In the first instalment of this two-part technical feature, Roofing specialists from SIG Design & Technology (now AccuRoof) and EJ Roberts Roofing discuss the design and specification of one of the most complex roofs ever constructed in London with Architecture Today’s Technical Editor John Ramshaw.

Located behind the Piccadilly Circus Lights in London’s West End, Lucent W1 is a major retrofit development comprising 13 buildings (ph: Fletcher Priest Architects).

Designed by Fletcher Priest Architects, Lucent W1 is a mixed-use retrofit development uniting 13 buildings sited behind the iconic Lights of Piccadilly Circus in London’s West End. Providing more than 13,000-square-metres of retail, restaurant, office and residential space, the project not only retains and celebrates the existing structures – many of which had lain empty for decades – but also unites them beneath a landmark ‘folded’ roofscape made of slate, zinc and hot melt.

SIG Design & Technology, in partnership with DATAC-accredited roofing contractor EJ Roberts Roofing and main contractor Wates Group, designed and supplied the roof, which includes 22 terraces, a rooftop restaurant, and multiple dormer windows. In the first instalment of this two-part technical article, Architecture Today’s Technical Editor John Ramshaw discusses the roof concept, design and specification with Trading Director Rob Edwards, Daniel Bosworth and Devon Brewin of the D&T Technical Office, Jeremy Cline of SIGA Slate, Simon Walker of SIG Zinc & Copper (soon to be SIG Hard Metals) and Emerson Bennett of EJ Roberts Roofing.

The complex ‘folded’ roofscape employs slate, zinc and hot melt, and is seamlessly integrated with the various masonry and ceramic façades below (ph: Dirk Lindner).

What was the project brief?

Rob Edwards: The architects’ original design was for the roof to be a ‘ceramic lattice mantle of complex folds’ that would unify the various façades into a seamless whole, as well as articulate the entrance on Sherwood Street and other significant points around the building, including Rainbow Corner: the famous nightclub at the corner of Denman Street and Shaftesbury Avenue, which was home to US servicemen in the 1940s.

However, as the design developed it became clear that the roof concept would not meet budgetary requirements, and a decision was made to execute the main areas in natural slate. Furthermore, the project was targeting both BREEAM Outstanding, the Gold WELL Building Standard and had to meet stringent fire safety requirements. These constraints restricted most hot works and prevented the use of lead, which is the traditional method for executing interfaces and rainwater details in slate.

Flashings, cladding and rainwater goods conventionally fabricated in lead would therefore have to be carried out in fully supported zinc, together with the dormer window surrounds and sections of the roof that would need to interface with the three historic façades. Areas of the roofscape at near zero pitch would require a hot melt waterproofing solution, and some of the slate planes would need to meet falls below those conventionally undertaken in slate.

Roof plan showing areas of slate, zinc, hot melt, and historic façades.

How did SIG Design & Technology become involved with the project?

Daniel Bosworth: Wates Group needed a design team, distributor and specialist roofing contractor to work together to design, supply and construct all three high-quality roof finishes, so that all the interfaces would be covered by a single team. We have a long history of working with Wates, and were to provide the roof design and supply, with EJ Roberts Roofing – one of our DATAC contractors – as the specialist contractor. Wates was confident  that we would be able to meet all the requirements of such a complex project and had experience working together on multi-material roofs.

What were the main USPs that SIG and EJ Roberts brought to the project?

Rob Edwards: SIG Design & Technology, currently rebranding as Accuroof, is part of SIG Plc, one of Europe’s leading suppliers of construction products. We provide a complete and impartial design and supply service with access to an unrivalled choice of roofing products. There would be few other teams able to take on such a project.

Emerson Bennett: EJ Roberts is a London-based multi-disciplinary roofing contractor with over 60 years experience, including hot melt, felt, slate, tiling, hard metals, lead and heritage projects. The slaters who worked on the project have more than 100 years joint experience in the trade.

The skilfully designed roof terraces employ hot melt on the flat areas with over-sailing roofs in slate with crisp zinc detailing (ph: EJ Roberts Roofing).

How important was the collaborative aspect of the roof design process?

Daniel Bosworth: It was hugely important. In undertaking the design liability for the roof, SIG Design & Technology needed to meet the original design intent of the architect and the planning consents. A steel-framed new-build element was to infill between all the existing buildings and needed to incorporate significant movement – up to 20mm horizontally and vertically – in places. Both requirements meant that the team would have to work collaboratively from the start.

SIG D&T’s technical team, supported by SIG Zinc and Copper and SIGA Slate, and working collaboratively with EJ Roberts, developed more than 300 details, working closely alongside Wates Group’s project manager, as well as the cladding and window manufacturers. With a range of rebuilt historic facades involved, some of the detailing also required on site resolution.

Private balconies overlooking Shaftestbury Avenue (ph: Dirk Lindner).

What type of slate was specified and why?

Jeremy Cline: The slate used in Lucent W1 is a beautiful dark blue-grey Welsh Slate from the historic Cwt-y-Bugail quarry in Ffestiniog. It covers a total of 1,450-square-metres and is designed to meet the requirements of BS 5534. The main roof areas are covered in 500x250mm slates in the heavier 7mm County grade, which has a distinct riven edge giving the geometric roofscape the familiar high-quality aesthetic of historic London buildings.

At Rainbow Corner the slate roof planes step down and merge seamlessly with the grey faience façade cladding (ph: Dirk Lindner).

How was zinc used on the project and what product was chosen?

 Simon Walker: The flashing, gutters, balcony edges and other infills where slate cannot go are executed in zinc, alongside the cladding and some areas of roofing, such as behind the historic façades. The Spanish zinc product elZinc Lava was chosen by the architect as the best match for Welsh Slate, enabling the roof planes to be read as an interconnected whole.

How and why was hot melt roofing used?

Daniel Bosworth: All 22 terraces, together with the flat roof at the top of the scheme and the waterproofing of the building maintenance units behind the illuminated billboard, are covered with IKO Permatec Hot Melt roofing. Hot Melt is a monolithic hot applied roofing membrane that is usually applied as part of an inverted roofing system. This was ideal for Lucent W1 where the waterproofing had to be concealed from view.

Devon Brewin: Hot melt was favoured because it produces a completely seamless waterproofing barrier without seams or lap joints and is formulated to last for the lifetime of a building. It also has the additional benefit of being able to be installed to zero falls, and in a complex project such as Lucent W1, also has the advantage of being simple to apply without the need for complex detailing.

A 360-degree flyover of the completed roofscape (video: Wates Group).

Coming soon…
The second part of this technical feature will explore key detailing challenges and solutions, how the roof was tested, and the construction process, including logistics, overcoming onsite problems and site visits.

Contact Details
if you require design assistance on a roofing project, please call 01509 505 714, email, or visit the SIG Design & Technology website.