Fibre Reinforced Facade Encyclopedia: Installation
Flashing is the formed metal or membrane element that collects water inside a facade cavity and discharges it back to the outside face wherever the cavity is interrupted. Openings, slab edges, parapets, bases, service penetrations and changes of plane are all interruptions, and each one is a place where water running down the back of the cladding would otherwise enter the building. The weather barrier sheds the water; the flashing is what gets it out.
A cavity tray with no stop end at its outer corner does not drain the cavity. It delivers the water it collected straight into the wall at the end of the tray.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Function | Collect cavity water and discharge it outboard | At every interruption of the cavity |
| Common materials | Aluminium, stainless steel, coated steel | Selected for exposure and adjacent metals |
| Typical gauge | 0.5 to 1.2 mm sheet metal | Heavier for long spans and exposed edges |
| Upstand height | Typically 75 to 150 mm minimum | Higher in severe wind driven rain exposure |
| Drip projection | Typically 25 to 40 mm past the face below | With a throat or turned-down edge |
| Fall on horizontal legs | Minimum 1 in 20 toward the outside | Nothing laid dead flat |
| Stop ends | Required at every open end of a tray | Turned up at least the upstand height |
| Expansion joints | Typically every 6 to 8 m in aluminium | Free movement plus a lapped or bellows joint |
| Aluminium thermal expansion | About 23 x 10 to the minus 6 per degree C | A 6 m run moves about 7 mm over 50 degrees |
The flashing types and what each one does
- Head flashing: a tray over a window, door or louvre head that catches cavity water above the opening and discharges it outboard, with stop ends turned up at both ends and an upstand taped into the barrier above
- Sill flashing: a tray under an opening, falling outwards, with a turned-up back and end dams, catching anything passing the window sill and draining it clear of the wall below
- Jamb flashing: the vertical closure at the sides of an opening, linking head and sill flashings to the barrier and closing the cavity against wind driven rain without blocking drainage
- Parapet flashing or capping: the horizontal cap at the top of a wall, falling toward the roof side, with drips on both faces and continuity into the roof waterproofing
- Base flashing: the discharge at the bottom of the cavity above finished ground or podium level, draining clear rather than into a rebate or planter
- Penetration flashings: collars, saddles or liquid-applied details at pipes, conduits, brackets, signage and anchors
- Cavity trays at slab edges and horizontal fire barriers, which interrupt the cavity and need a drainage path
Upstands, drips and stop ends
Three geometric features do almost all the work, and the same three are omitted in almost every failed detail. The upstand is the vertical leg at the back that stops water running off the back edge; it has to be tall enough to stay above any water that can pond or splash in the cavity under design wind conditions, which is why 75 to 150 mm is a common minimum. It also has to be lapped with the weather barrier so the barrier drains onto the flashing and not behind it. A flashing lapped the wrong way is worse than no flashing, because it collects water and delivers it inboard.
The drip is the projecting, turned-down or grooved edge that makes water leave the flashing rather than tracking back along the underside by surface tension, which it will do for a surprising distance. A projection of 25 to 40 mm past the face below, with a turned-down lip or throat, stops it, and it is the same detail that controls streaking.
The stop end is the turned-up closure at the end of a tray. A tray without stop ends is a gutter with its ends open and it discharges into the wall. Stop ends must be formed or welded rather than made with a bead of sealant, because a sealant joint at the end of a tray is a service life item in a location nobody can reach.
Material selection and compatibility
Galvanic compatibility is the constraint that catches people. Where two dissimilar metals are in contact, or where run-off from one lands on the other, the less noble metal corrodes preferentially. The combinations that matter are copper run-off onto zinc, aluminium or steel, which should be avoided entirely; aluminium in contact with uncoated steel or stainless fasteners, which needs separation or a compatible fastener specification; and aluminium in prolonged contact with wet cementitious materials, where the alkalinity attacks it. Keep the number of metals small, separate dissimilar metals with a durable isolating layer rather than a tape with a shorter life than the building, specify fasteners of the same or a more noble metal, and check the run-off path as well as the contact points.
Thermal movement in long runs
Flashings are long, thin, exposed metal elements that see the full diurnal and seasonal temperature swing, and dark coated metal on a west elevation reaches surface temperatures well above air temperature. Aluminium expands about 23 micrometres per metre per degree Celsius, so a 6 metre run through a 50 degree service range moves roughly 7 mm. Steel moves a little over half as much.
If that movement is restrained, something gives: the flashing buckles between fixings, the fixings elongate their holes and leak, or the sealant at a lap tears. Break long runs into lengths of the order of 6 to 8 metres for aluminium with a deliberate movement joint between them, formed as a generous lapped joint with a loose cover piece or a purpose-made expansion joint, and fix with one fixed point per length and slotted holes elsewhere. Never rely on a sealant bead alone, because sealant at a lap has a service life measured in years while the flashing is expected to last decades. Movement joints should also align with those in the structure and cladding.
Standards and references
- AS/NZS 2904 Damp-proof courses and flashings
- AS 1562.1 Design and installation of sheet roof and wall cladding, metal, for fixing, laps and movement
- AS/NZS 2179.1 Specifications for rainwater goods, accessories and fasteners
- AS/NZS 2312.1 and 2312.2 Guide to the protection of structural steel against atmospheric corrosion
- AS/NZS 4680 galvanized coatings, and manufacturer data for coated steel compatibility
- AS/NZS 4284 Testing of building facades, water penetration testing including flashings
- NCC 2022 Volume One Part F1 weatherproofing of external walls (check the edition adopted in your jurisdiction)
Related terms
Questions specifiers ask
What does a flashing actually do on a rainscreen?
It collects water running down the inside of the cavity wherever the cavity is interrupted, at openings, slab edges, parapets, bases and penetrations, and discharges it back outboard of the cladding. The weather barrier sheds water down the wall; the flashing is what gets it out of the wall.
Why do flashings need stop ends?
Because a tray without turned-up ends is a gutter with its ends open. It collects water along its length and discharges it at the ends, which are inside the wall. Stop ends should be formed or welded, not made with sealant, because that joint is unreachable for the life of the building.
Which metals can be used together?
Keep the number small and check run-off paths as well as contact. Copper run-off onto zinc, aluminium or steel is aggressive and should be avoided. Aluminium needs isolating from uncoated steel and from wet cementitious materials. Fasteners should be of the same or a more noble metal than the flashing.
How much do long flashings move?
Aluminium expands about 23 micrometres per metre per degree Celsius, so a 6 metre run through a 50 degree range moves roughly 7 mm, and steel a little over half that. Break long runs at around 6 to 8 metres, use one fixed point and slotted holes elsewhere, and align movement joints with the structure and cladding.
Working through the flashing set for a moulded panel facade? We resolve panel geometry, cavity and flashings on one set of shop drawings. Talk to us about detailing
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.