What is a walk-on rooflight?
A walk-on rooflight is a glazed unit designed to carry foot traffic as well as weather, set flush into a terrace, a balcony, a courtyard or a path, so that the surface you are standing on is also the ceiling of the room below. It is the answer where a lower storey has no external wall to put a window in, which in practice means basements, lower ground floors, and side returns built out under an upper terrace.
Structurally it has more in common with a floor than with a rooflight. The glass is thicker, the build-up is laminated in several layers, the frame carries a different set of loads, and the structure underneath is designed rather than assumed. This page sets out what defines one and how they are specified. Whether an ordinary flush unit will take your weight is a separate question, answered on can you walk on a flat rooflight.
The definition, and the two duties it serves
Two things have to be true at once. The unit has to weather the building, which is the rooflight duty: a sealed unit, a thermally broken frame, an upstand or a flush kerb detail, and a drainage path. And it has to act as a floor, which is the structural duty: a designed imposed load, a limited deflection, a slip resistant surface and a defined post-breakage behaviour.
That second duty is what separates it from everything else in the range. A conventional flat rooflight is designed for wind, snow and a maintenance operative with a harness. A walk-on unit is designed for people standing on it in normal use, without thinking about it, in bare feet in August and in wet shoes in November.
The glass build-up, layer by layer
A walk-on pane is a laminated assembly of three or more sheets bonded with structural interlayers, then combined with a sealed unit below it where the roof is heated.
A typical domestic build-up runs a 10mm toughened top sheet, then two further 10mm toughened or heat strengthened sheets beneath it, bonded with SentryGlas or a similar stiff ionoplast interlayer rather than the softer PVB used in ordinary laminated glass. The ionoplast matters: it is roughly a hundred times stiffer than PVB and it retains far more strength after a sheet cracks, which is what allows the remaining plies to carry the load. Below that laminate sits a cavity and an inner pane, so the finished unit is commonly 60mm to 80mm thick and weighs 150 to 200 kilograms per square metre.
Point loads against distributed loads
Two load cases govern, and they test different things. The distributed load is the crowd case, taken from BS EN 1991-1-1 as 1.5 kilonewtons per square metre for a domestic balcony or terrace and higher where the area is accessible to the public.
The point load is more demanding on glass. It represents a single concentrated force, typically 1.5 kilonewtons over a 50mm square, and it stands in for a stiletto heel, a chair leg or a dropped tool. Glass is strong in compression and weak in local bending, so it is the point load that usually sets the thickness of the top sheet, not the crowd. That is why walk-on glass is thick even in a small pane where the span is short.
Anti-slip surfaces, and how they are applied
Wet glass in the open is a slip hazard, and the top surface is treated rather than left plain. Three methods are common.
- Ceramic frit, a fired-on pattern of dots or bars applied before toughening. Permanent, will not wear off, and it reads as a visible pattern from below.
- Acid etch, a chemically roughened surface giving an even matt finish across the whole pane. Subtle, and slightly less effective when very wet.
- Sandblasted texture, mechanically abraded to a defined depth, often in a band pattern.
The reference figure is a pendulum test value, and a PTV of 36 or above wet is the usual target for an external walking surface. Ask for the tested figure rather than the phrase “anti-slip”, which on its own means nothing.
The frame, and how the glass is carried
The pane is supported on all four edges, seated on a continuous structural gasket or a setting block arrangement that spreads the bearing rather than concentrating it. Point supports and bolted fixings, common in commercial glazing, are avoided in a walk-on floor because a bolt hole is a stress raiser in a sheet that is already working hard.
The frame profile is deeper than a standard rooflight frame and it is designed to transfer the load into the surrounding structure at defined bearing points. In a terrace it is set so the glass finishes flush with the paving, which means the frame is buried in the build-up and the water has to be taken away by a linear drain running alongside rather than by a raised upstand.
| Property | Standard flat rooflight | Walk-on rooflight |
|---|---|---|
| Outer sheet | 6 to 10mm toughened | 10mm+ toughened, top of a 3 or 4 ply laminate |
| Interlayer | PVB, 0.76mm typical | Ionoplast, 1.52mm or more |
| Total thickness | 28 to 40mm | 60 to 80mm |
| Weight | 30 to 45 kg/m² | 150 to 200 kg/m² |
| Design load | Wind and snow | Wind, snow, 1.5 kN/m² crowd, 1.5 kN point |
| Surface | Clear or self cleaning | Fritted, etched or blasted to a tested PTV |
Part K, guarding and the level threshold
Because a walk-on unit is a floor, Approved Document K applies to it in the way it applies to any other floor. Where there is a change in level alongside, guarding is required, and the rooflight frame is not guarding.
The more common domestic case is simpler and more attractive: a walk-on panel set into a terrace that is level with the surrounding surface, so there is no edge and no guarding question at all. That is one of the reasons the flush detail is worth paying for. It also keeps the threshold level, which matters where the terrace is reached through a set of sliding doors.
Drainage across a flush walking surface
A flush unit gives up the upstand, which is the standard defence, so the drainage strategy has to do the work instead. The paving around the panel is laid to a small fall away from the glass, and a linear channel drain runs along the low side to take the water off before it reaches the frame.
Below that, the frame carries its own perimeter gutter with weep openings into the roof drainage, so anything that does reach the seal has a route out. The critical maintenance item is that channel: leaves and grit collect in it, and a blocked linear drain on a terrace turns the whole surface into a shallow tray. It wants clearing twice a year, autumn and spring.
Where these appear on Essex houses
The clearest case is a basement or lower ground floor in a Victorian property in Chelmsford, Colchester or Maldon, where a rear lightwell has been covered over to gain floor area and the room below would otherwise be dark. A walk-on panel in the yard above returns the daylight without losing the space.
The second is a side return infill under a first floor terrace, and the third is the top of a garden studio or garage conversion where the roof doubles as a seating area. In all three the glass is genuinely the expensive half of the job, which is unusual, and the structural design is done before anything is ordered. Sunspire installs walk-on rooflights alongside standard flat roof units, and the survey settles the loading case first.
Working life, and looking after the surface
The glass itself does not wear out. The sealed unit below it has the same twenty to twenty five year expectation as any other, and the laminate above it is effectively permanent.
What does change is the surface treatment. A fritted pattern is fired into the glass and will outlast the building. An etched or blasted finish holds grime in its texture and wants washing more often than plain glass, particularly under a tree. Grit tracked across it is the one genuine risk, because hard particles under a shoe can scratch, and a scratch in the top ply is cosmetic but permanent. A doormat at the door onto the terrace does more for a walk-on rooflight than anything else you can buy.
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