Window Wall Slab-Edge Interfaces: Firestopping, Movement and Aluminium Glazing Coordination

Every window wall meets a concrete slab twice on every storey: once above the head and once below the sill. Those two lines look unremarkable on a drawing, and yet they carry more coordinated risk than almost any other part of the building envelope. The slab edge is where the fire engineer, the structural engineer, the facade contractor, the waterproofing installer, the insulation subcontractor and the certifier all have a legitimate interest, and where each of them can quietly undo the work of the others.

The purpose of this article is to explain that interface in plain terms. It deals with the decision sequence rather than a catalogue of products, because the correct detail on any given project is a project-specific outcome, not a transferable standard detail. It explains how window walls differ from curtain walls at the slab line, why the perimeter fire barrier cannot be treated as a late trade, how movement has to be divided between joints that each do one job, and why drainage, insulation, access and inspection all compete for the same narrow zone.

Nothing in the following text should be read as a specification. Fire resistance levels, acoustic performance, thermal values and dimensions are deliberately absent, because those numbers belong to tested systems, project documentation and the determinations of the responsible consultants. What is offered instead is a framework for asking the right questions early, in the right order, with the right people in the room.

Window Wall versus Curtain Wall: Setting the Context

In everyday construction conversation the two terms are used almost interchangeably, and on many elevations the visual result is similar. Technically, though, they describe different structural relationships between the glazed assembly and the floors behind it, and that difference changes everything at the slab edge.

Where the Terms Overlap

A curtain wall is generally understood as a non-loadbearing envelope that hangs outside the structural frame and spans across floor slabs, so that the slab edge is hidden behind the facade rather than expressed at it. A window wall is generally understood as an assembly that sits between the slabs, storey by storey, so that the floor edge is visible from the outside and the glazing is framed by it. Both are aluminium-framed, both are glazed, both must resist wind, water and heat, and both must satisfy the same building code. The structural philosophy, however, is inverted: the curtain wall carries its loads back to the frame through anchorages that do not bear on the slab above, while the window wall is usually supported at the slab below and restrained, rather than carried, at the slab above.

Why the Interface Is the Real Subject

That distinction matters because the slab edge is a horizontal band of structure interrupting a vertical plane. On a curtain wall, the band is concealed behind spandrel panels and the fire barrier is installed in the gap between the slab edge and the back of the facade. On a window wall, the band is part of the visible composition, and the head and sill details have to deliver weathertightness, fire separation, movement accommodation and a finished appearance in the same shallow depth. When a project mixes both systems on one elevation, or transitions from one to the other at a podium or a setback, the transition itself becomes the highest-risk location on the building.

Good coordination therefore begins with an honest description of what is being built. A specification that says “curtain wall” while the drawings show a window wall, or the reverse, will produce anchor details, fire details and flashing details that belong to the wrong system. The first deliverable on any project is not a detail; it is agreement on terminology, recorded in the facade performance documentation and understood by every trade that touches the line.

The Slab Edge as a Coordinating Datum

Treat the slab edge as a datum rather than as a surface. A datum is something that other work is set out from and measured against; a surface is something that other work is simply fixed to. The moment the slab edge becomes a datum, the questions change from “how do we cover this” to “what does everything else need from this line, and in what order”.

What the Slab Edge Actually Delivers

The slab edge provides structural support for the window wall, a fire separation between storeys, a thermal break between the interior and the exterior, a fixing zone for anchorages and fire stops, and a starting plane for flashings and membranes. It also, unavoidably, provides an uneven surface. Concrete slabs deflect under load, they move with shrinkage and creep, they are cast within tolerances that are generous by facade standards, and their edges are rarely as true as the aluminium that will sit against them.

As-Built Reality and Tolerance Stack-Up

The practical consequence is tolerance stack-up. The slab has a tolerance, the cast-in or drilled anchorage has a tolerance, the bracket has a tolerance, the window frame has a tolerance, and the fire stop and flashing each need a minimum practical gap to be installed correctly and inspected afterwards. Individually each allowance may seem reasonable; accumulated across the interface they can consume the entire available depth, leaving the installer to compress a fire stop, thin a flashing or omit a movement joint in order to close the building.

The remedy is early survey. A measured survey of the as-built slab edges, taken before fabrication is released, converts an assumption into a fact. Where the survey shows variation beyond what the detail can absorb, the design must change, not the installation. Adjustable brackets, slotted connections and explicit packer zones exist precisely so that the frame can be set true without transferring the slab’s irregularities into the glazing line, and they are only useful if the design allows enough adjustment to cover the measured range.

Fire and Smoke Boundaries at the Slab Edge

Compartmentation is the organising principle of fire safety in multi-storey buildings, and the slab edge is where compartmentation is most easily compromised. The floor slab is a horizontal separating element; the window wall is a vertical element that passes in front of it. Between them lies a gap, and that gap is a potential path for fire, hot gases and smoke from one storey to the next.

Compartmentation Logic for Window Walls

The fire strategy for a building establishes where fire-resisting construction is required, and the facade must not undermine it. At a window wall, the requirement normally falls into two related parts: maintaining the horizontal separation at the floor line, and controlling the risk that fire breaking out of a window will re-enter the storey above. The second part is a function of spandrel geometry, the extent of any vertical projection, and the proximity of openings, and it is a matter for the fire engineer rather than for the glazing contractor. What the glazing contractor must do is provide accurate geometry and honest installation, so that the engineer’s assumptions about the built form remain true.

The Perimeter Fire Barrier as a System

The perimeter fire barrier is the assembly that closes the void between the slab edge and the facade. It is not a single product. It comprises the substrate preparation, the attachment or support method, the fire-resisting material itself, the sealant or topping, and the way the assembly terminates at mullions, at columns, at slab penetrations and at the ends of a run. It also includes the restraint that keeps the material in place when the facade moves and when a fire creates pressure differences across it.

Because the barrier is a system, it must be selected and installed as a system. Substituting a different density of mineral wool, a different sealant, a different fixing spacing or a different substrate condition can invalidate the basis on which the assembly was chosen. The relevant information comes from the tested system documentation and the project’s fire engineering report, and the correct response to an unfamiliar condition is a request for direction, not an improvised detail.

Smoke Control and the Storey Line

Smoke is the more immediate problem in most fires, and smoke moves through gaps far smaller than those that admit flame. A fire barrier that looks continuous may still leak smoke at the head of a mullion, at the junction between a fire stop and a flashing, or where a membrane is dressed over a profile without a sealed termination. Smoke sealing and fire stopping are related but distinct functions, and a detail that satisfies one does not automatically satisfy the other. Where the fire strategy relies on smoke separation at the facade line, that reliance must be visible in the documentation and verified on site.

Project-Specific Determination, Not a Catalogue Number

There is a strong temptation, particularly under programme pressure, to nominate a fire resistance level from a previous project or from a supplier’s brochure. This is exactly the wrong move. The required fire resistance level depends on the building classification, the effective height, the presence of sprinklers, the fire compartments, the distance to boundaries and the concessions applied by the fire engineer. Two buildings with identical elevations can legitimately require different treatments at the slab edge. Fire resistance levels and system selections belong to the project’s fire documentation; this article deliberately states none, because none could be correct without that documentation.

Movement: Three Movements That Must Not Compete

Facades move. They move because the structure deflects, because the aluminium expands and contracts with temperature, because the building settles, and because wind pushes against the whole assembly. The slab-edge detail has to absorb all of it without breaking a seal, cracking a fire stop or loading the glass. The common failure is not that movement is ignored; it is that two different movements are forced to share a single joint, so that each one uses up the allowance the other needed.

Structural Movement

Live load deflection of the slab above a window head is a routine occurrence, and it is often the largest single movement at the interface. Long-span slabs deflect more than short-span slabs, and the deflection that matters at the facade is the differential movement between the slab carrying the window and the slab restraining its head. Column shortening, creep and shrinkage add a slower, cumulative component that becomes significant over many storeys. Wind-induced sway adds a reversible, cyclic component, particularly on tall or slender buildings.

The detail must therefore allow the head of the window wall to translate relative to the slab above without transferring that translation into the glazing. Slotted connections, sliding sleeves and compressible backings are the usual vocabulary. What matters is that the allowance is explicit, that it is stated in the shop drawings, and that it is not consumed during installation by a well-intentioned attempt to make the frame rigid.

Thermal Movement in Aluminium

Aluminium has a comparatively high coefficient of thermal expansion, and a dark anodised or powder-coated profile exposed to direct sun on a hot day will reach temperatures well above ambient. A long mullion run therefore changes length measurably between a cold morning and a hot afternoon, and between summer and winter. That movement is reversible, but it is not negligible, and it acts along the length of the member rather than across the storey height.

Thermal movement is managed by allowing the members to expand and contract within their connections, by using expansion joints at appropriate intervals in long runs, and by avoiding details that clamp a long member at both ends. It is also managed by colour and finish selection, which is a design decision with structural consequences rather than a purely aesthetic one.

Movement Joints and the One-Joint-One-Function Rule

The most useful discipline at the slab edge is to give each joint one job. A joint that is expected to accommodate slab deflection, thermal expansion of the mullion and building settlement simultaneously will be designed for the sum of three allowances nobody has calculated. Separating the functions, even when it costs depth, produces a detail that can be inspected and reasoned about: this gap takes deflection, that sleeve takes thermal movement, this sealant line takes nothing but weather.

Where separation is impossible, the combined movement must be quantified by the structural engineer and the facade designer together, and the fire stop, flashing and sealant selected on that combined basis. That conversation is far cheaper at documentation stage than during a site inspection when a compressed fire stop has already been covered by a spandrel panel.

Spandrel Panels and the Fire Stop Zone

The spandrel is the opaque band at the floor line, and on a window wall it is the element that converts an awkward junction into a controlled one. It conceals the slab edge, houses the fire barrier and the edge insulation, and provides the visual transition between storeys.

Spandrel Geometry and Glazing Selection

Spandrel geometry is set by the architect, but it has performance consequences that must be resolved early. The depth of the spandrel affects how much of the slab edge is visible and how much space exists for the fire barrier and insulation. It affects the risk of fire re-entering the storey above, because the relationship between the opening above and the spandrel below is one of the parameters the fire engineer considers. It affects thermal performance, because a large opaque band changes the balance between glazed and solid areas. And it affects movement, because a spandrel panel is a comparatively stiff element fixed across a zone where the slab is moving.

Glazing selection for spandrels is often treated as an afterthought, with tinted or opacified glass chosen for appearance alone. In practice, the glazing type, the coating and the treatment of the cavity behind it all influence how heat builds up in the spandrel zone, which in turn affects thermal stress in the glass and the temperature of adjacent components. Those decisions belong with the facade designer and the glass supplier, working from the project’s own performance requirements rather than from a default assumption.

Backpans, Insulation and the Void Behind

Behind the spandrel there is normally a backpan or liner tray that closes the cavity, and behind that a layer of insulation against the slab edge. The order matters: the backpan provides the air and water control, the insulation provides the thermal control, and the fire barrier provides the separation. Where the fire barrier is installed in the same void, the three functions must be arranged so that none of them depends on the others being perfectly executed, and so that each can be inspected before it is covered.

Backpans should drain rather than pond. A backpan that collects water and holds it against a fire stop or an insulation face will degrade both, and the resulting failure will be invisible until it presents as a stain on a ceiling or a soft patch in a barrier. Sloped pans, weep paths and a clear route to the outer face are part of the fire detail as much as the weatherproofing detail, because a wet barrier is not the barrier that was specified.

Mullions, Transoms and the Vertical Stack

At the slab edge the mullion does the hardest work in the system. It carries the dead load of the glazing above, resists wind load, provides the vertical line to which fire stops and flashings terminate, and must do all of it while the slab behind it moves.

Anchorage and Dead Load Path

Dead load should travel down, not sideways. A window wall is normally supported on the slab below, with the head connection providing restraint against wind and seismic action rather than vertical support. Where a mullion is supported at its head instead, the dead load path changes and the whole assembly must be designed for it, including the anchors, the embedment and the local reinforcement in the slab.

Anchor design is a structural matter that is frequently delegated to the facade contractor without adequate information. The contractor needs to know the design actions, the concrete strength, the edge distance available, the location of reinforcement and the fire exposure of the anchorage. Where any of that is missing, the anchor selection becomes an assumption, and assumptions about anchors at slab edges have a way of becoming project issues.

Terminations at Head and Sill

The head and sill terminations are where the mullion meets the fire stop, the flashing and the movement allowance. A mullion that terminates hard against a slab soffit transfers deflection directly into the frame. A mullion that terminates with a generous but unsealed gap provides a smoke path. The correct termination is a designed one: a specified gap, a specified backing, a specified seal and a specified fire stop termination, all shown on a detail that the installer can actually build from a swing stage or an internal scaffold.

Continuity between adjacent mullions deserves equal attention. Each bay is usually fabricated separately, and the joints between bays are the places where fire stops stop, flashings lap the wrong way and sealant is applied in a hurry at the end of a shift. Bay-to-bay continuity should be a named item on the inspection checklist, not an assumption.

Drainage and Water Management at the Slab Edge

Water management and fire stopping compete for the same void, and the resolution of that competition is one of the defining skills of facade coordination. A drained and ventilated cavity is good for weathertightness; a fully packed void is good for fire separation. Both cannot win in the same place without careful design.

Pressure Equalisation and Weep Paths

Rain-screen and drained-cavity principles rely on pressure equalisation behind the outer face, with drainage and ventilation openings sized and positioned to let water out and air in. At the slab edge, the fire barrier typically interrupts that cavity. The design must therefore either preserve a drainage path around the barrier or accept a different water management strategy at that line, and that decision cannot be left to the installer.

Weep paths should be unambiguous: a route, a fall, a discharge point and a clear space in front of it. Water that leaves the cavity and lands on a horizontal ledge, a backpan or the top of a fire stop has not been drained, it has been relocated. The sill and head details should show where water goes next, and the answer should be the outside face of the building.

Flashings, Membranes and the Fire Stop Conflict

The classic conflict is a flashing that must pass through or beside a fire barrier. Cutting the barrier to install the flashing compromises the separation; installing the barrier over the flashing traps water. The resolution is usually a sequenced detail in which the flashing is installed first, the barrier is formed around it with a tested termination, and the whole assembly is inspected before the spandrel closes it in.

Membranes add a further layer. A vapour-permeable or vapour-closed membrane dressed across the slab edge must be continuous with the wall membrane and must terminate in a way that does not create a capillary path. Where the membrane passes behind a fire stop, the fixing pattern and the sealant must be compatible with both. Compatibility is not a formality: some sealants are not suitable for use against certain insulation facings, and some fire stop materials will not accept a membrane dressed over them.

Thermal Insulation, Condensation and the Slab Edge

The slab edge is a thermal bridge by construction. Concrete is continuous from the interior floor plate to the exterior face, and unless the edge is insulated, the slab becomes a fin that conducts heat out of the building in winter and into it in summer. On a window wall the bridge sits directly behind the spandrel, which makes it easy to conceal and equally easy to forget.

Where the Insulation Belongs

Edge insulation belongs on the outside of the slab edge, continuous with the wall insulation, so that the slab is kept within the insulated envelope. Insulation placed only on the inner face leaves the slab edge cold and does not interrupt the bridge. Insulation placed in the cavity behind the spandrel must be continuous around the fire barrier, around anchors and around the sill and head profiles, or the bridge simply moves to the gaps.

Continuity is the whole point. A well-insulated slab edge interrupted at every mullion by an uninsulated bracket is a series of thermal bridges with insulation between them, and its performance will not match the design intent. Where brackets and anchors must penetrate the insulation, the design should include a specified treatment, and the installation should be inspected before the spandrel closes it in.

Condensation, Interstitial Moisture and Hidden Failure

A cold slab edge inside a warm, moist interior is a condensing surface. In humid climates and in buildings with high internal moisture loads, the risk is not limited to visible surface mould at the reveal; it extends to interstitial condensation within the spandrel cavity, where moisture can accumulate against the back of the glazing, on the fire stop, or on the cold face of the insulation. Because the zone is concealed, the first sign is often corrosion of fixings, degradation of the barrier, or water staining at a joint some distance away.

Vapour control, air control and thermal control have to work together at this line. Air leakage is often the dominant moisture transport mechanism, which is why the air barrier continuity at the slab edge deserves the same attention as the insulation. Airtightness detailing at the head and sill, sealed penetrations through the membrane and continuous tapes at laps are unglamorous items that determine whether the thermal design delivers what it promised.

Access, Sequencing and Installation Practicalities

Details that cannot be built are not details. The slab-edge zone is narrow, congested and often accessed from awkward positions, and the sequence in which trades arrive determines whether the finished work matches the drawing.

Sequencing the Trade Interface

A workable sequence usually runs from the structure outwards: survey the slab edge, install anchors and brackets, set the frame and confirm the alignment, install flashings and membranes, install the fire barrier and its terminations, install insulation and backpans, test and inspect, then close the spandrel and complete the seals. Each step creates the substrate for the next, and each step is difficult to revisit once the following one has been completed.

The fire barrier and the flashing trade in particular need to be sequenced deliberately rather than by availability. If the fire crew arrives before the flashing is complete, the barrier will be formed around an incomplete substrate. If the flashing crew arrives after the barrier, they will cut it. Naming the sequence in the project programme, and naming who signs off each step, is more effective than any instruction issued on site.

Access for Installation and Inspection

Access must be designed, not improvised. Internal access from the floor plate is preferable for inspection but may not reach the outer face of the barrier. External access from swing stages or elevated work platforms imposes its own constraints on reach, on the size of components that can be handled, and on the time available in a shift. Where access is limited, the detail should be simplified, or the inspection should be moved to a stage where the work is still visible.

Maintenance access is a separate question. Sealant joints, weeps and drainage paths at the slab edge need periodic attention, and a detail that makes them permanently inaccessible guarantees that they will be neglected. Removable spandrel liners, accessible joint lines and documented maintenance procedures are cheap compared with the cost of discovering a blocked weep path through a failed ceiling.

Quality Assurance and Verification

Fire stopping and facade sealing are both trades where the finished product is hidden, and where the quality of workmanship varies more than the quality of materials. Verification therefore has to be structured, recorded and independent of the installer’s own assurances.

Hold Points and Inspection Records

Effective quality assurance at the slab edge uses hold points: the work cannot proceed until a nominated person has inspected and recorded it. Typical hold points include slab edge survey and acceptance, frame alignment and anchorage, flashing and membrane continuity before covering, fire barrier installation including terminations at mullions, insulation and backpan completion, and final sealant and drainage verification.

Records should be specific. A photograph of a completed barrier is of limited value without knowing which storey, which bay, which elevation and which detail it represents. Photographic records tied to a drawing reference, a bay number and a date allow later investigation and give the certifier something to rely on. Where a defect is found, the record should show the rectification as well as the original condition.

Testing and Documented Evidence

Laboratory and mock-up testing of facade systems provides the performance evidence on which the specification rests, and site testing provides evidence that the installed work resembles the tested work. Water penetration testing, air infiltration testing and structural performance testing are normally carried out on a mock-up or a representative sample before widespread installation, and the results should be reviewed against the project’s stated requirements rather than filed unread.

Testing is not a substitute for inspection, and inspection is not a substitute for testing. A tested system installed with a compressed fire stop, a reversed flashing lap or a missing seal will fail in service regardless of what the mock-up achieved. The two forms of evidence answer different questions and both are needed.

The Regulatory and Standards Framework

In Australia and New Zealand, the slab-edge interface is governed by a combination of building code requirements, referenced standards and project-specific engineering. Knowing which document does what prevents the common error of treating a product standard as a compliance certificate.

NCC, ABCB and the Performance Pathway

The National Construction Code, maintained by the Australian Building Codes Board, sets out the performance requirements that a building must satisfy, including structural stability, fire safety, waterproofing, energy efficiency and safe movement. Compliance can be demonstrated through deemed-to-satisfy provisions, through a performance solution, or through a combination of both. At the slab edge, performance solutions are common, because the interaction of fire, movement and weathertightness rarely fits neatly into a single deemed-to-satisfy detail.

A performance solution must be documented, and its assumptions must survive construction. If the fire engineer’s assessment assumes a continuous barrier, a particular spandrel depth or a specific movement allowance, then the built work must deliver those assumptions. The facade contractor is, in effect, the custodian of someone else’s engineering, and that responsibility should be made explicit in the contract documents.

Australian Standards in the Specification Chain

Several Australian standards appear repeatedly in window wall and curtain wall specifications. AS 2047 addresses windows and external glazed doors in buildings and is the reference most often cited for product performance. AS 4284 covers the testing of building facades and provides the framework for the mock-up testing described above. AS 1288 governs glass in buildings, including selection, structural adequacy and safety glazing, and is central to any decision about spandrel and vision glass. AS 4666 sits alongside these in the specification chain for window and facade assemblies and is commonly nominated where interface performance must be demonstrated.

Each of these standards defines a method or a set of requirements. None of them tells a project what fire resistance level to adopt, how much movement to allow, or which system to install. Those decisions come from the building code, the fire engineering report, the structural design and the project specification, with the standards providing the vocabulary and the test methods by which the decisions are verified.

New Zealand References

New Zealand projects work within the New Zealand Building Code, where Clause B1 addresses structure, the fire clauses and their acceptable solutions address separation and means of escape, Clause E2 addresses external moisture and Clause H1 addresses energy efficiency. NZS 4223 provides the glass selection framework, NZS 3604 governs light timber framing and is relevant wherever a window wall meets a timber-framed wall, and SNZ TS 4211:2022 is used in the specification of window and door system performance. Seismic design is a more prominent consideration than in most Australian locations, and the slab-edge detail must accommodate inter-storey drift without transferring it into brittle elements.

Where a project spans both countries, or uses products tested to Australian standards in a New Zealand building, the equivalence of evidence must be established rather than assumed. Test reports, appraisals and certifications carry scope limitations, and those limitations are exactly where slab-edge details tend to fall outside the tested configuration.

Common Coordination Failures

The same failures recur across projects, and they are worth naming because each has a known early intervention.

Fire stop installed before flashing. The flashing is then cut or omitted, and neither the weathertightness nor the fire separation survives intact.
Movement allowance consumed on site. Slotted connections are packed tight and sealant is applied rigid, converting a designed movement joint into a crack.
Barrier not terminated at mullions. The run is continuous in the middle of each bay and discontinuous at every junction, which is where smoke will travel.
Insulation interrupted at brackets. Thermal performance falls short of the design intent and condensation appears at the very point that was meant to be protected.
Backpans that pond. Water is held against the barrier and the insulation, and the resulting deterioration is invisible until it is expensive.
Survey not done before fabrication. Frames arrive that cannot be set true within the available adjustment, and the facade line becomes a negotiation.
Documentation borrowed from another project. A detail that suited a different slab, a different fire strategy or a different movement regime is applied without re-checking its basis.

Specification and Documentation Checklist

The following items are worth confirming before any window wall package is released for fabrication.

Terminology agreed: window wall or curtain wall, and the structural relationship stated explicitly.
As-built slab edge survey completed and reviewed against the available adjustment.
Fire engineering report identifies the required separation at the slab edge and any spandrel-related conditions.
Fire barrier system nominated by tested reference, with terminations at mullions, columns, penetrations and run ends detailed.
Movement strategy documented: deflection, thermal, settlement and seismic components allocated to specific joints.
Head and sill details show the gap, the backing, the seal and the fire stop termination together.
Drainage path from cavity to exterior shown continuously, including around the fire barrier.
Insulation continuity and bracket penetration treatment specified, with airtightness detailing at the slab line.
Access strategy for installation, inspection and maintenance recorded, including any removable components.
Hold points, inspection records and photographic referencing agreed with the certifier before work starts.
Testing regime defined for mock-up and site, with acceptance criteria traceable to the specification.
Standards cited with their scope limitations understood, and no performance value transferred between projects.

Frequently Asked Questions

Is a window wall simply a cheaper curtain wall?

No. They are different structural systems with different load paths, different slab relationships and different fire detailing. A window wall can be economically efficient in the right application, but treating it as a substitute curtain wall leads to details that belong to neither system.

Can the fire stop be installed after the facade is complete?

In most cases it cannot be installed correctly after the spandrel is closed, because the terminations at mullions and flashings become inaccessible. Sequencing the barrier before concealment, with a hold point for inspection, is the practical approach.

Who decides the required fire resistance level?

The project’s fire engineer, working from the building classification, the fire strategy and the applicable code, and confirmed by the certifier. The facade contractor’s role is to install a system that achieves the determined outcome, not to determine it.

How much movement should the detail allow?

That is a calculated value, not a default. It depends on slab spans and deflection limits, building height, seismic and wind demands, member lengths and temperature range. Ask the structural engineer and the facade designer for the figure, and record it on the detail.

Does a tested system guarantee a compliant installation?

No. Testing demonstrates what the system can do when installed as tested. Compliance depends on substrate condition, fixing pattern, terminations, sequencing and workmanship, all of which require inspection.

How does the slab edge affect condensation inside the building?

A slab edge that is not insulated continuously becomes a cold surface, and cold surfaces condense moisture. Continuous edge insulation, a continuous air barrier and controlled internal moisture are the three levers, and all three are needed.

What is the most common cause of leaks at the slab line?

Incomplete or reversed sequencing, followed by sealant applied over wet or dirty substrates. Water that cannot drain will find a path, and a compressed fire stop or a trapped flashing gives it one.

Conclusion

The slab edge is a small band of a large building, and it concentrates an unusual number of responsibilities into a shallow, congested space. Fire separation, movement accommodation, weathertightness, thermal continuity, structural support and finished appearance all meet there, and each of them is delivered by a different trade working to a different drawing.

The projects that get this interface right tend to share the same habits. They agree on terminology before detailing. They survey the structure before fabricating. They resolve the fire strategy, the movement strategy and the drainage strategy as a single coordinated exercise rather than three parallel ones. They sequence the trades deliberately and inspect at hold points that are hard to skip. And they refuse to import performance values from other projects, insisting instead that every number on the drawing is traceable to the documentation for this building.

For owners and designers, the practical message is that the slab edge deserves design attention disproportionate to its area. For contractors and installers, it is that the quality of this interface depends on access, sequence and inspection as much as on materials. For certifiers, it is that the evidence trail at the slab line is the clearest indicator of whether the facade has been built as designed. Get the interface right and the rest of the envelope has a chance; get it wrong and no amount of quality elsewhere will compensate.

References

National Construction Code, Australian Building Codes Board (performance requirements, fire safety, waterproofing and energy provisions).
AS 2047, Windows and external glazed doors in buildings.
AS 4284, Testing of building facades.
AS 4666, referenced in the specification chain for window and facade assemblies.
AS 1288, Glass in buildings.
New Zealand Building Code, including Clause B1 (structure), the fire clauses, Clause E2 (external moisture) and Clause H1 (energy efficiency).
NZS 4223, Glass in buildings.
NZS 3604, Timber-framed buildings.
SNZ TS 4211:2022, window and door system performance specification.

Window Wall Slab-Edge Interfaces: Firestopping, Movement and Aluminium Glazing Coordination