There is no single number, because the glass is designed to the span rather than the other way round. What limits it in practice is that the build up gets thicker and heavier as the span grows, until the panel is too heavy to handle, too deep for the frame or simply uneconomic. Beyond that point, the answer is two panels rather than one.
Why thickness rises with span
Because deflection grows faster than length.
A longer panel bends more under the same load, and since deflection is what governs the design, the build up has to get considerably deeper to keep it within limits.
Why deflection rather than strength
Because a floor that moves feels unsafe.
A panel is rarely near breaking under ordinary use, and the design limit is how much it may move underfoot, which is a far tighter constraint.
The figures are on how much weight can a walk on rooflight take.
Support on two edges
The weaker arrangement by a wide margin.
A panel bearing on two opposite sides spans in one direction only, so all the load has to be carried across that single span.
Support on four edges
Much stiffer, for the same glass. A panel held all round spans in both directions, and the load finds the shorter path, so the same build up covers a considerably larger area.
Why the short dimension matters most
Because that is the span it will use.
On a four edge supported panel, the shorter dimension does most of the work, so a long narrow panel can be surprisingly capable.
What a long narrow panel gives you
A practical strategy on a terrace. A strip of glass running across a terrace, supported on its long edges, gives a good deal of light with a modest span and a sensible build up.
The practical ceiling on a single panel
Weight, before anything else. Beyond a certain size the panel needs a crane and specialist handling, and that turns a straightforward installation into an operation.
What that means is on how long does a walk on rooflight take to fit.
The frame depth
The other practical ceiling. A very deep build up needs a deep frame with proper bearing, and that has to be accommodated within the terrace build up without losing the upstand height.
Cost as the real limit
Rising faster than the area. Because thickness increases with span, the cost of a large single panel grows disproportionately, and there is a point where two panels are simply cheaper.
Where that sits is on how much does a walk on rooflight cost.
Two panels with a member between
The usual answer on a large opening.
A structural member across the middle halves the span, which reduces the build up dramatically and keeps each panel handleable.
What that member looks like
A line rather than a beam, ideally.
A slim steel or a structural mullion between two panels reads as a joint in the terrace surface rather than as an obstruction.
Whether the join is a problem
Not visually, and it is a detail.
The junction between two panels is a weathering and a drainage detail, and it is designed rather than left to a bead of sealant.
Why that matters is on do walk on rooflights leak.
Whether a member costs light
Very little. A slim line across an opening removes a small fraction of the glazed area, and the difference in the room below is imperceptible.
The insulated version
Reduces the span for a given depth.
Where the panel is also a sealed unit, part of the build up depth is the cavity and the inner pane, so the walk on laminate has less depth to work with.
What the manufacturer needs
The two dimensions and the support. Clear span in each direction, whether it is two or four edge supported, and the imposed loading, produce a specification rather than a discussion.
Standard sizes
Where the sensible spans already are. Manufacturers hold standard walk on panels at sizes that work economically, and designing the opening to one of those is worth doing where the terrace allows.
Designing the opening to the glass
The cheapest approach on a new terrace.
Where the structure is being built, setting the opening to a standard panel size rather than the reverse saves a great deal.
On a retro fit
The existing structure sets the span. An opening between existing joists or beams has a span you did not choose, and the build up follows from it.
That case is on can a walk on rooflight be retrofitted.
Where the answer is a different product
A very large area. Beyond a certain size, a glazed floor system with a designed grid of members and multiple panels is the right answer rather than a single rooflight.
The comparison is on is a glass floor the same as a walk on rooflight.
Panels of unequal size
Possible, and usually worth avoiding. Two different sizes means two different build ups and two different thicknesses to reconcile at the joint, where equal panels share one specification.
Setting out from the room below
Worth doing before the structure is fixed.
Where a member falls across the opening is visible from the room beneath as a line across the light, and centring it deliberately looks far better than letting it land where the joists suggested.
The joint and the drainage
Designed as a channel rather than a seam.
The line between two panels collects water from both, so it is drained rather than merely sealed, and it discharges into the perimeter channel.
What we propose on walk on glass
We propose two panels with a member between wherever a single one would need an unusual build up, because the light is the same and everything else about the job is easier.
The whole picture is on the walk on rooflight installation page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.