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Roof flashings and sealing: residential installation guide

Apron, step, valley, ridge and penetration flashings for Australian residential roofs. AS 1562.1 requirements, lap minimums, sealant rules, and common defects.

By Dan Waring. Last updated 10 May 2026. Verified 10 May 2026.

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TL;DR

Flashings are the last line of defence at every roof-to-wall junction, penetration, and change of plane. A missing or incorrectly lapped flashing is one of the most common causes of residential warranty claims and consequential water damage. The governing standard is AS 1562.1:2018 for metal sheet roofing, with NCC 2022 Housing Provisions Part 7.2 setting the DTS path for Class 1 buildings. The hard rule: flashings must achieve water-tightness by mechanical means (correct lap direction, minimum 75 mm overlap in the direction of fall for sheet-to-sheet joints) rather than relying on sealants as a primary barrier. Sealant supplements laps where capillary action is a risk, it does not replace them. Penetrations must use purpose-made boots or collar flashings, not just sealant rings.

When you do this

Flashing installation runs across multiple stages of a residential build. Install flashings at each stage as the relevant work is completed; do not leave them all to the end:

  1. Frame complete, roofing underway: apron and step flashings at roof-to-wall junctions, valley flashings as sheets are laid, ridge and hip cappings after the last sheet course is fixed.
  2. Penetrations: plumbing vent boots, exhaust flue collars, and skylight flashings installed before the certifier’s lock-up inspection or before any lining work begins. Cutting retroactively through installed sheets creates extra leak points and is poor practice.
  3. Pre-PCI walk: verify all flashings are seated, lapped correctly, and free of open joints before calling for the PCI.

Flashings must be installed by or under the direct supervision of a licensed roofer. In most states, flashing work on a roof is part of the licensed roofing trade scope.

Who’s involved

RoleResponsibility
Roofer (licensed)Primary installer of all roof-plane and junction flashings
Plumber (licensed)Pipe penetration boots for plumbing vent stacks (licensed plumbing work in all states)
Builder (principal contractor)Coordination, hold-point sign-off, pre-PCI inspection
CertifierChecks flashing systems at lock-up and PCI inspections

Flashing types and installation

Apron flashings

An apron flashing runs horizontally across the low side of a roof-to-wall junction: where a roof slope meets a vertical wall below the slope (for example, where a lower skillion meets a wall rising above it, or where a verandah roof meets a house wall).

Key requirements per AS 1562.1:2018 and NCC 2022 HP 7.2:

  • The flashing must turn up the wall (upstand) and be secured into the wall. For masonry walls, the flashing is chased into mortar joints or behind the cladding; for lightweight walls, it laps behind the sarking and wall wrap.
  • An anti-capillary fold (10 mm fold at 30 degrees minimum on flat surfaces, or 10 mm fold at 90 degrees on other surfaces) must be formed at the uphill edge of the flashing to prevent water tracking back by capillary action under the flashing (NCC 2022 HP 7.2.7(1)(g), verified 2026-05-10).
  • The flashing must lap over the roofing sheets below, with the lap minimum 75 mm in the direction of fall (NCC 2022 HP 7.2.7(1)(d), verified 2026-05-10).
  • Over-flashing (where a separate piece covers the top of the upstand and is chased into masonry) must cover the upstand by a minimum of 50 mm and be chased at least 25 mm into the wall.
  • Where the over-flashing is used with a sloping wall cut, the upstand must be at least 100 mm high.
  • Joints in apron flashings (where lengths are butted) must lap a minimum 75 mm in the direction of fall, with appropriate sealant between lapping faces (NCC 2022 HP 7.2.7(1)(d), verified 2026-05-10).

Source: NCC 2022 ABCB Housing Provisions Part 7.2, ABCB (verified 2026-05-10).

Sidewall step flashings

Step flashings run up the side of a roof slope where it abuts a vertical wall: the classic pattern at a gable wall or at a parapet beside a hip roof. Each individual step flashing piece is fixed to the roof and interlocked with the wall cladding or masonry above.

Installation requirements:

  • Each step flashing piece must overlap the piece below it by at least 75 mm in the direction of fall (NCC 2022 HP 7.2.7(1)(e), verified 2026-05-10).
  • For masonry walls, step flashings are fastened into the mortar joint with galvanised or zinc/aluminium sheet metal wedges at each end of each length and at intermediate intervals of no more than 500 mm (NCC 2022 HP 7.2.7(1)(e), verified 2026-05-10).
  • A counter-flashing (over-flashing) chased into the wall face must cover the top of each step flashing by at least 50 mm.
  • Where the roof sheet rib or corrugation abuts the wall, the gap must be sealed with appropriate sealant or profiled foam closures to prevent wind-driven rain entry. Sealant alone is not a substitute for the lap; it is used in addition to the mechanical joint.
  • Minimum clearance from the roof surface to the top of the step flashing upstand: the flashing must turn up the wall enough that in any anticipated rain event, water shed from the roof does not overtop the flashing. 75 mm upstand above the roof cladding surface is the practical minimum for a standard residential pitch; steeper pitches with high rainfall may require more.

Source: NCC 2022 ABCB Housing Provisions Part 7.2, ABCB (verified 2026-05-10); QBCC Roof flashings, Queensland Building and Construction Commission (verified 2026-05-10).

Valley flashings

Valley flashings (valley gutters) run along the internal angle where two opposing roof slopes converge. They collect and channel rainwater to the eaves gutter. On high-rainfall sites or roofs with a large contributing catchment area, valley gutters carry significant hydraulic load.

Installation requirements:

  • Valley gutters must be formed from material compatible with the adjoining roof cladding. Galvanic corrosion occurs where dissimilar metals contact in the presence of moisture: no copper valleys under a Colorbond/Zincalume roof; no lead valleys on any roof that forms part of a drinking water catchment (NCC 2022 HP 7.2.3, verified 2026-05-10).
  • The open channel of the valley (between the turned-up edges of the valley gutter and the roofing sheets laid over the valley) must remain clear to allow debris to wash free. A minimum open channel width is necessary to handle anticipated rainfall intensity without backing up. Valley gutters are sized in accordance with the hydraulic requirements of the catchment area; for a standard suburban residential valley, 200 mm overall valley width is a common starting point.
  • Roofing sheets are cut at an angle to the valley and turned down into the valley, not butt-jointed. The lap of the sheet over the valley edge must be sufficient to prevent wind-driven rain entry.
  • Joints in valley gutter lengths must lap minimum 75 mm in the direction of fall (NCC 2022 HP 7.2.7(1)(d), verified 2026-05-10), with sealant between lapping faces.
  • Valley gutters should not rely on sealant as a primary watertight element. The lap and fall are the primary means; sealant addresses residual capillary risk at the lap joint.
  • In bushfire-prone areas rated BAL 12.5 and above, valley gutters must be screened or self-cleaning to prevent ember accumulation. AS 3959:2018 sets requirements per BAL rating.

Source: NCC 2022 ABCB Housing Provisions Part 7.2, ABCB (verified 2026-05-10); QBCC Roof flashings, Queensland Building and Construction Commission (verified 2026-05-10).

Ridge and hip cap flashings

Ridge cappings run along the apex of a gabled or hipped roof. Hip cappings cover the external hip angle where two roof planes meet at an upward angle.

Installation requirements:

  • Ridge and hip caps must be purpose-made, machine-folded sheet metal sections compatible with the roofing material (NCC 2022 HP 7.2.7(1)(a), verified 2026-05-10).
  • Lead flashing caps must not be used with prepainted or zinc/aluminium-coated steel (NCC 2022 HP 7.2.3, verified 2026-05-10).
  • Joints in ridge and hip caps must lap minimum 75 mm in the direction of fall (normally, this means lapping from the low end toward the high end so that the upper cap overlaps the lower one), fastened at intervals no more than 40 mm from the lap edge (NCC 2022 HP 7.2.7(1)(d), verified 2026-05-10).
  • The cap must bed down into the corrugation or rib profile of the sheeting. Where the profile creates gaps between the cap underside and the crest of the sheet, purpose-made profiled foam closures (infill strips) must be installed to seal those gaps. An open gap between the ridge cap and the sheet profile allows wind-driven rain and embers (in BAL areas) to enter.
  • Fixings for ridge and hip caps must comply with the same corrosion protection and material compatibility requirements as roofing fasteners. In coastal and marine environments, stainless steel fasteners are required.
  • Expansion joints are required in ridge and hip cap runs exceeding 12 m to allow for thermal movement (Lysaght Flashing Guide, verified 2026-05-10).

Source: NCC 2022 ABCB Housing Provisions Part 7.2, ABCB (verified 2026-05-10); Lysaght Metal Flashing Guide, BlueScope (verified 2026-05-10).

Vent and pipe penetration boot flashings

Every plumbing vent stack, exhaust flue, solar panel penetration, and conduit entry through the roof surface requires a purpose-made penetration flashing. Sealant rings alone are not a compliant primary barrier.

Installation requirements per NCC 2022 HP 7.2.7(2):

  • Collar or boot flashings must be designed to permit total drainage of the area above the penetration: no pooling of water behind the collar is acceptable (NCC 2022 HP 7.2.7(2)(a), verified 2026-05-10).
  • Where a penetration requires cutting through rafters or purlins, the roof structure must be restored to its original load-carrying capacity using trimmers and appropriate structural support (NCC 2022 HP 7.2.7(2)(b), verified 2026-05-10).
  • Purpose-made malleable aluminium-edged EPDM rubber boot flashings (the Dektite-type product common in Australian residential work) are the standard solution for round pipe penetrations on metal sheet roofing. These are fixed over the pipe, cut to suit the pipe diameter, and fastened to the roof sheet with screws at the base flange, with sealant under the flange.
  • For square or rectangular penetrations (flue boxes, curb mounts), custom-folded metal collars are required.
  • The flashing must form a continuous weatherproof collar around the penetration. A sealant-only bead around the base of a pipe emerging through a hole in the sheet is not compliant; it is the most common penetration defect found at inspections.
  • Plumbing vent stacks are licensed plumbing work in all states. The plumber is responsible for the penetration boot installation on the stack.

Source: NCC 2022 ABCB Housing Provisions Part 7.2, ABCB (verified 2026-05-10); QBCC Roof flashings, Queensland Building and Construction Commission (verified 2026-05-10).

Sealants: supplement, not substitute

The QBCC principle captures it directly: “methodologies described aim to achieve water-tightness by mechanical means rather than an over-reliance upon the use of sealants.” (QBCC Roof flashings, verified 2026-05-10).

Sealant is used:

  • Between lapping faces of flashings where capillary action could draw water back through the lap under wind pressure
  • Under penetration boot base flanges
  • At the interface between a cap and a profiled sheet where foam closures cannot fill every gap

Sealant is not a substitute for:

  • A correctly lapped and mechanically fixed flashing
  • A purpose-made boot or collar at a penetration
  • An anti-capillary fold at an apron or step flashing upstand

Use a neutral-cure silicone sealant compatible with the flashing material. Acetoxy-cure (acid-cure) silicones can corrode some metals. On Colorbond and prepainted surfaces, confirm sealant compatibility with the manufacturer’s technical data.

Tolerances and acceptance

ElementReferenceRequirement
Flashing joint lap (sheet roofing)NCC 2022 HP 7.2.7(1)(d)75 mm minimum in direction of fall
Step flashing overlapNCC 2022 HP 7.2.7(1)(e)75 mm minimum in direction of flow
Over-flashing coverage of upstandNCC 2022 HP (general detail)50 mm minimum cover over upstand
Masonry wall wedge spacingNCC 2022 HP 7.2.7(1)(e)Maximum 500 mm centres
Flashing fastener lap clearanceNCC 2022 HP 7.2.7(1)(d)Fixed at not more than 40 mm from lap edge
Anti-capillary fold (flat surfaces)NCC 2022 HP 7.2.7(1)(g)10 mm / 30-degree fold minimum
Anti-capillary fold (other surfaces)NCC 2022 HP 7.2.7(1)(g)10 mm / 90-degree or 135-degree fold
Ridge/hip cap lapNCC 2022 HP 7.2.7(1)(d)75 mm minimum
Workmanship (flashing alignment, seating)HIA Guide to Materials and WorkmanshipPer current HIA Guide to Materials and Workmanship. Verified numerical value pending HIA member access. [HIA-101]

Documents needed

  • Engineering drawings confirming wind classification (AS 4055:2021)
  • Manufacturer’s product data sheet for flashing materials (confirm corrosion compatibility)
  • Roofer’s SWMS (working at heights is an HRCW under WHS Regulations)
  • NCC 2022 HP 7.2 compliance confirmation via certifier’s inspection schedule

Common holds

Wrong lapping direction. Flashings lapped with the upper piece under the lower piece are reverse-lapped: water tracks directly into the building rather than being shed away. Check lap direction before each section is covered by the next course. Reverse laps are the single most common flashing defect found at inspection.

Sealant in lieu of lap. A continuous sealant bead around a penetration pipe, or between abutting flashing ends without a mechanical lap, is not a compliant installation. Sealant fails by UV degradation, thermal cycling, and adhesion loss. It has a service life measured in years; the roof has a service life measured in decades. The mechanical lap must be there first.

Missing anti-capillary fold. At any apron or step flashing where the upstand meets a wall surface, without the anti-capillary fold, wind-driven rain rides the surface tension of the water film back up and under the flashing. A single 10 mm fold at the uphill edge breaks the film.

Incompatible metals at flashing and roof. Galvanic corrosion between dissimilar metals creates rapid localised pitting at the contact zone. Lead with prepainted steel, copper with Zincalume, steel fasteners with aluminium flashings: all are prohibited combinations. Confirm material compatibility before ordering flashings.

Penetration boot improperly fixed. EPDM boots that are not screwed down at the base flange, or where the flange is simply bedded in sealant without mechanical fasteners, lift in high wind events. Sealant failure then leaves the penetration fully open.

Ridge cap not bedded into profile. A ridge cap that bridges the corrugation crests without foam closures leaves a gap under the cap of 20 to 30 mm on standard corrugated profiles. In heavy rainfall with any wind, water tracks through this gap into the ceiling space.

Valleys blocked by debris. Valley gutters that collect leaf litter form a dam that backs water under the roofing sheets. Include valley gutter access and maintenance in owner handover notes.

References

See also

Last updated: 2026-05-10. Verified: 2026-05-10. Quarterly review for currency.

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