Key takeaways
- Flashing at an opening works as a drainage system, so every layer must lap over the layer below it and discharge to the outside face of the wall.
- The sill is set and flashed first, because head and jamb flashings must lap onto it and any rework of the sill later destroys the layers above.
- Head flashings must include a drip edge clear of the wall face so water leaves the building rather than tracking back onto the head.
- Sealant is a complement to flashing, not a substitute, and a fully sealed perimeter with no drainage path will hold water against the frame.
- The window and door schedule, the flashing manufacturer’s installation instructions and the NCC waterproofing provisions are the documents to check when a detail is not obvious on site.
Most water that enters a building around a window or door does not come through the glass. It comes through the opening — at the sill, at the jamb-to-sill junction, at the head where the cladding meets the frame, or at the point where a fixing has been driven through a flashing to hold a reveal trim. The glazing unit itself is usually the most watertight component in the assembly.
That is why flashing an opening is less about sealing and more about sequencing. A window or door opening is a drainage system. Water that gets behind the cladding has to be collected, directed down and out, and discharged clear of the wall face. Every layer in that system has to lap over the layer below it, and the whole system has to finish outside the building.
Get the order wrong and the detail can look perfect on the day of installation and still leak in the first decent southerly. This article covers the installation and flashing sequence for openings, complementing the glazing performance material in our article on window and glazing energy performance.
Why the sequence matters more than the product
Flashing products are generally well made. The failures come from the order of installation and from what happens after the frame is fixed. A sill flashing installed after the frame is in place is very hard to lap correctly under the frame, and it usually ends up relying on sealant at exactly the junction where sealant performs worst.
The same applies to the head. A head flashing installed before the jamb flashings means the jamb flashings have to be dressed up behind the head, which creates a reverse lap. Water that runs down the jamb then sits on the head flashing instead of passing over it and out.
The general principle is simple: work from the bottom up, and make sure each element laps over the one below it. Water runs downhill, so the detail should be built so that gravity does the work and sealant only handles what gravity cannot.
This is the same logic that governs metal roof flashing details. It applies to openings regardless of whether the wall is brick veneer, lightweight cladding, render on foam, or a combination.
Set out the sill before the frame arrives
The sill is the first element and the one that constrains everything above it. Before the frame is delivered, confirm the sill line, the fall, and the relationship between the frame, the structural opening and the finished wall face.
The fall on a sill or sill pan is not optional. A sill that is level, or worse, falls back into the building, will pond water against the frame and the weatherproofing layer until something gives. The amount of fall is set by the flashing manufacturer and by the relevant standard, and it should be confirmed against those documents rather than assumed from the last job.
Set out also needs to allow for the cladding and any trim that will sit over the flashing. If the sill flashing projection is worked out before the cladding thickness is known, the flashing will either finish behind the cladding face or project so far it looks wrong. Both outcomes are expensive to correct once the frame is fixed.
Check the structural opening against the schedule
The window and door schedule should state the product, the opening size, the fixing method and the flashing system. If the opening as built differs from the schedule, resolve it before installation. Packing a frame to fit an oversized opening can leave the flashing laps too short to be effective, and no amount of sealant recovers that.
Flashing the sill and the jamb-to-sill junction
The sill flashing is installed before the frame. It should extend up the jambs far enough to receive the jamb flashings, and it should discharge to the outside face of the wall with a drip edge. Where the wall has a drained cavity, the sill flashing has to connect to the cavity drainage path so that water in the cavity reaches the same exit.
The jamb-to-sill junction is the most common leak point in the whole opening. It is a corner where two flashings meet, it is usually where the frame is packed and fixed, and it is often hidden behind a reveal trim by the time anyone looks at it.
Jamb flashings are installed after the frame is set, and they lap over the upturned leg of the sill flashing. They should not be tucked behind it. Where a proprietary corner piece is available, use it, because formed corners handle the geometry better than a folded or cut joint.
If the frame is fixed through the sill flashing, the flashing is compromised. Fixings should be positioned so they do not penetrate the drainage layer, or the penetration should be sealed in the manner the manufacturer requires.
Flashing sequence at a typical opening
- 1Set and level the sillEstablish the sill line and fall before anything else, so the frame sits on a known plane.
- 2Flash the sillInstall the sill flashing with the required fall and lap it over the wall’s weatherproofing layer.
- 3Install and fix the frameSet the window or door into the opening, pack and fix it without puncturing the sill flashing.
- 4Flash the jambsLap jamb flashings over the sill flashing and seal the vertical junctions.
- 5Flash the headInstall the head flashing with a drip edge, lapping over the jamb flashings at both ends.
- 6Close out the claddingInstall cladding and trims so each layer laps over the layer below and drains outward.
Head flashings and the drip edge
The head flashing is what stops water running down the wall face and into the top of the opening. It goes over the top of the frame, under the cladding above, and it must project clear of the wall face with a drip edge so water leaves the building rather than tracking back along the underside of the flashing.
Two details cause most head flashing failures. The first is a flashing that finishes flush with the cladding face, so surface tension pulls water back onto the head. The second is a flashing that has been installed with the lap reversed, so water running off the cladding above passes behind the flashing rather than over it.
Head flashings should lap over the jamb flashings at both ends. That means the head is installed after the jambs, not before. Where the opening is wide and the flashing is in sections, the laps between sections must run in the direction of fall, and the joints should be sealed in the manner the manufacturer specifies.
Where the cladding is discontinuous
Openings in walls with a change of cladding, a control joint, or a balcony above need the flashing detail resolved with the adjacent details, not in isolation. The head flashing may need to connect to a cavity drainage path, a sub-sill drainage system, or a separate weatherproofing layer. These junctions should be drawn and agreed before installation, because retrofitting a connection through a finished wall is expensive and rarely watertight.
Sealant, air seals and what each one actually does
Sealant around a window or door has two jobs, and conflating them causes problems. The outer seal manages the weather at the junctions the flashing system does not cover. The inner seal, or air seal, manages air movement and is part of the building’s thermal and moisture performance. They are not the same bead, and they do not go in the same place.
A common mistake is to run a continuous bead around the entire outer perimeter of the frame. That looks thorough, but if it closes off the drainage path at the sill, water that gets in cannot get out. The sill should be left to drain, with sealant used at the junctions the manufacturer nominates and drainage left open where the system requires it.
Sealant is also not a fix for a flashing lap that is wrong. Sealant has a service life, it moves with temperature, and it is applied on site in conditions that are rarely ideal. A lapped, draining detail will still work when the sealant has failed. A detail that relies on sealant will not.
Checks before the opening is closed up
- Sill flashing has the fall the manufacturer specifies and discharges clear of the wall face.
- Jamb flashings lap over the sill flashing, not behind it.
- Head flashing has a drip edge positioned clear of the cladding face.
- Fixings through the frame do not penetrate the sill flashing or the weatherproofing layer.
- Sealant beads are continuous at the junctions the manufacturer nominates, with drainage paths left open.
- Air seal on the inside face is continuous and independent of the outer weather seal.
- The installed product matches the window and door schedule for the opening.
Interaction with cladding, cavity and weepholes
The opening flashing system has to work with the wall’s weatherproofing layer, not sit outside it. Where the wall has a drained cavity, the sill flashing connects to the cavity and the cavity drains through weepholes or a drained base detail. If the weepholes are blocked, the cavity fills and water finds the path of least resistance, which is often the opening.
Masonry walls introduce their own requirements, and the detailing around weepholes and base flashings is covered in our article on masonry flashing and weephole detailing. The opening sequence is unchanged, but the connections to the cavity and to the base of the wall have to be made deliberately.
Lightweight cladding systems typically rely on a drained and ventilated cavity behind the cladding, with the opening flashing discharging into that cavity or through it to the outside. The cladding manufacturer’s installation instructions set out how the flashing should interface with the cladding, including the trim and the fixing pattern around the opening.
Where the cladding is fixed through the flashing, check that the fixings are the type and length the manufacturer requires and that they do not create a path through the drainage layer. A screw that penetrates the flashing and the weatherproofing layer is a leak waiting for the right wind direction.
Worked example: a builder in western Sydney
A builder in western Sydney is completing a two-storey dwelling with lightweight cladding on the upper level and a large sliding door opening onto a balcony. The window and door schedule specifies the products and the flashing system, and the cladding manufacturer’s instructions set out the interface details.
During installation, the frame is set and fixed before the sill flashing is installed, because the flashing delivery is delayed. The frame is packed and fixed to the structural opening. When the flashing arrives, the installer dresses the sill flashing up to the frame and seals the junction with a continuous bead.
The detail looks acceptable. The cladding is installed, the reveal trims go on, and the job progresses. Six months later, after a sustained easterly weather event, water appears at the junction of the floor and the wall inside the sliding door opening. The flooring is damaged, and the repair requires removal of the cladding around the opening.
The cause is the sequence. The sill flashing could not be lapped under the frame because the frame was already fixed, so it relies on a sealed joint at the most water-loaded part of the opening. The sealant has since failed. Correcting it means removing the frame, reinstalling the sill flashing, re-flashing the jambs and head, and reinstating the cladding and internal finishes.
The cost is not just the flashing. It is the cladding, the internal lining, the flooring, the scaffolding, and the delay to the next stage of the programme. It also surfaces at the worst time, after practical completion, when the builder is carrying the defect and the homeowner is living in the space.
What to check before the opening is closed up
The window or door opening is one of the few parts of the building that becomes inaccessible as the job proceeds. Once the cladding and lining are on, confirming the flashing sequence means removing work. That makes the pre-close-up inspection the most valuable check on the job.
Walk the opening before the cladding goes on and confirm the laps, the fall, the drip edges and the fixings. Photograph the detail. If a later defect claim arises, the photograph is the evidence of what was installed, and it is far more useful than a recollection.
Where the detail is not clear on the drawings, check the window and door schedule, the flashing manufacturer’s installation instructions, and the waterproofing provisions of the NCC. If the answer still is not clear, get it in writing before proceeding. A written direction from the designer is cheap; a failed detail is not.
Builders working towards their continuing professional development requirements will find the weatherproofing and detailing content relevant to this work across the PointCPD course catalogue, and the guide to how NSW builder CPD works explains how the requirements are structured. If you are new to the platform, start at the PointCPD home page.
Frequently asked questions
Does the sill flashing have to be installed before the frame?
Yes, in almost every system. The sill flashing has to extend under the frame and lap up the jambs so the jamb flashings can lap over it. Installing it after the frame means the flashing cannot be dressed correctly under the frame, and the junction ends up relying on sealant.
Can I seal the whole perimeter of the window?
No. The outer seal is used at the junctions the manufacturer nominates, and the sill is left to drain. Sealing the full perimeter closes off the drainage path, so water that gets behind the cladding cannot escape and will find its way into the building.
What is the minimum fall on a sill?
The fall is set by the flashing manufacturer and by the relevant standard, and it varies with the product and the exposure. Check the manufacturer’s installation instructions and the current NCC waterproofing provisions rather than working from memory or from the last job.
How far should a head flashing project past the wall face?
Far enough that water leaving the flashing clears the wall face and drips free, with a drip edge formed or fitted to the flashing. The projection depends on the cladding and the flashing profile, and it should be confirmed against the manufacturer’s details.
What happens if a fixing penetrates the flashing?
It creates a direct path for water through the drainage layer. Fixings should be positioned to avoid the flashing, or sealed in the manner the manufacturer requires. A penetrated flashing is a defect even if it does not leak immediately.
Where do I check the flashing requirement for a specific opening?
Start with the window and door schedule and the flashing manufacturer’s installation instructions. Then check the cladding manufacturer’s interface details and the waterproofing provisions of the NCC. If the detail is still not clear, get a written direction from the designer before you proceed.
Questions builders ask
Does the sill flashing have to be installed before the frame?
Yes, in almost every system. The sill flashing has to extend under the frame and lap up the jambs so the jamb flashings can lap over it. Installing it after the frame means the flashing cannot be dressed correctly under the frame, and the junction ends up relying on sealant.
Can I seal the whole perimeter of the window?
No. The outer seal is used at the junctions the manufacturer nominates, and the sill is left to drain. Sealing the full perimeter closes off the drainage path, so water that gets behind the cladding cannot escape and will find its way into the building.
What is the minimum fall on a sill?
The fall is set by the flashing manufacturer and by the relevant standard, and it varies with the product and the exposure. Check the manufacturer’s installation instructions and the current NCC waterproofing provisions rather than working from memory or from the last job.
How far should a head flashing project past the wall face?
Far enough that water leaving the flashing clears the wall face and drips free, with a drip edge formed or fitted to the flashing. The projection depends on the cladding and the flashing profile, and it should be confirmed against the manufacturer’s details.
What happens if a fixing penetrates the flashing?
It creates a direct path for water through the drainage layer. Fixings should be positioned to avoid the flashing, or sealed in the manner the manufacturer requires. A penetrated flashing is a defect even if it does not leak immediately.
Where do I check the flashing requirement for a specific opening?
Start with the window and door schedule and the flashing manufacturer’s installation instructions. Then check the cladding manufacturer’s interface details and the waterproofing provisions of the NCC. If the detail is still not clear, get a written direction from the designer before you proceed.
Sources
- Windows, doors and skylights · Your Home (Australian Government)
- Window Energy Rating Scheme · WERS
- Retention money held by head contractors · NSW Government
Published 24 August 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
