The answer
It depends on the unit, because there is no standard walk-on rooflight. Each one is designed and tested for a stated load, set by the glass build-up, the size of the panel and what it rests on. A panel made for occasional foot traffic on a flat roof is not the same product as one made for a busy terrace or for something heavier. The condition that changes the answer is the intended use. Ask for the load rating in writing, matched to what will stand or walk on the glass, and do not rely on the word walk-on alone.
Why there is no one number
People expect a single weight limit, like the plate on a lift. Walk-on glass does not work that way, because the load it can carry is the result of a design, and the design changes with the job.
Four things set it. The first is the glass build-up: how many laminated sheets there are, how thick each is, and whether they are toughened or heat-strengthened. The second is the span, the distance the glass crosses between supports. A longer span carries less, all else equal, because the glass flexes more. The third is how the edges are supported. Glass held on all four sides behaves differently from glass held on two. The fourth is the frame and the structure underneath, which must pass the load on to something solid.
Change any one and the rating changes. That is why a supplier will give you a figure for a particular unit at a particular size, not for walk-on glass in general.
The layers behind a rating are laid out in walk-on rooflight glass explained.
Foot traffic, furniture and anything heavier
Loads on a floor are described by use, and the useful question is what will really happen on top of the glass.
Occasional foot traffic is the lightest case: someone stepping across to reach a vent, clean the glass or inspect the roof. A garden terrace panel takes regular walking and perhaps chairs and a table. A heavier setting adds a barbecue, a hot tub or a crowd.
There is a difference between a load spread over a wide area and one concentrated on a small one. A person standing still presses through the soles of their shoes, and a chair leg or a heel presses through a much smaller contact area. Point loads like those are tougher on glass than the same weight spread out, and a good specification takes them into account as well as the overall figure.
Impact is separate again. A dropped tool is a sudden load, not a steady one, which is one reason walk-on glass is laminated.
So when you describe the job to a supplier or to us, say what you really intend: how many people, what furniture, whether anything is ever wheeled across, and whether children or a dog will be running over it. A rating chosen for the real use is better than the biggest figure on a data sheet.
The part below the glass
The glass gets the attention, but the structure under it is just as much a part of the rating. Glass can carry only what its frame and support can pass on.
A walk-on unit sits on an upstand or a kerb. That kerb rests on joists, a slab or masonry, and the opening in the floor or roof has been cut and trimmed around. If the surrounding structure is weak or has moved, the glass will not make up for it. A rating on a data sheet assumes a proper support, and that assumption has to be true on site.
This is why we look at the structure at the survey. On a timber roof or terrace, we check the joists and how the opening will be trimmed. On a concrete slab, we consider the thickness and condition. Over a basement, we look at what the patio rests on and whether the existing structure is what the drawings suggest.
A larger opening or an unclear structure can call for an engineer, and we say so early.
Water plays its part. A kerb that crumbles after years of leaks will not carry what it carried on day one. Our walk-on rooflights page covers how the whole assembly goes together.
What the regulations say about loads
Building Regulations do not give a table of weights for rooflights, but they do set the standard the work must meet. Part K covers protection from falling, and glazing that people walk on is expected to be safe for the loads it will carry. Part A covers the structure, which matters when an opening is cut in a floor or a roof.
In practice that means the rating is not a marketing figure. It should come from calculation and testing for the unit, and the person who specifies the unit should be able to show it. The installer's job is to fit the unit as designed, onto a structure that suits.
We do not quote load figures from memory, because each unit has its own, and the right number is the one on the supplier's documents for the panel being fitted. What we can do is make sure the figure on those documents matches the use, put it on the written quote, and check the structure to match.
Notification is a separate matter, covered in do walk-on rooflights need building regulations?
What to ask for before you order
A short list of questions will tell you whether a rating has been properly thought through.
- What load is the unit designed for? Ask for the figure in the supplier's own units, with the size it applies to.
- What use does that cover? Foot traffic only, a terrace with furniture, or something heavier.
- Does it allow for point loads? A heel or a chair leg is not the same as a standing crowd.
- What does it rest on? The kerb, the frame and the structure should be named.
- What happens if a sheet breaks? The answer should describe laminated layers that stay in place.
- Is it in writing? The rating belongs on the quote, not just in a conversation.
If a quote says only toughened safety glass or heavy-duty, ask again. Those words describe a material, and they say nothing about a load.
Tell us if the use changes later. A planter moved onto a panel specified only for walking is the sort of change that causes trouble.
We use the survey to bring the use, the size and the structure together. You then receive a fixed written quote that records the rating, and our 10-year workmanship guarantee is in writing too. For glass over a patio or lower-ground room, the terraces and basements guide adds the water and structure side.