Roof Flashing Installation for Australian Residential Builds

What it is

Roof flashing is the sheet metal or formed metal detail that closes the junction between two roof planes, or between the roof and a wall, chimney, vent pipe or skylight. It carries water away from joints that the cladding cannot weatherproof on its own. On a residential build, flashings sit between the gutter, the cladding, the wall and any roof penetration.

Flashing work is governed by several Australian Standards and the National Construction Code. Roof drainage including gutters and downpipes follows AS/NZS 3500.3. Sheet metal roof cladding and accessories follow AS 1562.1, and concrete and clay tile roofing follows AS 2049 and AS 2050. The NCC 2022 Volume Two Part 7.3 sets the performance requirements that all flashing details need to satisfy on Class 1 and Class 10 buildings.

What the standards require

NCC 2022 Volume Two clause 7.3.3 requires flashings at every junction between the roof and a wall, parapet, chimney or penetration where water could enter the building. The flashing needs to be continuous, secured to both surfaces and dressed in a way that sheds water away from the joint.

AS/NZS 3500.3 sets out the design of roof drainage including the flashings at gutter to fascia and gutter to wall junctions. Box gutters need to be fitted with overflow devices that discharge clear of the building. Apron flashings at chimneys and wall abutments need a minimum cover over the cladding of 100 mm. Step flashings at side walls need to overlap by at least 100 mm at each tile or pan course.

AS 1562.1 for sheet metal roofing requires flashings to be the same material grade as the roof cladding. Mixing dissimilar metals causes galvanic corrosion at the fixing line. A Colorbond steel roof needs Colorbond steel flashings. A Zincalume roof needs Zincalume or compatible stainless steel fixings. Copper and lead flashings are restricted on potable water catchment roofs because the runoff carries metal ions into the rainwater tank.

Apron flashings

Apron flashings sit at the upslope side of any roof to wall junction. The vertical leg needs to extend at least 150 mm up the wall behind the cladding or render. The horizontal leg dresses down over the roof cladding by at least 75 mm. The leading edge needs a 6 mm hemmed return or kick to break surface tension and stop capillary creep.

On masonry walls, the apron needs to be turned into a brick mortar joint at least 25 mm deep. Surface mounted aprons fixed only with sealant fail within 5 to 10 years as the sealant ages.

Step flashings

Step flashings cover the cross slope junctions between a tile or sheet roof and a side wall. Each step laps the next by 100 mm vertically and 75 mm horizontally. The top edge tucks behind the wall cladding or under a counter flashing chased into masonry.

For tile roofs, the step flashing sits over the tile pan and under the next course up. The flashing leg should not be visible from below. For sheet roofs, step flashings sit on the ribs of the sheet so water cannot pond behind the upslope edge.

Penetration flashings

Vent pipes, flue penetrations and skylight curbs all need site formed or proprietary flashings. The flashing needs to be sized to match the pipe and pitched to suit the roof slope. Most failures occur at the top edge where the flashing collar meets the pipe. Stainless steel band clamps with a soft EPDM sleeve outperform silicone sealed collars by a wide margin.

Skylight curbs need apron flashings at the downslope side and step flashings up the sides. The upslope side needs a back tray that discharges either side of the skylight.

Common defect modes

Capillary creep is the most common cause of flashing failure on residential roofs. Where the flashing edge sits flat against the cladding without a hemmed return, water tracks back under the flashing by surface tension. A 6 mm kick at every leading edge prevents this.

Galvanic corrosion at fixings is the second most common issue. Stainless steel rivets in a Colorbond flashing accelerate corrosion of the surrounding steel. The fix is to use class 3 or class 4 painted rivets in the same colour and metal grade as the cladding.

Sealant only flashings fail when the sealant cures and loses elasticity. Silicone reaches end of useful life in 7 to 10 years on a roof that sees daily thermal cycling. Any flashing detail that relies solely on sealant needs to be redesigned with a mechanical lap and a sacrificial sealant bead behind it.

Builder checklist

Specify the flashing material in the same grade and colour as the roof cladding. Detail every flashing junction on the working drawings before the roofer arrives on site. Insist on hemmed edges at every leading face. Use mechanically fixed laps with a sealant bead behind, not in front of, the lap. Inspect penetration collars before the scaffold comes down because access is hard to recover later.

Test gutters and box gutters by flooding with a hose for 5 minutes and check ceiling cavities below for any sign of water. Document the test in the handover pack.