Unitised curtain walling

How does Unitised curtain walling work, step by step?

1. 1

   Step 1: Engineer the system and freeze the design

   The facade engineer and the specialist contractor develop the system from the architect's intent: unit sizes, the joint arrangement, movement provision, the bracket and anchor concept, thermal and acoustic performance and the interfaces with roof, base, corners and any feature elements. Loads on the facade and the movement the building will impose on it are established by the designers, and the fixing back to the structure is engineered accordingly. This all has to be resolved and approved before manufacture, which is why the design freeze on a unitised facade sits earlier than on almost any other package.

2. 2

   Step 2: Prove the system in mock-ups and testing

   A visual mock-up settles appearance, colour, reflectivity and joint lines with the design team, and a performance mock-up representing a typical bay, a corner and a joint intersection is tested for air permeability, water tightness and structural performance against the criteria the specification sets. Testing is done before production release so that a failure changes a prototype rather than a facade. On most projects the tested arrangement becomes the benchmark for the whole package, and factory sampling and site inspection are judged against it.

3. 3

   Step 3: Survey the structure and set out the brackets

   The frame is surveyed as built, floor by floor, and the results are compared against the theoretical grid so the setting-out for the brackets accounts for real deviation rather than drawn intent. Cast-in channels are positioned before the floor pours, or anchors are drilled and fixed to the engineered detail afterwards, and every bracket is set to line, level and offset within the tolerance the system allows. Brackets provide adjustment in three directions precisely because frames move and drift, and that adjustment is used deliberately from the survey, not chased unit by unit up the building. This stage is the single biggest determinant of whether the installation runs smoothly.

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   Step 4: Manufacture, sample and deliver in installation sequence

   Units are assembled in the factory under controlled conditions, with glazing, sealing and any structural bonding done indoors to a quality regime that could not be achieved on a scaffold. Units are marked, sampled against the benchmark and protected for transport, then delivered in the order they will be installed, because a unitised facade is installed in a strict sequence and cannot easily take a unit out of turn. Storage on a commercial site is limited, so just-in-time delivery is normal and the delivery programme is managed as tightly as the manufacturing one.

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   Step 5: Hoist and hang the units

   Units are lifted by tower crane, by a monorail or davit arrangement mounted at the floor above, or by a mini crane, guided in and hung onto the brackets. Crews typically work from within the floor plate rather than from outside it, which is the main safety benefit of the system - very little of the work happens on the outside of the building. The lift plan, the edge protection and the exclusion zone below are all agreed before lifting starts, and wind limits for handling large flat units are set and observed. Each unit is engaged into the units below and beside it as it is set.

6. 6

   Step 6: Align, engage the joints and make the interfaces

   Each unit is adjusted on its bracket to line, level and plumb, and the interlocking gaskets are engaged so the drained and pressure-equalised joint works as designed. The four-way intersections where four units meet are the critical points of the whole facade and are made up exactly as detailed and inspected individually. Movement joints are left free to move rather than packed solid, and interfaces at the slab edge, the base, the parapet and around any opening or louvre are completed to the approved details, with fire stopping and smoke sealing at the perimeter installed and inspected as the facade rises.

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   Step 7: Test on site, protect and hand over

   Site hose testing or a similar water test is carried out on a sample of joints as the facade goes up, so that a defect is found on one floor rather than on twenty. Any failure is opened up, investigated and the cause corrected in the installation rather than patched with sealant. Protective films and coverings stay on until the risk from following trades has passed, then the facade is cleaned down, inspected and handed over with the maintenance and access strategy - because a facade that cannot be reached cannot be maintained, and the cleaning and replacement route is part of the design.