Glass box extensions.

Glass boxes are the most common bespoke ask in our structural glazing range. The geometry is straightforward — a small glazed volume attached to the host building on one elevation, open on three sides and the roof — and the result reads as a deliberate architectural moment against an existing wall. The hard work happens at the corners.

A frameless corner is a structural silicone bond between two perpendicular glass panes, with no aluminium post at the intersection. The bond carries the structural action of both panes — wind load, thermal expansion, the weight of any glass roof above — and seals the weather face simultaneously. Done well, the corner is invisible from outside; what reads is the glass on each side, meeting at a silicone-thin line.

The roof is the engineering case that drives most of the design work. A glass roof on a glass box has to span between the two side walls (or between the side walls and the host building wall), carry snow and live loads, and resolve at the gutter or capping detail at the boundary. The lay-up is typically 21.5 mm triple-laminated (6 + 1.52 + 6 + 1.52 + 6 mm with SentryGlas), sized to the span; for larger boxes the roof becomes a glass-fin assembly with vertical glass beams supporting the horizontal pane.

Glass boxes are not appropriate for every brief that asks for one. The structural cost is real, the lead time is longer than for a framed system, and the buildability constraints — site access, lift requirements, programme co-ordination with the contractor — are tighter than for sliding doors. We tell clients honestly when a framed glass extension would resolve the brief at lower cost; the cases where it would are not the cases where the brief specifically wants a frameless reading.

When a glass box makes sense. The architectural brief calls for a small structural glazing volume read as pure glass — three glass walls and a glass roof, silicone-bonded at the corners with no visible aluminium frame; the structural opening can absorb the load case (the engineered beam above the box, the slab beneath); the project budget supports the per-project engineering and the controlled-conditions fabrication that a glass box requires.

When something else makes sense. Where the architectural priority is a wide flat opening rather than a small architectural volume, opening-corner sliding doors deliver a similar moment with a different structural strategy. Where the brief is a glazed extension that prioritises floor area and daylight rather than the pure-glass moment, a framed structural glass extension is materially less expensive — and Maxlight glass boxes are deliberately limited as a configuration; we recommend the right scale for the right brief.

Detail considerations. The corner detail is a structural-glazing-rated silicone bead, sized against the load case and the glass thickness on each side. The edge clamp into the structural opening is engineered against the calculated forces, coordinated with the project structural engineer for the beam above and the slab beneath. The thermal weak points are the silicone joints at the corners and the edge clamps; we engineer those with thermal breaks where the project U-value target requires it. the structural calculation case (live load, snow load, occasional walk-on) is run at design stage.

Regulatory context. Glass-box engineering is calculated against BS EN 16612 (overhead glazing where the roof is part of the box) and CWCT (Centre for Window and Cladding Technology) technical notes for structural glazing performance. Toughened laminated glass per BS EN 12150 and BS EN 14449 is the canonical build-up. The BFRC whole-window U-value rating applies to the assembled box; the architect should expect the published rated figure, not the centre-pane number.