal Gutters): Drip edge installs directly on the bare wood deck, beneath the underlayment. This ensures that any incidental condensation or drainage tracking down the surface of the underlayment discharges cleanly over the metal flange into the gutter.
• Gable Rakes (Sloping Edges): Drip edge installs on top of the underlayment. Water flowing down the shingle plane cannot penetrate sideways under the underlayment edge because the metal flange caps and seals the membrane against wind-driven lateral rain.
• Lapping Joints: Overlap adjoining drip edge sections by a minimum of 2 inches. At eave-to-rake corners, neatly miter the rake flange over the eave metal to create a clean water-shedding fold.

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2. Valley Construction and Hydraulics: Open, Closed-Cut, and Woven

Valleys channel high volumes of concentrated runoff from intersecting roof planes. Selecting the appropriate valley profile depends on roof slope, material thickness, architectural design, and tree debris conditions.

Open Valley Hydraulics and the W-Profile

In high-volume open valleys, water rushing down a steep slope can carry sufficient velocity to wash up under the shingles of an opposing shallower slope. To prevent this, code specifies a W-valley profile:

• A center splash rib (inverted V) rising 1 inch high at the valley centerline acts as a hydraulic baffle, dissipating water velocity and redirecting runoff down the channel.
• Shingles are cut straight along chalk lines parallel to the valley, exposing 4 to 6 inches of open metal at the top and widening by 1/8-inch per foot toward the eave to handle accumulating water volume.
• Metal is fastened solely at its outer edges using metal cleats or nails driven outside the water-flow zone. Never drive fasteners through the central valley pan.

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3. Wall Transitions: Step Flashing, Aprons, and Reglet Counterflashing

Where sloping roof planes intersect vertical walls (dormers, second-story walls, chimneys), water flows parallel to the junction.

Step Flashing at Sidewalls (CRC § R905.2.8.3)

• Dimensions: Individual L-shaped sheet metal pieces measuring at least 4 inches high by 4 inches wide by 7 inches long (or 2 inches longer than the shingle exposure).
• Integration: Installed progressively, one piece per shingle course. The horizontal leg sits on the shingle and roof deck, covered by the next shingle course; the vertical leg extends 4 inches up the wall behind siding or building paper.
• Shingle Lap: Each step flashing piece overlaps the lower piece by at least 2 inches in shingle fashion.
• Crucial Attachment Rule: Fasten each step flashing piece with a single nail into the roof deck only near the top corner. Never nail step flashing into both the roof deck and the vertical wall framing. Differential thermal expansion and structural settling between walls and rafters will tear the metal or rip out fasteners.
• Siding Clearance: Wood, fiber cement, and stucco siding must maintain a minimum 1-inch to 2-inch clearance gap above the finished roof surface. Siding touching shingles wicks moisture, causing dry rot and paint failure.

Continuous Headwall and Apron Flashing

At horizontal intersections where a roof plane slopes downward away from a vertical wall (dormer fronts, clerestories):

• Install a continuous apron flashing with a minimum 4-inch vertical wall leg and 4-inch horizontal roof flange.
• The bottom metal flange rests directly over the lower shingle course, discharging water to daylight. Siding and water-resistive barriers overlap the vertical leg by at least 2 to 4 inches.

Counterflashing and Reglets

On masonry or concrete walls, step and apron flashings cannot tuck behind siding. Instead, a two-piece flashing system is employed:

• Base Flashing: Step or apron metal installed on the roof deck.
• Counterflashing (Cap Flashing): Sheet metal tucked into a 1/2-inch to 3/4-inch deep saw-cut groove (reglet) in the masonry mortar joint, secured with lead wedges and elastomeric polyurethane sealant, overlapping the base flashing by at least 3 inches.

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4. Chimney Flashing, Cricket Sizing, and Pipe Penetrations

Four-Part Chimney Flashing Assembly

Complete chimney weatherproofing integrates four coordinated components:

1. Apron Flashing (Downhill Face): One-piece metal flange overlapping lower shingles.
2. Step Flashing (Both Sidewalls): Interwoven with each shingle course up the incline.
3. Backpan or Cricket (Uphill Face): Diverts descending water around the chimney corners.
4. Counterflashing (All Four Faces): Overlaps all base flashings, set into masonry reglets.

Chimney Cricket / Saddle Sizing Rules (CBC § 1503.2 & CRC § R903.2.2)

When a chimney or penetration measures more than 30 inches (762 mm) in width perpendicular to the roof slope, a cricket or saddle is mandatory on the ridge side. (Unit skylights installed per CRC § R308.6 and flashed to the manufacturer's instructions are exempt.)

• Purpose: Prevents water and debris from damming behind the flat vertical uphill curb.
• Geometry: A ridge running from the center of the chimney uphill to the deck, forming two pitched triangular planes with valley transitions on either side.
• Cricket Ridge Height: Must be sized proportionally to roof slope to maintain positive drainage outward to the sidewall step flashings.

Plumbing Vent Jacks and Pipe Boots

• Flange Layering: The lower portion of the pipe flange must sit on top of the shingles below it, while the sides and top of the metal flange are lapped under the shingles above it.
• Fasten outer corners of the flange, seal the underside with flashing cement, and ensure the rubber elastomeric collar snugly hugs the PVC or cast-iron stack.