Walk-on rooflight installation in Exeter

A standard rooflight is designed to keep weather out and light in, and to survive nothing heavier than snow, wind and the occasional maintenance visit. Walk-on glass is designed as a floor. It must carry a person, a few people, and in some cases a chair or a planter, without cracking and without deflecting enough to feel alarming underfoot.

To do that the glass is built up in layers. A typical walk-on panel is laminated: several sheets of toughened or heat strengthened glass bonded together with clear interlayers. If one sheet cracks, the others and the interlayers hold the panel in place while it is replaced. The top sheet is often treated as the wearing surface, because it takes scuffs, grit and the occasional dropped tool. Where the panel is also the weather skin of the roof, a sealed insulating unit sits beneath that laminated stack, which is what gives the whole assembly its heat performance.

The frame is different too. A rooflight fixed to a kerb is mostly resisting wind lift and holding the seals together. A walk-on frame has to pass every footstep down into a structure that can take it, and it has to support the glass around its whole edge or at carefully designed points. That support detail is as important as the glass thickness, and it is why we survey the structure before we ever talk about a glass price.

Our guide to walk-on rooflight glass specification goes through the layers one by one, and how much weight a walk-on rooflight can take explains why the answer is set by the design and not by a single number.

Every footstep on walk-on glass travels down through the panel, into the frame, and from the frame into the building. If any link is weak, the glass will not fail loudly, but you will feel it move, hear it creak or see a seal open up. So the first job at survey is to understand the structure, and only then to choose a panel.

There are three common support arrangements.

Kerb and upstand

On a flat roof, the panel often sits in a frame fixed to an insulated upstand, much like a flat rooflight. The upstand is built wide and stiff enough to carry load as well as weather, and the roof covering is dressed up its sides. The glass finishes a little above the walking surface or flush with a terrace deck, depending on the detail.

Flush frame in a deck

On a terrace, the frame can sit level with the decking or paving, so the glass reads as part of the floor. This looks clean, but drainage becomes the challenge: water must run off the glass and away from the frame without lying at an edge. The falls in the deck are set to suit.

Supported glass in a floor

For an internal glass floor, or for a panel set into a solid floor, the glass is carried on steel or timber trimmers with a continuous seat around the edge. The load goes into the trimmers, and the trimmers are sized by calculation.

Where the opening is cut into an existing roof or floor, the joists are trimmed around it, and that is structural work. In a Georgian house the timbers may be old, irregular or not quite where the drawings say, and the ceiling below may be lath and plaster that must be protected while the opening is made. On a 1930s flat roof extension the deck may be thin plywood on joists that were never meant to carry people. We check each of these in advance. Our guide to walk-on rooflight sizes and supports sets out how the span, the thickness and the supports relate.

A structural engineer is usually involved in a new opening, because the Building Control officer will want to see how the cut timbers are carried. If calculations are needed, the written quote says so before any work starts, and the engineer's details form part of what we build to.

Walk-on glass is available in sizes from a small square, the size of a paving slab, to large single panels that cover a stretch of terrace. The larger the panel, the thicker it must be, the heavier it is to handle and the more carefully the supports have to be designed. At some size it becomes better to divide the opening into two or three panels with a supported bar between them.

Three things set the practical limit. The first is the span, the distance between the supports, because glass deflects more as the span grows and thickness rises quickly to compensate. The second is the support method: a panel supported on all four edges can be larger than one supported on two. The third is the access, because a big heavy panel has to get from the van to the roof, and on a Georgian terrace with no side passage that may mean a crane or a careful carry through the house.

The shape also counts. Rectangles are the simplest. Squares are stiff and economical. Circles and unusual shapes are possible but need bespoke glass and bespoke framing, which adds time and cost. If you are planning an opening in an existing structure, it often makes sense to choose a size that sits between the existing joists, so that the trimming is minimal and the load is carried where the structure is already strong.

We decide the size with you at survey, looking at where the light is needed, how the ceiling below is shaped and where the existing timbers run. The guide on walk-on rooflight sizes and supports gives more detail on the relationship between span, thickness and framing.

Planning and Building Regulations ask different questions. A walk-on rooflight can need one, both or neither, depending on where it is and what it is used for.

For planning, rooflights on a house are usually permitted development under Class C of Part 1 of Schedule 2 to the General Permitted Development Order, provided they project no more than 150mm beyond the plane of the roof and sit no higher than its highest part. A flat, flush panel often meets that test comfortably. Side-facing rooflights must be obscure glazed and non-opening unless they are more than 1.7m above the floor, which rarely applies to a panel in a flat surface.

The use of the glass is a separate question. If the panel forms part of a roof terrace or balcony, the terrace itself is generally not covered by householder permitted development, and planning permission is usually needed for a raised platform or roof terrace. The usual considerations are privacy for neighbours and how the structure looks from the street. Check with the local planning authority before you commit to a design, and consider a Lawful Development Certificate if you want the position confirmed in writing.

Some homes have no permitted development rights to rely on. Flats and maisonettes do not have householder permitted development. A listed building needs listed building consent for external changes, and a conservation area may have an Article 4 direction that removes rooflight rights. Parts of our area fall within Dartmoor and Exmoor National Parks, where some rights are restricted. The authorities concerned are Exeter City Council, East Devon, Teignbridge and Mid Devon district councils, depending on where you live, along with the national park authorities. Our guide to rooflight planning and Building Regulations in Devon covers the wider picture.

For Building Regulations, cutting a new opening in an existing roof or floor means cutting and trimming joists, which is a structural alteration, so it needs Building Control approval. The new glazing must also meet Part L for heat loss where it forms part of the thermal envelope. Replacing a whole rooflight, frame and glass together, is notifiable too, because a rooflight is a controlled fitting. Replacing only a sealed glass unit in an existing frame is not notifiable. Part B can matter near boundaries and over escape routes, and Part K applies the principle of safe glazing where people can fall, collide or stand. We handle the Building Control notification wherever the work is notifiable, and our answer to does a walk-on rooflight need Building Control goes through the cases.

We price every walk-on rooflight from a survey, because two panels of the same size can cost very different amounts on different structures. The written quote breaks the job down line by line, and the costs page shows how a typical rooflight quote is built up.

The glass is the obvious starting point. Its size, the number and thickness of its layers, the anti-slip finish, any obscuring interlayer, solar control coatings and the insulating unit beneath all move the price. Glass of this kind is a made-to-order structural product, and a larger panel costs disproportionately more than a small one.

The structure is the second driver. A new opening in an existing roof or floor needs trimmed joists, perhaps a steel beam, and an engineer's calculations. If the deck or covering is tired, the area around the new frame may need renewing so that the weathering is sound. Access counts as well: a large panel to a rear roof with no side passage may need a crane or a scaffold.

The finish inside is the third. A straight reveal costs less than a splayed one, lime plaster takes more time than gypsum, and decoration is a separate line if you want the reveal painted before we leave. Building Control fees and any lighting channel are on the quote as well.

Because the finish schedule records the reveal, the plaster and the decoration in writing, you can compare our quote with another line by line. The 10-year workmanship guarantee on our guarantee page covers our work on the roof and in the room alike.

Living with walk-on glass is straightforward once the specification is right. Two of our guides cover the practical side: can you put furniture on a walk-on rooflight, and is a walk-on rooflight safe for children. The questions below come up most often at the quote stage.

It is a one-page written note for your room.

It can be if it is left smooth, which is why a terrace panel is given an anti-slip surface. The finish may be a fired-on pattern, an etch or a textured print, and the right one depends on how much grip you need and how much light you want to keep. We note the finish on the quote so it is agreed before the glass is made.