How much weight can a walk-on rooflight take?

Glass that people walk on is checked against more than a single weight. Two kinds of load matter, and a specification should state both.

Distributed load

This is weight spread across the whole surface, such as several people standing on the panel, furniture, or snow on an external unit. It is expressed as a force per square metre.

Point load

This is weight concentrated in a small area, such as a heel, a table leg or a dropped tool. It is expressed as a force over a small contact patch, and it is often the more demanding of the two for glass, because the stress is concentrated.

The design load is the larger of what the space is likely to see, with a safety margin on top. A panel that passes the distributed check can still fail the point check if the build is wrong, which is why both are asked for.

The weight a panel can carry is the result of several things working together, and changing any one of them changes the answer.

• The make-up of the glass. Walk-on units are multi-layer laminates, with a load-bearing stack of toughened or heat-strengthened panes bonded with interlayers. The number and thickness of the layers is chosen by calculation.
• The size of the panel. A larger pane bends more under the same load, so it needs a thicker build.
• The support. Glass resting on all four edges behaves differently from glass supported on two. The frame, kerb or steelwork under it must carry the load too.
• The setting. Packers and bedding material spread the load. Hard point contact between glass and metal is avoided.

Our guide to walk-on rooflight glass specification sets out the layers in more detail, and walk-on rooflight sizes and supports explains how span and support change what is possible.

This is also why a weight quoted for glass alone, with nothing about the frame, is of little use. The panel, the packers, the frame and the structure beneath form one system, and the system carries the load. The weakest part sets the limit, and a strong pane on a weak bearing is no stronger than the bearing.

The use of the space sets the design load. A rooflight in a first-floor landing seen only by the household has a lower allowance than one in a room where guests gather. A rooflight on an external terrace or balcony needs more again, because of crowding and because snow and standing water add weight. If the use might change, such as an office becoming a party room, say so at the survey, because the design should cover the likely future and not only today.

On an external roof terrace, the glass also has to deal with weather and with surface grip, so the finish and the load go together. See walk-on rooflights for roof terraces and balconies for the extra points that apply outside.

Glass that is walked on is designed against structural guidance for trafficable glazing, and the supplier should be able to name what they have used. Expect the floor loads in BS EN 1991-1-1 (Eurocode 1) and its UK National Annex, and often the Institution of Structural Engineers' guidance on structural glass. A non-fragility rating to the CWCT technical notes covers maintenance access only, so it is not a walk-on rating. We do not give a figure here, because the right one depends on your opening and use. The point is that the figure is calculated, written down and checked against a recognised method.

A calculation covers the glass, but a person is not a static weight. Stepping, turning and the occasional jump all load the panel dynamically, and a design load allows for that with margin. Even so, a walk-on rooflight is a floor, and it should be treated like one: no climbing onto it with ladders, no standing on the frame edge, and no using it as a work platform without checking the specification first.

When you compare quotes for a walk-on rooflight, these are the questions that tell you whether the weight limit has been worked out.

1. What is the design load, and is it given as both distributed and point load?
2. What is the make-up of the glass, layer by layer?
3. What span and support does the calculation assume, and does it match the opening?
4. What standard or guidance was used?
5. Does the frame or kerb carry the same load?

If a quote only says "walk-on" with no further detail, ask. A written answer is easy to give when the work has been done.

Walk-on glass is strong, but it is not indestructible, and the way it is used affects how long it lasts. Grit under shoes scratches the surface, and a deep scratch can weaken the top layer, so a mat at the nearest door helps. Heavy items dragged across it, or a concentrated load such as a stack of paving slabs, are beyond a normal design. Our note on putting furniture on a walk-on rooflight goes into everyday objects.

If a panel chips at the edge or the top layer is damaged, stop using it and have it looked at. The laminated build is designed to stay in place after damage, but the design load no longer applies.

A new walk-on rooflight in an existing floor or roof is building work, and the structure that carries it is part of the notification. Where the notification is required, we submit it. Our page on whether a walk-on rooflight needs Building Control covers the detail. Inside, the ceiling below is part of the job, with the lining or soffit formed and finished so the opening looks right from underneath.

To have the load worked out for your own opening, ask for a quote and we will specify the glass against the use you describe.