Working out the weight from the glass
You can estimate the weight of any walk-on skylight yourself, because glass has a steady density of about 2,500kg per cubic metre.
That density gives a handy rule: every millimetre of glass thickness adds about 2.5kg to each square metre of pane. So 10mm of glass is about 25kg/m², 20mm is about 50kg/m², and so on. To estimate a unit, add up the glass plies only (the argon in the cavity weighs next to nothing, and the plastic interlayers add very little), multiply by 2.5 to get kg per square metre, then multiply by the area of the pane.
Take a build-up of the kind published across the market: a walkable top laminate of three 10mm toughened plies, a sealed cavity, and an inner laminate of two 6mm plies. That is 30mm plus 12mm, or 42mm of glass in total. At 2.5kg per millimetre per square metre, the glass alone comes to roughly 105kg/m². A lighter build with two 12mm top plies and a thinner inner pane might land nearer 80kg/m²; a heavier one with three 12mm plies over a thicker inner laminate could approach 125kg/m². These are illustrations of the arithmetic, not anyone's product data. The layer-by-layer picture is on how thick is walk on glass.
Worked estimates for common sizes
Multiply the per-square-metre figure by the pane area and the numbers climb quickly, which is why size matters as much as build-up.
The table below uses the middle example of about 105kg/m² of glass. Treat every figure as an order of magnitude to plan around, not a specification. Frames, perimeter trims, fixings and any heating film add a little more, and a real unit sized by a manufacturer may use thicker or thinner plies than this for the same span.
| Pane size | Area | Glass weight, as a guide |
|---|---|---|
| 600 x 600mm | 0.36m² | around 38kg |
| 1000 x 1000mm | 1.0m² | around 105kg |
| 1500 x 1000mm | 1.5m² | around 160kg |
| 2000 x 1000mm | 2.0m² | around 210kg |
Larger panes also tend to need thicker plies, because a wider span bends more under the same load. So weight grows faster than area once you go past a modest size. That is one reason a long run of walk-on glass is often split into several panes on intermediate supports; see what is the largest size for a walk on rooflight. A triple-glazed unit adds another cavity and another laminated pane, so expect it to be heavier again; the trade-off is set out in can a walk on rooflight be triple glazed.
What that weight asks of the roof below
The glass is a permanent load that sits on the edges of the opening every day, on top of the people, furniture and snow the unit is rated to carry.
It helps to compare the two. Domestic walk-on glazing is commonly designed for a uniform imposed load of 1.5kN/m², which is about 153kg per square metre, plus a separate concentrated load for a heel or a chair leg. The glass itself at around 105kg/m² is a large share of that again, and unlike a person it never leaves. Put together, the kerb or upstand is carrying something in the region of a quarter of a tonne per square metre of glass when the terrace is in use. How those ratings work is covered in walk-on rooflight load ratings.
A second comparison is useful on a roof terrace. A 50mm concrete paving slab weighs roughly the same per square metre as the glass example above, because concrete is only a little less dense than glass. So swapping an area of paving for walk-on glass does not change the load on the terrace very much. The difference is where that load goes: paving spreads its weight across the whole deck, while a rooflight gathers it onto a narrow ring around an opening that has had joists cut away. Trimmers, bearings and a level, continuous seat for the glass all have to be designed for it, which is what the structural opening for walk-on glass explains. The page on whether walk on glass needs a structural engineer sets out when calculations are needed.
This is especially relevant when glass goes into a roof that was never built for it. A timber flat roof over a 1970s extension in Cherry Hinton, or the rear addition of a Mill Road terrace, may have been sized for maintenance access and a light covering, not a terrace with glass in it. Whether an existing deck can take the new loads is a question for the engineer, and can a walk on rooflight be fitted to an existing flat roof goes through what is usually checked.
Getting a heavy pane onto a Cambridge roof
A unit of 100kg or more cannot be carried up a ladder, so the route from the van to the opening is planned before the glass is ordered.
Central Cambridge makes this interesting. A Romsey or Petersfield terrace often has no rear access at all, so the glass has to come through the house, down a narrow hall and out to the back, or be lifted over the roof from the street. We measure doorways, turns and stairs at the survey, and if the pane will not pass, the options are a lifting route from the front, a small crane or a glass lifter, or splitting the opening into panes that can be handled. Newer homes at Eddington or Trumpington Meadows usually have easier side access but can have their own limits, such as shared courtyards or managed parking.
On the day, glass of this weight is moved with vacuum lifting cups and enough people to control it, set down onto the prepared kerb without dragging, and bedded on the maker's specified packers and seals. Laminated edges are protected throughout, because a knock to an exposed edge is one of the easiest ways to damage a new pane. Where a crane or lifting equipment is needed, access, parking and any permits are sorted in advance. How this fits into the wider programme is covered in how long does walk on rooflight installation take.
Weight has a small effect on the light, too. All those plies of glass carry a faint green tint from iron in ordinary float glass, and a heavy build-up shows it more; low-iron glass keeps the patch of sunlight in the room below closer to its true colour. We note that in the written light plan alongside where the sun will fall, as described on how we plan the light. The full picture of how we specify and fit these units is on our walk-on rooflight installation page, and you can book a light survey or message us on WhatsApp when you are ready.
Questions people also ask
How much does 10mm glass weigh per square metre?
About 25kg. Glass has a density of about 2,500kg per cubic metre, so each millimetre of thickness adds roughly 2.5kg to every square metre of pane.
Is a walk-on rooflight heavier than a standard flat rooflight?
Yes, by a wide margin. A standard flat rooflight has much thinner glass because it is not designed for foot traffic, so a walk-on unit of the same size can weigh several times as much.
Can two people carry a walk-on rooflight?
Only the smallest panes. Once a unit reaches around 100kg it needs vacuum lifters and more hands, or mechanical lifting, and the exact method is decided from the maker's stated weight and the access route.
Does the frame add much to the weight?
Less than the glass on a typical unit, though it varies by product. Aluminium frames and trims are light next to 40mm or more of glass, but the manufacturer's data sheet gives the true total.
