Walk-on rooflight sizes and supports

Glass bends under load, and the longer the unsupported span, the more it bends. For a pane you only look through, a little movement does no harm. For a pane you stand on, the movement has to stay small enough that it feels solid underfoot, never rocks on its seating and never stresses the edge seal. So the maker limits the span for each glass build-up, and the build-up gets thicker as the span grows.

Four things work together to set the largest pane you can have.

The load

A rooflight in an upper floor or a landing is designed for the loads a floor carries, which is people, a few pieces of furniture and the occasional dropped weight. One on a flat roof that is only used for maintenance access is designed for less. One on a roof terrace or balcony is designed for more, because people gather there. The brief you give us changes the size before anything else does.

The glass build-up

Walk-on glass is made of several layers, with toughened or heat-strengthened glass laminated together, and a wearing surface on top that is treated to grip. A thicker build-up spans further, but it also weighs more and costs more, and the extra thickness can make the unit deeper than the floor or roof it has to fit into.

How many edges are supported

A pane resting on all four edges can be larger than one resting on two. Most domestic units are supported all round, which is also the better arrangement for keeping the weathering seals under even pressure.

The structure around it

The opening has to be framed with members stiff enough that they do not flex before the glass does. Glass will not forgive a flexing support, so the frame often sets the limit rather than the pane.

Walk-on glazing is specified against structural loading guidance, with British Standard and trade body documents behind it, and the maker supplies the calculation for the unit they have sized. That calculation, not a rule of thumb, is what we work from.

Most of what we fit in houses falls into a handful of sizes, and it helps to think of them by what the opening is doing.

A small opening over a stair or landing is the easiest to engineer and the least disruptive to the ceiling below. Medium and large openings are where the structure becomes the project. Once the opening is wide enough that a single pane would need glass of very great thickness, it is usually split into two or more panes with a supported joint between them, rather than made as one heavy sheet. A split also helps when the glass has to be carried through a doorway or up a stair, which matters more than people expect on an old Devon townhouse with a narrow passage.

Because every manufacturer publishes its own limits, we do not quote a maximum size here. The largest pane possible for your opening depends on the unit chosen, the load and the supporting frame, and you will find it written on the quote and the maker's drawings.

There are three common ways to carry the glass, and the right one depends on whether the unit goes into a floor, a flat roof or a garden.

Steel frame

A fabricated steel frame, usually galvanised or powder-coated, sits in the opening and the glass rests on it all round. It is stiff, it can be made to almost any size, and it gives the glass a flat, level seating. This is the usual choice for larger openings and for roof terraces, and it often arrives with its own upstand so the roof membrane can be dressed up to it.

Masonry or concrete upstand

In a garden, a basement lightwell or a new-build with a concrete roof slab, the glass can sit on a formed masonry or concrete upstand with a steel angle or aluminium frame on top. The weight goes straight into the wall or slab, which suits heavier panes. The upstand has to be waterproofed and level, and the glass supplier will want the finished dimensions confirmed on site before the pane is made.

Timber trimmed opening

In a timber floor, such as a landing or the floor of an upper room, the opening can be trimmed in joists with the glass unit sitting in a frame fixed to them. It is the most modest arrangement and suits smaller openings. The trimmers have to be sized for the extra load, and the floor has to be checked to see that the joists beside the opening are sound and properly bearing.

Each has a different thickness, and thickness matters when you are working inside a floor or roof build. A glass unit with its frame and upstand can be much deeper than a standard roof window, which affects the kerb height, the insulation and the way the lining is formed underneath. Our guide to walk-on rooflights for roof terraces and balconies covers how the drainage and falls work when the glass sits in a walking surface.

A walk-on rooflight takes more from the structure than a roof window does, because the glass is part of the floor or deck rather than a window sitting on a slope. Cutting a hole and framing it correctly is the part that has to be right before the glass arrives.

In a timber floor or flat roof, joists crossing the opening are cut and the cut ends are carried on doubled trimmer joists either side, or on a new header across the opening. Those trimmers pick up the load from the joists that have been cut, so they are usually thicker or deeper than the joists around them, and they are connected with joist hangers rather than nailed. The size of each member comes from a calculation, and in many homes that calculation is done by a structural engineer.

In a masonry or concrete slab, the opening is cored or cut and a steel lintel or edge angle may be added. A slab is very stiff, but it carries reinforcement that has to be understood before it is cut, and the edges need to be sealed to keep water out of them.

Because this is a structural alteration, it is building work and goes through Building Control. We handle that notification as part of the job, and a drawing and calculation are usually part of what goes with it. For the fuller picture, see whether a walk-on rooflight needs Building Control.

Glass that is flat on its seating feels solid underfoot and keeps its edge seals working. Glass that rocks, even slightly, wears them.

Walk-on units sit on a gasket or setting block, and the gasket has to be continuous and evenly compressed. A frame that is out of level by more than the maker allows will put point loads on the glass edge, which is how stress cracks start. So the frame is levelled and checked before the glass goes in, and shims or packers are used as the supplier specifies rather than whatever is to hand.

Movement matters too. Steel expands and contracts, timber moves with moisture, and the roof deck beneath changes a little under load. Good details leave a small, controlled gap that a flexible seal fills, so the glass is never wedged tight between two things that move at different rates. A hard filler in that gap is the quickest way to a cracked pane.

Upstands and drainage are the last part of this. A rooflight set in a walking surface needs the surface to fall away from it, so water runs off instead of lying against the glass edge. Where a unit is let into a terrace, the waterproofing is dressed up the frame and under the cover trim, and the layers are lapped so water follows the fall out and away.

A walk-on rooflight is the only kind that you see from both sides in daily use. People look down through it from above, and look up through it from below, so the underside matters as much as the top.

The frame depth, the glass edge and the joists around the opening all show from below unless they are covered. There are two ways to deal with that. A boxed lining, formed in plasterboard and skimmed, hides the structure and gives a clean reveal that meets the ceiling at a crisp edge. Or the steel is left exposed and painted, which can look right in a converted barn or a contemporary kitchen extension but needs the welds, bolts and edges to be tidy.

Light shapes the choice. A walk-on pane is made of several layers of glass, so it can look slightly greener or cooler than a single sheet. A white or pale reveal throws that back into the room and keeps the glass from looking like a dark hole. A splayed reveal is worth considering where the structure is deep enough, as it spreads the light further than a straight box.

Under a ceiling that is old, the join where new lining meets existing plaster is the delicate part. On a lath and plaster ceiling in a Georgian terrace in St Leonards or Southernhay, or a lime-plastered cottage ceiling in the Exe valley, the opening is supported and cut cleanly and the new lining is built up in lime where the ceiling is lime. Our guide to how a rooflight is finished inside sets out those choices.

With a walk-on rooflight there is more to agree before the work starts, because the structure, the glass and the room below all interact. So the finish schedule supplied with each Brightview quote sets down the following for yours.

• the room above and the room below, and the floor or roof build between them;
• the opening size, the number of panes, and the way each edge is supported;
• the frame and any exposed steel or upstand, and how it is finished;
• how the underside is formed, straight or splayed, and what it is finished in;
• the wearing surface on the glass, and any anti-slip treatment;
• how dust is managed and how the room below is protected during the work.

Writing it down prevents the common surprise, where the glass is exactly what was ordered but the underside was never discussed, and you are left looking up at raw timber or an unfinished steel box.

A few questions settle most size decisions, and they are worth asking of any installer.

1. What is the glass unit built for: occasional foot traffic, regular use, or a terrace?
2. Who has calculated the frame and trimmers, and can I see it?
3. How are the edges of each pane supported, and what happens at any joint between panes?
4. How deep is the whole unit, and what does that do to my ceiling or roof build?
5. How will the glass get into the building, and does it need a crane or an extra pair of hands?
6. What will I see from the room below, and who finishes it?

If you would like those answered for your own house, ask for a quote and we will survey the floor or roof, measure the opening and send a written quote with a finish schedule. For the load side of the question, see how much weight a walk-on rooflight can take, and the whole subject is gathered on our walk-on rooflights page.