If you've spoken to friends or neighbours about installing a skylight, you've probably heard someone say:
"I'd love one, but don't they all end up leaking?"
It's a common concern, and it's understandable. Many Australians have seen older skylights that leaked, stained the ceiling or eventually had to be replaced.
The good news is that a properly designed, correctly installed modern skylight should not leak.
So why do some skylights develop problems?
Having manufactured and supplied skylights for decades, we've found there are several common causes. Surprisingly, in many cases the skylight itself isn't actually the problem.
1. The leak isn't actually the skylight
This is probably the most common situation.
Water can enter the roof through many different points, including:
- deteriorated roof penetrations
- rangehood flues
- plumbing vents
- antenna or satellite mounts
- damaged ridge capping
- cracked roof tiles
Once inside the roof, water often travels along the underside of roof sheets or follows the roof sarking before eventually reaching the skylight opening.
The skylight shaft simply becomes the first place where the water can drop into the room below, making it appear that the skylight is leaking.
Before replacing a skylight, it's always worth checking whether the source of the leak is actually higher up the roof.
2. Ageing glazing materials
Many skylights installed 20–40 years ago used acrylic domes or twin-wall polycarbonate sheets because, at the time, plastic glazing was considerably more economical than glass.
After decades of exposure to Australia's harsh UV conditions, older plastic glazing can begin to deteriorate.
Depending on the material and age, it may:
- lose clarity
- become brittle
- develop surface pitting
- crack
- eventually require replacement
Modern acrylic and polycarbonate products contain significantly improved UV stabilisers, giving them a much longer service life.
For many standard roof windows, insulated safety glass has also become a cost-effective option and is now commonly used because of its excellent durability, clarity and thermal performance.
3. The flashing has been damaged
The flashing is an integral part of the skylight.
It is not simply decorative trim—it forms part of the roof's waterproofing system.
Flashings are carefully designed, assembled and sealed so they direct water safely around the skylight opening.
They are not intended to be walked on.
Walking on the flashing, bending it during roof maintenance or disturbing factory-sealed joints can damage the waterproofing system and eventually create a leak.
4. Blocked drainage around the flashing
A skylight flashing works much like a shallow roof gutter.
Rainwater needs to flow freely around all four sides before returning to the roof surface.
For this reason, adequate clearance should always be maintained between the flashing and surrounding roof tiles.
If tiles are cut too tightly against the flashing, leaves, twigs, bird nests or other debris can become trapped.
Over time, this blockage may cause water to pond against the flashing. During severe weather, water can overflow the flashing upstand or be forced sideways over the edge.
Like house gutters, skylight flashings benefit from occasional inspection and cleaning.
5. Incorrect installation
Even the highest-quality skylight can leak if it is installed incorrectly.
A skylight should become part of the roof—not simply be sealed to it.
Correct installation ensures the flashing integrates with the roofing material so water naturally drains around the opening and back onto the roof.
Quality sealants play an important role, but they should never be relied upon as the primary waterproofing system.
6. High roof pitches and heavy water flow
Not every roof carries the same amount of water.
On steep roofs, rainwater accelerates rapidly as it travels downhill. The larger the roof area above the skylight, the greater the volume and speed of water reaching the uphill flashing.
Where roof pitch or catchment area is significant, additional detailing such as back flashings or water diverters may be recommended. These spread the water flow and direct it safely around the skylight rather than allowing a concentrated stream to impact one point.
Some corrugated roofs benefit from this extra protection, while on deck roofs it is generally considered best practice.
7. Low-pitch roofs require sufficient upstand height
Low-pitch roofs present a different challenge.
Instead of fast-moving water, the concern is temporary ponding during heavy rainfall.
To prevent standing water entering the skylight, the flashing upstand must extend sufficiently above the finished roof level.
Industry guidance commonly refers to a minimum upstand of around 70 mm above the finished roof surface. Splendid Skylights are manufactured with a 90 mm upstand, providing an additional margin of protection.
On metal deck roofs, the skylight also sits on top of the roof ribs, further increasing the effective height above the roof's drainage plane.
8. Metal roofs require specialised detailing
Corrugated steel roofing introduces another consideration.
Sometimes the skylight opening crosses an overlap between two roof sheets. Under certain conditions, water can travel upwards along this overlap by capillary action.
Correct installation ensures these overlaps are treated appropriately so water cannot migrate beneath the flashing while still allowing any water within the sheet overlap to drain freely.
Experienced installers understand where sealing is required—and just as importantly, where it should not be applied.
Similarly, deck roofs require specialised flashing details and should generally be installed by experienced roof plumbers.
The bottom line
Modern skylights should not leak.
When leaks do occur, they are often caused by factors outside the skylight itself, such as roof defects, ageing materials, damaged flashings, blocked drainage or incorrect installation.
A quality skylight should be:
- designed for the specific roof type
- fitted with the correct flashing system
- installed in accordance with the manufacturer's instructions
- inspected occasionally as part of normal roof maintenance
When these principles are followed, a modern skylight should provide decades of reliable weatherproof performance.
Note: This article refers to modern, quality skylights installed using purpose-designed flashing systems. Like any roof penetration, performance depends on correct product selection, installation and ongoing roof maintenance.
