Before we understand racking (a key component in curtain walling), let’s discuss curtain walling itself. Curtain wall façades are exterior cladding systems used in various types of buildings, including commercial, industrial, and institutional structures.
Typically classified as a non-structural element, its main purpose is to resist external forces and provide an envelope (not to bear the structural loads of the building itself).
When designed by professionals, an aluminium curtain walling system should be able to account for certain critical factors imposed by the environment (and the supporting structure). These are:
- Loading: In-plane and out-of-plane loading from wind and seismic activity.
- Movement accommodation: The system must accommodate movement generated by the main structure, including:
- Thermal expansion/contraction
- Long-term creep, deflection from supporting structural elements (like floor slabs due to gravity loads)
- Seismic inter-storey drift (building sway)
- Structural integrity: Components, such as mullions, must be designed with sufficient inertia values (resistance to bending) to prevent failure mechanisms, including mullion buckling or glass racking.
What is racking?
In the context of façades, racking describes a phenomenon of deformation experienced by walls and cladding panels, particularly during lateral movements of a building’s structure. They become misaligned as they are subject to external forces.
It’s mainly caused by the relative lateral movement between adjacent floors, known as inter-storey drift. This drift is a key parameter in seismic design and is typically induced by seismic activities (earthquakes), wind loads and thermal effects (expansion and contraction).
Why buildings move
But why do high-rise structures actually move? It’s driven by two main categories of loads: dead loads and live loads.
Dead loads
This term describes the inherent weight of all materials added to the superstructure and façade during construction.
Live loads
Live loads encompass everything else, including environmental conditions (as listed above), as well as the movement of people or the placement of heavy equipment in a specific area.
What UK standards and guidance say about movement
From a UK policy perspective, racking and other movements in curtain walling aren’t dealt with in a single code. Instead, they sit within a web of performance-based standards and guidance – with CWCT and BS EN 13830 being the most relevant.
CWCT (movement without loss of performance)
The CWCT Standard for Systemised Building Envelopes is the UK’s go-to reference for curtain walling, rainscreen cladding, and overhead glazing.
It outlines performance criteria for weathertightness, loading and movement, thermal, acoustic, and fire performance, and the tests used to demonstrate compliance.
For movement, CWCT guidance is explicit that dead and live loads must be transferred safely to the structure without permanent deformation, and that movement must be accommodated without any reduction in performance.
In fact, CWCT Sequence B testing, which many UK façades undergo, subjects full-scale mock-ups to static and dynamic wind pressures, air and water testing, and impact to prove that the system remains airtight, watertight, and structurally sound after repeated loading.
BS EN 13830 – the product standard for curtain walling kits
Alongside CWCT, BS EN 13830:2015+A1:2020 (and its UK adoption) is the product standard for curtain walling kits. It defines the essential characteristics that a system has to be assessed against when used as part of the building envelope, including:
- Resistance to loads (especially wind load resistance and structural stability)
- Air permeability and watertightness under static pressure
- Safety in use (including impact)
- Thermal transmittance and other energy-related characteristics.
The standard ties these characteristics to a family of test methods (EN 12152/12153 for air, EN 12154/12155 for water, EN 13116 for wind load, etc.), which are now routine requirements in UK project specifications.
While EN 13830 itself is primarily a product standard, recent European work has expanded its scope to include seismic considerations as well (meaning it’s now increasingly being incorporated into project briefs).
Façade contractors are increasingly asked not just for wind resistance classifications, but for evidence that their systems can accommodate specified drift and racking movements.
Very few hard deflection limits for glass
A subtle, but important point from CWCT is that UK standards give surprisingly little explicit guidance on how much glass is allowed to move. CWCT’s own Technical News notes that there are no established limits on the deflection of glass used in windows and curtain walls.
The only clear movement limit you’ll find is in BS 6180, which caps the displacement of glass in certain protective barrier situations at 25 mm, including any movement of the supports.
Beyond that, most of the familiar guidance (BS 6262) is really based on stress checks rather than on how far the glass is actually allowed to move. So, in practice, engineers can’t hide behind one ‘magic number’ for glass deflection.
Instead, movement must be treated as a performance issue. Combine structural frame deflections, floor edge rotation, differential shortening, and thermal movement, then test whether the system as a whole still behaves acceptably.
Choosing curtain walling with confidence
Racking is one of those behaviours that only really shows itself when a façade is under real-world building movement. Treat it as an afterthought, and you risk water ingress, glazing damage, noisy joints and unhappy occupants.
As we’ve seen, there isn’t a single go-to deflection limit for glass in UK standards. Good curtain wall design means understanding how the whole system racks and moves, with mullions, transoms, brackets, anchors, seals, gaskets, and glass all sharing the load.
At Pops Facades, this is exactly how we approach unitised curtain wall installation.
We’re used to delivering large installation projects for leading system manufacturers such as Schüco, Kawneer and Reynaers, all with the help of our experienced in-house unitised teams.
