Structural glass is glass that carries a load rather than simply filling an opening. In a frameless roof, wall or floor there is no frame to act as the beam, so the glass itself becomes the structure: it spans between supports, resists wind and floor loads, and holds the weather out through joints made of structural silicone instead of aluminium.
That changes the questions a project has to answer. The engineering is no longer about whether the glass is strong enough — almost always it is — but about how far it moves, how it is restrained at its edges, what happens when a pane breaks, and how the joints between panes are sealed and drained. Each of those is decided long before anything is ordered.
The limit is usually deflection, not strength
Glass is a brittle material with high compressive strength and useful flexural strength, so a roof pane that carries the design load comfortably is normal. What governs the design is how much that pane bends under load. A glass roof that flexes visibly reads as unsafe to everyone standing underneath it, and excessive movement works the seals and the fixings until they fail.
We therefore calculate deflection first. Glass roof members are generally limited to span/200 or 20mm, whichever is less, under service load. Walk-on floors are held tighter, at span/300, because a floor is felt as well as seen. Where those limits cannot be met in glass alone, the answer is a glass beam or fin, a steel primary structure, or a shorter span — decided at concept stage, not on site.
Build-ups, redundancy and heat-soak testing
Structural glass is laminated, so a breakage stays in the opening rather than falling. Roofs typically use heat-soak tested laminated toughened glass, with glass beams from 10+10mm to 12+12mm depending on span. Heat-soak testing removes the small proportion of toughened panes that would otherwise fail spontaneously through nickel sulphide inclusion — a risk that matters most overhead, where a break would drop glass onto people below.
Walk-on floors go further: three-ply laminated toughened assemblies from around 33.7mm to 44.4mm, designed so the floor keeps its capacity after one ply has failed. Slip resistance is verified by pendulum test to BS 7976, with the class chosen for the environment rather than assumed from a catalogue. Low-iron glass is used where the green edge tint would spoil the detail, and solar control or acoustic interlayers are built into the make-up where the room behind needs them.
Standards the work is verified against
Structural glass in the UK is not designed by opinion. The following are the standards that govern the assemblies we deliver, and each appears in the drawings and calculations issued for a project.
- BS EN 1991-1-4
- Wind actions — the load case every external glass wall and roof is designed against, with site exposure, altitude and height factors applied.
- BS 6180
- Barriers in and about buildings — the line load and impact criteria for glass balustrades, screens and walls acting as protection.
- BS 7976
- Pendulum test for slip resistance, used to select and verify the surface on walk-on glass floors and external walkovers.
- CWCT TN92 / ACR[M]001
- Non-fragility and safe access on glass roofs where maintenance access is required.
- BS EN 12150 / BS EN ISO 12543
- Heat-soaked thermally toughened glass and laminated safety glass, the two product standards behind every pane we specify.
One team, from the first drawing to the final seal
Frameless work fails at the interfaces. The head, base and abutment details, the fall of every roof pane, the drainage path and the route of every cable have to be fixed before the foundations are poured, because there is no frame to absorb a late decision. That is why we design and engineer in house, manufacture with named partners to our drawings, and install with our own teams across London and the Home Counties.
If you are specifying a glass roof, a frameless extension, a walk-on floor or a structural façade, bring us in at concept stage. Send the drawing and we will tell you what the glass can do, what it cannot, and what it will cost to build it properly.