Placing Expansion Gaps in Long Vertical Aluminium Glazing Mullions

Long vertical aluminium mullions behave differently from the shorter sticks used in low-rise storefronts. A 6 to 9 metre run between floor slabs on a Brisbane office tower, a Perth apartment block or a Sydney CBD refit will move noticeably between a cool winter morning and a 40°C summer afternoon, and that movement has to be allowed for in the joint layout. Get the spacing, sizing or detailing wrong and the glass picks up the strain: edge pressure builds up, gaskets extrude, sealants split, and the facade starts creaking or weeping within a couple of seasons.

Treating expansion gaps as a finishing detail is one of the most common causes of call-backs on Australian facade work. The Building Code of Australia expects movement-tolerant construction through the glazing package, and the lessons that apply in temperate European projects need to be pushed harder here because of larger diurnal swings, stronger UV exposure and coastal salt loads.

Why Thermal Movement Matters for Aluminium Mullions

Aluminium has a linear coefficient of thermal expansion of roughly 23 × 10⁻⁶ per °C. That sounds modest until you multiply it by an 8 metre stick of mullion and a 55°C swing, which is plausible between a chilly Adelaide dawn and a black-painted western facade at the same site in February. The arithmetic gives a thermal movement in the order of 10 mm, and that figure has to be absorbed somewhere in the joint stack.

Three factors push Australian projects further than equivalent jobs in milder climates. Interior-to-exterior temperature differentials on fully glazed facades can exceed 70°C during a heatwave, especially on the western elevations seen across new Sydney and Melbourne towers. Dark anodised or powder-coated finishes absorb more solar radiation and run hotter than light colours. Mullions fixed rigidly at both ends can experience combined thermal and structural movement at the same time, which is rarely the case in design calculations done from a desk in winter.

Sizing the Gap Using the Linear Expansion Formula

The basic equation is straightforward: ΔL = α × L × ΔT, where α is 0.000023 for aluminium, L is the unrestrained length of the mullion between fixed points, and ΔT is the maximum temperature range the section will see in service. In practice the inputs should reflect the worst credible case rather than the annual average.

For a 7.2 metre mullion on a Brisbane project with a ΔT of 60°C, the calculated movement is 0.000023 × 7200 × 60 = 9.9 mm. Designers usually add a safety factor and split the movement between the top and bottom of the sliding section, so a 12 mm overall gap with 6 mm of travel at each end is a reasonable starting point. In cyclonic regions around Cairns, Townsville and the Top End, the same calculation should also factor in the elevated temperatures reached when dark frames sit under cyclone-rated screens for extended periods.

Two practical points are often missed. Allowances for fabrication tolerance, typically ±1.5 mm per stick, need to sit on top of the thermal calculation. The gap should also be checked against the gasket and sealant compression limits specified by the system supplier, because a gap that is too wide can lose its weatherseal long before the aluminium reaches its travel limit.

Stack Joints Versus Running Mullions

A stack joint terminates one stick of mullion at a horizontal floor line and starts a fresh stick above it. A running mullion continues past the floor, with the slab anchored to its side via a slip bracket. Both approaches are widely used across Australian commercial construction, and the choice drives how the expansion gap is created and where it ends up.

Stack joints are simpler to detail and easier to install because each piece can be prefabricated, lifted in and bolted off without complex bracketry. They also make the movement calculation more transparent, since the engineer knows exactly where each length of aluminium starts and stops. Running mullions give a cleaner external shadow line and reduce the number of horizontal cover plates, which architects often prefer for premium apartments on the Gold Coast or in inner Melbourne. The trade-off is that the slip brackets must be designed to carry the wind load while still allowing the aluminium to slide, and the brackets themselves sit inside the cavity where they are hard to inspect after handover.

For most projects above 30 metres in Sydney and Melbourne, design teams end up with a hybrid: stack joints at major floor breaks and short running mullions between to manage the line of the glazing.

Where to Position the Joints Along the Elevation

Spacing rules of thumb vary by system, but a 6 to 8 metre maximum between sliding connections is a safe working figure for Australian projects. Beyond that, the cumulative thermal growth starts to exceed what standard gaskets and sealants can absorb without distress.

Joints should sit as close as practical to a structural floor line so that the slab can act as a wind anchor for one stick while the other is free to move. Where this is not possible, an intermediate bracket fixed back to the primary structure can do the same job. The key is to avoid creating a long lever arm: if the sliding connection is more than about 600 mm away from the structural anchor, the mullion will bow sideways under wind load as it heats up, and that movement is what cracks the perimeter seals first.

Corners deserve extra care. A right-angle mullion running up the corner of a Perth or Adelaide building will be restrained on one leg and free on the other during thermal cycling, which produces twisting that is rarely visible in static calculations. Designers usually split the corner into two short sticks with a stack joint within 300 mm of the change of direction.

Detailing Joints for Harsh Australian Conditions

Once the position and size of the gap are settled, the joint itself has to survive local conditions. In BAL-40 or BAL-FZ bushfire zones along the New South Wales coast and into the Adelaide Hills, the expansion gap cannot be a route for ember entry. Non-combustible backing rods, intumescent sealants and stainless steel mesh at the slip bracket are all common responses, and the detailing needs to be agreed with the project's bushfire consultant well before shop drawings go out.

Coastal projects on Sydney Harbour, around Port Phillip Bay or along the Fremantle coast need corrosion-resistant fixings throughout the joint. Standard zinc-plated brackets fail quickly in salt air, so 316 stainless steel or marine-grade aluminium slip connections should be specified, with isolation tapes where dissimilar metals meet. The same sheathing board installation logic that covers metal compatibility at the wall build-up applies just as directly at the slip bracket detail.

UV exposure is a quieter but steady threat. EPDM gaskets that would last 25 years in a European climate can harden and shrink within a decade on a north-facing Brisbane or Darwin facade. Specifying silicone gaskets or silicone-compatible EPDM at the expansion gap is worth the modest premium, particularly for buildings with limited access for future replacement.

Coordinating the Glazing Package with Other Trades

Even a perfectly detailed expansion joint fails if the trades on either side of it do not know it is there. Glaziers need to be told which mullions are running and which are stacked, otherwise a glass unit can be wedged in tight against a moving stick and shatter the moment the sun comes around. The sealant applicator needs to know the expected joint movement so they can pick a Class 25 or Class 50 sealant rather than a non-moving fillet.

Coordination extends to the wall build-up behind the mullion. If the slab edge, the sheathing board and the cladding zone are not aligned with the joint, the slip bracket cannot perform as designed. This is where cross-discipline shop drawing reviews pay for themselves, particularly on mixed-use projects in inner Sydney where the facade contractor is often brought in late. Following well-established steel frame cladding practices on the structure side makes the movement story on the glazing line much easier to get right.

Every project should also have a written movement joint schedule that lists each expansion gap, its design travel, the sealant used and the inspection hold points. Hand that document to the facade maintenance team at handover, and the curtain wall will keep performing long after the warranty inspections are over.