How Thick Is Walk-On Rooflight Glass?

A walk-on rooflight is made from several sheets of glass bonded together with clear interlayers. The sheets are toughened or heat-strengthened, which makes each one stronger than ordinary glass and changes how it breaks.

The reason for stacking is safety. If the top sheet is scratched or chipped, the sheets below still carry the load. If one sheet fails, the interlayer holds the pieces in place, so the glass does not drop away underfoot.

• Top layer. Takes wear and carries the anti-slip finish. It is often described as the sacrificial or wearing layer.
• Layers beneath. Carry the design load across the span; the calculation often leaves the top layer out.
• Bottom layer. Faces the room and can be specified for clarity, colour or privacy.
• Interlayers. Bond the sheets and hold the glass together after a failure.

The stack is why quoting a thickness for walk-on glass in a single figure misses the point. Two units with the same overall depth can have different layers and different ratings.

Laminating also changes how the glass behaves in use. A single toughened sheet that fails breaks into small granules and can drop out. A laminated stack keeps its shape and stays in the frame, so the floor remains usable until the unit is replaced.

Several things drive the build-up, and the maker takes each into account.

Span is the clear distance the glass bridges between supports. Glass flexes more as the span grows, so a wide pane needs a stiffer stack than a narrow one. Load rating is the next factor: a unit meant for a terrace with furniture needs more margin than one over a rarely used loft hatch. The load ratings guide explains how those ratings are stated.

Heat loss comes next. A unit over a heated room must also insulate to the Part L figure for its type in an existing home (2.2 W/m²K at most where the unit counts as a rooflight; where it counts as a roof window, 1.4 W/m²K unless it carries a Band B energy rating), so it is built as a sealed unit with a gap and extra glazing. The walking layers sit on top and the insulated unit sits below, which adds depth.

Edge support matters as well. Glass held on all four sides spans further than glass held on two, because the load is shared round the perimeter. The maker is told which edges are supported, which is another reason the survey records the structure under and around the opening.

The overall depth of a walk-on unit decides several details that are settled at the survey.

• The recess. The frame and glass need a recess deep enough to take the whole stack and still sit at the right level.
• The finished level. For a unit that sits flush with a floor or paving, the glass depth sets the build-up of the surfaces around it.
• The support. A deeper, heavier unit puts more weight on its bearing, and the structure beneath has to carry it.
• The view from below. Edge lines of the layers are visible from beneath, which is why the trim and the lining are specified together.

These are measured on the roof or floor itself. A figure taken from a catalogue leaves the recess and finished level unconfirmed, which is the sort of gap that is easy to miss and expensive to correct once the glass has arrived.

The glass maker does not choose a build-up by eye. They calculate the stress and deflection of the stack for the span, the support conditions and the load, then check the result against the rating. The calculation gives the layer thicknesses, the interlayer type and the edge detail.

A unit that looks like an earlier one does not simply copy its build-up. A slightly longer span on the next job can call for a different stack, which is why every order is worked from its own measured survey sheet.

We pass the measured opening, the way the glass is supported and the intended use to the maker, and the figure comes back on the order. The measured survey sheet records the span and the use, so you can see what the build-up was designed for.

A heavier stack costs more, weighs more and needs more support. It can also look different from below, because thick glass shows a green or grey cast at the edges and cuts light a little. Low-iron glass reduces that cast, and some makers offer it as an option. Going well beyond the rated need adds weight without adding anything you will use.

The aim is a stack matched to the span and use, with a margin set by the rating. The way that figure is agreed with the maker is set out under walk-on rooflight installation, and the answer on daily use covers what the glass then faces in service.