Walk-on rooflight installation in SouthamptonGlass the structure waits for.

A walk-on rooflight is a sheet of structural glass built into a floor that happens to be outdoors. People step across it, put a chair on it, carry a tray over it and, in time, stop noticing it is there. Below, the same glass is a ceiling with a view of the sky. It is one of the few building elements that does two jobs in two directions, and that is why it asks for more agreement between trades than a roof window or a flat rooflight does.

The commonest homes for walk-on glass are the places where a room sits under an outdoor surface: a flat-roofed extension with a terrace on top, a patio laid over a garage, a utility room or a converted cellar, or a garden that slopes away so that the lower floor of the house has a wall of earth where a window should be. In each case there is a room that would be brighter with a hole in the ceiling and an outdoor area that would be poorer if that hole were left open.

The glass is only one piece of the package. Under it sits a supporting frame, around it sits the waterproofing, and over the frame sits whatever finish the garden or terrace has: paving slabs, porcelain, timber boards, resin or gravel. Each of those belongs to a different trade, and each is built to a size that has to match the panel.

Why the glass is ordered late and sized early

The structure around a walk-on panel has to be right first, because a made-to-measure sheet of laminated glass cannot be trimmed on site. If the opening is a few millimetres out, it is the glass that waits while a new one is made. We work to a fixed sequence. The builder or engineer receives the opening size and the bearing detail as a written sheet. The structure is formed and checked. Only then does the glass order go in, against the date the terrace or patio will be ready.

That ordering is what stops a half-finished terrace sitting under tarpaulin for weeks. The paviours, the decking carpenter or the roofer can all lay the surface up to the opening and move on, with the panel arriving as the last piece of the floor.

Bare glass is smooth, and a smooth surface wet with rain is slippery. So the walking face of a panel is treated, and the treatment is both a safety decision and a design one. The most familiar finish is a pattern of small ceramic dots fired into the glass, which gives grip and is easy to recognise. Others are an etched or sandblasted face, a fine textured coating, or a printed pattern. They look different from above and they feel different underfoot, and each has its own habits when it gets wet and when it gets dirty.

Maker data for slip resistance is commonly reported from a pendulum test, which measures how a rubber slider grips a wet surface. It is worth asking for the result for the exact finish being quoted, rather than for the product range in general, and we pass that information on in the quote. For a surface by a pool, a spa or a north-facing yard where moss grows, the question has more weight than on a sunny terrace. The answer page on whether walk-on glass is slippery covers the finishes side by side.

A walk-on rooflight sits in the one place on a terrace where water must not be allowed to stay. Flat glass in a flat floor can collect a thin film, and a thin film can find a way past a poorly dressed edge. This is a drainage question, settled by the fall of the surface and the detail at the edge.

The surface around the glass should fall away from it, so rain runs off across the paving rather than towards the frame. On a pedestal terrace this is usually built into the pedestals. On a patio laid on a bed it is built into the bed. On timber it is in the joists and the gaps between boards. Where the fall cannot go away from the glass on every side, a channel or a drain outlet takes the water instead.

Under the paving, the waterproofing layer is the real barrier. The roofer or waterproofing contractor dresses the membrane up to the frame or the upstand that holds it, and that junction is the detail we agree with them before anyone lays a slab. For a unit with a raised upstand the membrane goes up the outside of it; for a flush frame it is lapped and bonded to the frame's flange to the maker's instructions.

Water that gets through the paving

On a pedestal terrace, rain passes through the joints and runs along the membrane below. That water needs somewhere to go as well, and the glass must not be sitting in its path. The roofer's fall design should carry it past the panel to an outlet, with a gap or a channel at the glass edge for it to run through.

On the underside of the glass, condensation can form on cold winter mornings, particularly in a room with a lot of moisture, such as a kitchen or a utility space. A good insulated unit reduces that, and a ventilated room below lets damp air leave. No glazing stays completely dry in every condition, but the build-up and the room's ventilation work together to limit it.

Maintenance is mainly keeping the edges clear. Leaves, grit and blown soil gather in the joint around a panel, and a quick brush each season keeps water running as it was built to.

On a typical job the builder forms the opening in the structure: a slab, joists or a steel and concrete deck, with the engineer's bearing detail. The structure is checked for size, squareness and level. If the room below is an existing one, its ceiling opening is formed at the same time and the temporary cover goes on.

The roofer then dresses the waterproofing up to the frame or upstand and, where the build includes insulation, the insulation is cut in tight. The frame is set and levelled, and the paviour or carpenter lays the surface up to it. The glass comes last. It is lifted into place, fixed to the maker's instructions and sealed, and the joint with the finish is completed.