A unitised façade (spelt unitized in the US) is a curtain wall built from large prefabricated panels, usually one storey high and one grid module wide, that are assembled, glazed and sealed in a factory, delivered to site and lifted onto brackets at each floor edge, where each unit interlocks with the units beside, above and below it. Because most of the work happens off site, a unitised façade goes up faster and more consistently than a stick system built piece by piece on the building. It is the usual choice for tall office and residential buildings and for programme-critical projects. This guide explains how unitised curtain walling works, how it compares with stick systems, and when it is worth the extra design and fabrication effort. For our own systems, see unitised façades.
How a unitised curtain wall works
Each unit is a complete piece of wall: an aluminium frame with split mullions on each side and split transoms at the top and bottom, glazed or fitted with opaque spandrel panels and insulation, and sometimes with louvre panels or opening vents. On site, brackets are fixed to the slab edge, into cast-in channels or with post-fixed anchors, and set out to line and level. The units are lifted into place, hooked onto the brackets and slid together so the male and female halves of each mullion engage.
Stack joints and interlocking mullions
The horizontal joint between floors, the stack joint, is where the top of one unit meets the bottom of the unit above. It is designed to take the vertical movement of the structure, such as slab deflection and thermal movement, without opening up or the units clashing, while the interlocking split mullions take movement between neighbouring units. Gaskets in the interlocking joints form the air and water seals, and each unit drains to the outside.
Unitised vs stick curtain walling
A stick system, or stick-built curtain wall, is assembled on the building piece by piece: mullions are fixed to the slab edges, transoms are fixed between them, and the glass and infill panels are then fitted and sealed, usually from scaffold or mobile platforms. A unitised system does most of that work in the factory. The main differences are:
| Unitised façade | Stick system | |
|---|---|---|
| Where it is built | Assembled, glazed and sealed in a factory; arrives as finished panels | Assembled on the building from individual mullions, transoms and glass |
| Site time | Much faster per floor; each floor is weathertight as soon as its units are in | Slower per floor, as each part is fixed, glazed and sealed in place |
| Access | Often installed from the floor plate, with little or no external scaffold | Usually installed from scaffold or mobile platforms outside the building |
| Lead time | More design, approvals and fabrication before anything reaches site | Can start on site sooner |
| Cost | More aluminium, because each mullion and transom is a pair of profiles, plus packing, transport and lifting; the savings come from site labour, scaffold and a shorter programme | Lower frame cost, with more site labour and longer access |
| Movement | Stack and mullion joints absorb building movement at every unit | Relies on movement joints at intervals |
| Geometry | Pays off where the same unit type is made many times | Adapts more easily to irregular shapes, one-off openings and small areas |
Unitised façade programme and cranage
The programme for a unitised façade is front-loaded. Design, structural calculations, interface details, a performance mock-up and approvals all have to be complete before fabrication starts, and fabrication has to run ahead of installation so units are ready in the right sequence. Once on site the payback is speed: installation can follow closely behind the structural frame, and each floor is weathertight as soon as its units are in, so fit-out trades can start sooner.
Lifting needs planning early. Units may be lifted by the tower crane, by a mini crane or monorail working from the floor above, or by a combination, and each option affects the crane programme, the floor loads where units are stored, the wind limits for lifting and the logistics plan. On tight London sites, with restricted delivery slots and no room to lay units out, deliveries are often called off in installation order rather than stored on site in bulk.
Quality control in unitised construction
The strongest technical argument for unitised construction is control. Glazing, sealing and assembly happen at a bench, under cover and at a comfortable working height, rather than at the slab edge in the weather. That makes it easier to inspect every unit, record checks against each unit number, and repeat the same details hundreds of times. At BDF Architectural, profiles are processed, fabricated, glazed and checked in our 60,000 sq ft West Drayton facility, then packed and despatched in the sequence the site needs them, floor by floor or zone by zone.
Testing, fire and thermal performance
A unitised façade is still curtain walling, so it is designed and tested to BS EN 13830, with evidence for air permeability, watertightness and wind resistance. Specifications commonly require a full-size mock-up tested to the CWCT test sequence before production, which is also the cheapest point at which to fix an interface problem. In England, Approved Document L Volume 2 (2021 edition) sets a limiting U-value of 1.6 W/m²K for curtain walling in new non-domestic buildings; the figure achieved depends on the frame, the glass and the proportion of glass to insulated spandrel.
Fire needs careful detailing at the slab edge, where a fire-stopping system closes the gap between the floor and the back of the units, and in the spandrel zones. On relevant buildings, broadly those with a storey at least 18 m above ground that contain flats or certain other residential uses, Regulation 7(2) requires materials in the external wall, including spandrel insulation, to achieve class A2-s1,d0 or A1, subject to listed exemptions such as glass, frames and seals. The fire strategy sets what each zone needs.
When a unitised façade makes sense
- Tall buildings, where external scaffold is expensive or impractical and work at height should be kept to a minimum
- Large, repetitive façade grids, where the same unit type is made many times
- Tight programmes, where weathering the building early lets internal trades start sooner
- Restricted city sites, where there is no room for scaffold or for storing materials outside the building
- Projects where consistent factory quality matters more than the lowest frame cost
Stick-built curtain walling is usually the better choice for low-rise buildings, podiums and entrance screens, irregular geometry and smaller areas, where the design and set-up cost of unitised work cannot be spread across enough units. Many projects use both, and where the solid parts of the building are clad rather than glazed, unitised curtain walling is often combined with rainscreen panels, explained in our guide what is rainscreen cladding. Our curtain walling guide covers system types and specification, our guide what is structural glazing explains the flush, silicone-bonded finish often used within unitised panels, and our façades guide for developers and contractors covers procurement.
Frequently asked questions
What is the difference between unitised and stick curtain walling?
A stick system is assembled on the building from individual mullions, transoms and glass. A unitised system is made from complete, factory-glazed panels, usually one storey high, that are lifted onto brackets at each floor and interlock with each other.
What is a stick system façade?
A stick system, or stick-built curtain wall, is a façade assembled on site from individual parts: mullions fixed to the slab edges, transoms fixed between them, then glass and infill panels fitted and sealed, usually from scaffold or mobile platforms. It suits low- and mid-rise buildings, irregular geometry and smaller areas.
Is a unitised façade more expensive than a stick system?
The façade itself usually costs more, because of the extra aluminium in split mullions, the factory work, packing and lifting. On tall or programme-critical buildings that is often offset by less scaffold, less site labour and a shorter programme, so compare the whole project cost rather than the façade rate alone.
How are unitised façade panels installed?
Brackets are fixed to each slab edge and set to line and level. Each unit is lifted into position by the tower crane, a mini crane or a floor-mounted hoist, hooked onto its brackets and slid into the unit beside it so the interlocking joints engage.
Which standard applies to unitised curtain walling?
Unitised façades are curtain walling, so BS EN 13830 is the product standard. Performance is usually proven on a full-size mock-up tested to the CWCT sequence for air, water and wind before production begins.
When should a unitised façade contractor be appointed?
Early, ideally by RIBA Stage 3, because the slab edge details, bracket positions, crane strategy and fabrication lead time all depend on the unitised design and are hard to change once the frame is under way.
What is a stack joint in a unitised façade?
The stack joint is the horizontal joint where one unit meets the unit above or below it. Its interlocking profiles and gaskets keep air and water out while allowing vertical movement of the structure, such as slab deflection and thermal movement, without the joint opening up or the units clashing.
Can unitised panels include structural glazing, spandrels and opening vents?
Yes. One factory-built unit can combine capped or structurally glazed vision areas, insulated opaque spandrel panels, louvres and opening vents, provided the system is designed and tested for that combination.