Walk-on rooflight installation in Cambridge, light through the floor above

The rooms that need it are easy to recognise once you look for them. A kitchen extension on a Victorian terrace in Romsey or Petersfield often carries a flat roof that the owners want to use as a small terrace off the back bedroom. Put an ordinary flat skylight in that roof and you lose part of the terrace, because nobody can step on it and it needs space around it. Put a walk-on unit there and the terrace stays whole, with a bright panel set into it that throws light onto the worktop underneath.

The second case is the room below ground. Basement conversions under larger Victorian and Edwardian houses in Newnham or De Freville Avenue, and lower-ground rooms in some new homes, have a front or rear lightwell and little else. A walk-on panel in the ground above extends the lightwell's reach without a railing or a raised kerb in the middle of the garden. Our guide to walk-on glass over basements covers the lightwell detail.

The third is the courtyard or side passage. Many terraces have a narrow side return that was built over to make a bigger kitchen, and the path that used to run along it now sits on top of the extension roof or has vanished. A strip of walk-on glass laid along that line keeps a path to the garden and puts light into the deepest part of the kitchen. We cover layouts like this in walk-on glass in courtyards and gardens.

Places where walk-on glass is the right call:

• Roof terraces over a kitchen, bathroom or stairwell, where the glass must sit level with decking or paving. See walk-on rooflights for roof terraces.
• Basement lightwells and front areas, where a larger opening would need guarding or would block access.
• Courtyards and paths laid over a single-storey extension.
• Internal floors, such as a landing over a double-height hall, where the glass acts as a floor panel rather than a roof. The difference is explained in is walk-on glass the same as a glass floor?

If nobody will ever walk across the roof, an ordinary flat unit usually does the job for less weight and less structure. Our comparison of walk-on vs standard flat rooflights sets out the trade, and our flat roof rooflight installation page covers the non-walkable option in full. The short test we apply at the survey: if the roof is, or will become, somewhere a person stands, a chair gets dragged or a ladder gets rested, the glass has to be designed as floor.

Clear, polished glass is slippery when it rains, and on an open terrace in the Cambridge climate it will be wet for much of the winter. So the walking surface is usually treated. The common finishes are a sandblasted or acid-etched pattern, a ceramic frit fired into the surface in dots or stripes, or a gritted coating. Each adds texture that a shoe can grip. Slip resistance is measured with a pendulum test, set out in BS 7976-2, and UK guidance generally treats a wet pendulum test value of 36 or more as low slip potential. When comparing products, ask for the wet figure, not the dry one. The detail sits in our guide to the slip resistance of walk-on glass.

Now look at the same finish from below. A clear pane lets you see the sky, the clouds and anyone standing on top. A fully sandblasted pane turns the rooflight into a glowing panel: the light is diffused and softer, the sun no longer throws a hard-edged bright shape on the floor, and nobody above can see into the room. A dotted frit sits between the two, with a partial view and a partly broken-up sun patch.

How we match the finish to the room:

• Bathroom or bedroom below: a full obscure finish, for privacy and even light.
• Kitchen or dining room below: a dotted frit or a border of obscure glass with a clearer centre, so there is some sky view and the terrace stays safe.
• Stairwell or hall: often fully obscure, because the light is for moving through, not for looking out.
• Glass where nobody treads, such as a pane surrounded by planters: it can stay clear, with a finish only on the walking route.

Privacy has more options than the walking surface alone. An interlayer can be made translucent, and some units offer switchable glass that turns from clear to opaque. Our guide to privacy finishes for walk-on glass weighs each one. Wear matters too: sand carried on shoes will mark any glass over time, and textured finishes hide fine marks better than polished ones. The answer on whether walk-on glass scratches covers that, and how to clean walk-on glass covers the textured surfaces, which need a brush rather than a squeegee.

Most flat roofs over Cambridge extensions are timber joists at 400 mm or so centres, with a deck, insulation and a membrane or paving on top. A walk-on rooflight wider than one joist bay means cutting joists and carrying their ends on new trimmers, with the load passed back to the walls or beams. The glass also needs the frame under it to deflect very little, because glass does not tolerate a sagging support. For those reasons, a structural engineer normally designs the trimming and checks the existing joists for the extra weight of the glass and the terrace finish. The answer on whether walk-on glass needs a structural engineer explains the cases where a manufacturer's standard detail is enough and the cases where a calculation is required.

Steel or concrete decks are a different case. On a concrete slab over a basement, the opening is formed with a cast upstand or a steel frame set into the slab, and the builder or engineer will want to know the exact glass size well before the slab is poured. On a steel-framed terrace, the glass often sits on an angle frame bolted to the steels.

What the opening has to provide:

• Continuous bearing on all four edges, or on the edges the manufacturer specifies.
• A level top, so the glass sits flat and flush with the finished surface around it.
• Square corners within tight tolerances, since the glass arrives made to size.
• Room for the insulation to wrap the upstand, so the kerb does not become a cold bridge.

Our guide to the structural opening for walk-on glass shows the common details for timber, steel and concrete. If you are adding walk-on glass to a roof that is already built, the answer on fitting a walk-on rooflight to an existing flat roof covers what has to be opened up and what can stay.

The opening is also where the light plan and the structure meet. If the ideal position for the light falls across a beam or a steel, the engineer and we agree between us whether to move the glass, split it into two panes, or re-route the load. That discussion happens on paper before the quote is fixed, so the price reflects the opening you will actually get.

Approved Document L sets a limiting U-value of 2.2 W/m²K for rooflights in existing dwellings, assessed with the unit lying horizontal, and new walk-on units designed for homes can do much better than that. Horizontal glass loses heat faster than the same glass standing upright, because warm air sits against it and moves more, so a unit's horizontal figure is the one to compare. Our page on energy efficient rooflights explains how the glazing choices trade heat loss against solar gain.

The ways to get the figure down are a low-emissivity coating, argon in the cavity, a warm edge spacer and, where the structure can take the weight, a second cavity. Triple glazing adds another pane and more weight, and it is not always possible within a standard kerb depth. The answer on whether a walk-on rooflight can be triple glazed covers when it makes sense.

Condensation forms on the inside of a rooflight when warm, moist air meets glass colder than its dew point. Better insulated glass keeps the inner face warmer, and a warm edge spacer keeps the edges from being the first place to fog. The frame and kerb also need to be insulated so there is no cold ring around the glass. In kitchens and bathrooms, extraction does as much as the glass. The detail is in our guide to the thermal performance of walk-on rooflights.

A walk-on rooflight can open, though it is a specialist unit. Hinged and sliding versions exist, usually driven by an electric actuator so the glass lifts clear of the surface, and they are used for access hatches to roof terraces as well as ventilation. The answer on whether a walk-on rooflight can open covers the options, and our electric and opening rooflight installation page explains the controls and rain sensors used on opening units generally.

In England, rooflights on a house are generally permitted development under Class C of the General Permitted Development Order, provided they project no more than 150 mm from the roof plane and do not rise above the highest part of the roof. A walk-on rooflight flush with a terrace projects nothing. Permitted development does not cover flats or maisonettes, and it can be removed by an Article 4 direction. Listed buildings always need listed building consent for this kind of work. Planning in the city and South Cambridgeshire is handled by Greater Cambridge Shared Planning, which publishes the conservation area boundaries.

The terrace is the part to check with care. Forming a roof terrace, balcony or raised platform on a house is not normally permitted development, because of overlooking, and on a terrace in Romsey or Mill Road the neighbours' gardens are very close. If the terrace needs permission, the walk-on glass is shown on the same drawings. The answer on whether a walk-on rooflight needs planning permission goes through the combinations. A permission can also come with conditions about obscure finishes where the glass would allow a view down into a neighbour's room, which brings us back to the surface finish.

Building Regulations are more straightforward. Cutting joists, adding a structural opening, fitting new glazing to an insulated roof and forming a terrace are all covered, including Part A (structure), Part K (guarding and safety) and Part L (energy). Where the work is notifiable, we handle the Building Control notification and arrange the inspections as part of the job, and the engineer's calculations form part of that submission. Party walls matter too: Approved Document B restricts rooflights close to a wall shared with next door, so on a terrace the glass normally sits back from the party wall line. The building regulations for walk-on rooflights guide sets out each part.

We never give a guarantee that permission will be granted, because only the council can decide that. What we can do is check the constraints on your address, draw the glass in a way that answers the obvious questions, and tell you plainly when a pre-application enquiry is worth the time.

Walk-on glass is made to order once the opening is built and measured, so there is a gap of some weeks between forming the opening and fitting the glass. During that time the opening is weathered with a temporary cover, so the room below stays dry. The answer on how long walk-on rooflight installation takes sets out the full timeline from survey to finished ceiling.

On the day, in order:

1. Access and protection. Scaffold or a tower where needed, protection laid inside over floors and worktops, and the lifting route checked. Heavier panes come up with a vacuum lifter, and on a tight terrace with no side access a small crane or a glass-handling robot may be needed. That is agreed in the quote, not decided on the day.
2. Kerb check. The upstand is checked level and square, the membrane upstand is inspected, and the bedding tape or gaskets are laid to the maker's detail.
3. Setting the glass. The pane is lowered onto its bearing and positioned so the gap around it is even. Nothing is forced.
4. Sealing. The perimeter is sealed with the specified structural or weather silicone, and the cover trims or flashings are fitted.
5. Terrace finish. The paving or decking is brought back up to the glass so the surface is flush, with open joints for drainage.
6. Inside. The reveal below is lined, insulated, boarded and plastered, then left ready for decoration.

Every quote comes with a written light plan showing the position, size, glass and finish, and why each was chosen, and the price is set against that plan before any work begins. The installation carries our 10-year workmanship guarantee. For an idea of how walk-on glass compares with other rooflights on budget, our costs page sets out the factors that drive the price. For a pitched roof over the same house, see our roof window installation page, and for a raised glass centrepiece on a larger flat roof, our roof lantern installation page. Our skylight and rooflight installation in Cambridge overview shows the full range.