Key takeaways
- Most skylight leaks come from the order in which the flashing and underlay were installed, not from the skylight unit itself, so the sequence deserves as much care as the product.
- Each layer around the kerb must lie over the one below it on the downslope side and under the roof covering on the upslope side, so that water is always shed outward.
- Use the manufacturer’s flashing system for the specific unit and roof covering, because substituting components changes the way the layers lap and can void the intended protection.
- Finish the ceiling side junction with a drained detail rather than relying on sealant alone, because any water or condensation that reaches the reveal needs a route out.
- Photograph each layer before it is covered and keep the manufacturer’s instructions with the job file, so the installation can be proven if the leak returns.
Skylight leaks are common enough that most builders have been called back to one. The stain appears on the ceiling reveal after a heavy downpour, the owner blames the glass, and the first inspection often concludes the unit is faulty. In most cases the unit is sound. The water found its way in at the kerb, the flashing or the junction with the ceiling, because one of the layers was installed in the wrong order or cut short.
A skylight is a hole in a roof, and water hates holes. The roof is designed to shed water in one direction, over the top of each layer and onto the next. A skylight interrupts that plane and creates a step, a joint and a pocket where water can pool. The flashing system exists to carry the water across that interruption and back out onto the roof, and it only does so when the layers lie in the right relationship to each other. This article explains that relationship and the order that produces it.
Why a skylight leaks where the rest of the roof does not
A roof covering on its own sheds water across its whole area, and the flashing at its edges is a small and well understood detail. A skylight takes a section of that roof away and replaces it with a raised kerb and a glazed unit. Water running down the slope now meets a vertical face on its uphill side, a horizontal lip on its downhill side and two side faces where the kerb meets the covering. Every one of those is a place where water can be driven sideways or upwards, or where it can run back under a layer that was meant to shed it.
That is why skylight leaks cluster in a few places: the head of the kerb, where the upslope flashing has to pass under the covering; the sides, where the joins between flashing and covering are made; the apron, on the downslope side; and the ceiling reveal, where water or condensation that has got past the outer layers comes through into the room. Each of these has a correct layering, and most faults are a layer that lies in the wrong order relative to the one beneath it.
Start with the manufacturer’s flashing system
The manufacturer of the unit usually supplies a flashing kit designed to suit its kerb, and the installation instructions describe the sequence the kit was tested with. Read those instructions before the roof is opened, and confirm the kit suits the roof covering in use. A metal sheet profile and a concrete or clay tile each need a different relationship between the flashing and the covering, and a kit designed for one will not necessarily work on the other.
Substitution is the most common cause of a preventable leak. A builder who has a spare apron from a different job, or who prefers the look of a different flashing, changes the lap lengths and the fixing positions without changing the layering rules. The result looks neat and works until the first heavy rain. If the manufacturer’s system cannot be used for a particular roof, get written advice from the manufacturer or a suitably qualified person before you improvise. Keep that advice in the file. It is far easier to defend a deliberate departure than an accidental one.
Lapping each layer over the one below it
The governing rule is simple to state and easy to get wrong. On the downslope side, each layer lies over the one below it, so that water running down the roof passes over the joint rather than into it. On the upslope side, the flashing passes under the roof covering above it, so that water running down the roof is directed over the top of the kerb rather than behind the flashing. The layers therefore behave like the tiles or sheets of the roof itself, each one shingled over the next.
The upslope head flashing is where most faults begin. It must slide under the covering above the kerb and then turn up or over the kerb face, so that any water arriving from above is guided over the top of the unit. If the head flashing is laid over the covering instead, water arriving from above runs straight under the flashing and into the joint. The fault is invisible once the covering is in place, which is why it is so common and so frustrating to find later.
Installing the flashing in sequence
- 1Set and check the kerbFix the kerb to the manufacturer’s requirements and confirm it is level and square before anything is fixed to it.
- 2Fit the upslope flashingSlide the head flashing under the roof covering so that it sheds water over the top of the kerb.
- 3Lap the side flashingsFit each side so its upper edge sits under the covering above it and its lower edge overlaps the layer below.
- 4Fit the downslope apronLay the apron over the roof covering on the downslope side, so water running down the roof passes over it.
- 5Seal the ceiling revealFinish the internal junction with a drained detail, then test the assembly with water before the roof is closed.
The side flashings and the joins
The side flashings carry water down the sides of the kerb and into the covering below. Each side should overlap the layer beneath it on the downslope side and sit under the covering on the upslope side, so the join behaves like a shingled edge rather than a seam. The lap length matters, and the manufacturer’s instructions will say what is needed for the covering in use. Where a side flashing is cut to fit, the cut edge must be turned or sealed so that water cannot wick into the end grain or the metal edge.
The corners are where the joins are most likely to fail. A corner that is folded but not properly lapped leaves a gap that water will find, and a corner that is sealed with sealant alone will crack as the building moves. Make each corner with the flashing itself, overlapping in the right order, and use sealant only as a secondary measure where the detail calls for it.
The apron and the downslope discharge
The apron sits on the downslope side and is the layer most exposed to the water that runs down the roof. It should lie over the covering below it, so that the water passes over the apron and onto the roof beyond. An apron tucked under the covering on its downslope edge reverses that relationship and collects water behind it. The fault is particularly hard to see from the ground, because the apron may look perfectly tidy from above while the water is entering beneath it.
Check the apron after the covering is fixed and before the roof is closed. Lift the edge of the covering if you can, confirm the apron is on top of the layer below it, and make sure there is a clear path for water to leave the apron without ponding. A short rainy test with a hose, run from the uphill side, is the simplest way to see whether the water goes where it should before anyone is standing in the ceiling below.
The underlay, the sarking and the kerb
The underlay under the covering is part of the flashing system, not a separate trade. It must be continuous across the roof and cut and lapped around the kerb so that any water that gets past the covering is caught and directed outward. Underlay cut short at the kerb leaves a gap that water can run into and under, and a strip of underlay that is not joined to the flashing does not do the job it was installed for. The article on sarking and roof underlay installation covers how the underlay should be laid across the roof before the kerb is set.
At the kerb, the underlay should run down over the outside of the kerb and be joined to the flashing so the two form one continuous layer. The join should be made in the direction that sheds water, and the underlay should not be trapped between the flashing and the kerb in a way that holds water against the timber. Check the join by eye before the next layer goes on, because it is much harder to inspect once the flashing covers it.
Correct and common faults at a skylight kerb
| Correct installation | Common fault |
|---|---|
| Upslope head flashing slides under the roof covering | Upslope head flashing laid over the covering |
| Downslope apron sits over the roof covering | Downslope apron tucked under the covering |
| Underlay lapped down over the kerb and joined to the flashing | Underlay cut short at the kerb |
| Ceiling reveal drained to the outside | Ceiling reveal relying on sealant alone |
| Each layer photographed before it is covered | Layers closed up with no record |
The ceiling side junction and the reveal
The ceiling side of a skylight is where water finally becomes visible, and it is where a poor installation is most often blamed on the product. The reveal is the internal lining around the opening, usually a timber frame lined with plasterboard. Any water that gets past the outer layers, and any moisture that condenses on the cold glass or the frame, can collect here. A reveal sealed only with sealant has no route out for that water, and it will eventually stain the lining, swell the frame or both.
A drained detail gives the water a path. The reveal should be finished so that moisture reaching it can run down the frame and out, rather than sitting against timber or plasterboard. Insulation around the reveal should be installed so it does not trap moisture against the cold surfaces. Sealant has a role at the internal joins, but it should be a finishing measure on top of a drained detail, not the detail itself.
Working on the roof safely
Skylight installation means working on a roof that is open, often for a long period, with the rain forecast as a constant risk. Plan the work so the roof is open for as short a time as possible, and do not open it at all when rain is coming that you cannot cover. Keep the opening protected with temporary covering whenever work stops, and have the materials for the flashing ready before the old unit is removed.
Overhead services and the roof edge are the other hazards. Check for overhead powerlines before you set up access, keep the required clearances and treat any roof edge as a fall risk. Safe Work Australia’s guidance on electrical safety is a useful reference for the overhead line side of the job, and the risk of contact with a live line is one that the builder must plan for before the ladder goes up.
Documenting the installation before the roof closes
The best protection against a future leak claim is a record of the installation made while it can still be seen. Photograph each layer before it is covered: the head flashing under the covering, the side flashings lapped in the right order, the apron on top of the layer below, the underlay joined at the kerb and the drained detail at the reveal. Keep the photographs with the manufacturer’s instructions and any written advice on a departure from them.
The record matters most when the leak returns. A builder who can show the layering at each point, and the manufacturer’s system used throughout, has a defensible answer. A builder who cannot show it will be asked to open the roof, and the cost of doing so is usually larger than the cost of the photographs would have been.
A worked example: a builder in the Illawarra
Consider a builder in the Illawarra who was called to a skylight that leaked at the ceiling reveal after a storm. The owner had been told the unit was faulty. The builder checked the reveal first, confirmed the water was entering at the upper edge of the frame, and then looked at the roof. The head flashing had been laid over the covering rather than under it, and the apron on the downslope side was tucked beneath the sheets. Water arriving from above was running straight under the head flashing and into the kerb.
The builder did not replace the unit. He lifted the covering around the kerb, removed the head flashing and the apron, and reinstalled them in the correct order, with each layer lapping the one below. He joined the underlay to the flashing at the kerb, finished the reveal with a drained detail and tested the assembly with water before closing the roof. He photographed each stage and filed the photographs with the manufacturer’s instructions. The owner had a dry ceiling, and the builder had a record that showed the fault was in the installation, not the product.
The useful point is that the leak was not mysterious. It was a layer in the wrong order, and it was visible to anyone who looked at the roof with the layering rule in mind.
The mistake that costs the most. Laying the upslope head flashing over the roof covering. It sends water behind every layer below it, and the fault stays hidden until the water has already reached the ceiling.
Where this sits in your contract and your records
A skylight leak is a defect claim as well as a building problem, and the contract determines much of what happens next. Check the written contract for what is included in the roof work, who supplies the flashing system, and what the defects period covers. The NSW Government guidance on contracts for home building work is the reference point for what the contract should say, and it is worth reading before you agree to replace a roof opening on a job that was not designed for it.
If you are building your own competence in roof flashing, the PointCPD guide to how NSW builder CPD works sets out how the learning is recorded for a licence holder.
The course catalogue at PointCPD courses is the place to check what is currently available.
Whatever you learn, the habit that matters on the roof is the same one: confirm the order of the layers before the covering hides it.
Common questions before the roof is opened
The questions below come up most often when a skylight is being replaced or newly installed. The answers are the ones a builder can apply directly to the job in hand, without needing to read the detail of every product in the market.
Questions builders ask
Why does my skylight only leak in heavy rain?
Heavy rain puts water across the kerb faster and at greater depth than light rain does, so a layering fault that is only marginal shows up under those conditions. A skylight that leaks only in heavy rain usually has a flashing or underlay layer lying in the wrong order at the kerb or on the downslope side.
Can I use sealant instead of flashing?
Sealant has a role at the joins, but it should not replace the flashing. A sealant-only detail cracks as the building moves and gives water no route out, so the flashing must do the work and the sealant should be a finishing measure on top of it.
Should I follow the manufacturer’s flashing kit or my own details?
Follow the manufacturer’s system for the specific unit and roof covering wherever it is available. If it cannot be used, get written advice on the alternative before you install it, and keep that advice in the job file.
Does the underlay need to run over the kerb?
Yes. The underlay should be continuous across the roof and run down over the outside of the kerb, joined to the flashing so the two form one layer. Underlay cut short at the kerb leaves a gap that water can run into and under.
How do I tell condensation from a leak at the ceiling reveal?
Condensation tends to appear on the cold glass and frame in cool, humid weather, even when it is not raining, and it often clears as the surfaces warm. A leak appears after rain and follows a path from the roof. Check the roof and the reveal at the same time to see which one is responsible.
What should I photograph before the roof is closed?
Photograph the head flashing under the covering, the side flashings lapped in the right order, the apron on top of the layer below, the underlay joined at the kerb and the drained detail at the reveal. Keep those photographs with the manufacturer’s instructions so the installation can be shown later.
Sources
- Continuing professional development for builders and swimming pool builders · NSW Government
- General building work licensing · NSW Government
- Home Building Act 1989 (NSW) · NSW Legislation
Published 14 October 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.
