Two numbers on every walk-on rating
A walk-on rooflight load rating is two figures, not one: a uniformly distributed load in kilonewtons per square metre, and a concentrated load in kilonewtons. Glass that meets one and not the other is not rated for a domestic walking surface.
The uniform figure describes weight spread evenly over the whole pane, the way a group of people standing on a terrace or a light layer of furniture loads it. It is written as kN/m², and one kilonewton is roughly the force that 102 kilograms exerts under gravity. A uniform load of 1.5 kN/m² therefore works out at a little over 150 kilograms on every square metre of glass, all at once, across the whole panel.
The concentrated figure describes weight pressed onto one small spot. It is written as plain kN, with no area attached, because the load is assumed to act on a very small area of the surface. A concentrated load of 2.0 kN is about 204 kilograms bearing down on something the size of a heel or a table leg. For a sheet of glass this is the harder test of the two, because glass resists a load spread along its supported edges far better than a load pushed into the middle of the pane.
Both figures are characteristic loads: the loads the design has to be able to take in normal use, before any safety margin is added. When a supplier quotes a walk on skylight load rating, you want to see both numbers, the standard they were taken from, and the pane size and support arrangement they apply to. A rating printed without those details tells you very little.
Where the domestic figures come from
For homes in England, the imposed loads a floor or accessible roof should be designed to come from BS EN 1991-1-1, the Eurocode for actions on structures, read with its UK National Annex. The National Annex sets the values used in the UK, and it groups spaces into categories by use.
Category A covers residential areas. Within a self-contained dwelling, the National Annex figures are 1.5 kN/m² uniform and 2.0 kN concentrated. Those are the numbers domestic walk-on rooflights are generally designed to, and they are the same figures a timber floor in a house is designed to. A walk-on pane rated this way is being asked to behave like a piece of floor.
Roofs have their own categories. A roof that is not accessible except for normal maintenance falls into Category H, which carries a much lighter imposed load. A roof that people use, such as a roof terrace, falls into Category I, and Category I takes its loads from the category of use it serves. For a private terrace belonging to a house, that points back to the residential values, although balconies are listed separately in the National Annex with a higher uniform figure. Which line applies to your roof is a decision for the structural engineer, not for the glass supplier.
| Use (UK National Annex) | Uniform | Concentrated |
|---|---|---|
| Within a dwelling (Category A) | 1.5 kN/m² | 2.0 kN |
| Balcony of a single dwelling | 2.5 kN/m² | 2.0 kN |
| Older BS 6399-1, dwelling | 1.5 kN/m² | 1.4 kN |
Treat the table as orientation rather than as a design sheet. The engineer works from the current published standard and the specific use of your roof, and the glass maker designs the pane to whatever load that engineer specifies.
Why the point load decides the glass
For walk-on glass the concentrated load usually governs the thickness. A pane that comfortably carries 1.5 kN/m² spread over its whole area can still be pushed hard by 2.0 kN pressed into its centre, so the build-up is sized with the point load in mind.
Think about how the glass is supported. A walk-on pane sits on a continuous bearing around its edges, and when you stand on it the glass bends like a very stiff plate between those supports. A load in the middle of the pane produces the largest bending and the largest stress on the underside of the glass. A load near the edge is carried almost straight down into the bearing. So the worst case for a walk-on rooflight is a single heavy point sitting at the centre of the longest free span.
In a real home that point load is not an abstraction. It is a heel as someone steps across, a ladder foot during window cleaning, the leg of a loaded garden chair, a barbecue on three small feet, or the feet of a planter full of wet compost. The rating exists so that the pane has enough strength and stiffness for those ordinary moments, with margin to spare.
Stiffness matters as well as strength. Glass that is strong enough can still flex enough to feel unsettling underfoot, and the maker designs to a deflection limit as well as a stress limit. That limit is set in the product design, and it varies between makers, so it is one of the things to ask about rather than assume. The thickness that results is explained in how thick walk-on glass is.
The trouble with older ratings
Before the Eurocodes, UK imposed loads came from BS 6399-1, which set the concentrated load for dwellings at 1.4 kN. That standard has been withdrawn, and a unit rated only to its point load is rated below the 2.0 kN figure in the current National Annex.
The uniform figure did not change for dwellings: both standards use 1.5 kN/m². That is exactly why older ratings can mislead. A datasheet that quotes 1.5 kN/m² looks current at a glance, and the lower point load underneath it is easy to miss. Because the point load is the one that usually governs the glass, a pane designed to 1.4 kN may be thinner or less stiff than one designed to 2.0 kN for the same size.
Older ratings turn up in a few ways. A supplier may still be circulating a datasheet written before the change. A rooflight you are replacing may carry a label or original paperwork that quotes the old figure. A product may have been sold for years on an old test and never re-rated. In each case the question is the same: what standard is the rating taken from, and what concentrated load does it state?
An older walk-on rooflight that has been in place for years has not failed simply because its rating is out of date. When it is time to replace it, though, the new unit should be specified to the current figures, and the engineer may want to check that the bearers suit the new glass. Our walk-on rooflight installation page covers how we handle a replacement from survey to finished terrace.
Which category your walk-on glass falls into
The same pane of glass can sit in very different places around a house, and where it sits decides the load it has to carry. A walk-on rooflight is designed to the use of the surface it forms part of, so it helps to be clear about that use early.
A flush pane set into a garden path or patio over a basement or a lower room forms part of the outdoor ground of a private house. Engineers will generally treat it as part of the dwelling, but a pane next to a drive or a shared path raises other questions. Glass that sits where a car could run onto it is a separate matter entirely, covered in whether you can drive over a glass rooflight.
A pane in a roof terrace above a kitchen extension, which is a common way to light the back of a Cambridge terrace with a first-floor deck above, forms part of an accessible roof. Here the engineer decides whether it should be treated like a floor within the house or like a balcony, and the uniform load can rise as a result. Walk-on rooflights for roof terraces looks at the terrace side of that decision.
Glass over a basement lightwell that opens onto a shared area, or a pane in a courtyard used by more than one household, may fall under a heavier category than a private garden. The loads for shared and public areas in the National Annex are higher than the domestic ones, and a product rated only for domestic use would not be right there. If the area is shared in any way, say so at the survey. The engineer's wider role in sizing the opening and its supports is set out in whether walk-on glass needs a structural engineer.
Safety margins, broken plies and what the glass must still do
A load rating is the starting point for the glass design, not the whole of it. The designer adds safety factors, checks deflection, and checks what the pane can still carry if one of its layers breaks.
The imposed loads in the National Annex are characteristic values. In structural design they are multiplied by a partial safety factor before the glass is checked, and for glass the material's own design strength is reduced further to allow for the way glass behaves under long-term load. The exact method depends on the design guidance the maker follows, and it is the maker's engineer who carries that responsibility. As a homeowner you do not need to run the numbers, but it is worth knowing that a 2.0 kN rating does not mean the glass breaks at 2.0 kN and a little more.
Walk-on glass is laminated, built from several toughened or heat-strengthened plies bonded with interlayers. A sound design checks that if the top ply is broken, by a dropped tool for example, the remaining plies still carry a meaningful load and the broken glass stays bonded in place until the unit is replaced. What load the unit carries in that damaged state, and for how long, is set by the maker's design and testing, so ask for it. There is more on this in whether walk-on glass is safe if it cracks.
A slip-resistant finish on the top surface does not change the load rating on its own, but some finishes are applied to a sacrificial top ply that is not counted in the structural build-up. That is one reason two units that look alike can have different thicknesses for the same rating.
Loads the rating does not cover
A domestic walking rating covers people and ordinary household use. It does not cover everything a terrace or garden might end up carrying, and a few loads need separate thought.
- Snow. Snow loads come from a different part of the Eurocode, BS EN 1991-1-3, and depend on location and altitude. The Eurocode does not require the imposed roof load and the snow load to be applied together, but a flush pane in a low, flat Cambridge terrace still has to carry whichever governs.
- Large planters and water features. A big container of wet soil or a water bowl can exceed both the uniform and the point figure over its footprint. Keep them off the glass unless the engineer has designed for them.
- Hot tubs and heavy fixed items. These are well beyond a domestic walking rating and belong on the structural roof, not on a rooflight.
- Vehicles. No domestic walk-on unit is a drive-over unit.
- Impact. A rating is about static load. A heavy object dropped from height is a different event, which is why the broken-ply check matters.
Everyday furniture is usually within what a domestic rating allows, provided the legs are not concentrated onto the centre of a large pane. Putting furniture on a walk-on rooflight goes into chair legs, tables and planters in more detail.
Reading a maker's declared rating
Every walk-on product carries its own declared rating, and that declaration is what the installation relies on. The figures in this guide explain the standard, but the maker's paperwork for the specific unit is the document that counts.
When we specify a walk-on rooflight, these are the points we check against the maker's declaration, and they are worth asking about if you are comparing quotes:
- Both loads are stated, uniform in kN/m² and concentrated in kN, and the concentrated load is 2.0 kN or more for domestic use.
- The rating names BS EN 1991-1-1 and the UK National Annex, not the withdrawn BS 6399.
- The rating applies to the size you are buying. A rating proven on a small pane does not carry over to a larger one of the same build-up.
- The support condition is stated, usually continuous bearing on all four edges, and the opening can provide it.
- The post-breakage behaviour is described, so you know what the unit does with one ply broken.
- Any higher category needed for a terrace, balcony or shared area is matched by the rating.
The size point is the one people miss. Glass thickness rises as the shorter span of the pane grows, so a maker's rating is tied to a maximum size for each build-up. Where a single pane would be too large, the answer is usually two or more panes on an intermediate support, as explained in the largest size for a walk-on rooflight. The support condition leads straight to the opening itself, which is the subject of the structural opening for walk-on glass.
How the rating shapes the light below
The load rating decides the glass build-up, and the glass build-up changes the light that reaches the room underneath. It is worth knowing that before choosing a pane size or finish.
A walk-on unit designed to 2.0 kN is thicker than a standard flat rooflight, and every extra ply and interlayer takes a little from the light passing through. Thick glass made from ordinary float glass also carries a green edge tint that becomes visible in deep build-ups. Low-iron glass reduces that tint and lets slightly more daylight through, which matters most under a north-facing terrace or a deep basement where every bit of sky light counts.
Splitting a large opening into smaller panes to keep within a rating also changes the light: the intermediate support casts a line of shadow across the floor below, and on a sunny day that line moves with the sun. We would rather plan where that line falls than discover it after fitting. A south-facing flush pane throws a sharp patch of sunlight into the room below at midday in summer, and a slip-resistant or obscure finish on the walking surface softens it. How we plan the light explains the method, and every written quote includes a light plan showing the pane position, size and glass we propose. When you are ready, book a light survey.
Questions people ask
Is 1.5 kN/m² enough for a roof terrace?
It is the residential floor figure in the UK National Annex, and a private terrace may be designed to it, but balconies are listed with a higher uniform figure and some engineers apply that to terraces. The engineer for your project makes the call, and the rooflight is then specified to match.
Can I tell a walk-on rooflight's rating by looking at it?
No. Thickness gives a rough hint, but two panes of similar thickness can be designed to different loads and sizes. The only reliable source is the maker's declared rating for that unit, backed by the standard it names.
My existing walk-on glass quotes 1.4 kN. Is it unsafe?
Not necessarily. It was designed to the standard in force when it was made. When you replace it, specify the new unit to the current 2.0 kN concentrated load and have the bearers checked for the new glass.
