The top layer breaks and the layers beneath it keep carrying. That is the whole design principle: the uppermost pane is sacrificial, it is not counted in the structural calculation, and a break in it leaves the panel safe to stand on while a replacement is arranged. What you get is a visible cracked surface rather than a hole in a floor.
What a break looks like
A crazed surface, held together.
Toughened glass fractures into small blunt pieces, and the interlayer holds them in place, so the pane looks shattered while remaining a continuous surface.
Why it does not fall
Because of the interlayer.
The tough plastic layer bonding the panes keeps broken glass adhered to it rather than dropping, which is what makes laminated glass safe overhead.
Why the panel still carries
Because the structural layers are beneath.
The design calculation is done on the layers below the sacrificial one, so a break in the top pane removes something that was never counted.
The principle is on is walk on glass safe.
Post breakage capacity
A designed requirement rather than a hope.
A panel forming part of a floor is expected to remain safe to stand on after a layer has broken, and that is part of what is specified and checked.
What causes a break
Impact, almost always.
A dropped tool, a heavy pot, a dropped scaffold component or something falling from above is what breaks a walk on panel, rather than the weight of people.
Why not weight
Because the margins are large.
A panel designed for imposed floor loading with a substantial factor of safety is nowhere near its limit under ordinary use.
The figures are on how much weight can a walk on rooflight take.
Edge damage
The other cause.
Glass is weakest at its edges, and a knock during installation or a hard point in the bearing can start a fracture that appears later.
Nickel sulphide inclusions
The rare spontaneous cause.
Toughened glass can very occasionally break without an impact due to a microscopic inclusion, which is why heat soaked glass is specified for critical applications.
Heat soaking
What reduces that risk.
A heat soak process puts toughened glass through a controlled cycle that causes suspect panes to fail in the factory rather than in a building.
It is worth asking whether it has been done.
What to do first
Keep off it and cover it. Even though the panel is designed to remain safe, the sensible response is to stop using that part of the terrace and cover the panel while a replacement is arranged.
Whether it is urgent
Prompt rather than an emergency. The panel is still doing its job, and it is not weathertight in the way it was, so it is a job for the next few weeks rather than the next few hours.
Whether water gets in
Generally not immediately.
A broken outer pane still sheds water because the interlayer is intact, though over time water can work into the laminate edges.
What the repair is
A new panel. Individual layers cannot be replaced, because the assembly is laminated in a factory, so a broken panel is replaced as a unit.
How long that takes
A manufacturing lead time. The replacement is made to the same specification, which is weeks rather than days, and the panel is covered in the meantime.
The programme is on how long does a walk on rooflight take to fit.
How it is replaced
Frame and structure stay. The broken panel is lifted out and a new one set on fresh blocks, which does not involve the structure or the waterproofing unless something else is wrong.
Whether the paving comes up
Around the perimeter only. Enough of the surface is lifted to reach the frame and get the panel out and in, rather than the whole terrace.
Whether insurance covers it
Frequently, and it is worth asking. Accidental damage to glass is a common policy inclusion, and a dropped object breaking a panel is exactly that kind of claim.
Keeping the specification
What makes a replacement straightforward. The original build up, the loading and the frit specification are what a manufacturer needs, and having them saves a survey and a delay.
Preventing one
Mind what happens above it. Most breaks come from something being dropped, so care during any work on the building above or beside the terrace is the practical prevention.
Scratches rather than breaks
Far more common, and not the same thing.
Grit underfoot marks the sacrificial layer over years, and those marks are cosmetic. A break is a distinct event with a distinct appearance.
Neither affects the structural layers beneath.
A crack rather than a shatter
Worth telling apart. Toughened glass shatters into small pieces rather than cracking in a line, so a single visible crack usually means an annealed or a laminated inner layer rather than the toughened top pane.
Either way it is looked at rather than watched.
What a break tells you about the design
Usually nothing.
A panel that has been hit hard enough to break its top layer has done exactly what it was designed to do, and it is not evidence that the specification was wrong.
What we keep
We keep the original specification on file and we cover a broken panel the same week, because the design means nobody is in danger and the appearance means everybody assumes they are.
The whole picture is on the walk on rooflight installation page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.