Design for decay: translucent roof sheets are a latent fall risk
A skylight sheet is the only part of a roof that gets weaker every year it sits there, and the only part that looks exactly like the bit either side of it. Design the layout for what the sheet will be in year fifteen, not what it is on handover day.
Why they are there in the first place
Nobody puts translucent sheets in a warehouse roof for fun. They are there for natural light. On a typical warehouse, industry guidance from the sheet makers (Ampelite, Laserlite and the like) puts translucent area at roughly 10–15% of the roof to hit internal light levels without running artificial lighting all day. That is a real design driver with a real running-cost saving behind it.
So the question is almost never "should we have skylights". It is "where do they go, what are they made of, and what happens to them over time".
The decay problem
Standard polycarbonate and fibreglass sheeting is not designed to be walked on, and it does not stay where it started. UV exposure, thermal cycling, dirt loading and hail all take a slice out of it. Fibreglass loses resin at the surface and exposes fibre. Sheets yellow, craze, and go brittle. The fixings work at the sheet. The impact resistance that might have caught a stumble at year one is not there at year fifteen.
The dangerous part is that none of this is visible from above. A degraded sheet under dust looks like roof. Someone doing a gutter clean, a solar install, an aircon service or a lease inspection walks a line, hits the sheet, and goes through. It is one of the most consistent fatal fall mechanisms on industrial roofs in Australia, and it is almost always a design decision that set it up years earlier.
That is why WHS codes push hard toward safety-rated or trafficable translucent sheeting where sheets sit in the walking plane — and why a mesh or grid protector under or over the sheet is not an accessory, it is the control.
Where the standards sit
- —AS 4256.3 — the manufacturing standard for plastic and fibreglass translucent sheeting in Australia. It governs how the sheet is made and rated, including load classifications.
- —AS 1562.3 — the design and installation standard for plastic roof cladding: fixing, spans, weatherproofing and wind resistance.
- —AS 1657 — fixed platforms, walkways, stairways and ladders. What you fall back on once you accept people will need to be on that roof.
- —WAHA IC-002.01, figure 3.16 — useful industry guidance on laying out roof access around fragile roof elements. It makes clear that when anyone is accessing, inspecting or maintaining roof assets within 2 m of a translucent roof sheet, protection is required. Read it here.
None of those standards will tell you where to put the sheets. That is a design decision, and it is the one that decides whether the roof is walkable for the next thirty years.
The layout we recommend
Keep translucent sheets 3 m clear of every roof edge, run them 6 m apart, and put a skylight protector at the end of every run. That gives you a continuous 6 m walking corridor between the runs and a clean way to move around the ends without ever being within 2 m of an unprotected edge. You get the light you designed for, and you keep the roof usable without a harness for routine work.
If the design needs more light, pair the sheet runs together rather than squeezing in extra singles. Two sheets side by side with the same 3 m / 6 m spacing keep the walkways intact and the fall risk where it can be managed. Adding more singles and closing up the gap is what turns the walking route back into a fragile-roof obstacle course.
Why those numbers
- —3 m from the edge — a fragile sheet hard up against a gutter line stacks two hazards in the same place. Push it in and the edge work stays a single, manageable problem.
- —6 m apart — wide enough for a genuine walking route with a person, a toolbag and a bit of wobble, not a tightrope between two fragile strips.
- —Protector at each end — the ends are where people cross the run. That is the crossing point, so that is where the load gets taken by something structural.
- —2 m edge zone — the layout is set so no one is pushed into it just to get around a skylight.
Where this sits in the hierarchy
Skylight layout is elimination and substitution work, done at the drawing stage, for nothing. Move the runs, change the sheet to a safety-rated or trafficable product, and the fall risk is gone before anyone owns the building. Leave it, and every future owner pays for it with static lines, permits, spotters and a fragile-roof management plan that has to survive four changes of facilities manager.
That is the designer's duty under s22 of the WHS Act working exactly as intended: the cheapest place to fix a fall risk is on the page. Our free height safety design assessment will score fragile roof tasks against the hierarchy of controls and produce a Design Report you can put in the project file.
What to ask for
- —The AS 4256.3 classification of the actual sheet being supplied, in writing.
- —Whether it is rated non-trafficable, or safety-rated / trafficable — and what that rating relies on (the sheet, a mesh, or the purlin spacing).
- —The design life of the product and what the maker says about it at that age. If the answer is vague, treat the sheet as fragile from day one.
- —The skylight protector detail and its fixing, shown on the roof plan — not left as a note for the installer to sort out.