If your skylight looks like this after a few weeks… it wasn't pitched correctly.
The photo above was taken on a commercial building in Sydney. The skylights are relatively recent. The staining, those brown oval rings in the centre of each panel, tells the whole story.
That pattern isn't random. It's the waterline left behind by pooled water that has sat, evaporated and sat again, leaving dirt and sediment in a ring each time. Every oval marks a puddle that never drained.
In most cases like this, the cause comes down to one thing: the skylight was installed without enough fall.
Before and After: Getting the Fall Right
Here's another job with the same problem. The skylights on the left were installed flat, and water sat on the glass until it left these rings behind. On the right, the same openings after the skylights were replaced and installed on a pitched hob, so water runs off.
BEFORE
AFTER
BEFORE
AFTER
BEFORE
AFTER
There's No Such Thing as a Truly Flat Roof
In construction, "flat roof" is a misnomer. Flat roofs that work are never perfectly level. They're designed with falls that move water toward drains and away from penetrations. A roof that holds water isn't performing correctly. It's a maintenance liability and a leak waiting to develop.
The same logic applies directly to skylights. A skylight installed without enough pitch becomes a low point in the roof plane, collecting water instead of shedding it. Even in moderate rain, that pooling leads to:
- Dirt and sediment build-up: carried in by water and left behind as it evaporates
- Algae and biological growth: stagnant water is an ideal place for it to grow
- Faster frame and seal wear: long contact with water shortens the life of seals, gaskets and frame finishes
- Higher leak risk: water sitting on joints and flashings finds pathways that moving water wouldn't
Flat vs 3° vs 5°: What Actually Happens
What the Standards Say: 3° Minimum, 5° Recommended
AS 1288:2021 Glass in Buildings sets a minimum fall of 3° for skylight glazing, and AS 4285:2019 Rooflights recommends 5°. Below 3°, the skylight is below the minimum AS 1288:2021 allows. Rainwater can't be relied on to clear the glazing, and normal glass deflection is enough to create a hollow where water collects.
Three degrees is the floor, not the target.
The 5° Rule
AS 4285:2019 recommends a pitch of 5°, and Skyspan recommends the same for all skylight installations. That's above the 3° minimum in AS 1288:2021, and the margin matters. At 5°, rain drains freely even with building tolerances and glass deflection, water doesn't pool on the glazing, and rain's natural cleaning effect keeps the unit performing as intended.
Five degrees is not dramatic. On a nominally flat roof, it's achieved through the hob design: the hob is built up at the high end to create the fall without affecting the roof membrane or the ceiling below. It's a detailing decision, not a structural one, and it costs very little to get right at installation.
How much taller should the hob be at the high end?
Most installers don't measure in degrees. This table shows how much higher the hob needs to be at the high end than at the low end, based on the skylight's length in the direction of the fall.
| Skylight length (in fall direction) |
Hob height difference at 3° (AS 1288:2021 minimum) |
Hob height difference at 5° (AS 4285:2019 recommended) |
|---|---|---|
| 1000 mm | 52 mm | 87 mm |
| 1500 mm | 79 mm | 131 mm |
| 2000 mm | 105 mm | 175 mm |
| 3000 mm | 157 mm | 262 mm |
| 4000 mm | 210 mm | 350 mm |
| 5000 mm | 262 mm | 437 mm |
Height difference = skylight length × tan(angle), rounded to the nearest millimetre.
Getting it wrong costs far more to fix. In some cases, the staining and seal damage in the photo above can't be fully reversed without replacing the unit.
What This Means for Specification and Installation
For architects and specifiers: Call out pitch explicitly in skylight specifications and show it in the hob details on the drawings. Specify 5°, and never less than the 3° minimum in AS 1288:2021. Don't assume a builder will add fall where none is shown. If the design calls for a low-profile hob, check that the required fall can still be achieved, and if it can't, design the hob to suit.
For builders and installers: If you're installing a skylight on a low-pitch or nominally flat roof and the drawings don't show a fall, raise it. A conversation at frame stage takes minutes. Fixing pooling and seal failure after handover takes much longer, and the cost of fixing it rarely falls on the right party.
A Simple Check Before You Install
The check takes thirty seconds and prevents a problem that can show up within weeks, and that takes longer still to explain to a client.
Frequently Asked Questions
Can I install a skylight flat?
A flat install is below the 3° minimum set by AS 1288:2021. Expect water to pool on the glazing, leading to staining, sediment build-up and faster seal wear. We recommend designing the hob to give 5° of fall, in line with AS 4285:2019.
My roof is already pitched. Do I still need a taller hob at one end?
If the roof itself falls at 5° or more in the direction the skylight will drain, the skylight can follow the roof plane. If the roof pitch is lower than that, build up the hob to make up the difference.
Why isn't 3° enough on its own?
All glass deflects slightly under its own weight, and larger panels deflect more. Add normal building tolerances, and a 3° install can end up with a shallow hollow where water sits. 5° gives enough margin for water to keep moving.
How do I measure the fall without a digital level?
Measure the skylight's length in the fall direction and the height difference between the high and low ends of the hob, then check it against the table above. For example, on a 2000 mm skylight the hob needs to be at least 105 mm higher at the high end for 3°, or 175 mm higher for 5°.
Want to talk through your next project?
Share:
Centre of Glass vs System Value: Why Architects Should Specify Beyond the Glass