Walk-on rooflights for roof terraces and balconies

Roof terraces and balconies tend to sit above the rooms with the least daylight. A flat roofed kitchen extension with a deck over it, a bedroom balcony above a deep ground floor plan, a small terrace over a utility room: in each case the room below has a ceiling and nothing else between it and the sky. A walk-on panel lets daylight drop through the floor of the terrace into that room.

It also changes how the terrace works. Where a conventional flat rooflight needs its own kerb and a keep-off zone around it, a walk-on panel can be set flush with the deck boards or pavers, so the terrace stays one continuous surface. That matters on a small terrace, where a raised kerb in the middle would use up the usable space and catch your toes.

The trade-off is thermal and structural. A walk-on unit is thicker and heavier than a standard flat rooflight, and its edges have to be built into the terrace make-up. That is why it is a job for a survey first. If nobody needs to stand on the glass, a flat rooflight or roof lantern will insulate better and cost less to build, and our walk-on rooflight installation page says when the simpler option is the better one.

The position of the glass changes almost everything else, so decide it early. There are four common places, and each has a different set of consequences.

Central, as a feature

A panel in the middle of the terrace is seen and walked across all the time. It brings the most daylight into the room below, and it is the position where the surface finish and the look from underneath matter most.

Against the house wall

Here the panel runs along the wall, often beside a door. It keeps the open part of the terrace clear for furniture, and the wall gives a clean edge to build the upstand and flashing against. The shadow of the house falls across it for much of the day, so a north-facing wall puts it in damp shade, which affects how quickly algae form on the glass.

Near the threshold

A panel just outside a glazed door sees the heaviest footfall, including wet feet and wet shoes, in and out. The grip of the glass counts for more here than anywhere else on the terrace.

Beside the balustrade

A panel set back from the guarding keeps the edge of the terrace solid. Glass right against a drop is possible, but guarding and loading near an edge are reasons to keep a margin.

Whichever you choose, keep the panel clear of any point where water naturally collects and clear of rainwater outlets. The terrace falls, usually a shallow gradient towards an outlet, and the glass needs to sit within that fall without creating a pond on its upstream edge.

A flat roof that was built only to keep weather out may not be a floor. Its joists were sized for a roof covering and snow, and a terrace asks for more: people, furniture, planters, and the standing weight of a wet deck. So the first question at survey is what is actually under the surface.

In timber flat roofs the joists run in one direction, and a walk-on panel interrupts them. The joists either side of the opening are trimmed and doubled, and the cut ends are carried on new members so that the load goes round the opening and not through it. In concrete or steel decks the opening is formed with its own support. The panel's frame then sits on the structure, not on the waterproofing, which should never be asked to carry weight.

The glass is supported around its edge or at set points, and that support detail is as important as the thickness of the glass. A panel resting on a poorly bedded or uneven edge can feel lively underfoot, and any movement works at the sealant over time. On an existing terrace, we look at the deck from below as well as above, because the condition of the joists, any old water damage and the depth available for the build-up are all easier to read from the ceiling side. Where there is doubt, a structural engineer's calculation is the sensible route, and it belongs in the quote.

The panel is specified to structural loading guidance for glazed floors and walkways, not to the standards used for an ordinary rooflight. For a terrace used by a household, the loading category and the likelihood of furniture, parties or planters go into that specification. Our guide to walk-on rooflight glass specification goes through the layers of the glass itself, and the answer on how much weight a walk-on rooflight can take explains why the figure comes from the design and not from a headline number.

The glass is the easy part to admire. The waterproofing around it is what keeps the room dry. A terrace panel sits in a roof, so the roof covering has to be dressed up to the frame and finished so that water running across the deck is turned away from the joint.

The build-up usually runs in this order from the structure up: the deck, a vapour control layer, insulation, the waterproof membrane, and then the walking surface of boards, tiles or pavers on supports. The walk-on frame sits into that sequence. The membrane is lapped up to a small upstand at the frame edge, a flashing or trim covers the top of that upstand, and the surface of the terrace finishes close to the glass without covering the joint.

Three details are worth asking about. The first is the height of the upstand, which should be enough to keep standing water away from the frame even when the terrace outlet is partly blocked with leaves. The second is the gap between the glass and the surrounding paving, which needs to be wide enough to clear debris and narrow enough that a heel or a chair leg does not catch. The third is how the deck covering stops short of the frame so the glass can be lifted out, or its sacrificial top sheet replaced, without breaking up the terrace.

Where the terrace is a warm roof, the insulation sits above the structure and the panel's thermal performance has to be matched to it. Where the terrace is over a room that has a cold void beneath, the dew point moves, and condensation risk goes up. Both are reasons to settle the whole roof build with the glass, not the glass alone.

This is where terraces differ most from ordinary rooflights. For a rooflight in a house roof, permitted development is granted by Class C (Part 1, Schedule 2, General Permitted Development Order) on two conditions: a projection of 150mm or less from the roof plane, and nothing above the roof's highest point. A flush walk-on panel is well within that.

The terrace itself is another matter. Householder permitted development does not cover a raised platform or balcony of the kind you would stand on, and a roof terrace or balcony usually needs planning permission, mainly because of overlooking of neighbours. Householder permitted development rights do not exist for flats and maisonettes. The sensible step is to ask the planning authority before spending anything on design. That may be Exeter City Council, East Devon District Council, Teignbridge District Council or another authority depending on the address, and a conservation area, an Article 4 direction or a listed building changes the position again. On a listed building, listed building consent is needed for any exterior change.

Building Regulations apply in addition. Cutting a new opening in a roof or floor structure is building work, and a new walk-on panel has to meet the thermal requirements of Approved Document L. Guarding at the terrace edge is covered by Part K, and the rules on fire spread matter where the terrace is near a boundary. Where the work is notifiable, the Building Control notice comes from us. Our answer on whether a walk-on rooflight needs Building Control sets out when it applies.

A terrace is outdoors, so the glass is rained on, shaded, leafed and walked on in wet shoes. Plain smooth glass is slippery when wet, which is why walk-on panels for terraces are made with a treated top surface. The usual treatments are a ceramic pattern fired into the glass, an acid etched finish, or a textured print, and they trade grip against cleaning and light. We cover the choices in anti-slip glass for walk-on rooflights.

On a terrace, grip is worth more than a few extra percent of light. Shaded positions grow a thin green film that can make any surface treacherous, so a panel on the north side of a house wall needs both a gripping finish and a routine of cleaning. A soft brush and plain water, or a mild cleaner of the kind suggested by the glass supplier, does the job.

Think about what is on the terrace as well. Planters leave rings of damp, furniture legs concentrate load on small areas, and barbecues drop heat and grease. Say at the start if the terrace will be used for dining, because it changes the loading category and the choice of surface. Our answer on putting furniture on a walk-on rooflight goes into the practicalities.

Standing on glass looks different from underneath. From the room below, a clear panel shows soles, shadows and the undersides of furniture, and anyone sitting under it gets a view of whoever is on the terrace.

There are practical answers. A ceramic pattern or etched finish partly obscures what can be seen through the glass without stopping daylight. Interlayers can be tinted or translucent, giving a softly diffused panel that shows movement but not detail. Where the room below is a bedroom or a bathroom, a fully frosted or fritted panel is normal. Choose this by standing under a sample, not by reading a description, because the same pattern looks very different seen from underneath in daylight and at night with the room lit.

The underside of a terrace panel is a hole in a ceiling that you will see every time you look up in the room below. In a flat roof the depth from the glass to the ceiling is the depth of the joists plus insulation, often more than a rooflight in a sloping roof, so the lining is a short shaft. It is made good in one of three ways. It can run straight, square to the glass, which suits a narrow opening in a shallow build. It can splay outwards towards the ceiling, so the opening looks larger and the plaster face catches the daylight that comes through the panel. Or it can be boxed in with a trimmed return where the ceiling is lath and plaster, in an older house, and the edge needs to be built up in lime to match.

The glass here is lit from above and often seen by a person looking up at it, so the plaster edge is under close inspection. Beads, trims and the paint line are set before the room is decorated, and the ceiling around the opening is blended into the rest. Our guide to how a rooflight is finished inside goes through each layer in order.

All of this goes into the written quote: the room, the ceiling and the roof build, how the lining is formed, and the state of the surface when we hand it over, bare for your decorator or already painted. On a terrace job it also records where the panel sits and how the surface is treated, so the choices you made at survey are still on paper when the glass arrives.

A short list keeps the survey focused. Take it with you when you speak to anyone about a glass terrace.

• What is the deck made of, and who has checked it for the load of people and furniture?
• Where does the panel sit, and how does the terrace fall around it?
• What is the upstand height, and how can the glass or its top sheet be replaced later?
• What surface treatment does the top sheet have, and have you stood on a sample wet?
• How will the room below be seen from the terrace, and the terrace from the room?
• Does the terrace itself need planning permission, and has the local planning authority been asked?

If those answers are clear, the rest is straightforward. The structure, the glass, the waterproofing and the inside finish are all part of one specification, and they are easier to get right when one team writes it. Everything about the service, from the survey to the guarantee, is on our walk-on rooflight installation page. When you are ready, ask for a quote and a finish schedule for your own terrace.