Architect Luke Dewey of Metropolitan Workshop, and Contracts Director Tim Coakley of All Metal Roofing discuss best practice design for zinc roofing and cladding at 83 Barchester Street in east London, with Architecture Today’s Technical Editor John Ramshaw
Designed by Metropolitan Workshop, 83 Barchester Street is an exemplar project that challenges expectations for social rented housing. Comprising 115 affordable homes, the development not only occupies a difficult brownfield site set within the Limehouse Cut Conservation Area of east London, but also incorporates elements of an existing warehouse building. Equally striking are the project’s saw-tooth-profiled upper storeys and roof – all of which are finished in elZinc Rainbow Brown zinc from SIG Zinc & Copper. So what were the reasons behind this unusual choice of form and cladding? And how were the complex roofing and cladding packages developed and installed? Project architect Luke Dewey of Metropolitan Workshop, and Contracts Director Tim Coakley of All Metal Roofing, discuss these questions and more with Architecture Today’s Technical Editor, John Ramshaw.
What is the project concept and how does this apply to the cladding and roofing?
LD: The development occupies a former industrial site, which included a disused factory and warehouse. The three new buildings are organised around a central landscaped courtyard, with a six-storey structure replacing the warehouse, and four- and five-storey elements set within the existing walls of the factory. Intended to evoke the saw-tooth profile of the factory, the new zinc-clad upper floors and roof form a series of ‘spines’ that progressively rise in height from Balladier Walk to Barchester Street. This element is also conceived as a distinct metal volume that has been ‘slotted’ into the existing brick facade.
Why did you choose zinc over other types of metal roofing and cladding?
LD: We initially explored various types of metal cladding, including aluminium and painted steel, but had concerns over durability and long-term maintenance. One of the most important considerations was that the cladding should retain its original appearance for as long as possible. This led to a choice between two materials: bronze alloy or pre-weathered coloured zinc. The design team ultimately favoured zinc for its sheen-like appearance, which is very much in keeping with the site’s industrial heritage. We also admired the material’s ability to subtly absorb and reflect changing light conditions – almost as if it has a life of its own.
Why did you specify elZinc Rainbow Brown from SIG Zinc & Copper?
LD: SIG Zinc & Copper’s elZinc Rainbow Brown zinc was chosen for several reasons. First, its warm, bronze-coloured tone was felt to perfectly complement the yellow and pink hues of the existing brickwork. Second, the material’s attractive yet durable coating provides excellent protection against corrosion, ensuring longevity in use. Last but not least, elZinc is cost-effective, sustainable and 100 per cent recyclable.
How did you approach the detail design of the zinc cladding and roofing?
LD: The project was procured using a Construction management contract. Initially, Metropolitan Workshop worked with SIG Zinc & Copper to produce design intent drawings for tender. The successful specialist contractor, All Metal Roofing, then completed the zinc roofing and cladding packages before issuing a set of construction drawings. We worked closely with All Metal Roofing to refine and then approve all the roofing and cladding details.
One of the most important aspects of the detail design process was to retain a seamless aesthetic with crisp lines and minimal overlaps at eave and verge conditions. All Metal Roofing was able to adapt traditional details to more closely meet the original design intent drawings. For example, typical 150-200mm verge overlaps were reduced to around 80mm, without compromising the performance of the envelope.
What construction methods were chosen? And what are the material build-ups?
LD: The zinc facade and roof employ two different forms of construction, both of which are A1-rated and fully non-combustible. From inside to outside, the facade comprises a steel framing system (SFS), a cement particle board deck, mineral wool insulation, aluminium rails supporting trapezoidal steel sheets, a breather membrane, and zinc standing-seam cladding. The latter is laid diagonally on the saw-tooth profile and vertically on the ‘book ends’.
The (warm) roof construction is formed of trapezoidal steel sheets fixed to timber rafters, then a layer of rigid PIR insulation, followed by acoustic matting, a breather membrane, and zinc standing-seam cladding.
What were the main technical challenges?
LD: The constructional differences between the facade and roof proved challenging where the two systems met. The facade is essentially a ventilated rainscreen, whereas the warm roof system does not require ventilation. The solution was to move the facade ventilation to the back of the parapet to obscure it from view.
TC: The diagonal standing-seam zinc cladding created quite a lot of technical challenges in terms of accommodating penetrations and balconies, as well as material expansion and contraction. This required a number of bespoke details to ensure both adequate performance and onsite buildability. Added to this, the roofs had to be constructed from the top down due to their tiered formation and the shape of the building. This necessitated careful sequencing, particularly with regards to the erection and fall of scaffolding.
What is the nature of the roofing and cladding warranty for the project?
TC: The zinc facades and roofs are covered by an SIG full-system 30-year warranty. This provides excellent peace of mind to the building owner and is a testament to quality of the materials used, as well as the detail design and installation processes employed.
How did the installation process work? And how was quality maintained?
TC: All Metal Roofing employed a full time manager, as well as an inspection test plan (ITP) and a quality assurance document to ensure that the project was delivered to the highest standard. The works were signed off at designated stages or ‘hold points’. Following completion of specific elements, the installation would be put on hold and the main contractor, together with the architect and client (if necessary), would join AMR in an onsite inspection and quality assurance checks. The zinc facades and roofs took approximately 12 months to install from start to finish.
LD: Metropolitan Workshop undertook site inspections with the construction manager every week. Onsite issues were limited, as there had been a long period of technical design development where all the roof and cladding details had been carefully worked through and drawn-up. This also helped to maintain quality during the installation process. Any queries that did arise were dealt with by the site team and quickly resolved.
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