7.2 Penetrations, Step Flashing, Counterflashing & Valleys

7.2 Penetrations, Step Flashing, Counterflashing & Valleys

Vertical wall abutments, chimneys, rooftop equipment curbs, penetrations, and intersecting roof planes (valleys) represent the primary transition zones where continuous roof coverings terminate. Over 90% of all reported roof leaks occur at these critical junctions rather than across the seamless field of the roof. To maintain structural envelope integrity, the CR-42 roofing contractor must master the installation of step flashing, two-piece counterflashing, low-slope curb base flashing, penetration seals, and roof crickets under the International Building Code (IBC), the International Residential Code (IRC), and National Roofing Contractors Association (NRCA) specifications.

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1. Wall & Vertical Abutment Flashing in Steep-Slope Roofs

Where a steep-slope roof plane intersects a vertical sidewall—such as at dormers, second-story exterior walls, and sidewalls of masonry chimneys—the intersection must be waterproofed using individual, L-shaped pieces of sheet metal known as step flashing.

Step Flashing Dimensions & Fabrication

• Vertical Leg: Minimum 4 inches (102 mm) extending vertically up the wall sheathing beneath the water-resistive barrier (housewrap or building paper) and exterior cladding.
• Horizontal Leg: Minimum 4 inches (102 mm) extending horizontally onto the roof deck over the underlying shingle course.
• Length: Equal to the shingle exposure plus 2 inches (typically 7 inches to 8 inches long for standard 5-inch or 5-5/8-inch exposure asphalt shingles). This 2-inch additional length guarantees that each step flashing piece overlaps the preceding piece by at least 2 inches.
• Material: Fabricated from corrosion-resistant metal—minimum 26-gauge galvanized steel, 0.019-inch aluminum, or 16-ounce copper.

The Interwoven Step Flashing Protocol

Step flashing is installed progressively in tandem with each individual course of asphalt shingles:

1. The first piece of step flashing is placed over the starter shingle course at the lowest eave edge.
2. A shingle is laid over the horizontal leg of the step flashing piece, leaving the bottom edge of the metal concealed while the vertical leg lies flat against the wall.
3. The next step flashing piece is placed directly on top of the newly installed shingle, aligned with the exposure line so that it overlaps the lower step flashing by at least 2 inches.
4. This weaving process is repeated up the entire length of the sidewall.

The Single-Fastener Deck Nailing Rule

[!IMPORTANT] Accommodating Differential Movement: Step flashing pieces must be secured with a single roofing nail driven into the roof deck sheathing near the upper corner of the horizontal metal flange. Under no circumstances should step flashing ever be nailed to both the roof deck and the vertical wall!

A building's roof framing (rafters or trusses) and vertical wall framing expand, contract, settle, and deflect independently under wind loads and thermal cycling. Nailing flashing to both substrates rigidly locks them together. As the framing moves, this rigid connection tears the metal flashing, shears fasteners, or rips holes through the shingles, creating catastrophic hidden leaks.

Cladding Clearance & Wall Wraps

The building paper or housewrap must overlap the 4-inch vertical leg of the step flashing by at least 2 inches. Furthermore, exterior wall cladding—such as stucco, fiber-cement siding, or wood lap siding—must terminate with a minimum 1-1/2-inch to 2-inch clearance above the finished surface of the roof shingles. Setting siding tight against shingles creates a capillary wick that rots wood siding, spalls stucco, and prevents future roof maintenance or shingle replacement without destroying the wall facade.

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2. Counterflashing (Cap Flashing) & Reglet Detailing

While step flashing or low-slope base flashing seals the roof deck interface, its exposed vertical top edge remains vulnerable to downward water penetration. Counterflashing (also called cap flashing) is installed over the vertical leg to provide a protective, shed-down weather shield.

Two-Piece vs. Surface-Mounted Counterflashing

1. Reglet-Mounted / Mortar-Joint Counterflashing: The gold standard in masonry and concrete walls. A horizontal joint or saw-cut kerf (reglet) is cut into the mortar joint to a minimum depth of 1 inch to 1-1/2 inches. The counterflashing is fabricated with a 1-inch return flange that inserts into the reglet, locked securely into place with lead wedges spaced 12 inches to 18 inches on center, and sealed with a non-sag polyurethane or silicone elastomeric sealant.
2. Surface-Mounted Counterflashing: Used on solid concrete or smooth masonry surfaces where saw-cutting is impractical. A metal receiver bar featuring an angled upper sealant flange is mechanically fastened to the wall with concrete drive anchors spaced 12 inches on center. The top reservoir is filled with a continuous, tooled 3/8-inch bead of polyurethane sealant.
3. Through-Wall Counterflashing: Embedded completely through the brick veneer to the structural backup wall during initial masonry construction, terminating with an internal drip lip to intercept cavity drainage and expel it through weep holes.

Engineering Dimensions & Capillary Hems

• Vertical Lap: Counterflashing must overlap the vertical leg of the base or step flashing by a minimum of 1-1/2 inches to 2 inches (NRCA recommends 3 inches in severe climates).
• Lower Hem: The bottom edge of the counterflashing must be folded under in a 1/2-inch hem (kick). This hem provides structural rigidity to keep the long metal edges straight and acts as a capillary break to shed water away from the base flashing.
• Clearance: Counterflashing must terminate at least 1 inch above the horizontal roof surface to prevent stagnant roof moisture from wicking upward behind the metal.

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3. Low-Slope Base Flashing at Parapets & Equipment Curbs

In low-slope commercial roofing (BUR, modified bitumen, single-ply), vertical walls and rooftop equipment curbs (HVAC units, skylights, exhaust fans) require heavy-duty base flashing assemblies.

The 8-Inch Vertical Height Rule

Under NRCA guidelines and commercial roofing manufacturer warranty mandates, base flashing must extend vertically up the wall or curb to a minimum height of 8 inches (203 mm) above the finished roof membrane surface (measured above the top of the insulation and drainage plain). In regions subject to heavy snow accumulation or severe desert monsoon ponding, specifications frequently mandate a 12-inch minimum flashing height. Installing base flashings shorter than 8 inches leaves rooftop equipment vulnerable to water intrusion during monsoon deluge conditions when temporary water heads surge across low-slope decks.

Cant Strips for Bituminous Systems

At all 90-degree internal angle transitions between the horizontal roof deck and vertical walls or curbs, Built-Up Roofing (BUR) and Modified Bitumen (APP/SBS) systems require the installation of a cant strip:

• Material & Geometry: Fabricated from treated wood fiberboard, perlite, or kiln-dried wood cut at a 45-degree angle, typically presenting a 3-inch to 4-inch diagonal face.
• Failure Prevention Mechanism: Bituminous felts, fiberglass plies, and heavy modified bitumen sheets are thick and relatively brittle. Bending these multi-ply membranes into a sharp 90-degree internal right angle creates severe tensile bending stresses that cause cracking, void delamination, and membrane bridging (where the membrane pulls away from the corner, creating an unsupported hollow void easily punctured by foot traffic). The 45-degree cant strip eases the membrane transition from horizontal to vertical, ensuring full continuous substrate adhesion.
• Single-Ply Membranes: Flexible single-ply sheets (TPO, PVC, EPDM) do not require cant strips; they are adhered directly into the 90-degree corner with mechanical termination bars or continuous angle securement at the base.

Termination Bar & Sealant Detailing

The top edge of the low-slope base flashing membrane must be mechanically clamped to the wall or curb using a rigid termination bar (an extruded aluminum bar, minimum 1/8 inch thick by 1 inch wide):

• Fasteners (corrosion-resistant expansion anchors or screws) must be spaced 6 inches to 12 inches on center (maximum 8 inches o.c. per NRCA specifications) to prevent membrane slippage.
• The termination bar must be completely capped by a metal counterflashing or sealed along its top beveled edge with a continuous bead of high-performance polyurethane sealant.

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4. Pipe, Conduit & Penetration Flashing

Rooftop penetrations—including plumbing vent stacks, electrical conduits, structural pipe supports, and guy wires—require specialized flashing to prevent water intrusion around cylindrical projections.

Prefabricated Pipe Boots

Plumbing vent pipes are flashed using factory-manufactured pipe boots made from molded elastomeric rubber (EPDM or silicone) or sheet lead:

• Flange Geometry: Features a wide horizontal deck flange extending at least 4 inches outward onto the roof plane in all directions. In steep-slope roofs, the flange is interwoven with shingles (lapping over downhill shingles and under uphill shingles); in low-slope roofs, the flange is hot-air welded or stripped-in with membrane plies.
• Cylindrical Sleeve: Extends vertically up the pipe a minimum of 8 inches.
• Top Sealing: For elastomeric boots, the top collar is secured with a stainless steel draw band (clamping ring) and sealed with polyurethane sealant. For lead boots (minimum 2.5 to 4.0 lb/sq ft sheet lead), the lead sleeve extends 2 inches past the top of the pipe and is rolled down inside the pipe's internal diameter, providing a maintenance-free mechanical seal that cannot dry out or degrade in the Arizona sun.

Pitch Pockets (Sealant Pans): The Last Resort

A pitch pocket (sealant pan) is a formed sheet metal cup (minimum 4 inches deep with a 4-inch deck flange) installed around structural penetrations that cannot be accommodated by standard cylindrical pipe boots (such as angle irons, wide-flange I-beams, or multiple clustered conduits).

• Engineering Restrictions: The NRCA classifies pitch pockets as a design method of last resort. They rely entirely on chemical sealants that degrade under desert ultraviolet radiation and thermal cycling, requiring mandatory semi-annual contractor inspection and resealing.
• Clearance Mandate: The inside perimeter of the metal pan must provide a minimum clearance of 2 inches from the penetration on all sides, and a minimum of 2 inches between adjacent clustered conduits. This 2-inch void ensures sufficient volume for pourable sealant to flow freely and form an unbroken moisture seal.
• Filling Protocol: The bottom of the pan is filled with non-shrink hydraulic cement or mortar grout up to 1 inch below the rim to consolidate the base. The top 1 inch to 2 inches is filled with a two-part, non-shrink pourable polyurethane or elastomeric sealant. The sealant must be mounded into a crown to shed standing water away from the penetration.

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5. Skylight, RTU Curb & Chimney Flashing: The Cricket Mandate

Large roof penetrations—such as masonry chimneys, skylights, and rooftop air-handling units (RTUs)—act as structural dams on steep-slope roofs, interrupting the natural downward flow of rainwater.

The Roof Cricket (Saddle) Code Mandate

Under IRC Section R905.2.8.3 and IBC Section 1507.2.8.2, the installation of a cricket (or saddle) is a non-negotiable legal requirement under specific dimensional criteria:

[!IMPORTANT] Code Threshold for Crickets: A roof cricket (saddle) is mandatory on the ridge side (high-slope side) of any chimney or penetration that is wider than 30 inches (762 mm), measured parallel to the roof ridge.

                  Roof Ridge
                      /\
                     /  \
                    /    \
                   / Cricket \  <-- Splits descending water runoff
                  /   Ridge   \
                 /  / \    / \ \
                /  /   \  /   \ \
               /  / Valleys    \ \
              +--+--------------+--+
              |                    |
              |  Chimney or Curb   |  (Wider than 30 inches)
              |                    |
              +--------------------+

Mechanics & Construction of Crickets

When a roof penetration exceeds 30 inches in width:

1. Rainwater cascading down the roof plane hits the flat, vertical back wall of the curb. Without a cricket, water dams against the curb, forming a temporary reservoir that submerges flashings and causes structural wood rot. Leaves, pine needles, and desert debris collect behind the wall, retaining destructive moisture.
2. A cricket is a small, double-pitched peaked wooden or metal roof structure built behind the penetration. The central ridge of the cricket splits the descending stream of water, diverting runoff smoothly into two secondary valleys around the left and right sides of the curb.
3. The pitch of the cricket should be designed equal to or steeper than the main roof pitch (typically 1/2 of the main roof slope or a minimum of 3:12) to ensure rapid water shedding. The secondary valleys formed by the cricket must be fully lined with self-adhering modified bitumen underlayment (ASTM D1970) and lined with metal valley flashing.

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6. Valley Flashing Engineering: Open, Closed-Cut & Woven Valleys

A valley is the internal intersecting angle formed where two sloping roof planes meet, creating a concentrated drainage channel that collects and channels high-velocity runoff from both slopes. Valleys carry the heaviest water volume of any component on a steep-slope roof.

Open Valleys

In an open valley, sheet metal flashing is left exposed in the center to form a durable, unobstructed water channel:

• Sheet Metal Specifications: Fabricated from minimum 24-inch wide (610 mm) sheet metal—minimum 26-gauge galvanized steel, 0.019-inch aluminum, or 16-ounce copper.
• The W-Profile Splash Rib: High-performance open valley metal features a central inverted "V" or "W" splash rib extending 1 inch high along the valley centerline. This rib serves a crucial hydrodynamic purpose: it acts as a baffle that breaks the kinetic momentum of water rushing down a steep slope, preventing high-velocity runoff from surging horizontally across the valley and washing underneath shingles on the opposing roof plane.
• Installation Detailing: The valley is first lined with a continuous 36-inch wide layer of self-adhering underlayment. The metal sheet is secured along its outer 1/2-inch return hems using sheet metal cleat clips spaced 12 inches on center (avoiding penetrating nails through the metal). Shingles are trimmed back neatly to leave a 4-inch to 6-inch wide open trough (tapering wider toward the bottom by 1/8 inch per foot). The upper corner of every trimmed shingle is clipped off at a 45-degree angle to guide water inward, and each shingle is bedded in a 3-inch wide continuous strip of plastic cement. Under no circumstances may fasteners be driven through shingles within 6 inches of the valley centerline.

Closed-Cut Valleys

In a closed-cut valley, shingles from one slope cross the centerline, while shingles from the intersecting slope are cut in a clean line parallel to the valley:

• Substrate Preparation: Lined with a minimum 36-inch wide layer of self-adhered underlayment or #30 felt centered in the valley.
• Sequencing: Shingles from the roof plane with the lesser slope or smaller drainage area are installed first, extending across the valley centerline onto the adjoining roof slope by at least 12 inches. No fasteners are placed within 6 inches of the centerline.
• Cutting the Intersecting Slope: Shingles from the steeper or larger slope are then laid across the valley. A chalk line is snapped 2 inches back from the centerline on the uncut side, and the shingles are cleanly trimmed. The top corners of all cut shingles are clipped at 45 degrees, and the edges are bedded in roofing cement. Closed-cut valleys present a clean architectural line and are ideal for heavy laminated (architectural) shingles.

Woven Valleys

In a woven valley, shingle tabs from opposing roof planes are interwoven across the valley centerline in alternating courses:

• Engineering Limitations: Woven valleys are strictly limited to standard three-tab asphalt strip shingles on roof slopes of 4:12 or greater.
• Prohibitions: Woven valleys are strictly prohibited for dimensional/laminated architectural shingles, concrete or clay tiles, slate, and wood shakes. Laminated shingles are thick and rigid; bending them back and forth across a valley centerline causes severe bridging, voids, cracked shingle backings, and bulky, unsealed humps that collect debris and funnel water into fasteners.

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7. Valley Construction Comparison