What is a walk-on rooflight?
A walk-on rooflight is a glazed panel engineered as a floor that happens to be transparent. It is set flush into a terrace, a balcony or a courtyard so that people walk across the glass as they would across paving, and daylight passes through it into the space beneath. The distinction from an ordinary rooflight is not a stronger version of the same product. It is a different item designed to a different set of loads, with different glass, a different frame and a different structure underneath. The full installation detail sits on the walk-on rooflight installation page. This one answers what the thing actually is.
The three properties that define one
A unit is a walk-on unit when all three of the following are true, and it is not one when any of them is missing.
- The glass is specified and tested to carry a distributed floor load and a concentrated point load, with a stated figure behind both.
- It retains its capacity after the top layer has broken, so a cracked panel still holds a person up while it waits to be swapped.
- The walking surface is set at or very near the level of the material around it, with a slip resistant treatment on the exposed face.
Nothing in an ordinary flat roof rooflight installation is asked to do any of it, which is why the two products look similar from a distance and share almost no engineering. A product described only as non-fragile satisfies none of these. Non-fragile means a body falling onto it will not go through, which is a safety property for people working on a roof. Regular pedestrian traffic is a separate rating, and the difference between the two is set out on the page asking whether you can walk on a flat rooflight.
The glass build-up, in plain terms
An ordinary flat rooflight might be 6mm toughened outside, a 16mm cavity and 6.8mm laminated inside: about 30mm overall and around 30kg per square metre. A walk-on unit is built differently from the top down.
The uppermost layer is a sacrificial toughened pane, usually 10mm, whose job is to take the scratching, the grit and the dropped furniture. Below it sits the structural laminate, commonly three plies of 10 or 12mm heat strengthened glass bonded with a stiff interlayer, and below that a cavity and an inner pane. Total thickness runs from 50 to 70mm and the weight lands between 100 and 160kg per square metre. A modest 1,200 by 1,200mm panel therefore weighs somewhere near 200kg on its own.
Interlayers, and the property nobody sees
The interlayer is what makes the difference between laminated glass and structural laminated glass. Standard PVB is soft and viscous, excellent at holding fragments together in a windscreen and poor at transferring shear between plies, which means three plies of PVB laminate behave close to three separate sheets.
Structural interlayers, ionoplast types in particular, are far stiffer and considerably stronger in tension. They make the three plies act closer to a single thick sheet, which is where the load capacity comes from, and they keep the broken pane rigid rather than letting it sag. They also hold up better at the exposed edge, which matters on a panel that spends its life horizontal and wet.
Fixed by design, and how the level finish is achieved
A walk-on unit is a fixed panel. There is no opening light, no hinge and no frame standing above the surface, because anything projecting above a floor is a trip hazard and a place for water to collect against.
The flush finish is achieved with a recessed sub-frame set into the terrace build-up, so the frame sits below the walking level and only the glass is exposed. Around it the paving, decking or stone is brought up to meet the panel edge with a consistent joint, and the drainage happens in the cavity beneath the walking surface rather than on top of it. Ventilation for the room below is provided elsewhere, typically by an opening unit on an adjacent roof area or by mechanical means.
Slip, and glass that spends its life wet
Polished glass wet is one of the more slippery surfaces available, so the exposed face is always treated. The two common approaches are a ceramic frit printed onto the surface in a dot or line pattern and fired in, or an acid etched or sandblasted finish.
Frit is more durable and gives a predictable pattern; etching gives a softer, more uniform look and takes marks from foot traffic more readily. Both are specified against a pendulum test value, and both reduce the light transmission a little, which is the trade being made. A fully sandblasted panel is effectively translucent rather than transparent, which is often exactly what is wanted where the room below is a bedroom or a bathroom.
What the structure underneath has to be
This is the part that surprises people who arrive thinking about the glass. A walk-on panel imposes a floor load on its supports, and floor loads are considerably higher than roof loads. A domestic roof is designed around 0.6 kN/m2 of imposed load plus snow. A domestic floor is designed at 1.5 kN/m2, and a balcony or terrace higher again.
So the deck around the panel, the trimmers either side of it and the joists carrying those trimmers all get calculated as floor structure. Deflection limits tighten too, because a panel supported on a frame that flexes underfoot feels alarming even when it is perfectly safe. Where a walk-on unit is being added to an existing roof, the structural check comes first and the glass specification second.
Where one earns its place in a house
The situation that produces a walk-on rooflight is almost always the same: a room that has no other route to daylight, with usable outdoor space directly above it.
A basement or lower ground floor room under a courtyard. A rear extension whose roof doubles as a terrace off a first floor bedroom. A cellar vault under a light well. A garden room cut into a bank where the ground behind arrives at roof level. In each case the choice is not between a walk-on panel and an ordinary rooflight; it is between a walk-on panel and the room staying dark, because the alternative is losing the terrace.
Walk-on panels in Gloucestershire houses
Three settings account for most of them here. Cheltenham’s Regency terraces in Lansdown, Montpellier and Pittville have lead flats behind parapets that are already at first floor bedroom level, and turning one into a small terrace with a glazed panel lighting the room below is a recurring project. The Stroud valleys produce the opposite geometry: houses built into the hillside at Chalford, Rodborough and Nailsworth where the garden arrives level with the first floor, so a rear extension roof becomes the only flat outdoor space on the plot.
The third is Gloucester’s Victorian terraces, where a narrow rear addition, a brick vault under the pavement or a small courtyard between the outrigger and the boundary wall leaves a room with a single window and a blank wall a metre away. A panel set into the courtyard flags is often the only daylight available. Cotswold stone flags can be laid up to the panel edge on the wolds so the glass reads as one missing slab in the terrace, which is the finish most conservation officers in the county are comfortable with.
What to establish before asking for a price
A walk-on enquiry gets a useful answer faster with four things settled. What is above: a terrace in daily use, a courtyard crossed occasionally, or an area that will only ever be walked on for cleaning. What is below: a habitable room, a store, or a stairwell, since that decides the post-breakage requirement and the fire question.
Then the finished level of the surrounding surface and what it is made of, because the recess depth follows from it. And access, which on a panel weighing a couple of hundred kilograms is the item most likely to change the answer. Severn surveys all four before any figure is fixed, and the process for that is described on the costs page.
Next on this subject.
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- Question How much does a flat roof rooflight cost? No figure is published on this site, and the reason is that the same glass unit can sit inside two…
- Question Do flat rooflights need to be sloped? Yes. A flat rooflight is laid to a slope, and most manufacturers set a minimum of about five degrees on…