Aluminium Window Expansion Joints: Movement, Seals and Façade Coordination



Every aluminium window, door and glazed façade is built from materials that change size. Aluminium expands when it warms and contracts when it cools, glass moves far less, concrete and steel move at their own rates, and the building as a whole deflects, settles and drifts. None of that movement is a defect. It is a physical fact that the envelope has to absorb, and the component that absorbs it is the expansion joint: the designed discontinuity that lets one part of an assembly move relative to another without tearing a seal, cracking glass or loading a fixing beyond what it was engineered to carry.

Expansion joints are also the least photogenic part of a window system, which is part of why they are so often under-designed. A mullion profile can be photographed and sold. A joint is a gap, a splice, a sliding bracket, a bead of sealant and a note on a drawing, and its quality only becomes visible when it stops working, usually as a leak, a cracked pane, a distorted frame or a sealant bead that has failed along its bond line. This guide sets out how movement behaves in aluminium fenestration, why the joint requirement is always project-specific, and how mullions, transoms, fixings, seals, glazing clearances, drainage and finishes have to be coordinated so that the joint does its job for the life of the building. It deliberately avoids nominal dimensions, because the right joint capacity, gap and fixing pattern on any project come from that project’s geometry, climate, structure and exposure, not from a generic detail.

What an Expansion Joint Actually Does

Thermal Movement, the Basic Mechanism

Aluminium has a relatively high coefficient of thermal expansion for a structural façade metal. The practical consequence is simple to state: a long aluminium member changes length when its temperature changes, and the change is proportional to both the length of the member and the temperature difference it experiences. A short window frame in a sheltered wall moves by an amount that the glazing clearances and sealant joints absorb without thought. A continuous run of framing across a large elevation, a ribbon window, a shopfront or a curtain wall ladder does not, because the movement accumulates along the length rather than staying local to one window.

The temperature that matters is not the weather forecast. It is the temperature of the metal itself, which depends on orientation, solar exposure, the colour and finish of the coating, whether the frame is shaded, whether it sits behind glass, and how quickly it can shed heat to the air around it. A dark powder-coated frame on a west elevation will run considerably hotter in summer sun than the ambient air temperature, and considerably colder on a clear winter night through radiative loss. The joint has to be sized for that surface temperature range, not for a comfortable indoor assumption, and the range is different on a Cairns elevation and a Christchurch one.

Movement That Is Not Thermal

Thermal expansion is only one of the movements a window assembly sees, and it is rarely the largest. Structural movement comes from the building: slabs deflect under load, concrete columns shorten progressively as a tall building is loaded, frames rack under wind, and inter-storey drift concentrates at floor lines. Settlement and long-term creep change the geometry of openings after the frames are installed. Moisture movement affects timber and masonry substrates far more than aluminium, but the frame is fixed to those substrates and inherits their movement. Seismic and wind events impose transient displacements that a joint may only see once in its life, but must still survive.

These movements act in different directions, at different rates and on different cycles. Thermal movement is diurnal and seasonal, and it reverses. Settlement is one-directional and permanent. Wind deflection is rapid and elastic. A joint that is generous in one direction and tight in another is not a movement joint; it is a gap that happens to work until the building does something the designer did not consider.

Where the Joint Lives

In practice, movement is absorbed at a small number of defined locations rather than everywhere at once. There is the interface between the frame and the structure, closed by a sealant joint that is a movement joint as much as a weather joint. There is the splice between two lengths of the same profile, where a long run is deliberately broken. There is the junction where one system meets another: window to curtain wall, window to shopfront, glazed screen to masonry, frame head to a structural soffit. And there is the point where the façade crosses a structural movement joint in the building itself, which the envelope must follow rather than resist.

Each of these locations has a different movement regime and therefore a different detail. Treating them as one generic condition, sealed the same way, is the origin of a large share of façade defects.

Long Runs and the Accumulation Problem

Why Length Changes the Question

A single window is a component. A run of windows coupled together is a structure, and it behaves like one. When frames are joined into a continuous band, the assembly stops being a series of independently fixed boxes and starts acting as a long member restrained at intervals. The thermal movement of that member is the sum of the movement of its parts, and the restraint is concentrated at whatever fixings hold it. If those fixings are all rigid, the member cannot expand where it wants to, so it expands where it can: it bows, it lifts, it distorts the frame geometry, it pinches the glass, and it opens the sealant at the very joints that were supposed to stay closed.

This is why long runs need to be divided into segments with designed joints between them. The division is a structural decision as much as an architectural one, because it determines where the assembly is fixed and where it is free to slide. It should appear on the elevation as a visible break or a deliberate shadow line, and on the section as a splice with a defined movement capacity. A run that looks continuous but is structurally discontinuous is perfectly achievable; a run that looks continuous and is rigidly fixed end to end is a future failure.

Where to Put the Break

Good joint locations are chosen for four reasons, usually in combination. They coincide with a structural movement joint in the building, so the façade and the structure move together rather than fighting. They sit at a natural visual break, such as a transom line, a pier, a column line or a change of system, so the joint reads as a deliberate detail rather than a mistake. They fall where access for inspection and maintenance is practical, because a joint nobody can reach is a joint nobody will maintain. And they divide the run into segments whose movement each individual joint can absorb comfortably, with margin, rather than at the limit of the sealant’s capability.

What should be avoided is the accidental joint: the one created because a profile length ran out, because a coupling sleeve was the only connector on the truck, or because the frame was installed in storey-height lifts without anyone asking how the lengths were meant to relate. Accidental joints have no designed capacity and no documented movement direction, and they are usually where water enters.

Design Discontinuity and Visual Continuity

Architects reasonably want unbroken sightlines, and manufacturers reasonably want to supply long lengths. Both wishes can be met, but only if the discontinuity is designed rather than denied. A spliced mullion can be detailed with an internal sleeve that carries the structural continuity while allowing longitudinal movement, with the outer faces aligned so the joint reads as a fine shadow line. A transom can be broken at a location where a horizontal line already exists. A coupling between two window units can be designed so that each unit moves within its own frame and the coupling accommodates the differential.

The failure mode to watch for is the joint that is visually resolved but structurally ignored: the faces align beautifully and the two members are then screwed hard together, which converts a movement joint into a rigid connection and transfers the accumulated movement into the nearest glass edge or anchor.

Why the Joint Is Always Project-Specific

What a Joint Calculation Needs

There is no such thing as a standard expansion joint for aluminium windows, and any claim otherwise should be treated with suspicion. The required movement capacity of a particular joint is derived from a small set of project inputs: the length of the run that the joint serves, the surface temperature range the frame will experience at that orientation and finish, the movement of the structure at that location as advised by the structural engineer, and the movement capability of the seal or splice that will bridge the gap. Change any one of those inputs and the answer changes.

A joint sized for a shaded southern elevation of a low-rise building is not the same joint as one on a dark, west-facing, unshaded run at height. A joint in a building with a post-tensioned slab behaves differently from one over a transfer structure. A joint detailed by a manufacturer for a tested system is valid for that system as tested, at the sizes tested, with the fixings and seals tested with it. It is not automatically valid for the same profile stretched into a different configuration, coupled to a different product, or fixed to a different substrate.

Why Catalogue Details Do Not Travel

Manufacturers publish standard details because they are useful starting points, and a good supplier will provide them with the assumptions stated. The risk arises when a standard detail is lifted from one project to another without checking whether the assumptions still hold. The copied detail may show a splice at a spacing that suited the original building’s structural grid, a sealant size that suited the original exposure, or a fixing pattern developed for a particular slab edge. On the new project those may be conservative, or they may be inadequate, and there is no way to tell from the drawing alone.

This is not an argument against standardisation. It is an argument for stating, on the drawing, the movement the joint is designed to accommodate and the direction in which it acts. A detail with a stated capacity can be checked against a new project in minutes. A detail with no stated capacity can only be copied on faith.

Who Owns the Decision

Responsibility is usually shared, which is exactly why it can fall through the gaps. The architect sets the visual and functional intent. The structural engineer defines how the building moves at each interface. The façade consultant or window supplier develops the system detail and confirms what the product can accommodate. The builder executes the setting-out and installation sequence. The certifier confirms the installed result against the applicable provisions of the National Construction Code and the referenced standards.

The practical discipline is the same one that governs every other difficult interface: draw it once, at a scale where the frame, the splice, the fixing, the seal, the flashing and the adjacent structure are all visible together, then have each party confirm the part they own before fabrication begins. A movement joint that exists only as a note saying “allow for movement” has not been designed.

Mullions and Transoms: The Grid That Moves

Mullion Continuity and Splice Design

The mullion is the vertical member that carries wind load from the glass to the structure, and in a long run it is also the member in which longitudinal thermal movement accumulates. Its splice is therefore doing two jobs at once: transferring bending and shear across the joint, and permitting longitudinal movement without transferring it. Those two requirements are met by different parts of the detail, and confusing them is a common error. A splice that is strong but rigid will crack its sealant; a splice that slides freely but cannot carry bending will allow the mullion line to hinge.

A well-resolved splice uses an internal sleeve or a matched engagement to maintain alignment and transfer load, combined with a connection that is fixed in the directions that need restraint and free in the direction that needs movement. The joint is then finished with a seal that is designed for the movement, not simply filled with sealant after the fact.

Transoms, Dead Load and Differential Movement

Transoms carry the dead load of the glass into the mullions, and they sit between mullions that may themselves be moving. A transom that is rigidly fixed at both ends to mullions that are expanding in opposite directions will be pushed or pulled out of position, and the glass it carries will rotate slightly in its rebate. That rotation is small, but glass edge clearances are also small, and a transom that lifts at one end can close a clearance that the glazing design assumed was open.

For this reason, transom connections in long runs are frequently detailed with one end fixed and the other able to accommodate movement, and the glazing clearances are set with that movement in mind. The horizontal line that looks continuous on the elevation is, in section, a series of connections with a defined structural hierarchy.

Coupling Windows Into a Run

When separate window units are coupled together, each unit brings its own frame, its own glazing and its own small movement. The coupling has to accommodate the sum of those movements plus any differential between adjacent units caused by shading, orientation or one unit being in a wall and the next being in a recess. A coupling that simply pulls two frames tight together with a mullion cover creates a rigid chain, and the chain transmits movement to the weakest element, which is usually the sealant at the end of the run or the glass at the corner.

Where a run is long enough to need a structural break, that break should be at a coupling rather than inside a unit, because a coupling can be detailed with a movement capacity while a factory-assembled unit cannot.

Fixings: Fixed, Sliding and the Difference That Matters

The Fixed and Sliding Pattern

The single most important idea in movement detailing is that an assembly needs a defined fixed point and defined sliding points. The fixed point locates the frame and resists the loads that must be resisted. The sliding points allow the frame to expand away from the fixed point in the directions the design intends. Without a fixed point the assembly wanders; without sliding points it cannot move at all, so it stores the movement as stress.

Sliding is achieved with slotted holes, oversize holes with a defined washer, sleeves, sliding clips or brackets that engage a track. Each of these has a movement range, and the range must be adequate for the calculated movement at that location with margin for installation tolerance. A slot that is nearly used up when the frame is installed has no capacity left for the summer that follows.

Packers, Shims and Over-Fixing

Packers are a quiet cause of joint failure. A packer installed to level a frame during installation and then left in place can become a hard point that the frame was never designed to have, changing the effective span of a mullion or locking a joint that was meant to slide. Non-structural packers should be removed where the system requires it; structural packers should be specified, sized and recorded. The distinction should be on the drawing and in the installation instructions, because on site a packer is just a packer unless someone has said otherwise.

Over-fixing is the other common error. Adding an extra anchor “for strength” is not conservative; it changes the structural model. Every additional rigid fixing removes a degree of freedom the design relied on, and the resulting restraint shows up as bowing, sealant failure, glass contact or anchor distress somewhere else in the assembly.

Different Substrates, Different Behaviour

Concrete, structural steel, timber framing and masonry each move differently and each accepts a different type of anchorage. A frame fixed to a concrete slab edge, a steel shelf angle and a timber stud wall is fixed to three movement regimes, and the joint details at each must reflect that. Masonry is particularly unforgiving: it shrinks, it moves with moisture and temperature, and it is brittle at the anchorage. A fixing pattern developed for a concrete edge should not be assumed to work in a brick pier without a specific check.

Seals: Movement Capability, Not Just Weatherproofing

Sealant Joints Are Movement Joints

Sealant at a window perimeter or a frame splice is routinely described as weatherproofing, but its primary structural job is to accommodate movement while remaining adhered to two surfaces that are moving relative to each other. That is a demanding role. The sealant must have a movement capability appropriate to the range the joint will see, must be installed at a depth and width that let it deform rather than tear, must be bonded to properly prepared substrates on both faces, and must have a bond breaker behind it so that it forms an adhesion joint rather than a three-sided block.

Most sealant failures are not failures of the sealant itself. They are failures of preparation, geometry or specification: sealant applied over dust, release agent, moisture or a poorly cleaned coating; a joint too shallow or too deep for the movement required; a bond breaker omitted so the bead is stretched from behind; or a low-movement product used in a high-movement location because it was already on site. Silicone, polyurethane, modified polymer and acrylic sealants differ markedly in movement capability, adhesion, paintability and UV resistance, and the correct choice follows from the joint’s function and the substrate, not from what is in the van.

Gaskets and Compression Set

Dry glazing relies on gaskets rather than wet sealant, and gaskets accommodate movement by compression and recovery. Their weakness is compression set: an elastomeric gasket that is held compressed for years at high surface temperature gradually loses the ability to spring back, so the seal force falls and water can track past it. EPDM and silicone gaskets are the common choices for aluminium systems because of their weathering behaviour, and the corners are the critical detail, whether vulcanised, moulded or mechanically joined.

A gasket that is stretched during installation rather than laid in with a small allowance will pull back over time and open at the corners. That is an installation error with a long delay before it becomes visible.

Where Seals Must Not Be Load-Bearing

There is a persistent temptation to use sealant to bridge a gap that should have been a designed movement joint, because a bead of sealant is cheap and can be applied late. The result is a seal doing structural work. It will hold for a while, then fail along the bond line, and the failure will be diagnosed as a sealant problem when the real problem is that the joint had no capacity. Separating the movement function from the weather function, with a properly sized joint and a properly specified seal, is the only durable answer.

Glazing Clearances and the Glass Inside a Moving Frame

Glass Must Never Be the Restraint

Glass is strong in compression but brittle at its edges, and it must never become the element that restrains the frame. Setting blocks position the unit and carry its dead load; edge clearances and face clearances keep the glass away from aluminium and from fixings; and the whole arrangement is specified by the system and by AS 1288, the standard for glass in buildings. Those clearances exist to accommodate thermal movement, frame deflection and installation tolerance, all at once.

When a frame closes up because it has been over-fixed or because its joints have run out of travel, the clearance disappears. The glass then bears on the frame, usually at a corner or against a setting block that has become a hard point. The result can be a cracked pane, a chipped edge that becomes a crack later, or a failed insulating glass unit where the edge seal has been worked by repeated small movements.

Frame Deflection and Unit Rotation

A mullion deflects under wind load, and a transom rotates slightly as it carries glass. The glazing detail must assume that the opening the glass sits in is not perfectly rigid. This is why glazing clearances are set with deflection in mind and why the structural performance of the frame and the glazing specification cannot be decided independently. A frame designed to deflect more will need correspondingly more clearance, and a unit with a stiff or brittle edge detail will tolerate less.

Safety Glazing and the Movement Interface

Where AS 2208 requires safety glazing, the glazed element is by definition in a location where human impact is foreseeable, which often means a door, a low-level pane or a full-height panel. Those are also locations with the most traffic, the most frequent operation and the most cleaning. The glazing must be correctly identified and marked, and the movement detail must still work: a safety-glass pane installed with no clearance is no safer than an ordinary pane, because it can still crack from restraint.

Drainage Across a Joint

Keeping the Water Path Continuous

A drained and ventilated window system works by separating the outside from the inside with an internal drainage path that collects any water that passes the outer seal and discharges it back outside. That path is designed as a continuous route. A movement joint cuts across it, and if the joint is detailed only as a structural and weather seal, it can become a dam: water collects behind the seal, finds a path through the joint, and appears inside the building far from the leak’s apparent source.

Good details maintain the drainage path across the joint with an overlap, a baffle, a saddle or a continuous liner that allows water to pass while the structural joint moves. The principle is that the water path and the movement path are separate functions, resolved in the same location without either one blocking the other.

Weeps, Baffles and Blockage

Weep paths and drainage slots must remain open after the joint is complete. Sealant applied generously around a joint can close a weep without anyone noticing, and the consequence appears months later as water standing in a sill chamber. Similarly, a baffle that is meant to stop wind-driven water from crossing a chamber must not be installed in a way that stops the chamber draining. The sequence matters: drainage components go in before the joint is closed and sealed, and the completed detail should be inspected with that in mind.

Falls and Ponding

Drainage depends on fall, and fall depends on the setting-out being right. A sill or a threshold that is installed level, or falls the wrong way, will hold water regardless of how well the joint is sealed. Standing water accelerates seal degradation, corrodes cut edges and grows biological matter that blocks weeps. The joint detail and the fall are therefore part of one water management strategy, and neither can compensate for the other.

Finishes at the Joint

Coatings, Cut Edges and Touch-Up

Powder coating and anodising protect aluminium from the environment, but a joint exposes surfaces that the coating line did not cover: cut ends, drilled holes, slots and the faces inside a splice. Those are the places where corrosion starts, particularly on coastal sites where salt-laden air reaches every unsealed edge. Cut edges should be treated as the system requires, and site-cut or site-drilled areas should be protected with a compatible repair coating rather than left bare.

The finish also affects sealant performance. Sealant adheres differently to a powder coat, an anodised surface and bare aluminium, and some coatings require specific preparation or a primer. Applying sealant to a coated surface without the preparation the sealant manufacturer requires is a common cause of adhesion failure that only appears after a season of movement.

Colour, Shadow Lines and Visual Tolerance

A movement joint is visible, and its visibility is a design decision. A shadow line, a contrasting gasket or a deliberate reveal can make the joint read as part of the architecture. Hiding it under a cover cap is also legitimate, provided the cap itself accommodates movement and does not bridge the joint rigidly. What rarely works is pretending the joint is not there: a butt joint between two coated members with a thin sealant bead will show every variation in alignment, and the sealant will discolour and crack as it takes the movement the joint could not.

Dissimilar Materials and Galvanic Risk

Aluminium in contact with steel, stainless steel, copper or lead in the presence of moisture forms a galvanic couple, and the aluminium becomes the anode. At a joint, where fixings, reinforcement, flashings and brackets converge, this risk is at its highest. The remedies are standard: isolate dissimilar metals with a suitable separator, use compatible fixings, avoid trapping water in the joint, and specify the finish system for the exposure. On coastal projects these are not optional refinements; they determine how the joint looks in ten years.

Quality Assurance: Proving the Joint Works

Documentation Before Fabrication

The cheapest place to resolve a movement joint is on the drawing. Before any profile is cut, the project should hold a single coordinated detail for each joint condition, showing the structural break, the splice or sleeve, the fixed and sliding fixings, the sealant or gasket, the bond breaker, the drainage continuity and the finish treatment. The detail should state the movement the joint accommodates and the direction of that movement, so that it can be checked against the structural engineer’s advice rather than assumed.

Fabrication drawings, shop drawings and installation instructions should all describe the same arrangement. When they disagree, site will follow whichever is easiest, and the joint will be built to neither.

Testing and Mock-Ups

Performance evidence for a window system comes from testing to AS 4284, which evaluates resistance to air infiltration, water penetration and wind pressure, and from product certification to AS 2047. A mock-up takes that evidence one step further by reproducing the project’s actual configuration, including the joints, couplings, interfaces and installation sequence, and testing it before the façade is fabricated in full. A mock-up that omits the movement joint tests everything except the part most likely to fail.

Where a project includes a laboratory or site mock-up, the joint detail should be part of it, and the test should include the movement the joint is designed to accommodate as well as the weather performance. A joint that passes a static water test but has no travel left is not a proven detail.

Site Hold Points and Records

Several stages of the joint are invisible once complete. The condition of the substrate and the preparation before sealant, the presence of the bond breaker, the position of packers, the slot allowance in each fixing and the continuity of the drainage path are all concealed by the next trade. These are the natural hold points for inspection, and each should be recorded with photographs and a signed check sheet.

Records matter for another reason: when a joint fails years later, the only way to establish whether it was built as designed is to have evidence of what was built. Projects with good records resolve disputes quickly; projects without them replace components by guesswork.

Maintenance and Inspection Over the Life of the Building

What to Look At

An expansion joint inspection is a short list of specific observations. Is the joint still open, or has it closed to nothing? Is the sealant adhered on both faces, or has it peeled along one bond line? Is the gasket still resilient, or has it hardened and taken a set? Is there debris, mortar, render or sealant bridging the joint and preventing movement? Is there evidence of water tracking at the joint, such as staining, efflorescence or corrosion product? Is the finish intact at cut edges, or has corrosion begun?

These checks are quick when access is available and almost impossible when it is not, which is why maintenance access should be considered at the design stage along with the joint itself. A joint that can only be reached by rope access will be inspected less often than one reached from a balcony.

When a Joint Runs Out of Travel

A joint that has closed completely has stopped being a joint. From that point, movement is transferred into the frame, the glass and the fixings, and the symptoms appear elsewhere: a cracked pane, a distorted opening sash, a popped sealant bead, a loosened anchor. Re-sealing such a joint without restoring its capacity simply repeats the failure on a shorter cycle.

The correct response is to investigate why the travel has been consumed. It may be that the joint was undersized, that the fixed and sliding points were reversed during installation, that a packer or a later trade has locked the joint, or that the building has moved more than the design anticipated. Each of those has a different remedy, and only the last requires structural review.

Planned Replacement

Sealant and gaskets are consumable components. They weather, they harden, they take a compression set, and they will need replacement within the life of the façade. That is not a defect; it is normal maintenance, and it should be in the building’s maintenance schedule with an inspection interval and a description of the condition that triggers replacement. Budgeting for it as planned work is far cheaper than discovering it as a water ingress event.

The Regulatory Framework: NCC, ABCB and the Standards

The National Construction Code and the ABCB

In Australia, the National Construction Code sets the performance requirements that building work must satisfy, and the Australian Building Codes Board maintains and develops it, including the Housing Provisions used for residential work. The Code is written in performance terms, supported by deemed-to-satisfy provisions and verification methods, and it is administered by state and territory authorities through the building approval and certification process.

For windows and glazed façades, the Code’s relevance to expansion joints is indirect but real. The envelope must resist weather, remain structurally adequate, and be installed so that it continues to perform. A movement joint that is omitted or built incorrectly compromises those outcomes, and the certifier is entitled to ask for the evidence that the installed assembly meets the performance requirement. Project documentation, test reports and installation records are how that evidence is provided.

AS 2047 and AS 4284

AS 2047 sets out requirements for windows and external glazed doors in buildings, and it is the standard against which product performance is generally specified in Australia. AS 4284 provides the test methods used to demonstrate performance against air, water and wind pressure, and it is the source of the test evidence a supplier presents for a system. Together they define what a product can do.

Neither standard removes the need for project-specific engineering. A tested system is tested in a particular configuration, at a particular size, with particular joints and fixings. Extending that configuration, or coupling it into a longer run, requires a check that the tested performance still applies, and that is where the manufacturer’s engineering support and the project’s façade consultant meet.

AS 4666, AS 1288 and AS 2208

AS 4666 addresses installation, and installation is where most movement problems are created or avoided: setting-out, fixing pattern, packers, sealing sequence and the treatment of joints. AS 1288 governs the selection and installation of glass in buildings, including the clearances and support conditions that keep glass from becoming a structural element. AS 2208 sets requirements for safety glazing materials where human impact is a foreseeable risk. A movement joint that is correctly designed but incorrectly installed fails all three in practice, whatever the drawings say.

New Zealand: SNZ TS 4211:2022 and SNZ 4223

New Zealand operates its own building regulatory system under the Building Act, with a Building Code expressed as performance clauses and administered through building consents. In that framework, SNZ TS 4211:2022 covers the specification and performance of windows and doors, and SNZ 4223 addresses glazing in buildings. Projects crossing the Tasman should not assume that a detail accepted under one regime will be accepted under the other, because the referenced standards, the testing evidence expected and the consenting process all differ.

A manufacturer working in both markets maintains separate certification evidence for each. Meichen International Windows & Doors, for example, reports 43 product series certified to Australian standards and 13 product lines certified for New Zealand, with independent testing through bodies including BV, CSI, NATA, AZUMA and INTERTEK, and CodeMark certification actively being pursued. For a specifier, the practical lesson is to ask for the certificate or test report that names the exact series being proposed, rather than accepting a general statement of compliance that may refer to a different product.

Failure Modes and Their Causes

The same failures recur across projects, and almost all of them originate in coordination rather than in the performance of any single component.

Specification and Installation Checklist

• Identify every location where movement must be accommodated: frame to structure, mullion splices, couplings, system interfaces and structural movement joints.
• Obtain the structural movement at each interface from the structural engineer, and state the design movement and direction on the detail.
• Define fixed and sliding points for every run, and show the slot or bracket allowance at each sliding connection.
• Detail splices to carry load in the required directions while permitting longitudinal movement.
• Specify sealants by movement capability, substrate compatibility and exposure, with bond breakers and preparation requirements.
• Confirm glazing clearances and setting block positions against the system and AS 1288, allowing for frame deflection.
• Maintain the drainage path across every joint, and keep weeps and baffles clear of sealant.
• Specify finish treatment for cut edges and isolation between dissimilar metals.
• Include the joints in the mock-up and in the performance test configuration.
• Set hold points for substrate preparation, bond breaker, packers, fixing slots and drainage continuity, and record each with photographs.
• Hand over a maintenance schedule covering joint inspection, seal replacement and drainage clearing.

Frequently Asked Questions

Does every aluminium window need an expansion joint?

Every installation needs a movement strategy, but not every window needs a dedicated expansion joint in its frame. A single window in a sheltered wall usually accommodates its movement through the frame-to-structure sealant joint, the glazing clearances and the fixing detail. The need for a specific joint in the frame itself arises with long runs, continuous framing, large units and interfaces between systems, where movement accumulates or where two different movement regimes meet.

How large should the gap be?

There is no universal figure, and any published number should be treated as valid only for the conditions it was calculated for. The gap follows from the movement the joint must absorb, the movement capability of the seal or splice, the installation temperature and the tolerances achievable on site. The correct approach is to state the design movement on the drawing and let the joint geometry follow from it, then confirm it against the product data for the specific system.

Can a joint be hidden behind a cover cap?

Yes, provided the cap accommodates the same movement as the joint beneath it. A cover cap that is fixed rigidly across a moving joint will either distort, detach or lock the joint. Caps are usually designed with a sliding engagement or a flexible connection for exactly this reason, and the cap should be included in the inspection regime because it hides the joint’s condition from casual view.

Why did the sealant fail when the correct product was used?

Because sealant performance depends on installation as much as formulation. The most common causes are inadequate surface preparation, absence of a bond breaker, incorrect joint geometry, application in unsuitable weather, or movement larger than the product’s capability. A correct product installed in a joint that is too shallow, or over a contaminated surface, will fail on schedule regardless of its specification.

Does a movement joint affect weather performance?

It does, and that is precisely why it must be designed as part of the weather envelope rather than added to it. A joint interrupts the continuity of every layer it crosses, including the water-shedding layer and the drainage path. The detail has to restore both, usually with an overlap, a baffle or a drained saddle, and the assembly’s weather performance should be demonstrated with the joint in place.

What evidence should a supplier provide?

Ask for the test report or certificate that names the exact product series, the standard it was tested against, the testing body and the configuration tested. For Australian projects this generally means evidence against AS 2047 and AS 4284, with installation guidance consistent with AS 4666; for New Zealand, evidence against SNZ TS 4211:2022 and SNZ 4223. Also ask for the standard joint details with their stated movement capacity, because a detail without a capacity cannot be verified against a project.

How often should joints be inspected?

The interval should be set in the building’s maintenance schedule according to exposure, height, access and the consequences of failure, and it should be reviewed after any event that could have moved the building: settlement, adjacent excavation, impact, fire or major refurbishment. The inspection itself is more important than the interval, because a joint that is checked and recorded is a joint whose condition is known.

Conclusion

Expansion joints are where the physics of a building becomes visible in a window system. Aluminium moves with temperature, the structure moves under load, glass tolerates almost no restraint, and water finds every discontinuity. A joint is the designed answer to all four, and it only works when the movement it must absorb has been quantified, the members around it have been given a clear fixed and sliding hierarchy, the seals have the capability to follow the movement, and the drainage path has been kept intact across the break.

None of that can be supplied as a catalogue item, which is why the joint is inherently project-specific. The length of the run, the orientation and finish, the structural regime and the exposure are all local inputs, and the detail has to follow from them. What can be standardised is the discipline: draw the joint once with its movement stated, check it against the structure, test it in the mock-up, inspect it at hold points, and maintain it on a schedule.

Get that right and the joint becomes what it should be: an unremarkable line on the elevation that quietly allows everything around it to move. Get it wrong and it becomes the point where the façade tells you, years later, exactly which assumption was never checked.

References

1. Australian Building Codes Board. National Construction Code 2025, Volume One and Volume Two, including the ABCB Housing Provisions. https://ncc.abcb.gov.au
2. Standards Australia. AS 2047, Windows and external glazed doors in buildings. https://www.standards.org.au
3. Standards Australia. AS 4284, Testing of building facades. https://www.standards.org.au
4. Standards Australia. AS 4666, installation of windows and external doors. https://www.standards.org.au
5. Standards Australia. AS 1288, Glass in buildings – Selection and installation. https://www.standards.org.au
6. Standards Australia. AS 2208, Safety glazing materials in buildings. https://www.standards.org.au
7. Standards New Zealand. SNZ TS 4211:2022, Windows and doors – specification and performance. https://www.standards.govt.nz
8. Standards New Zealand. SNZ 4223, Glazing in buildings. https://www.standards.govt.nz
9. Ministry of Business, Innovation and Employment. New Zealand Building Code. https://www.building.govt.nz

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