Gloucester · Cheltenham · Stroud · the Cotswolds

Walk-On Rooflight Installation

A walk-on rooflight is a piece of structure that happens to be transparent. Everywhere else in this trade the glass is a lid over an opening and the only thing standing on it is weather.

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15 years in Gloucestershire
10-year workmanship guarantee
Building Control notification handled
The roof is never left open overnight

Tell us about the roof

We will come and look at it, and tell you honestly what it needs.

The enquiry form needs JavaScript. Email info@severnskylightsgloucester.co.uk with a photograph of the roof and rough measurements and you will get the same answer back.

What the job covers

The decisions that get made before anything is ordered.

Why installations fail

The unit is the small half of the job.

A walk-on unit is a structural floor that happens to be transparent. It is specified from the load it has to carry and the way water leaves a surface that cannot shed sideways, and both are settled before anybody talks about glass.

The load case. Foot traffic, point loads, and what the deck beneath is rated to carry.
The upstand. Level with the finished surface, which is harder than standing proud of it.
Drainage around the frame. A flush unit sheds nothing sideways, so the falls have to take it away.
The walking surface. Anti-slip specified for glass that will be wet and walked on.

That is what the ten-year workmanship guarantee is written against.

A flat roof is laid to a minimum 1:80 fall, drawn here exaggerated so it reads. Water standing more than 48 hours after rain is ponding, and ponding around an upstand finds every weakness in the detail. On the Severn Vale flat roofs we replace, the fall has usually been lost rather than never built.

How it runs

Two figures, and no surprises between them.

Send photographs A few pictures of the roof and the room, and rough measurements. An indicative range comes back by email.
The survey Forty minutes on site. Rafter centres, pitch off a digital level, the deck, the fall, and how a scaffold reaches the elevation.
A fixed price Written against what was measured, not against the drawing. It does not move unless you change what you asked for.
The installation Scaffold, opening, kerb, unit, weathering, reveal and making good. The roof is never left open overnight.
Notification and guarantee The Building Control notification is submitted and closed out as part of the job, and the workmanship carries ten years.

Asked before every job

The questions that come up first.

Can you walk on a flat rooflight?

No, not on an ordinary one. A standard flush glazed flat rooflight is designed to carry wind, snow and its own weight, and nothing else. It is not designed for a person to stand on, and that includes standing on it briefly to clean it or to reach something beyond it. A walk-on rooflight is a different product with a…

The full answer

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…

The full answer

Can a rooflight be fitted to a flat roof?

Yes. A flat roof takes a rooflight readily, and on most houses it is a more predictable job than putting a roof window into a pitched slope. The unit is a made-to-size glass panel that sits on a timber kerb built up above the roof surface, so the covering never has to be interleaved with courses of tile or slate.…

The full answer

What is a rooflight upstand?

An upstand is the raised edge that lifts a rooflight frame clear of the roof surface, and on a flat roof it is a timber box called a kerb, built on the deck around the opening, with the waterproofing dressed up its outside face and over its top edge. The standard height is 150mm measured from the finished surface of…

The full answer

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 jobs that have almost nothing in common. A made-to-size rooflight dropped onto a kerb formed by somebody else on a brand new extension, with a driveway to park on, is a fraction of the work involved in cutting an opening…

The full answer

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 the glass. It looks flat from the garden and it is not: the top edge of the kerb is cut to a rake so the four corners sit at four different heights, and the glass drains to one side. Below…

The full answer

Why is my flat rooflight leaking?

Almost always because of something at the perimeter, not because of the unit itself. Water arriving at a flat rooflight is coming through the joint between the glass and the roof in the great majority of cases, and the four usual culprits are an upstand that is lower than 150mm above the finished surface, water standing against the kerb because…

The full answer

How high should a rooflight kerb be?

150mm, measured from the finished surface of the waterproofing at the highest point of the fall, on the outside face of the kerb. That is the working standard on a domestic flat roof in this country and it is the figure a survey checks first on any existing installation. The number that surprises people is the length of timber it…

The full answer

The technical detail

Glass you can stand on, and the structure under it.

A walk-on rooflight is a piece of structure that happens to be transparent. Everywhere else in this trade the glass is a lid over an opening and the only thing standing on it is weather. Here it is a floor: somebody carries a tray of drinks across it, a child runs over it, a window cleaner puts a ladder foot on it, and the design has to survive all of that with one pane already broken. That single change of duty rewrites the specification from the interlayer outwards, and it also rewrites the perimeter detail, because a walk-on unit finishes level with the surface around it instead of sitting proud on a 150mm kerb. Severn Skylights Gloucester installs walk-on units into roof terraces, courtyards, light wells and internal floors across Gloucestershire and thirty miles by road around the city, and the rest of the rooflight range is set out on the rooflight and skylight services page.

30 sections 34 minute read

A floor first and a rooflight second, and everything that follows from it

Start with the duty and the rest of the specification falls out of it. A rooflight is designed against wind uplift, snow and the weight of a person who might fall on it once in its life. A floor is designed against people standing still, walking, jumping, dropping things, and dragging furniture, repeated for decades. The loads are larger, they are applied downwards rather than upwards, and stiffness matters as much as strength because glass that flexes underfoot feels unsafe long before it is anywhere near failure.

That is why a walk-on panel is not a thicker version of a flat roof unit. The outer pane is a laminate of three plies rather than a single toughened sheet. The glass is thicker, heavier, and set into a bearing rather than clamped to a frame. It sits level rather than at five degrees. And the whole assembly is designed so that if the top ply is broken by a dropped bottle, the panel still carries a person off it. Made-to-size units on a kerb, without any of that, are on the flat roof rooflight installation page.

Non-fragile, occasional access and walk-on: three ratings that get confused constantly

These three phrases are used interchangeably by suppliers and they mean quite different things.

Non-fragile describes a unit that will arrest a fall. The industry test is the roof safety drop test: a 45 kilogram sandbag dropped from 1200mm onto the assembly, twice, and the bag must not pass through. It is a one-off survival test, not a service condition. A non-fragile unit protects a roofer who slips while clearing a gutter. It is not a floor and nobody should plan a route across it.

Occasional access sits in the middle. The panel is designed for maintenance traffic: someone crossing it a handful of times a year with a bucket, on a roof that is otherwise not used. Build-ups are lighter than a full walk-on and the surface is often left clear.

Walk-on means the panel is designed as a floor to the same imposed loads as the surrounding terrace, with a slip resistant surface, a post-breakage check, and a deflection limit. It is the only one of the three that belongs in a terrace off a bedroom or in a garden path. Getting this wrong is the commonest error on the whole subject: a non-fragile unit specified for a terrace looks identical from the garden and is a different product entirely.

A unit that will stop a person falling through is not the same as a unit a person can stand on, and the difference is a survey question rather than a catalogue one.

The loads a terrace over a room actually has to carry

Domestic floor loading in the current Eurocode is 1.5 kilonewtons per square metre spread evenly, with a separate check on a concentrated load of 2.0 kilonewtons applied over a 50mm square. Balconies and terraces are rated higher, commonly 2.5 kilonewtons per square metre, because people gather on them. A roof accessible for normal use is designed for the occupancy it serves rather than as a roof, so a flat roof that has become a sitting-out area is a floor in loading terms whatever it was originally built as.

The concentrated load is the one that governs glass, and it is worth understanding why. A 2.0 kilonewton point load over a 50mm pad is roughly 200 kilograms standing on an area the size of a playing card. That is not an unusual event: it is a woman in a narrow heel, a table leg, or the corner of a plant pot being slid across. Uniform load spreads the bending across the whole pane. A point load concentrates it into one spot, and if that spot is near an unsupported edge the stress climbs sharply. It is the reason walk-on panels are always supported on all four sides and never left with a free edge.

Deflection, and the number that decides how the glass feels underfoot

Strength is rarely what sizes a walk-on panel. Stiffness is. Glass can be strong enough to carry a load and still move enough under it to be frightening, and a homeowner who feels 6mm of bounce in the middle of a panel will never use the terrace again regardless of what the calculation says.

The practical limit is a deflection of the span divided by 360, capped at around 8 to 10mm whichever is the smaller. On a 1.2 metre pane that means about 3mm of movement at the centre under full load, which reads as solid. Meeting it usually means more glass than the strength check alone would demand, and that is money well spent: it is the difference between a feature people walk across without thinking and one they edge around.

Support conditions matter as much as thickness. A panel bearing on a continuous 30 to 50mm setting shim on all four sides behaves very differently from one sitting on packers at the corners. The bearing is bedded on a structural gasket or a silicone strip so the load transfers evenly, because glass in point contact with a hard frame chips at the edge and edge damage is where a laminated panel begins to fail.

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Post-breakage capacity, and the interlayer that provides it

A walk-on rooflight set flush into a terrace

The design case that separates walk-on glass from every other unit is the broken one. The assumption is that the top ply will eventually be broken, by a dropped tool, a stone from a mower, or thermal shock, and the panel must still hold a person while somebody arranges a new one. Two plies cannot do that reliably. Three can, which is why a walk-on outer pane is a three-ply laminate: a sacrificial top ply and two structural plies below it.

The interlayer decides how much the plies act together. Standard polyvinyl butyral, the material in a car windscreen, is soft and it gets softer as it warms. Above about 30 degrees its shear stiffness falls away, so on a July afternoon on a south facing terrace a PVB laminate behaves closer to two separate sheets than one thick one. An ionoplast interlayer is one to two orders of magnitude stiffer and holds most of that stiffness at 50 degrees, which is exactly the temperature a horizontal panel with a dark room under it reaches. It also holds broken fragments in place far better, which is the whole point of the post-breakage check.

Ply type is a related decision. Fully toughened glass dices into small fragments when it breaks and a diced ply contributes almost nothing afterwards. Heat strengthened glass breaks into larger pieces that continue to bridge the span, so a common build-up is a toughened sacrificial top ply over heat strengthened structural plies, laminated with ionoplast.

Walk-on build-ups against weight, span and what each is for

Build-up	Approximate thickness	Weight per square metre	Suits	What to watch
Single toughened pane, non-fragile only	10 to 12mm	25 to 30kg	A roof nobody uses, where the classification exists to protect a roofer	Not a floor. No post-breakage capacity worth counting
Two-ply laminate, occasional access	21 to 25mm	50 to 60kg	Maintenance traffic a few times a year on an unused roof	PVB softening in summer. Modest spans only
Three-ply ionoplast laminate, single glazed	30 to 36mm	75 to 90kg	An internal glass floor or landing, where there is no thermal duty	Sound transmission into the room below. Nothing insulating about it
Three-ply outer over a cavity and a laminated inner	55 to 70mm	110 to 135kg	The standard external walk-on unit over a heated room	Weight at installation. The frame bearing is a cold bridge
Four or five-ply outer, deep laminate	70 to 90mm	150 to 190kg	Wider spans, public or commercial traffic, drive-over panels	Mechanical lifting is compulsory. Panel sizes get small
Multiple panes on structural intermediate beams	As above per pane	As above, plus the beams	Areas larger than a single pane can span	Every beam is a line of shadow and a thermal path

Slip resistance, and glass that spends its life wet

Clear glass, wet, is genuinely slippery. Measured on the pendulum to the British standard method, a smooth wet glass surface returns a pendulum test value in the low twenties, which is classed as a high slip potential. The threshold generally accepted as low slip potential is 36. That gap is not a detail: it is the difference between a terrace surface and an accident.

So the top face of an external walk-on panel is always treated. The durable method is a ceramic frit, an enamel screen printed onto the glass and fired into the surface during the toughening cycle, which makes it part of the glass rather than a coating on it. Sandblasted and acid etched patterns work by texturing the glass itself. Applied anti-slip films and paints exist and they wear, which on a surface that gets scrubbed twice a year is a short life.

Barefoot use is a separate test and a separate conversation. A terrace off a bathroom or a bedroom will be crossed by wet bare feet, and the ramp test classes used for pool surrounds are the right reference there. A coarse frit that grips a shoe can be uncomfortable underfoot without one, so the pattern gets chosen for how the terrace is actually used rather than for the highest number available.

Surface treatments against grip, daylight and bare feet

Treatment	Wet grip	Daylight cost	Barefoot comfort	Notes
Clear, untreated	Poor	None	Fine when dry, dangerous when wet	Internal floors under cover only
Fired ceramic frit dots, around 30 per cent coverage	Good	Roughly 25 to 30 per cent of transmitted light	Comfortable	The usual external specification. Permanent and cleanable
Fired frit, 50 per cent or banded	Very good	Up to half the light	Comfortable	Used where the panel is a main circulation route
Acid etched full surface	Moderate to good	Modest, and the light becomes diffuse	Good	Diffusing the light suits a room that would otherwise get glare
Sandblasted pattern	Good	Similar to frit at the same coverage	Slightly rougher	Texture holds dirt and moss on a shaded roof
Applied anti-slip coating	Good when new	Varies	Varies	Wears with traffic and cleaning, so it is a maintenance item

Zero pitch: the one unit in the family that cannot be laid to a fall

Every other rooflight on this site is set at a minimum of about five degrees so that rain sheets off it and dirt goes with the water. A walk-on panel is part of a floor, so it is level. That is not a compromise anybody chose, it is a consequence, and three things follow.

Rain does not clear the glass, it evaporates off it, and whatever was in it stays behind. On a Gloucester terrace that means a fine grey film within a fortnight and a visible tide line where the surface tension holds water against the frame. Self-cleaning coatings rely on water running off a slope and they do very little on a level pane. And standing water sits against the edge seal for long periods, which is the condition that shortens the life of any sealed unit, so the edge detail and the drainage around the panel matter more here than on a sloped unit.

The design response is to make the panel the highest point of its immediate area, so the surrounding terrace falls away from it rather than towards it, and to accept that the glass is cleaned rather than self-clearing. On a first-floor terrace that is a five-minute job with a squeegee. Over a light well reached only by ladder it is worth thinking about before the panel goes in.

The walking surface is not the waterproofing, and this is the detail that decides the job

The largest conceptual difference between a walk-on installation and everything else in this trade is that the surface people stand on is not the surface that keeps the water out. On a roof terrace, the waterproof membrane sits below the paving, at deck level, and it is the membrane that is continuous, dressed up every upstand and turned over every kerb. The paving above it is a wearing surface that water passes straight through at the joints.

That gives a two-stage drainage system. Rain falls on the pavers, most of it runs off their faces, and the rest goes down the open joints into the void beneath. It then runs on the membrane, down the designed falls, to the outlet. The void has to be able to drain and to breathe, which is why pedestal paving is laid with open joints rather than pointed.

The walk-on panel takes its place in that system as the one element that is both surfaces at once. Its top face is the wearing surface, level with the paving. Its frame sits on a kerb that rises from the deck, and the membrane is dressed up that kerb and turned over it exactly as it would be on any other rooflight. The paving simply comes up around the outside of the kerb and hides it. Get that right and the unit is protected by a full upstand while reading as a flush panel in the floor.

Pedestals, the drained cavity, and a finish that is genuinely flush

Adjustable pedestals are what make the whole arrangement work. A pedestal is a plastic support with a screw thread, adjustable typically between 20 and 200mm, with a head carrying four spacer tabs that set the joint width between pavers. The pavers or the decking sit on the heads, the membrane below is never penetrated, and the height of the pedestals is dialled in so the finished surface lands where the design wants it.

Two features earn their keep on a Gloucestershire terrace. Self-levelling heads correct for the fall in the deck below, so the membrane can be laid at 1:40 while the paving above stays dead level, which is what makes a level walking surface possible over a properly falling roof. And acoustic pads under the head reduce the hollow knock that pedestal paving otherwise makes, which the room below hears.

The gap around the walk-on panel is set out rather than left to chance. A perimeter of cut pavers or a slim aluminium trim runs against the frame with an open joint, so water leaving the glass goes into the void immediately rather than sitting against the frame. Where the panel is set in timber decking, the joists are stopped short and a hardwood or aluminium edge trim carries the last board, because a decking board running tight to a glass frame will swell and load the edge of the pane.

A flush glazed rooflight set into a flat roof

Door thresholds, terrace levels, and where the 150mm has to come from

This is the arithmetic that decides whether a terrace scheme is achievable, and it works downwards from the door rather than upwards from the roof.

A new door onto a terrace is required to have an accessible threshold, which in practice means no more than 15mm of upstand at the door. That sets the finished paving level within a few millimetres. The paving sits on pedestals, so the pedestal height plus the paver thickness gives the distance down to the membrane. The kerb under the walk-on panel has to reach from the deck up to within a few millimetres of the paving, and the membrane has to be dressed up that kerb with a proper upstand behind the panel frame.

Work it through and the numbers are tight. A 40mm porcelain paver on a 100mm pedestal puts the paving 140mm above the membrane. A walk-on frame is typically 70 to 100mm deep. There is enough there for a workable upstand, and it is why terraces of this kind are designed around a build-up of at least 150mm above the waterproofing. Where the existing door threshold is already low and the deck cannot come down, the honest answer at survey is that the level is short and either the door threshold changes, the deck is re-laid lower, or the panel finishes slightly proud of the paving with a chamfered trim around it.

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The terrace build-up, layer by layer

Layer, from the ceiling upwards	Typical dimension	What it does here
Plasterboard and skim	12.5 to 15mm	The room’s ceiling, and the surface any failure shows on first
Service void and vapour control layer	25 to 50mm	Taped to the walk-on frame, not trimmed short of it
Joists with insulation between, or a warm deck above	195 to 250mm	Carries floor loading now rather than roof loading
Structural deck, plywood or OSB	18 to 22mm	Trimmed around the opening on doubled trimmers
Insulation over the deck on a warm roof	100 to 150mm	Has to be a grade rated for foot traffic under the paving
Waterproof membrane and the kerb upstand	Membrane 1.5 to 2mm, upstand 150mm	The real waterproof line, dressed up and over the kerb
Pedestals	20 to 200mm adjustable	Sets the paving level and corrects the fall
Pavers or decking, and the glass	20 to 60mm	The wearing surface, with open joints that drain into the void

Span limits, and the point at which one pane becomes three

Glass gets heavy faster than it gets stiff. Doubling the span of a walk-on pane does not double the thickness needed, it roughly quadruples the bending moment, and because the deflection limit is proportional to span the required thickness climbs steeply. There is therefore a practical ceiling on a single pane, and for a domestic walk-on unit it sits somewhere around 1.2 metres in the short direction and around 2.4 metres in the long one.

Beyond that the panel becomes several panes with structural support between them. That support is either an aluminium or steel intermediate beam under the joint, or a glass beam where the specification justifies it. Each beam is a visible line across the panel, a shadow on the room below, and a thermal path from the terrace to the ceiling, so the layout is worth setting out on paper against the room beneath before anything is ordered.

A useful alternative on a wide opening is to break the glass area up deliberately: three narrower panes running the same way as the joists, with the beams over the joist lines, gives a set-out where structure and glazing agree with each other instead of fighting. It also keeps individual pane weights inside what can be lifted by hand and a suction frame rather than needing a crane.

The deck underneath, carrying a floor rather than a roof

A flat roof over a Gloucestershire kitchen extension was designed for roof loading. Turning it into a terrace changes the design case, and adding a walk-on panel adds a concentrated dead load on top of that.

Run the arithmetic on a typical case. A 1.4 by 1.0 metre insulating walk-on unit at 125 kilograms per square metre is about 175 kilograms of glass, plus the frame, and all of that bears on the two trimmers around the opening rather than being spread across the deck. On the joists either side of the opening it appears as two point loads. Meanwhile the terrace as a whole is now carrying 2.5 kilonewtons per square metre of imposed load where it was designed for perhaps 1.5 for maintenance access, plus a paving build-up of 40 to 80 kilograms per square metre that was never there before.

The usual result on a rear extension built in the last thirty years is that the joists need checking and often doubling, or a beam under the trimmer line. On an older lean-to structure over a Victorian outrigger the honest answer is sometimes that the deck is re-built from the joists up. That gets established at survey with the joist size, span and centres measured, and where the numbers need a calculation it goes in with the Building Control application rather than being decided on site. The general structural approach on flat roof openings is covered under flat roof rooflights, and the equivalent for a pitched slope under roof window installation.

Frameless, framed, and the recessed sub-frame that makes flush possible

Three configurations turn up, and they look very different from above.

The framed version has a visible aluminium border, usually 60 to 100mm wide, sitting proud of the paving by the thickness of its top capping. It is the most straightforward to build, the easiest to make watertight, and it reads as a hatch rather than as a floor panel. Where the terrace is a working surface rather than a showpiece, it is perfectly good.

The flush framed version sets that same frame down inside the build-up so the glass and the paving arrive at one level with a slim trim between them. This is the common domestic answer. It needs the level arithmetic described above to work out, and it needs a drainage joint around the frame so water does not stand in a channel.

The frameless or structurally glazed version bonds the glass into a recessed sub-frame with structural silicone and shows nothing but glass and a joint line. It is the most convincing visually and the least forgiving: the sub-frame must be dead flat and dead square, the bonding is a factory operation with a cured joint rather than a site-applied bead, and replacing a pane later is a bigger exercise. On a Cotswold stone terrace, where the panel needs to disappear, it is often the version worth the extra.

Footfall, and the room that hears the terrace above it

Nobody thinks about this until they are lying in bed underneath it. A walk-on rooflight is a large stiff plate bearing directly on a frame that is fixed to the structure of the room below, and it transmits impact sound extremely well. Heels on glass, a chair scraping, a dog crossing: all of it arrives in the room as a sharp knock rather than the dull thud that a paving slab on a pedestal makes.

Three things reduce it. A resilient bearing under the glass, a structural gasket or a cellular silicone strip rather than a hard shim, breaks the direct path from pane to frame. An acoustic interlayer in the inner laminate damps the pane itself, and because a walk-on inner pane is laminated anyway the upgrade to an acoustic grade interlayer is a small change. And asymmetric glass thicknesses across the cavity stop the two panes resonating together.

Where the terrace is over another dwelling, in a converted Cheltenham townhouse for instance, impact sound between dwellings is regulated and the whole floor construction has to satisfy it. That is a design-stage conversation, because a glass panel is not the element that will deliver it and the surrounding build-up has to do the work.

Looking up from below, and looking down from above

A walk-on panel is a window in both directions and people are usually surprised by the view upwards. From a kitchen, the panel above gives a view of the soles of shoes, the underside of a table, and a good deal of grit and leaf debris on the glass. From above, standing on glass with a room visible underneath produces genuine unease in a proportion of people, particularly on a first-floor terrace where there is an obvious drop.

Both problems have the same answer, which is why the frit pattern gets chosen for the room rather than for the glass. A dot pattern at 30 per cent coverage on the top face breaks up the view in both directions while passing plenty of light. An acid etched inner pane diffuses the view completely and turns the panel into a light source rather than a window, which suits a bathroom underneath or a terrace where somebody is nervous. A clear panel is the right answer where the room below is a hallway, a stair or a store, and where the effect wanted is a shaft of daylight in a dark space.

Where the aim is only daylight into a windowless room and nobody needs to stand on glass at all, a reflective tube through the roof void is a far cheaper way to get it, described under sun tunnel installation.

Thermal and solar performance through 60mm of glass

A walk-on insulating unit is a competent thermal element and a poor one at the same time, depending on which part you look at. The glass itself does well: a 33mm three-ply outer, a 16mm argon filled cavity, a low emissivity coating and a laminated inner pane will land around 1.1 to 1.4 W/m2K measured in the vertical plane, which is comparable with a good made-to-size flat unit. The extra mass of laminate adds a little useful resistance and a lot of thermal inertia, so the panel warms and cools slowly.

The frame is the weak point, and unavoidably so. A walk-on frame is a structural bearing carrying floor loads, so there is a limit to how much of it can be interrupted by an insulating break, and it sits inside a paving build-up where insulation is difficult to keep continuous. That perimeter is where condensation appears on a cold February morning, on the ceiling immediately around the opening rather than on the glass, and the answer is insulation carried up the outside face of the kerb and lapped into the deck insulation with no gap at the corner.

Solar gain runs the other way. The panel faces straight up, it has no shading from a frame projection, and it is over a room that often has no other glazing. A frit pattern is the most effective control available because it blocks light before it becomes heat, and 30 per cent coverage takes a useful slice off the solar factor as well as providing the grip. A solar control coating on the cavity face does the rest. The physics of overhead glazing and the numbers behind it are set out on the energy efficient rooflights page.

Getting a quarter of a tonne of glass onto a roof terrace

Weight is the practical constraint that decides how a walk-on job is planned. A 1.5 by 1.2 metre insulating walk-on unit is around 230 kilograms of glass before the frame. That is beyond hand lifting by any safe crew, so the panel arrives with a lifting method already decided: a vacuum lifter on a spider crane, a telehandler with a glazing attachment, a mechanical glass manipulator, or a gantry and slings where a machine cannot reach.

The site survey therefore includes questions that have nothing to do with glass. Can a lorry get within reach of the elevation, and is the lane wide enough. Is there a hardstanding a machine can be set up on, or a garden that will take ground protection. Is there a route through the house as a last resort, and what would it cost in protection and reinstatement. On a terrace behind a Cheltenham townhouse with a shared rear access, or a Chalford cottage reached down a flight of steps, this question decides the price and sometimes decides the pane size, which is one more reason to break a large area into three panes rather than one.

Sequencing follows from it. The kerb is built and the membrane dressed and made watertight before the glass arrives, so the roof is complete and weathertight in its own right and the panel is set into a finished opening. The roof is never left open overnight, and on a walk-on job that is easier to honour than on most, because the glass and the waterproofing are separate operations on separate days.

Guarding at the edge, and the reason a walk-on unit gets chosen at all

The reason people specify walk-on glass rather than an opening rooflight is almost never the daylight on its own. It is that they want the roof area and the daylight from the same square metres. A conventional unit on a 150mm kerb in the middle of a small terrace makes the terrace unusable: it is an obstacle, it collects furniture against it, and somebody will eventually stand on it.

A flush walk-on panel gives the floor back. It also simplifies the guarding, because the protection can go where it belongs, at the edge of the terrace, rather than being wrapped around a kerb in the middle of it. Where a terrace is more than 600mm above the surface below, guarding is required at the edge, at 1100mm high on a roof terrace, and designed so that it cannot be climbed by a small child and has no gap a 100mm sphere passes through.

Two interactions matter. The guarding has to be fixed to something structural, and if that fixing line runs close to the walk-on panel the sub-frame and the trimmer have to accommodate both. And a panel immediately adjacent to a glass balustrade produces a great deal of reflected glare in the room below, which is worth predicting rather than discovering. Where the terrace sits behind a masonry parapet, as most Cheltenham examples do, the parapet is usually already doing the guarding job and the whole question goes away.

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Cheltenham parapet roofs used as terraces off a first floor bedroom

Central Cheltenham produces more walk-on enquiries than anywhere else in the county, and the reason is architectural. The Regency and early Victorian houses through Lansdown, Montpellier, Pittville and the Bath Road have deep rear closet wings and later additions, and behind the parapets there are flat lead or asphalt areas at first-floor level that are reached by a window or a door from a bedroom. A great many of them have been used as informal terraces for a century.

Three things make them good candidates. The parapet already provides the guarding, at a height that generally satisfies the requirement without adding a rail visible from the street. The area is invisible from the pavement, which changes the consent conversation considerably on a listed building or in a conservation area. And the room underneath is usually a ground-floor rear room made dark by the addition itself, which is exactly the room a walk-on panel helps.

The technical catches are the parapet gutter and the build-up depth. These roofs drain through a parapet gutter to an outlet or a chute, often at a level that leaves very little height between the existing lead and the top of the parapet, so the level arithmetic is tight and the falls in the gutter cannot be compromised. Listed building consent covers alterations whether or not they are visible, so pre-application advice with photographs is the sensible first move, and the conservation-specific units used on visible historic slopes are covered under conservation rooflight installation.

Stroud and Chalford, where the garden arrives at first floor level

The Stroud valleys produce the most interesting walk-on projects in Gloucestershire because of how the houses sit in the ground. A cottage built into the side of the valley at Chalford, Rodborough, Brimscombe or Thrupp is commonly one storey at the front and three at the back, with the garden path arriving at what is internally the first floor. The lower rooms are cut into the bank, they have a retaining wall behind them and neighbours tight on both sides, and they have no elevation left to put a window in.

What that produces is a flat roof, often over a rear or side room, that is at the same level as the terrace or path outside and has always been walked across. A walk-on panel set into that surface is the only realistic daylight for the room underneath, and it changes a store or a utility space into a room somebody would use.

Two local realities come with it. Access is genuinely difficult: shared lanes, footpaths, flights of steps, no hardstanding for a machine, and a 230 kilogram pane that cannot be carried. That drives the panel layout towards several smaller panes and it drives the price, and it is priced from the survey with the access looked at properly so the figure holds. The other is water. These roofs sit below a bank, they take run-off from the slope above, and the drainage design has to deal with water arriving from uphill as well as rain landing on the terrace.

Gloucester outriggers, cellar vaults and glass set into a courtyard

Gloucester’s Victorian and Edwardian terraces through Tredworth, Barton, Kingsholm and Linden give two quite different walk-on situations, and the second one surprises people.

The first is the rear outrigger. A two-storey addition one room wide, with a single-storey extension across the yard behind it, gives a flat roof at first-floor level immediately outside a bedroom window. Turning that into a small terrace with a walk-on panel lighting the kitchen below is a common and sensible project, subject to the deck being brought up to floor loading and to overlooking, because a first-floor terrace looks straight into the neighbouring yards on both sides and that is a planning consideration rather than a technical one.

The second is the cellar. A great many of these houses have a vaulted cellar extending under the front path or the pavement, originally lit and coaled through cast iron pavement lights with small glass insets. Where those have been concreted over, a modern walk-on panel set into the path is a way of getting daylight into a cellar that has been dark for a century. It is a small panel in a heavy frame at ground level, it takes foot traffic and sometimes a wheelbarrow, and it is drained around all four sides because a panel set flush in a path is at the bottom of everything that runs across that path.

Cotswold stone flags, and a panel that reads as part of the terrace

On the wolds and through Painswick, Bibury, Northleach and Chipping Campden, the walk-on question almost never involves the main roof. Those roofs are stone slate in diminishing courses and the answer there is a conservation unit, for the reasons set out on the skylight and rooflight installation page. What comes up instead is the ground: a stone flagged terrace, a courtyard between a house and an outbuilding, or a light well against a wall, with a cellar, a plant room or a converted store underneath.

A flush glass panel is often the most acceptable intervention available in these settings, precisely because it is horizontal and at ground level, so it is invisible from any distance and from any public view. The detail worth getting right is the match to the flags. Cotswold flagstones run 50 to 60mm thick and they are laid with a wide joint, so a walk-on panel with a frit surface set flush and trimmed with a slim aluminium edge sits in that floor without announcing itself. Frameless glazing is usually worth the extra here for the same reason.

Two practical notes. Stone terraces on these houses are often laid on a bed rather than on pedestals, so the build-up has to be designed rather than assumed, and where the panel is over a cellar with a lime mortared stone wall around it the bearing detail keeps the frame load off the historic masonry. Consent still applies on a listed building even at ground level, and the officers in this county have seen the argument before and are generally receptive to it.

Forest of Dean hillside plots, and a room cut into the bank

Around Coleford, Cinderford, Mitcheldean and Lydney the ground moves as much as it does in the Stroud valleys, and the housing stock includes a lot of former mining terraces on sloping plots with workshops, garages and stores built into the bank behind or below the house. Those buried rooms are the classic candidate for a walk-on panel in a drive, a yard or a path above them.

The Forest changes two things about the specification. It is wetter and windier than the vale, with driving rain arriving from the south west, so falls are checked more carefully, upstands get the full height without argument, and the drainage around a panel set in a yard is designed for run-off from the slope above rather than for rainfall on the panel area alone. And the panels here are frequently over unheated spaces, a garage or a workshop, which changes the unit: a single three-ply laminate rather than an insulating unit is often the right specification where there is no heated room below, and it is lighter, cheaper and easier to get into a difficult position.

Sandstone and local stone detailing is worth respecting in the surround. A frit patterned panel with a stone edging reads as part of a yard. A wide aluminium border in a stone setting does not.

Drive-over glass, and where walk-on rating stops being enough

A car is a different order of load. Domestic vehicle loading is checked against wheel and axle loads rather than a spread figure, and a single wheel of a family car puts several kilonewtons through a contact area smaller than a sheet of paper. A walk-on panel designed for 2.5 kilonewtons per square metre and a 2.0 kilonewton point load is not in that territory.

Drive-over panels exist and they are a distinct product. The glass runs to four or five plies and 70 to 90mm of laminate, weights climb past 150 kilograms per square metre, the panel is supported in a steel frame on a generous continuous bearing, and pane sizes come down accordingly: a set of small panels in a grid rather than one large sheet. The surface treatment is heavier, because a tyre needs more grip than a shoe, and the frame has to be set in a bedded ground construction that will not settle differentially.

The reason this matters on a domestic job is that the situation changes over time. A panel set into a side path is a walk-on panel on the day it goes in, and five years later somebody parks a car on it or wheels a skip across it. Where a panel sits anywhere a vehicle could plausibly reach, that gets settled at design stage, either by specifying for it or by making the surrounding layout physically prevent it with a bollard, a planter or a level change.

Building Control, the structural calculation and the fire question

Forming a new opening in a floor or a roof is notifiable work, and a walk-on installation touches more parts of the regulations than any other unit in the range. The notification is submitted and closed out as part of the job, and the completion certificate comes back in the homeowner’s name.

Structure. The floor is designed to the imposed loads for its use, and the trimmers, joists and any beam are calculated. Glass acting as a floor is designed with a post-breakage check, and the calculation and the glass specification go in together. The recognised reference is the Institution of Structural Engineers guidance on the structural use of glass in buildings.
Fire safety. Where a terrace is over another dwelling, or forms part of a means of escape, the floor is required to resist fire from below for a stated period. A standard glass unit does not provide that, so the position and the extent of the panel are settled with Building Control at design stage rather than after ordering.
Conservation of fuel and power. The panel is a rooflight for thermal purposes and is assessed against the limiting standard, along with the continuity of insulation around the kerb.
Protection from falling. Guarding to the terrace edge, its height, and the requirement that it is neither climbable nor gapped.
Resistance to the passage of sound. Where the floor separates dwellings, impact sound performance applies to the construction as a whole.
Access. The threshold at any new door onto the terrace, which as described above sets the level for everything below it.

Planning is a separate question from Building Control. A flush panel in a terrace or a path is not a projection above a roof slope, so the permitted development test that governs a roof window behaves differently here, and the issue is more often overlooking from a raised terrace than the glass itself. A raised platform in a garden or on a roof is treated differently from a rooflight, and the local planning authority is the only body that can confirm the position for a specific address.

What the survey records, and the annual check afterwards

A walk-on survey takes longer than any other unit on this site because it is measuring a floor and a roof at once. What gets recorded: the joist size, span, centres and condition, and how the deck is built up; the existing membrane type and its falls, taken with a level rather than by eye; the position of the outlet and where the water currently goes; the finished level of the door threshold and the existing paving; the height available between deck and threshold, which is the number the whole scheme lives or dies on; what the room below is used for, since that decides the glass and the frit; the access route for a machine and for a pane; and the guarding, existing or required.

Afterwards the maintenance is short but it is real, and a glass floor is the one rooflight that genuinely benefits from an annual look. Clear the drainage joint around the frame, because a level panel with a blocked perimeter joint sits in standing water. Wash the surface with something that will not attack the frit or the frame finish, and avoid anything abrasive on the anti-slip pattern. Check the edge of the laminate at each corner for any sign of clouding or separation in the interlayer, which is the earliest indication that the edge seal is taking on water. Lift a paver or two nearby and look at the membrane, since that is far easier now than after a mark appears on the ceiling. And clear moss out of the paving joints, because a blocked joint turns the drained void into a reservoir.

What moves the figure, and how a walk-on enquiry starts

No figures are published on this site, because a small single-glazed panel over an unheated Forest workshop and a frameless flush insulating panel in a Cotswold flagged terrace with hand-carried access are not the same job at any price. What actually drives it:

The loading class. Occasional access, full walk-on and drive-over are three different pieces of glass with very different weights.
Whether the panel is insulating or single laminated, which follows from whether the room below is heated.
Pane size against the span limit, and whether the area needs intermediate structural beams.
Framed, flush framed or frameless, in that order of cost, and the accuracy the sub-frame needs for the last of them.
The state of the deck, and whether joists need doubling or a beam is required to carry floor loading.
The terrace build-up, since pedestals, insulation, a new membrane and paving are usually part of the same project rather than separate from it.
Access for a pane weighing a couple of hundred kilograms, which in the valleys and the older town centres is the single biggest variable.
Surface treatment and glass specification, including frit coverage, acoustic interlayers and solar control.
Guarding, drainage alterations and any consent work where the building is listed or in a conservation area.

Then two figures, in that order: an indicative range by email from photographs and a rough size, then a fixed and itemised price after a survey, which holds unless the scope changes or something structural is uncovered that was flagged as a risk beforehand. The process is set out on the costs page. The workmanship carries a ten-year guarantee, written against the perimeter details that decide whether the panel stays dry, and the glass carries the manufacturer’s own warranty as a separate document.

To start, email info@severnskylightsgloucester.co.uk. Four photographs make the first reply genuinely useful: the flat area from above, the room underneath, the door or window that opens onto it, and the access route from the road. Add a rough measurement of the flat area and a note of what the room below is used for. Severn covers Gloucester and thirty miles by road around it, listed town by town on the areas pages, with individual questions answered in the written guides and the background to the company on the about page. Where a walk-on panel turns out to be the wrong answer for a particular roof, the alternatives are a kerb-mounted unit on a flat roof, an electrically operated unit where the aim is ventilation, a roof lantern where the aim is height and volume, or a new unit into an existing opening as set out under replacing a failed or misted rooflight.

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