What Is Structural Glazing? How SG Curtain Walling Works

Structural glazing is a way of fixing glass to a curtain wall frame with high-strength structural silicone instead of an external pressure plate and cover cap, so the outside of the building reads as a near-continuous plane of glass. The silicone bond carries the wind load on the glass back to the aluminium frame behind it, while the weight of the glass is usually taken by mechanical supports. It is used on office façades, entrance screens, atria and corners where a flush, frameless look matters. This guide explains how structurally glazed (SG) curtain walling works, the standards behind it and what to check when specifying it. For the systems we install, see structural glazing and capped curtain walling.

How structural glazing works

In a conventional curtain wall, each pane sits in a rebate between the mullions and transoms and is clamped from outside by an aluminium pressure plate, which is then hidden by a snap-on cover cap. In a structurally glazed system the clamp is replaced by a bond. The glass, or the insulating glass unit, is bonded around its edges to an aluminium carrier frame with a bead of structural silicone of a calculated width and depth. The carrier is fixed back to the mullions and transoms, and the joints between neighbouring panes are closed with a silicone weatherseal over a backer rod. From outside you see glass and narrow silicone joints; the frame sits behind.

The structural silicone is designed to resist the wind pressure and suction on each pane, and to accommodate the small movements between glass and frame caused by temperature and wind. Many systems carry the self-weight of the glass on setting blocks or mechanical supports rather than leaving the silicone to hold it permanently, and some add concealed mechanical restraints as a back-up in case the bond fails, which clients and insurers often ask for on taller buildings or over public areas.

Structural glazing curtain wall details

A structurally glazed curtain wall detail has four key parts at each glass edge: the structural silicone joint, the spacer that sets its size, the support for the weight of the glass, and the weatherseal between panes.

• Structural silicone joint: its bite (width of contact with the glass) and glueline (thickness) are calculated from the wind load, glass size and movement
• Spacer tape: holds the glass at the right distance from the carrier while the silicone is applied and cures
• Dead-load support: setting blocks or a metal toe carry the self-weight of the glass so the silicone does not
• Weatherseal: a weatherproofing silicone over a backer rod closes the joints between panes
• Insulating units: the secondary edge seal is structural silicone sized for the load from the outer pane, and units are specified for SG use
• Spandrels: opaque zones at floor edges are built as insulated, fire-stopped spandrel or shadow-box panels so the glass face looks consistent

Structural glazing vs capped curtain walling

Types of structural glazing: 4-sided, 2-sided and toggle

• 4-sided structural glazing: all four edges of each pane are bonded, giving the flush, all-glass face most people mean by structural glazing
• 2-sided structural glazing: two opposite edges are bonded and the other two are captured with a pressure plate and cap, giving a façade with strong vertical or horizontal lines; this is sometimes called semi-structural glazing
• Toggle glazing: concealed mechanical toggles engage the edge of each unit, with silicone weatherseals between panes; the look is close to structural glazing, but the glass is held mechanically rather than by a structural bond alone

The choice depends on the look, budget and client risk appetite: 4-sided SG gives the cleanest face but the most demanding bonding and replacement, while 2-sided and toggle systems keep some mechanical retention.

Our structural glazing work is on Schüco FWS 50 SG, the structurally glazed version of the 50 mm FWS 50 mullion and transom system, with SG and SG.SI variants for fully or semi-structurally glazed elevations, and on Jansen VISS SG, a steel façade system that supports structural glazing and large glass formats. The same systems are used for capped curtain walling, so a building can combine SG on its main elevations with capped glazing elsewhere, with matching frames and finishes. See our structural glazing systems.

Structural glazing vs structural glass and glass fins

Structural glazing and structural glass are different things: in structural glazing the glass is bonded to a metal frame that carries the loads, while in structural glass the glass itself is a load-bearing element. Frameless glass fins, glass beams and bolted point-fixed glazing are structural glass. They are designed by specialist glass engineers using laminated and heat-treated glass, often with testing, and are usually procured as a separate package. The two are often combined, for example where a structurally glazed façade meets a frameless glass entrance.

Structural glazing standards and testing in the UK

No single UK regulation is written for structural glazing. It is covered by the curtain walling product standard and a group of European standards for the bond itself. The main references are:

• BS EN 13830: the product standard for curtain walling, covering air permeability, watertightness, wind resistance and other characteristics of the whole wall
• BS EN 15434: the product standard for structural and UV-resistant sealants used in structural sealant glazing and in insulating glass units with exposed edge seals
• BS EN 13022: Part 1 covers the glass products used in structural sealant glazing, and Part 2 sets out the assembly rules, including bonding conditions and quality control
• The European Assessment Document for structural sealant glazing kits, which replaced the older ETAG 002 guideline and is used to assess a complete SG kit
• Centre for Window and Cladding Technology (CWCT) standards and guidance, which most UK façade specifications refer to for design and testing

In practice, the checks that matter most are compatibility testing between the silicone and every material it touches, adhesion testing on the actual frame finish, bonding in a clean, controlled environment with batch records, and full curing before units are moved. Insulating glass units for structural glazing need a UV-resistant silicone edge seal, because the edge of the unit is exposed to sunlight and in many designs it carries load from the outer pane.

Building Regulations for structural glazing

Structurally glazed curtain walling has to meet the same Building Regulations as any other curtain wall. In England, Approved Document L Volume 2 (2021 edition) sets a limiting U-value of 1.6 W/m²K for curtain walling in new non-domestic buildings. Glazing in critical locations, such as low-level panes and doors, must meet the impact safety guidance in Approved Document K. On relevant buildings, broadly those with a storey at least 18 m above ground that contain flats or certain other residential uses, Regulation 7(2) requires materials in the external wall to achieve class A2-s1,d0 or A1. Glass, window frames and seals are among the listed exemptions, but spandrel panels, insulation and other opaque parts of the wall still need to be checked against the rule and the fire strategy.

Design checks for structural glazing

A structural glazing design is checked for the loads on the glass, the loads on the silicone, movement and thermal performance before anything is fabricated. The main checks are:

• Wind load for each zone of the building to BS EN 1991-1-4 and its UK National Annex, with higher suction at corners and edges
• Glass strength and deflection under wind, plus barrier loads where the glass protects a drop
• Silicone bite and glueline, using the sealant manufacturer's design values
• Dead load of large, laminated or triple glazed units on their supports
• Movement from temperature, slab deflection and building sway
• U-value, condensation risk at the edges and thermal stress in the glass
• Breakage risk: toughened glass is commonly heat soak tested to reduce spontaneous breakage from nickel sulphide inclusions
• Access for cleaning, and a method for removing, re-bonding and replacing a broken pane

Structural glazing installation and quality control

Structural glazing is installed in two stages: the glass is bonded to its carriers, normally in a factory, and the bonded units are then fixed to the curtain wall frame on site. Most of the quality control happens in the first stage, because the bond cannot be inspected once the unit is in place.

• In the factory: compatibility and adhesion testing of the silicone against every material it touches, cleaning and priming to the sealant manufacturer's procedure, bonding in a clean, controlled area, adhesion tests and batch records, and full curing before units are moved
• On site: set-out of brackets and frame to line and level, fixing of bonded units and any mechanical restraints, weatherseal joints over backer rod, water testing of sample areas, commonly a hose test following CWCT guidance, and handover of test and batch records

Where structural glazing is used

• Office and headquarters façades, where an uninterrupted glass face is part of the design
• Entrance screens and double-height lobbies, often combined with glazed commercial doors
• Building corners, where two panes can meet at a silicone joint without a visible corner post
• Atria and feature elevations, alongside capped curtain walling on the rest of the building

Structural glazing is less suited where frequent glass replacement is expected, where the budget is tight for the area involved, or where the grid of a capped system is part of the architecture. SG panels can also be built into unitised façades, explained in what is a unitised façade, and our curtain walling guide covers U-values and fire performance.

BDF Architectural fabricates curtain walling in a 60,000+ sq ft facility in West Drayton with CNC machining, installs across London and the South East, and supplies UK-wide. Our curtain walling projects include Prescot Street in Whitechapel and Pentonville Road in Islington. To price structural glazing, send elevations, sections and the specification to our commercial team.