When a metal roof leaks, the instinct is to blame the sheet. In practice, the sheet is rarely the problem. Water finds its way in at junctions: where roof meets wall, where ridge meets sheet, where a valley collects two planes of runoff. These are flashing zones, and getting them wrong is the most common source of roof failure in Australian residential and commercial construction.
The underlying principle is straightforward. Water must be mechanically led out of a building by directing it from one surface onto the next in the direction of flow, the same way roof tiles or weatherboards overlap. This is called shingle-lapping into the drainage plane. Sealant is a secondary measure to close gaps against wind-driven rain; it is never a substitute for correct mechanical overlap. When a flashing relies on sealant as its primary defence, that flashing will fail. The only question is how soon.
Why Silicone Fails as a Primary Seal
Silicone and polyurethane sealants degrade under UV exposure. In Australian conditions, particularly in Queensland, Western Australia and the Northern Territory, surface temperatures on metal roofing regularly exceed 70°C. Sealant applied over an exposed flashing junction will typically show cracking and adhesion loss within three to five years. Once the bond breaks, capillary action draws water into the joint faster than an open gap would.
There is also the movement problem. Metal expands and contracts with temperature. A 6-metre COLORBOND sheet can move several millimetres across a day-night temperature cycle. Sealant bridging a lapped joint between two metal surfaces is being stretched and compressed continuously. Even sealants rated for metal-to-metal applications have a finite fatigue life under this kind of cyclic loading.
The correct approach is to design the flashing so that water cannot reach the sealant line in the first place. Sealant then acts as a secondary barrier against wind-driven rain, not the primary one.
Apron Flashings
An apron flashing sits at the base of a wall or upstand where it meets the roof surface below. Its job is to collect water running down the wall face and direct it onto the roof sheet, then into the gutter.
The apron must lap over the top of the roof sheet below it by a minimum distance that accounts for the profile height and the roof pitch. On a low-pitch corrugated or Trimdek roof, this lap needs to be generous because water moves slowly and wind can push it back up the profile. AS 1562.1 provides minimum lap requirements by pitch, and these should be treated as absolute minimums rather than targets.
The top edge of an apron flashing must be fixed to the wall, not left floating. On a masonry wall, this means either chasing it into the render or mortar joint, or holding it with a separate over-flashing above. The junction between the apron and the wall face is where most apron leaks originate.
Over-Flashing and the Chased-In Debate
An over-flashing, sometimes called a pressure flashing or cap flashing, covers the top edge of the apron or counter-flashing below it. On brick veneer walls, the standard approach is to chase the over-flashing into a mortar joint, bed it in mortar and seal the exposed edge. This creates a mechanically fixed, weather-resistant termination.
On rendered masonry, the situation is more contested. Two approaches are used in practice.
The first is to chase the flashing into the substrate before rendering, so the render finishes over the top of the flashing edge. This looks clean and eliminates the exposed sealant line, but it makes future flashing replacement difficult and can cause cracking if the flashing moves.
The second is a surface-fixed pressure over-flashing, mechanically fixed to the wall with masonry anchors and sealed at the top edge with a compatible sealant. This is easier to replace and inspect, but the sealant line is exposed and requires periodic maintenance.
For new construction, chasing in is generally preferred where the wall substrate allows it. For remedial work on existing rendered walls, a well-fixed pressure over-flashing is often more practical. Either way, the flashing must overlap the apron below by at least 75mm, and the overlap must be oriented so that water cannot track behind it.
Ridge Flashings
A ridge flashing caps the apex of a pitched roof where two opposing roof planes meet. It must cover both sheets adequately and be fixed through the crown of the profile, not through the pan, to avoid creating a fastener penetration in a water-collection zone.
The ends of ridge flashings at gable walls need to be mitred or fitted with stop ends to prevent water entry along the ridge length. Where two ridge sections join, the upper section laps over the lower in the direction of prevailing weather. This is a detail that gets skipped on site more often than it should.
On roofs with a hip, the hip flashing intersects the ridge flashing at the apex. The geometry here requires careful cutting and fitting; a poor junction at this point is a reliable source of leaks.
Barge Flashings
A barge flashing runs along the raked edge of a gable roof, covering the edge of the sheet and the top of the barge board or fascia. Its primary function is to stop water from tracking back under the sheet edge in wind-driven rain and to protect the substrate below.
The barge flashing must turn down over the fascia face far enough to shed water clear of the timber. Where the barge meets the ridge flashing at the apex, the junction needs to be properly weathered, typically with a mitred cap or a formed stop end. At the eave end, the barge should terminate into or over the gutter fascia flashing to complete the drainage path.
Valley Flashings
Valleys concentrate runoff from two roof planes into a single channel. The volume of water in a valley during a heavy rain event is substantial, and the flashing must be wide enough and deep enough to contain it without water overtopping the sides and running under the sheets.
AS 1562.1 specifies minimum valley widths by roof pitch and catchment area. On low-pitch roofs with large catchments, a standard valley flashing may be undersized for the design rainfall intensity. This is worth checking against the Bureau of Meteorology IFD data for the project location.
Valley flashings should be installed before the roof sheets, with the sheets lapping over the valley flanges. The sheets must not be cut so close to the valley centreline that the flange is exposed. Fasteners should not be placed in the valley flange itself.
Weep Holes and Drainage
Flashings that sit against masonry walls often trap water behind them. Weep holes at the base of the flashing allow this water to drain rather than accumulate. Blocked weep holes are a common cause of water ingress that looks like a flashing failure but is actually a drainage failure.
On cavity wall construction, the weep holes in the flashing must align with the cavity drainage path. Mortar droppings in the cavity can block this path and redirect water to the interior. During construction, this is worth checking before the wall is closed.
Galvanic Corrosion and Metal Matching
Flashings must be made from metal that is compatible with the roof sheet. Placing copper or lead flashings against ZINCALUME or COLORBOND steel creates a galvanic cell. Runoff from the copper or lead carries ions that accelerate corrosion of the zinc-aluminium coating on the steel sheet below.
For COLORBOND and ZINCALUME roofing, flashings should be made from the same base metal: ZINCALUME steel or COLORBOND steel in a compatible colour. Aluminium flashings are also compatible. Copper and lead should not be used in contact with or upslope of zinc-coated steel products.
This is not a minor concern. Galvanic attack on ZINCALUME can progress quickly in coastal or industrial environments where the electrolyte (salt or pollution) is present in rainwater. BlueScope's technical literature addresses this directly, and the guidance should be followed on any project within the marine or industrial corrosion categories defined in AS 4312.
Bushfire Zones and Non-Combustible Flashings
In Bushfire Attack Level (BAL) rated zones under AS 3959, flashings must meet non-combustibility requirements consistent with the wall and roof assembly. Where the roof system is required to be non-combustible under NCC 2025 provisions for external walls, any flashings forming part of that assembly must also comply. COLORBOND and ZINCALUME steel flashings satisfy AS 1530.1 non-combustibility requirements. Painted or foam-backed products need to be checked against the specific BAL rating and NCC deemed-to-satisfy provisions.
Getting the Detail Right
Flashing failures are almost always detail failures: wrong lap direction, insufficient overlap, reliance on sealant where mechanical fixing was needed, or incompatible metals. None of these are difficult to get right if the principle is understood. Water flows downhill, and every junction in the building envelope must be designed to keep it moving in that direction.
For roofers and builders working through a leak diagnosis, the flashing junctions are the first place to look, in order: apron and over-flashing at wall abutments, ridge ends and hip intersections, valley widths, and barge terminations. The sheet itself is almost never the source.
ACS supplies COLORBOND and ZINCALUME flashings cut to length, including apron, ridge, barge, valley and custom profiles, along with compatible fasteners and sealants. For project quantities or non-standard profiles, the request-a-quote function at acsupplies.com.au is the quickest way to get accurate pricing and lead times.