A flat roof has been turned into a terrace, and the kitchen or study underneath it has no daylight at all. The owners want to sit outside in the evening and they would also like to stop switching on the lights at noon. A glass panel set into the terrace answers both wishes, but it only works if it was part of the roof from the first drawing. A walk-on rooflight is not something to cut into a finished terrace on a whim.
The reason is that the panel touches everything else on the roof at once. It has a frame that must be supported, a surface that must line up with the deck, an edge that water has to be kept out of, and a position that decides where the furniture can go. The roofer, the carpenter, the paviour and the engineer each own one of those, and every one is cheaper to settle on paper than on site. What follows is written for the homeowner and for the builder or architect who has to coordinate the sequence, in the order the decisions tend to arrive.
Light through the terrace
Start with the room below, not the terrace above. The question is which part of that room is darkest and how much of the ceiling above it is free of beams, ducts and rainwater routes. A panel placed over the dark corner will do more for the room than a larger one placed where the structure happens to allow it. Walk through the room with the architect's section drawing in hand and mark where daylight from the sides runs out.
Then look at the same spot from above. A terrace is usually organised around a seating area, a route from the door and perhaps a planter or screen. A walk-on panel in the middle of the main route will be crossed every day, while one tucked beside the seating is seen more than it is stepped on. Neither is wrong, but the choice changes how the glass is specified and how often it needs cleaning, so make it deliberately.
Sunlight matters in both directions. A south-facing panel can bring strong light and warmth into the room below at midday, and solar control glass is one way to manage that; our guide to solar control rooflight glass explains the options. A north-facing panel gives softer, steadier light and rarely needs it. The glass build-up, its coating and its loading class are covered in our guide to walk-on rooflight glass.
Levels and the deck
The finished terrace surface and the top of the glass should meet at the same level, or close to it, so nobody trips on the edge. That sounds simple, but a terrace has several layers between the structural deck and the surface: insulation, the waterproof layer, a drainage or support layer, and the paving, boards or tiles on top. The thickness of that build-up varies with the system, and the panel's frame has to be specified to suit it.
This is why the surface type has to be chosen before the unit is ordered. Porcelain paving on adjustable pedestals, timber boards on battens, and a bonded surface laid directly to falls each leave the frame at a different height relative to the structure. We ask for the build-up in section, with each layer named, so the kerb or opening dimensions we send in writing match the finish that will actually be laid.
Choosing the edge detail
Where paving meets the glass frame, there is a choice. A tight joint with a slim edge trim looks neat, though it leaves little tolerance if the paving is slightly out of square. A small margin of exposed frame gives the paver room to cut cleanly. Pedestal paving is the most forgiving, because each slab can be levelled individually to the frame.
If you are considering boards, our answer on setting a walk-on rooflight into decking covers the framing and ventilation points that timber brings. Wherever the panel sits, the structural opening below needs trimming with the right members, and that is a structural engineer's decision rather than a fitter's.
Drainage and falls
Terraces over rooms are flat roofs in law and in practice, and flat roofs need falls: a gentle slope that carries water to an outlet. A walk-on panel is a flat plane of glass, so the falls have to be arranged around it. The risk is a shallow dish forming against the upslope edge of the frame, where water would sit and leaves would collect.
The usual answer is to treat the panel like any other upstand or obstruction. The falls run past it, with the outlet positioned so that the water is drawn away on both sides rather than trapped behind. On a roof with a single fall, that means looking at where the panel sits relative to the outlet. If the panel is directly upslope of the outlet, check that the water has room to part around it.
- Outlet position. Mark it on the same drawing as the panel, and mark the direction of fall with arrows.
- Overflow. A terrace with a parapet needs a way for water to escape if the main outlet blocks. Confirm where it is relative to the frame.
- Surface water on the glass. The glass itself should be flat with the surface, so water crosses it without pooling. Keep it free of leaves and grit, because a slippery film is more likely when debris sits and breaks down.
- Gutters and downpipes. Check they can take the flow, since a terrace collects water from a wider area than the roof it replaced.
None of this is specific to rooflights, which is the point. The panel should not make the drainage more complicated than the roof already is. If the falls on the drawing do not work, change the falls before ordering, because the frame cannot be adjusted once it is fixed.
Waterproofing at the frame
The waterproof layer of the terrace has to be taken up to the frame, and it has to be taken up properly. In a single-ply or liquid system, the membrane is dressed up the upstand, or the frame sits on a kerb that the roofer has already covered. On a built-up system, the layers are lapped and sealed to the maker's instructions. How this is done depends on the covering, and the covering is chosen by whoever is laying the roof.
Our approach is to agree with the roofer, before anything is bought, who forms the upstand, who dresses the membrane and who fixes the frame. In writing, with the covering named. A frame that is lowered onto a freshly dressed upstand fits cleanly. A frame that arrives after the membrane has been finished to a flat opening means cutting into a completed roof, which no one wants to do on a terrace.
The usual advice for raised kerbs does not carry over, because a walk-on panel is set to suit a level surface, so the frame is normally part of a flush or near-flush build-up that the maker details. Follow the maker's drawings for the specific model, and treat any difference between those and the roofer's usual detail as something to resolve before the deck goes on. Our guide on who builds the kerb sets out how that split is usually handled.
Finally, think about movement. A terrace flexes slightly with temperature and load, and so does a glass frame. The junction between them is where the maker's recommended sealants and tapes earn their place, and where a mismatch between products from different suppliers causes trouble later. Keep the products compatible, and keep the paperwork.
Furniture and planters
A terrace is used, and the glass will be used with it. Chairs will be dragged, planters will be moved to follow the sun, and someone will carry a barbecue across it. A walk-on panel is designed to carry foot traffic and a rated load, so the first job is to read the rating for the specific unit and match it to what you plan to put on it. Our glass and load guide explains how those ratings are described.
Heavy planters are the usual difficulty. A large container full of wet soil is far heavier than it looks, and its weight sits on a small footprint. Keep planters, water features and fixed furniture on the surrounding deck, where the structure has been designed for them, and leave the glass for walking. If anything must stand near it, ask the structural engineer about the loads in advance.
- Furniture feet. Use wide, soft feet or glides so a dragged chair does not scratch the surface.
- Hot items. Keep barbecues and fire bowls off the glass and clear of the frame's seals.
- Grip. Walk-on glass is made with an anti-slip finish; our answer on whether walk-on glass is slippery explains how that works in wet weather.
- Cleaning. Soft brushes and non-abrasive cleaners suit the surface; avoid pressure washers aimed at the edge seals.
Lighting is worth planning at the same time. If you want lamps or low-level lights near the panel, the electrician needs to run cables before the surface goes down. It is much easier to fit a sleeve in the deck than to chase a finished terrace afterwards.
Programme with the builder
A terrace build has an order, and the rooflight has to find its place in it. The structure and trimming come first, then insulation and the waterproof layer, then the frame, then the surface. The glass itself usually arrives late and is lifted into place near the end, to keep it clear of dust, scaffolding and dropped tools.
Walk-on units are made to order, so waiting for glass is the thing most likely to catch a builder out. The maker sets the lead time, our quote states it, and the order confirmation fixes it. Counting back from the day the terrace surface is due, we place the order early enough for the frame and glass to be on site when wanted, and the delivery date is passed on to you and to the site.
- Survey, then a quote in writing that sets out the panel, its glass and what the builder must supply.
- Opening and frame dimensions for the unit being fitted go to the architect or builder on paper while the deck is still a drawing.
- Structural trimming and the waterproof layer, with the upstand detail agreed with the roofer.
- Order placed against the programme.
- Frame fixed, then surface laid, then glass fitted and cleaned at the end.
- Electrician's first fix for any lighting, done before the surface; plasterer's reveals in the room below once the roof is closed.
If you want to understand how this fits with the rest of the building work, our page on working with your builder describes the paperwork we exchange. The wider options for this kind of glazing are on the walk-on rooflights page, and if budget is the next question, the cost answer explains what moves the price.
Terrace questions that come up late
- Can I add a walk-on rooflight to a terrace that is already built?
- It can be possible, but it means cutting through the surface, the waterproof layer and the structure, and re-forming each one. The structural opening needs an engineer, and the roofer needs to remake the membrane to the new frame. It is planned in the same way as a new build, just with more making good, so ask for a survey first.
- Will the room below get too warm in summer?
- A large area of glass facing the sun will bring in heat as well as light. The glass specification can include solar control, and the position can be chosen with the sun's path in mind. The right balance depends on the room, so it is part of the quote conversation.
- Does the panel need Building Regulations approval?
- The terrace and the opening in the roof are building work, and the thermal performance of the glazing is covered by the energy efficiency requirements in Part L. Notifiable work goes to Building Control through us. Your architect or builder will say whether the wider terrace works need their own application.
