Flashing & Details: Penetrations, Curbs, Walls & Roof Edges

Welcome

Hello, and welcome. This is Super Structures General Contractors — a national general contractor headquartered in Powhatan, Virginia — here to help you and your clients build something that lasts. We're glad you're with us, and we look forward to connecting with you.

Alright, Flashing & Details: Penetrations, Curbs, Walls & Roof Edges. Don't let the plain title fool you. Here's the part that actually matters on the job: Roofs rarely leak in the field — they leak at the details, so every penetration, curb, wall, and edge gets a flashing built in the right order, to the right height, and held mechanically rather than by sealant. Learn it well and you become the person the service department never has to visit.

Flashing is where water tries to go sideways or down something that sticks up through the roof. The field of a membrane or a shingle roof almost never fails on its own; the detail is the roof.

The rules every flashing follows

• Height — NRCA's long-standing recommendation is a minimum of 8 inches of base flashing above the finished roof surface, measured after the membrane and any pavers are in place.
• Shingle fashion — every piece laps over the one below it in the direction water runs. A reversed lap is a funnel.
• Mechanical termination — the top of every base flashing is held by a termination bar, a counterflashing, or a nailer. Sealant is a secondary line, never the only one.
• Counterflashing — a separate metal that covers the top of the base flashing and sheds water onto it, ideally removable so the base flashing can be replaced without opening the wall.
• Compatibility and movement — asphalt against PVC, copper runoff onto aluminum, and a rigid flashing across a joint that moves are all slow failures.

Walls and parapets

On a low-slope roof the membrane or a separate base flashing runs up the wall onto a wood nailer or solid substrate, is fastened at the top through a termination bar set in sealant, and is covered by counterflashing let into a reglet, surface-mounted with a sealed top edge, or built into masonry as through-wall flashing. At a parapet, the membrane goes up and over onto blocking and is covered by a coping with continuous cleats on the outside face, cover plates at the joints, and a slight slope back toward the roof. On steep-slope roofs the wall detail is step flashing: a piece per course, at least 4 by 4 inches, lapped at least 2 inches, run up behind the housewrap and siding, with a kickout at the bottom to throw water into the gutter instead of behind the siding, and an apron at headwalls.

Curbs and equipment supports

Every rooftop unit, skylight, hatch, and large duct sits on a curb at least 8 inches above the finished roof, built before the membrane, fastened to the structure, and flashed with the membrane run up the curb face, terminated at the top, and covered by the unit's cap or a counterflashing. Small equipment, conduit, gas piping, and condensate lines sit on pipe supports with protection pads, never directly on the membrane and never on wood sleepers nailed through it. Where several pipes come through close together, they are gathered into one box curb with a hood rather than flashed individually.

Pipes, vents, and odd-shaped penetrations

Round pipes get pre-molded boots of the membrane material or a compatible elastomer, clamped at the top with a stainless band and sealed, set over a target patch. Pitch pockets filled with pourable sealer are the last resort for angle iron and guy wires, because they need periodic refilling; a bolted umbrella cover or a redesign to a curb is better. On shingle roofs, pipe boots go with the top under the course above and the bottom over the course below, sized to the pipe rather than stretched to fit. The most useful document for all of this is the penetration schedule: every roof penetration listed by trade, so the mechanical, electrical, and plumbing contractors know nothing goes through the roof without the roofer flashing it.

Roof edges, drip edge, and gutters

On a low-slope roof the edge is gravel stop or fascia metal over a wood nailer, with the membrane stripped in under it, continuous cleats on the outside, and joints with cover plates and a gap for movement. The IBC requires edge metal on membrane roofs to be tested under ANSI/SPRI ES-1, which governs the cleat and nailer design. The edge must also be lower than the field so water does not pond behind it; a thick tapered build-up can leave the edge metal as a dam.

Expansion joints and area dividers

Where the structure has an expansion joint, the roof gets one: a raised curb on both sides with a flexible bellows cover, never a flat membrane bridged across the gap. Area dividers split large roofs so thermal movement in the membrane and insulation is relieved.

Going Deeper (Intermediate)

Details fail for one of three reasons: wrong height, wrong sequence, or reliance on sealant. A base flashing 4 inches high gets overtopped by snow. A counterflashing installed after the siding cannot be tucked behind it. A surface-mounted counterflashing with a caulk bead as its only top seal lasts as long as the caulk. The other cause is the other trades: the HVAC installer who sets a unit on sleepers and the electrician who runs conduit through a boot never flashed. A roof with twenty undocumented penetrations has twenty future leaks.

Advanced / Pro-Level

Pros build details from the manufacturer's published detail drawings, not from memory, and submit any variation for approval before building it. They shop-fabricate edge metal, coping, and counterflashing from a sheet-metal schedule so joints, cleats, and cover plates match the ES-1 listing, and keep a mockup of the base flashing and curb detail on site for the crew and the manufacturer's technician to accept. The pro superintendent walks the roof with the roofer and every other trade before the membrane goes down and refuses any penetration added later without the roofer present.

Practice Challenge

A new rooftop unit is being added to an existing fully adhered PVC roof. The mechanical contractor proposes setting it on four treated wood sleepers screwed through the membrane and running the refrigerant lines and conduit flat on the roof to the wall. What does the roofer require instead? (Answer: a curb at least 8 inches above the finished roof, fastened to the structure and flashed with PVC base flashing, a termination bar, and counterflashing under the unit cap; the lines and conduit on pipe supports with pads, never laid on or screwed through the membrane; a boxed curb for anything that passes through the roof; and all of it added to the penetration schedule and reported to the manufacturer so the warranty stays intact.)

In Practice

A roofer who builds every curb 8 inches high, terminates every base flashing mechanically, and refuses the sleeper detail hands over a roof the service department never sees. A roofer who flashes to 4 inches and relies on a caulk bead at the top gets the leak call the first winter.

Common Mistakes to Avoid

• Base flashing or curbs below 8 inches, or counterflashing that cannot be removed
• Flashing that depends on sealant instead of a termination bar or counterflashing
• Wood sleepers, conduit, or piping set directly on the membrane
• Pitch pockets used where a boot, hood, or curb would work, then never maintained
• Step flashing installed after the siding, or a sidewall with no kickout

Takeaway: Roofs rarely leak in the field — they leak at the details, so every penetration, curb, wall, and edge gets a flashing built in the right order, to the right height, and held mechanically rather than by sealant.

Educational content only — not roof design, engineering, legal, or safety advice, and not a substitute for hands-on training under a licensed roofing contractor or the judgment of a licensed design professional. Follow the IRC, IBC, and plumbing code as adopted and amended in your jurisdiction, OSHA 29 CFR 1926, the manufacturer's installation instructions and warranty requirements, and your company's procedures.