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newSchoolWorks.com › VoTechWorks › Building & Construction › Roofing › Flashing: Waterproofing Transitions and Penetrations › Lesson 4
BLD-ROOF12-05 · Lesson 4 · 10 clock hours · demo

Penetration and Transition Flashing: Pipes, Skylights, Parapets, and Expansion Joints

Objective. Students will install a lead or EPDM pipe boot, a curb-mounted skylight flashing kit, a parapet cap with through-wall scupper, and an expansion-joint cover on the appropriate substrate mock-ups, meeting manufacturer installation requirements.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Penetration and Transition Flashing Matters

Every pipe, skylight, parapet wall, and expansion joint that interrupts a continuous roof membrane is a potential leak point. Water finds gaps with remarkable efficiency—capillary action can draw moisture through a seam as small as a hairline. Flashing is the system of formed metal, rubber, or composite components that bridges the joint between the roof membrane and whatever passes through or abuts it. Doing it right the first time prevents structural rot, interior damage, mold growth, and costly callbacks.

Pipe Penetration Flashing: Boots and Clamping Rings

A pipe boot (also called a pipe flashing or pipe collar) is a pre-formed sleeve that fits around a pipe and laps onto the surrounding roof surface to shed water away from the penetration. Boots come in lead and EPDM rubber as the most common materials on low-slope and steep-slope roofs.

Sizing is the first critical decision. The boot's inside diameter must match the pipe's outside diameter closely enough that the material can be compressed tightly against the pipe with a clamping ring (also called a draw band). A boot that is too large will not seal; one that is too small cannot be installed. Always read the manufacturer's sizing chart—do not guess.

Installation sequence for a pipe boot:

1. Clean the pipe surface and the roof deck or membrane in the work zone.

2. Slide the boot down over the pipe so the base flange lies flat on the membrane. The flange is typically set in a bed of compatible sealant and fastened to the deck through the membrane with roofing fasteners at the manufacturer's specified spacing.

3. Fold or press the boot collar snugly around the pipe.

4. Seat the clamping ring over the collar at the height specified by the manufacturer. Tighten the ring evenly to compress the rubber uniformly around the pipe—over-tightening can tear the boot; under-tightening allows water infiltration.

5. Apply a manufacturer-approved sealant bead at the top of the ring if specified.

Safety note: Lead boots require gloves and hand-washing discipline; wash hands thoroughly before eating or touching your face. EPDM boots may be supplied with adhesive primers—work in a ventilated area.

Skylight Curb Flashing

A curb-mounted skylight sits on a raised wood or metal frame called a curb. The curb must extend a minimum height above the finished roof surface as required by the skylight manufacturer and by applicable building codes—typically at least 4 inches for low-slope applications, though always confirm the specific manufacturer's requirement. An under-height curb traps debris and water against the skylight frame.

A curb-mount flashing kit is engineered by the skylight manufacturer to match that unit's dimensions. Do not mix components from different manufacturers or models. The kit normally includes: head flashing, sill flashing, step or side flashings, and a saddle or cricket flashing for the uphill side of larger units to divert water around the curb.

Installation sequence:

1. Confirm the curb is square, level side-to-side, and pitched correctly to drain.

2. Apply the sill flashing first (low side), then each side piece with proper lap—upper pieces always lap over lower pieces so water sheds outward.

3. Apply the head flashing last (high, uphill side).

4. Seal all laps and fastener heads with compatible sealant per manufacturer instructions.

5. Integrate flashing into the surrounding membrane or shingle system so water can never travel beneath the flashing base.

Parapet Cap and Through-Wall Scupper

A parapet is a low wall at the roof edge. Water that accumulates on a flat or low-slope roof surrounded by parapets must be routed through the wall via a scupper—an opening lined with a formed metal or plastic sleeve that carries water through the parapet to a downspout or open discharge point.

The coping cap (also called a coping cover) is the cap that runs along the top of the parapet, protecting the wall from direct rainfall and directing water inward to the roof drain system rather than into the wall cavity. Coping sections are lapped for drainage: on a parapet that runs parallel to the slope, each uphill section laps over the downhill section so water flows across the lap, not into it. On a level run, caps lap in the direction of prevailing wind where possible.

Scupper openings must be sized to handle the roof drainage load. The sleeve is set in sealant, fastened, and then the membrane or base flashing is integrated around it so water entering the scupper cannot escape beneath the lining.

Expansion-Joint Covers

Buildings shift due to thermal expansion, seismic movement, and structural loading. An expansion joint is a designed gap that allows controlled movement without cracking the structure. On a roof, that gap must be covered with a prefabricated expansion-joint cover—a flexible, bellows-style or flat cover assembly—that can move with the building while remaining watertight.

The cover is attached to blocking or nailers on each side of the gap, not across the gap itself. Fasteners go into the flanges on each side; the center bellows or flexible membrane bridges the gap freely. Adhesive is sometimes used in addition to mechanical fasteners per the manufacturer's instructions. Never caulk or rigidly adhere both sides of an expansion joint—that defeats its purpose and causes the cover to tear.

Water Testing and Diagnosing Failures

A low-pressure garden-hose water test simulates wind-driven rain. Observe from inside or mark wet spots on top with tape. Common failure causes include: boot not sealed at the base flange, clamping ring under-tightened, flashing laps reversed (water runs beneath the upper piece), insufficient sealant at scupper penetration, or expansion-joint cover fastened rigidly across the gap. For any leak, stop water flow, dry the area, identify the exact entry point, make the repair per manufacturer instructions, and retest.

PPE and General Safety

Wear safety glasses and gloves at all stations. On elevated mock-ups, maintain three points of contact and use fall protection as directed by the instructor. Keep the work area clear of trip hazards. When using sealants or adhesives, follow the product's SDS requirements for ventilation and skin/eye protection.

Key terms8

Pipe bootA pre-formed lead or rubber sleeve installed over a pipe penetration and lapped onto the roof surface to prevent water entry at the pipe; also called a pipe flashing or pipe collar.
Clamping ringA metal draw band tightened around the collar of a pipe boot to compress the boot material uniformly against the pipe and create a watertight seal.
CurbA raised wood or metal frame on which a curb-mounted skylight or other rooftop unit sits; must extend above the finished roof surface at the height specified by the manufacturer and applicable codes.
Coping capA formed metal or masonry cover that runs along the top of a parapet wall to shed water and protect the wall assembly from direct rainfall.
ScupperAn opening through a parapet wall, lined with a formed sleeve, that drains accumulated roof water through the wall to a downspout or open discharge point.
Expansion jointA designed gap in a building structure that allows controlled movement due to thermal, seismic, or load forces; on a roof it is covered with a flexible prefabricated cover that moves with the building while remaining watertight.
Flashing kitA manufacturer-supplied set of matched metal or composite components—head, sill, and side pieces—engineered to waterproof the perimeter of a specific skylight or rooftop unit.
Lap directionThe orientation of overlapping flashing sections so that each upper piece covers the top edge of the piece below, ensuring water sheds over laps rather than being directed beneath them.

Worked examples3

1. A lead pipe boot is available in three sizes with inside diameters of 1.5 in, 2.0 in, and 3.0 in. The pipe being flashed has an outside diameter of 1.875 in. Which boot size should be selected, and why?
  1. Step 1: Identify the pipe's outside diameter: 1.875 in.
  2. Step 2: Compare to available boot inside diameters: 1.5 in (too small—cannot fit over pipe), 2.0 in (slightly larger than 1.875 in—can fit and be clamped snugly), 3.0 in (too large—excessive gap, clamping ring cannot compress boot tightly enough to seal).
  3. Step 3: Select the 2.0 in boot because it is the smallest boot that fits over the 1.875 in pipe, allowing the clamping ring to compress the lead material evenly and create a watertight seal.
Answer: Select the 2.0 in inside-diameter boot. It is the closest fit above the pipe's 1.875 in outside diameter, allowing the clamping ring to compress the boot material securely around the pipe.
2. A curb-mounted skylight manufacturer specifies that the curb must stand at least 4 inches above the finished roof surface. The rough framed curb currently measures 6.5 inches tall. The roofing membrane and insulation added to the deck will raise the finished surface by 3 inches. Will the curb meet the 4-inch requirement after roofing is complete?
  1. Step 1: Determine the effective curb height above finished roof surface. Finished surface rises 3 in, so the curb height above finished surface = 6.5 in − 3 in = 3.5 in.
  2. Step 2: Compare 3.5 in to the 4 in minimum requirement: 3.5 in < 4 in.
  3. Step 3: The curb does NOT meet the requirement. The curb must be raised or blocking added to achieve at least 4 in of exposure above the finished roof surface before the skylight is installed.
Answer: No. The curb will only clear the finished surface by 3.5 inches, which is less than the required 4 inches. The curb height must be increased by at least 0.5 inch before proceeding.
3. A parapet coping run is 36 feet long and uses coping cap sections that are 9 feet long with a required 3-inch overlap at each joint. How many sections are needed, and what is the total coverage provided by each joint (the net coverage per section after accounting for the lap)?
  1. Step 1: Determine net coverage per section. Each 9 ft (108 in) section loses 3 in to the overlap at its uphill end, so net coverage = 108 in − 3 in = 105 in = 8.75 ft per section.
  2. Step 2: Divide total run by net coverage per section: 36 ft ÷ 8.75 ft = 4.114 sections.
  3. Step 3: Round up to the next whole section because partial sections still require a full piece: 5 sections needed.
  4. Step 4: Verify: 5 sections × 8.75 ft net = 43.75 ft of net coverage ≥ 36 ft. ✓
Answer: 5 coping cap sections are required. Each section provides 8.75 feet of net coverage after the 3-inch lap, and 5 sections give 43.75 feet of total coverage for the 36-foot run.
In class: Instructor rotates through four flashing stations (pipe penetration, skylight curb, parapet/coping, expansion joint) performing a complete installation at each while narrating critical decisions: boot sizing to pipe diameter, clamping ring torque, curb height above finished roof surface, coping lap direction for drainage, and expansion-joint cover attachment method; students follow along on a station note card and ask questions at each station. · Station rotation practice: students work in groups of three, each group spending approximately 20 minutes at each of the four stations to install the flashing type assigned using manufacturer instructions; instructor circulates and provides corrective feedback in real time; groups document each installation with a dated photo and a brief written note on what they found most difficult. · Whole-class water test: instructor uses a low-pressure garden hose to simulate wind-driven rain on each finished installation while the class observes; any leak location is marked with tape, the group that installed that section diagnoses the failure cause and describes the repair, and the class evaluates whether the diagnosis is correct.
Materials: Pipe boot station: various diameter lead and EPDM boots, clamping rings, screwdriver, silicone sealant. Skylight station: manufacturer curb-mount skylight flashing kit, flat roof mock-up, step bits. Parapet station: coping cap sections, through-wall scupper, sealant. Expansion-joint station: prefabricated expansion-joint cover, adhesive and fasteners. All stations: tape measure, chalk line, safety glasses, gloves, low-pressure garden hose, marking tape, manufacturer installation instructions, station note cards
Check: Water test is the primary check; a passing installation shows zero leaks during a minimum two-minute continuous water-flow test at each penetration or transition. Students who have a leak must identify and repair the failure point and retest; pass requires a clean retest.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Roofing – Flashing Installation
1. When installing a pipe boot, the boot's inside diameter should be selected so that it:
NCCER Roofing – Flashing Installation
2. Why must the coping cap sections on a sloped parapet run be lapped so that the uphill section overlaps the downhill section?
NCCER Roofing – Flashing Installation
3. An expansion-joint cover begins leaking after one winter season. The most likely installation error is that the installer:
NCCER Roofing – Flashing Installation
4. During a water test, a leak is found at the base of a curb-mounted skylight's sill flashing. The most likely cause is:
NCCER Roofing – Flashing Installation
5. A through-wall scupper is installed in a parapet. Which of the following best describes the correct integration of the scupper sleeve with the roof membrane?
OSHA Fall Protection / Safety – Hazard Recognition and PPE
6. Which PPE is required at minimum when working with lead pipe boots at all stations?
NCCER Roofing – Flashing Installation
7. A curb-mounted skylight manufacturer requires the curb to stand 4 inches above the finished roof surface. The rough framed curb is 5 inches tall, and the finished membrane and insulation system will add 2.5 inches to the deck surface. What is the curb's exposed height above the finished surface, and does it meet the requirement?
NCCER Roofing – Flashing Installation
8. During the whole-class water test, a group's pipe boot installation leaks at the base flange. After drying the area, what is the correct repair sequence?
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