Aluminium Curtain Wall Corner Junctions: Mullions, Gaskets, Movement and Drainage

Curtain wall corners concentrate many of the decisions that make a facade durable. At a straight bay, a mullion, pressure plate, cap, gasket, glass edge and drainage path can follow a repeated detail. At a corner, those parts must turn, stop, overlap or connect to another elevation while the building continues to move. The external corner is exposed to wind, rain and solar change. The internal corner must accommodate finishes, insulation, structure and sometimes a change in fire or smoke compartmentation. Both locations need a deliberate junction rather than an improvised meeting of two typical bays.

This guide explains how to think about aluminium curtain wall corner junctions in Australia and New Zealand. It covers external and internal corners, mullion geometry, pressure plates and caps, gaskets, glass and panel interfaces, movement, thermal bridges, drainage, sealants and spandrels. It also sets out a practical quality-assurance and testing process, with reference to the National Construction Code and the Australian Building Codes Board, AS 4284, AS 2047, AS 1288 and AS 4666, together with relevant New Zealand references. No generic detail can establish project compliance. The system supplier, facade engineer, glass supplier, building designer and installer must coordinate the actual configuration and its evidence.

Why a Corner Junction Needs Its Own Detail

A curtain wall corner is not simply two frames placed at an angle. The angle changes the way loads travel through the mullions, the way a pressure plate is returned, the way gaskets meet, and the way water is collected and released. It can also change sightlines and the space available for anchors, thermal isolators, fire stopping and internal linings. A corner detail therefore begins with the intended geometry and the performance obligations of the complete facade, not with a sealant line drawn over a gap.

The design team should identify the structural grid, slab edges, wind exposure, orientation, corner angle, glass or panel build-up, internal finish and drainage direction before the shop-drawing stage. A corner that appears identical in elevation may have different backing walls, parapets, soffits or spandrel zones on its two sides. Recording those conditions early prevents a standard corner component from being used where a bespoke transition is required.

External Corners

An external corner projects into the weather and receives pressure from more than one facade direction. The pressure field around the building corner can be different from that on a central bay, so the mullion, anchors, glass, panel and pressure-plate arrangement must be checked for the project design actions. The external cover also has to turn cleanly without creating a ledge that traps dirt or a discontinuity that exposes the gasket line.

At an external corner, the drawing should show the primary weather line, the pressure-equalised cavity where applicable, the outer cap or cover, the glass edge clearances, and the path from any collected water to an approved outlet. It should also show how the corner interfaces with adjacent caps, flashings, sunshades, parapets and sealant joints. A visually neat mitre is not evidence of a watertight or structurally adequate assembly; the concealed interfaces matter as much as the visible cover.

Internal Corners

An internal corner turns into the building and is often protected from direct rain, but it is not automatically low risk. Condensation, air leakage, cleaning water, roof or balcony interfaces and movement at the structure can all affect the junction. Internal corners can also have limited access for installing pressure plates, tightening fixings, placing gaskets or inspecting a sealant joint. If a liner, column casing or return wall closes the access too early, the installer may be forced to substitute an unapproved sequence.

Coordinate the internal corner with the air barrier, insulation continuity, internal vapour-control strategy where relevant, fire-resisting construction and finished floor or ceiling lines. Where a corner meets a fire or smoke barrier, the nominated fire-stopping system must be designed for that exact interface. Do not assume that a general perimeter seal, backer material or mineral-fibre product provides a tested fire-stopping solution without the required evidence and installation conditions.

Mullion Geometry and Load Paths

The mullion is the primary vertical member that transfers facade actions to the anchors and supporting structure. At a corner, the geometry may use a dedicated corner mullion, two intersecting mullions, a structural post with separate pressure plates, or another arrangement permitted by the chosen system. Each option changes the clear span, torsional behaviour, glass bite, cover alignment and access for drainage. The engineering must follow the actual section and connection rather than an assumed generic corner.

Shop drawings should distinguish the reference face, the structural axis, the glazing pocket, the pressure-plate line and the external cover. They should identify which member is continuous and which member terminates or is fixed to it. A drawing that shows only external sightlines can conceal a problem with anchor eccentricity, screw access or glass edge clearance. Section cuts through both elevations and a plan cut through the corner are usually needed to explain the load path and installation sequence.

Alignment, Tolerances and Interfaces

The supporting structure will have construction tolerances, and the curtain wall will have fabrication and installation tolerances. The corner detail must provide a controlled way to absorb those tolerances without forcing the mullions out of alignment or closing a movement gap. Survey points, setting-out references and permissible adjustment directions should be stated in the installation information. The installer should not be expected to discover the tolerance strategy by trimming covers on site.

Pay particular attention to the transition between a corner mullion and a transom. The transom end, end dam, gasket return, sealant pocket and pressure-plate connection all need to be compatible. If a transom is allowed to drain toward a corner, the corner detail must receive that water and direct it to the intended outlet. If it is not allowed to drain that way, the detail must prevent water from being trapped in a closed pocket.

Pressure Plates, Caps and the Weather Line

In a pressure-equalised curtain wall, the pressure plate helps retain the infill and compress the external gasket, while the cap protects the assembly and creates the external appearance. At a corner, the plate and cap may need to be mitred, returned, butted against a corner extrusion or arranged as separate members on each elevation. The selected arrangement must preserve the designed gasket compression, fastener engagement, drainage openings and access for inspection.

Pressure plates should not be treated as decorative trims. Their fastening pattern, bearing condition and continuity contribute to the way the infill is retained and the gasket is compressed. The corner drawing should show the plate joint, any required joint treatment, the relationship with the vertical and horizontal plates, and the method used to avoid an open path into the pressure-equalised cavity. The cap must be secured and aligned without crushing the gasket or blocking weep openings.

Where caps are mitred, the joint should be fabricated and sealed in a controlled manner compatible with the system instructions. Where separate caps meet at a corner, the joint should be located and detailed so that it does not become a water shelf or a source of sharp exposed edges. Avoid adding site holes, cuts or unapproved fasteners that could alter the tested or engineered configuration.

Gaskets, Glass and Panel Interfaces

Gaskets are functional components, not simply strips that fill a visible gap. They provide compression, retain the glass or panel, manage differential movement and help maintain the weather line. At a corner, a gasket may need to turn around a corner, terminate into a gasket pocket, meet a splice, or change to a compatible seal at a transition. The manufacturer’s installation method should govern the joint, corner preparation and permissible stretching of the gasket.

Gasket length, corner forming, joint location and compression should be controlled. Stretching a gasket around a tight corner can cause later shrinkage and open the joint. Cutting a gasket short can create a thin or discontinuous line. A splice that looks closed during installation may open after temperature cycling. The corner inspection should therefore include the concealed gasket return where access allows, not only the external appearance.

Glass Edges

Glass must be selected, sized, supported and installed as part of the complete curtain wall configuration. AS 1288 is a key reference for glass selection and installation in Australian projects, but the project team must use the applicable edition and any other requirements identified by the NCC and the design documentation. The glass supplier should confirm the build-up, edge treatment, markings, clearances, setting blocks and compatibility with the system. Corner geometry must not reduce required edge clearance or create point contact between glass and aluminium.

At a corner, reflections and sightlines can make a small alignment error particularly visible. More importantly, a glass edge that is forced into a pocket, rests on an unsuitable support or is exposed to unplanned restraint can be damaged during installation or service. Setting blocks and packers should be positioned as documented, with materials that are compatible with the glass, gaskets and sealants. Never solve a corner tolerance issue by wedging the glass against the frame.

Opaque Panels and Other Infill

Metal composite panels, insulated panels, stone-look panels and other opaque infills may have different thicknesses, edge details and movement behaviour from vision glass. The corner detail should identify the approved panel type, retainers, gaskets, liners, back pans and sealants. A panel that is compatible with a straight bay may require a different edge closure or support at a corner. The designer should also consider panel replacement access and the possibility that adjacent glass must be removed to service the panel.

Movement, Thermal Bridges and Continuity

Aluminium expands and contracts with temperature, while the supporting structure can deflect, shorten, sway and move at joints. Glass and opaque panels respond differently again. The corner must permit the movements allowed by the design without transferring unintended loads into the infill, tearing gaskets or breaking sealant bonds. Movement should be considered in the vertical mullion, transom, corner connection, anchors, pressure plates, caps and perimeter interfaces as one system.

Movement joints are not a licence to leave a gap. They need a defined width or range from the project engineer, a backing arrangement where required, and a sealant configuration that can accommodate the expected movement while maintaining adhesion. The joint must remain accessible enough to install and inspect. If the corner cover bridges a movement joint, the cover itself may need a slip or split arrangement. The exact solution belongs in the engineered and system-specific details; this guide intentionally does not invent dimensions.

Thermal bridges arise when conductive aluminium paths bypass insulation or when a corner creates a continuous metal connection between interior and exterior conditions. The corner can be more critical than a straight mullion because several members and plates meet in a compact area. Coordinate thermal breaks, isolators, pressure plates, panel back pans, slab-edge interfaces and internal liners. AS 4666 may be relevant to the project’s thermal insulation and condensation risk assessment, but it does not turn a generic corner into a tested thermal result. Project calculations should use the actual sections, materials, junctions and climate assumptions.

Continuity is the practical test. Trace the air-control layer, water-control layer, insulation layer and structural support around the corner. If one layer stops at the angle, identify how it resumes. A thermal break that is interrupted by a steel bracket, a sealant line that terminates at a cap joint, or an insulation blanket that is omitted behind a spandrel can undermine the intended performance even if the visible corner is precise.

Drainage and Pressure Equalisation

Water management should be drawn as a path, not described only as a seal. The outer gasket and cap shed bulk water; any water entering the pressure-equalised zone is collected and directed through the system’s drainage chambers; and outlets discharge it to the exterior. At a corner, the drainage path may need to turn, stop or cross from one elevation to another. The detail must prevent water from bypassing an end dam, flowing into a closed mullion pocket or being discharged onto a lower gasket.

Weep holes, slots, baffles and end dams must remain open and correctly positioned. Sealant applied across a drainage opening can defeat the system. Conversely, an unsealed corner joint can connect the exterior directly to an internal cavity. Mark drainage openings on fabrication and installation drawings and inspect them before covers, flashings or adjacent trades conceal them.

Pressure equalisation depends on controlled compartments and openings. An unplanned hole drilled for a bracket, a continuous seal where a vent is required, or a missing internal plug can change the pressure behaviour. The corner should therefore be reviewed as part of the full mullion and transom drainage strategy, including interfaces with the sill, head, parapet, sunshade and adjacent cladding. Water testing should include the corner and the transitions on both sides, not only a representative straight bay.

Sealants and Joint Preparation

Sealant is an important part of many curtain wall junctions, but it should not compensate for incorrect geometry, blocked drainage or missing gaskets. Select the product for the substrates, movement, exposure, ultraviolet conditions, compatibility and required cure environment. Follow the sealant manufacturer’s preparation, primer, backing, tooling and curing instructions. The project specification should identify the responsible party for adhesion testing and records.

At a corner, avoid three-sided adhesion where a movement joint is intended to work. Use appropriate bond-breaker or backing arrangements where specified, and keep the sealant depth and width within the approved system and product requirements. Substrates must be clean, dry and sound. Powder coating, anodising, glass, rubber, aluminium and membrane materials can have different preparation needs. A neat surface bead is not enough if the concealed bond line is contaminated or the sealant has skinned before the joint is closed.

Sealant joints should be sequenced so that one trade does not cover a surface that another trade must inspect. Record batch details, weather conditions where relevant, substrate preparation and any adhesion or compatibility checks. Remove failed sealant rather than applying a second bead over an unprepared joint. At a corner, a small unsealed return can allow water behind a large apparently continuous joint, so inspection should follow the junction around every change of plane.

Spandrels and Corner Fire or Smoke Interfaces

Spandrel zones often combine opaque glass or panels, insulation, back pans, slab edges, fire stopping and internal finishes. A corner spandrel can have a different slab-edge condition on each elevation, with limited room for the backing system to turn. The shop drawings should show the complete build-up, including the relationship between the curtain wall, slab edge, perimeter fire stopping, insulation, panel retainers and internal lining.

Do not infer fire or smoke performance from the appearance of a spandrel closure. The applicable NCC provisions, project fire strategy, tested system and approved installation detail must govern. Any fire-stopping product should be installed in the substrates and joint conditions covered by its evidence. Coordinate the sequence with the curtain wall installer and fire-stopping contractor so that the corner is not left with an inaccessible void or an unsealed discontinuity.

Standards, NCC and New Zealand References

In Australia, the National Construction Code provides the regulatory framework, subject to the edition, volume, building classification and applicable state or territory requirements. The Australian Building Codes Board publishes the NCC and supporting material; the project team should confirm the current provisions rather than relying on a summary in an article. Curtain wall design must also respond to the project’s structural, weatherproofing, glazing, energy, fire and condensation requirements.

AS 4284 is commonly used as a reference for testing building facades and should be considered where the project specification or compliance pathway calls for facade testing. AS 2047 concerns windows and external glazed doors and may be relevant to components or interfaces depending on the product scope; it should not be assumed to cover every curtain wall arrangement. AS 1288 provides an important reference for glass selection and installation. AS 4666 is relevant to thermal insulation and condensation considerations in the circumstances covered by the project documentation. Confirm editions, scope and applicability with the facade engineer, building surveyor or certifier.

For New Zealand work, coordinate the New Zealand Building Code, the project consent documentation and the relevant Acceptable Solutions or Verification Methods. NZS 4211 is a key reference for performance of windows, while the NZS 4223 series is relevant to glazing in buildings and should be checked for the applicable glass selection and installation requirements. Wind, moisture, durability, fire, energy and seismic considerations must be assessed for the building and location. Australian standards should not be substituted for New Zealand requirements without a documented compliance pathway and professional acceptance.

Across both countries, the useful question is not whether a corner resembles a tested image. It is whether the supplied corner, including its glass or panel, gaskets, pressure plates, fixings, sealants, drainage, supports and installation, is within the scope of the applicable evidence. Keep the system supplier’s instructions, engineering calculations, shop drawings, test reports and site records aligned to the same configuration.

Quality Assurance and Testing

Quality assurance starts before fabrication. Review the corner drawings against the architectural plans, structural grid, glass schedule, panel schedule, fire strategy, thermal assessment and waterproofing details. Confirm the corner angle, member continuity, fastener access, gasket path, cap joint, drainage route, movement allowance and perimeter interfaces. Resolve clashes before the first extrusion is cut.

Factory Checks

Verify extrusion identification, finish, orientation and cut length against the approved schedule.
Check corner joints, mitres, end dams, drainage openings and pressure-plate preparation before assembly.
Confirm gasket type, corner forming, splice location and installed compression against system instructions.
Check glass and panel identity, edge condition, setting blocks, packers and protective handling.
Record sealant batch, preparation and cure requirements, and protect joints from contamination.
Photograph concealed areas before covers or liners make inspection difficult.

Site Checks and Performance Testing

At site, verify survey and setting out before anchoring. Check plumb, line, level, corner angle, anchor installation, adjustment range and movement joints. Inspect gaskets, pressure plates, caps, sealants, flashings, membranes and drainage openings before adjacent work conceals them. The installer should record deviations and obtain a documented disposition; trimming, drilling or sealing a corner in the field should not be an undocumented design change.

Where specified, conduct facade performance testing with the corner represented in a meaningful way. The test plan should identify air, water and structural or serviceability checks as applicable to the project and should follow the selected standard and agreed procedure. A successful test on a straight bay does not automatically validate a corner, a spandrel transition or a different infill. Inspect the test specimen afterward for gasket displacement, glass distress, sealant failure, blocked drainage and permanent deformation, and keep the report with the project records.

Frequently Asked Questions

Can two standard curtain wall bays simply be sealed together at the corner?

Not without confirming the system and project design. Two bays may have incompatible pressure plates, gasket returns, drainage paths, cover joints or anchor conditions. A dedicated corner detail or engineered connection should define how the members, infills, weather line and movement strategy meet.

Is a corner cap mainly an appearance item?

No. The cap affects protection, access, visible alignment and the treatment of the pressure-plate and gasket area. Its joint must not block drainage or create an unplanned opening into the curtain wall cavity. Use the system’s approved cap and corner method.

Why can a corner leak when the straight bays do not?

Common causes include a missing or blocked weep, an incorrect gasket return, an unsealed cap joint, a drainage path that terminates at the corner, a panel or glass interface that does not match the pocket, or a sealant joint with poor preparation. The investigation should trace the water path and inspect both elevations rather than adding sealant at the first visible mark.

Do thermal breaks remove all corner condensation risk?

No. Thermal performance depends on the complete junction, including mullions, pressure plates, corner posts, anchors, spandrels, slab edges, liners, glass or panels and internal finishes. Use project-specific calculations and condensation assessment where required, based on the actual configuration.

Which standards should be written into a specification?

That depends on the building, jurisdiction, product scope and compliance pathway. The specification may need to address the NCC and ABCB material in Australia, AS 4284, AS 2047, AS 1288 and AS 4666 where applicable, and the New Zealand Building Code, NZS 4211 and NZS 4223 references for New Zealand work. Confirm the current editions, scope and project requirements with the responsible professionals.

Conclusion

A reliable aluminium curtain wall corner is a coordinated junction of structure, weather management, glass or panels, gaskets, pressure plates, caps, sealants, insulation and movement. External and internal corners need different reviews, but both require a continuous load path, a deliberate air and water strategy, accessible installation, compatible materials and evidence that matches the supplied configuration. The strongest process is to draw the corner in plan and section, trace every control layer, coordinate spandrels and fire interfaces, inspect concealed work, and test representative details where the project requires it. Treat the corner as a system rather than a finishing line, and the facade team can resolve the risks before they become leaks, cracked infills, thermal complaints or difficult maintenance work.

Aluminium Curtain Wall Corner Junctions: Mullions, Gaskets, Movement and Drainage