15.2 Flashing, Parapets, Copings & Roof Penetration Detailing

Flashing & Penetration Detailing

Roofing leaks rarely occur in the field of the membrane; over 90% of envelope failures originate at penetrations, parapets, transitions, and edges. Flashing detailing must accommodate differential movement while maintaining continuous watertight seals.

Parapet Base Flashings & Reglet Counterflashings

• Base Flashing Height: IBC Section 1503 and NRCA guidelines require vertical base flashings to terminate a minimum of 8 inches (203 mm) above the finished roof surface (above the drainage plane or top of ballast). This height prevents water from overtopping the flashing during seasonal snowdrifts, torrential rains, or temporary drain clogs.
• Cant Strips: For traditional bituminous membranes (BUR, Mod-Bit) that crack if bent at sharp 90-degree angles, a 45-degree treated wood fiber or perlite cant strip (typically 3" x 3" or 4" x 4") must be installed at the deck-to-wall intersection. Single-ply membranes (EPDM, TPO, PVC) do not require cant strips but require continuous mechanical fastening or peel stops at all vertical angle changes.
• Two-Piece Counterflashing: A durable base flashing requires a two-piece counterflashing system. The base flashing is mechanically fastened to the wall above the 8-inch mark. A separate sheet metal counterflashing overlaps the top of the base flashing by at least 4 inches. The counterflashing is secured into a continuous reglet—a sheet metal receiver cut into a masonry mortar joint or cast directly into a concrete wall. This two-piece assembly accommodates vertical building settlement and thermal expansion without stressing the primary roof membrane, while permitting reroofing without dismantling the exterior masonry facade.

PARAPET FLASHING DETAIL
  │ Masonry / Parapet Wall
  │ ┌─────── Reglet cut into mortar joint
  │ └───┐
  │     │ ◄── Counterflashing (4" overlap min.)
  │   ┌─┴──┐
  │   │    │ ◄── Base Flashing (8" height min. above roof)
  │   │    │
  │  /     │
  │ / 45°  │
  │/ Cant  │
──┴────────┴───────────────────── Finished Roof Surface / Membrane

Parapet Copings & Roof Edge Details

• Parapet Coping Caps: Parapet walls must be capped with continuous sheet metal copings sloping back toward the roof surface at a minimum of 1/4 inch per foot. Coping joints must be sealed with standing seams or concealed splice plates with butyl sealant. The exterior coping skirt must extend down over the wall cladding with a continuous drip edge bent outward at 45 degrees with a 1/2-inch kick-out, held off the wall face to break capillary action.
• Continuous Cleats (Hook Strips): Per ANSI/SPRI ES-1 (Wind Design Standard for Edge Systems Used with Low Slope Roofing Systems), perimeter metal drip edges, gravel stops, and fascia caps must lock over a continuous, heavier-gauge sheet metal cleat (typically one gauge heavier than the fascia) anchored to treated wood nailers at maximum 6 inches on center. Face-nailing edge metal without a continuous cleat causes catastrophic peel failure under wind uplift.
• Penetration Flashings: Pipes, conduits, and structural tubes must not be grouped into single penetrations or sealed with unmaintained "pitch pockets" (pourable sealer pans). Each round pipe penetration must receive a prefabricated elastomeric or lead boot flashing with a stainless steel draw band, covered by an umbrella counterflashing clamped above the boot.

Primary Drains & Secondary Emergency Overflow Systems

IBC Section 1503.4 mandates that every roof section capable of trapping water possess both primary roof drainage and an entirely independent secondary (emergency overflow) drainage system sized for catastrophic 100-year, 1-hour storm events.

• Primary Roof Drains: Cast-iron drain bodies recessed into tapered sump pans (depressed 1 to 1.5 inches below the main roof plane over a 4' x 4' area) to ensure swift gravity flow. Membranes are clamped under a bolted cast-iron clamping ring with an integral gravel stop and dome strainer.
• Secondary Drainage Methods:
  1. Secondary Overflow Drains: Dedicated overflow drains located adjacent to primary drains, connected to independent piping that discharges visibly above grade (to alert building occupants of primary drain failure).
  2. Parapet Overflow Scuppers: Open sheet metal scuppers passing through the parapet wall.
• Elevation Rule: The invert (bottom flow line) of the overflow scupper opening or the top weir of the secondary overflow drain must be installed not less than 2 inches (51 mm) and not more than 4 inches above the low point of the primary roof drain. Positioning the overflow 2 inches above the primary drain prevents nuisance drainage during normal rain while ensuring that if primary drains clog, water depth cannot accumulate to levels exceeding structural roof live-load capacities.

Equipment Curbs, Supports & Roof Expansion Joints

• Equipment curbs. Rooftop mechanical units, skylights, hatches, and vents sit on prefabricated insulated curbs that raise the flashing plane a minimum of 8 inches above the finished roof, matching the base flashing rule. Curbs must be square, level, and continuous; a curb set out of level produces a flashing lap that runs uphill. Where a unit is wider than its curb, the unit's own skirt or a sheet metal counterflashing must lap the curb flashing by at least 4 inches rather than relying on sealant.
• Sleepers and pipe supports. Conduit, gas piping, and duct runs must never bear directly on the membrane. They ride on manufactured supports or sleepers set on protection pads that spread the load and let the membrane be inspected and replaced beneath them.
• Roof expansion joints. Where the structure has an expansion joint, the roof must carry one directly above it — a raised double-curb assembly with a flexible bellows spanning between the two curbs. A membrane carried continuously across a structural joint will tear. Roof expansion joints are also required where the deck material or deck span direction changes, and where wings of a building move differentially.
• Area dividers. An area divider is a raised curbed assembly that accommodates thermal movement of the membrane itself across long runs. Unlike an expansion joint, it does not span a structural joint — a distinction the exam tests directly.

Compatibility, Sequencing & the Warranty

Flashing assemblies fail for two reasons that have nothing to do with geometry:

1. Material incompatibility. Asphaltic products attack EPDM; flashing chemistry must match the membrane; and dissimilar metals in contact — copper against aluminum or bare steel — corrode galvanically. The specification should name compatible accessories from a single manufacturer's system rather than leaving the pairing to the installer.
2. Sequencing. Reglets must be cut or cast before counterflashing is needed, through-wall flashing must be coordinated with the mason's coursing, and the roofer must be on site when the parapet is capped. A detail that is correct on paper but installed out of sequence produces a reverse lap, and a reverse lap leaks regardless of how much sealant follows it.

A manufacturer's total system warranty typically covers membrane, flashings, and accessories only when every component came from that manufacturer's approved list and the installer holds current certification. That is why substitution requests on roofing accessories deserve far more scrutiny than their dollar value suggests.