In the second instalment of this two-part technical feature, Roofing specialists from SIG Design & Technology (now AccuRoof) and EJ Roberts Roofing, in conversation with Architecture Today’s Technical Editor John Ramshaw, discuss how one of London’s most complex and dynamic roofs was detailed, tested and installed on site.

The complex Lucent W1 roof skilfully combines slate, zinc and hot melt to create a harmonious yet visually-striking form (ph: Dirk Lindner).

In Roofing Lucent W1: Part 1 – Design and Specification, the concept, design development, and specification of the landmark roof uniting Fletcher Priest’s Lucent W1 development in London’s Piccadilly Circus, was explored in detail. In this, the concluding instalment of the article, Trading Director Rob Edwards, Daniel Bosworth and Devon Brewin of the SIG D&T Technical Office (soon to be rebranded AccuRoof), Jeremy Cline of SIGA Slate, Simon Walker of SIG Zinc & Copper (soon to be SIG Hard Metals), and Emerson Bennett of EJ Roberts Roofing, discuss key detailing challenges and solutions, how the roof was tested, and the construction process itself, with Architecture Today’s Technical Editor John Ramshaw.

View of concealed box gutters in zinc above high-level dormers, as well as bespoke snow guard.

What were the key detailing issues relating to the roofing materials used on the project?

Jeremy Cline: To eliminate any possible risk to pedestrians from the swooping multifaceted roof planes, all the slates are double nailed and hook fixed and incorporate bespoke snow guards. Fire-retardant battens and counter battens – sourced from SR Timber – are used throughout. Some of the roof areas required a pitch of just 15 degrees, which is outside the British Standard. SIGA worked with specialist manufacturer Permavent to incorporate its Easy Slate system in these areas. This includes a unique soaker between each slate, which enables natural slates to be installed as low as 12 degrees.

Simon Walker: Unlike lead, zinc needs to be fully supported and incorporate a ventilation space. Indicative details prepared by the architect were all in lead, so the team developed hundreds of new details in zinc for the different junctions and abutments, with inspiration and support from both the Federation of Traditional Metal Roofing Contractors (FTMRC) and the Lead Contractors Association. Along the perimeter of the zinc elements, thermal movement is allowed for by bespoke stainless steel support systems. Due to concern over hot works, the zinc was designed to be applied with minimal soldering.

Detail of concealed box butter behind dormer (dotted line) showing how the slate roof abuts the side of the dormer.

How is the roof drained and how challenging was the design process for this?

Daniel Bosworth: Shedding rainwater effectively is one of the main challenges with such a complex roof. SIG Design & Technology worked with a specialist drainage consultant to ensure that arrangements for rainwater would meet British Standards. All the details were to some extent bespoke, with some requiring 3D design and the manufacture of bespoke zinc hoppers to discharge rainwater either into concealed downpipes or onto the hot melt flat roofing areas. At all times the design team had to work to meet the architect’s design intent, the wish for waterproofing to be concealed, and the need for robustness in all details.

3D view of slate roof over restaurant (translucent grey) discharging into a gutter which discharges onto a hot melt roof behind (light grey) via a bespoke zinc hopper (green).

What were the key details in this respect?

Devon Brewin: A good example is the rooftop restaurant, where rainwater from the restaurant roof and several other slate and zinc roof planes flows through a zinc gutter onto the hot melt flat roof at the end. This detail was particularly difficult due to the height restrictions and the requirement not to interfere with the restaurant interior.

Another example is the Denman Street façade where there is a series of new dormers. Here we detailed a concealed box gutter behind the dormers. This gutter is penetrated by zinc chutes which allow much of the rainwater to terminate onto flat roofs behind, minimising the need to discharge rainwater between the dormers.

Extract of BIM model showing a bespoke 3D fabricated hopper in green.

At Rainbow Corner, on the corner of Shaftesbury Avenue and Denman Street, where the original intent was for a ceramic roof to step right down to ground level, the slate roof planes step down and become grey faience façade panels. Rainwater is collected in a gutter at the top of the dormer, and additional gutters terminate into a bespoke zinc hopper, which discharges through a concealed outlet.

Daniel Bosworth: The terraces are also all waterproofed in hot melt. These were particularly challenging due to height restrictions. SIG Design & Technology developed a detail incorporating a drainage channel and specialist insulation, which met the contractor’s requirements and the necessary U-values.

Section through bespoke hopper at Rainbow Corner.

Was the roof design tested prior to construction, and if so how was this undertaken?

Emerson Bennett: The project’s city centre location is subject to high winds and the roof design had to be capable of handling wind speeds of up to 100mph. To ensure that the design would meet this requirement, we built a test rig – based on the Shaftesbury Avenue elevation – at the Vinci Test Technology Centre in Leighton Buzzard and tested to European standards of weathertightness. The rig went through several iterations and it took nearly a year before the team was able to prove the robustness of the details.

View looking down on cascading slate roof showing concealed zinc gutters and bespoke snow guards (ph: EJ Roberts Roofing).

What were the main logistical and technical challenges when it came to constructing the roof?

Emerson Bennett: Construction had to take place above and around three in-use flagship retail units, as well as behind the Piccadilly Lights, in one of the busiest parts of London. Installation often had to be reprogrammed to meet the tight construction programme. Furthermore, the complex nature of the project meant that details often had to be changed to allow for changes to the project on site. On several occasions construction deviated from the architectural and structural details modelled in BIM. Much of the roof was bespoke and had to cope with existing fabric, new steel structure and several existing historic façades which were dismantled, refurbished and reconstructed. The design team had to measure up on site as they went along, sometimes developing new details to ensure they would meet the necessary design requirements.

Roof terrace incorporating sharply-angled slate roofs with zinc cladding to the soffits/reveals (ph: Dirk Lindner).

All the roofing materials were supplied through SIG Roofing’s extensive supply chain, largely via it’s Kentish Town branch. However, the West End site location meant a separate holding area was designated for materials storage, and roofing materials were delivered from there on an as-needed basis. All our materials were craned onto the site, but the single crane was dismantled part way through the roofing works, so all 28,000 slates had to be stored on top of the building weeks in advance of their installation and worked around in the meantime.

Just some of the huge crew of EJ Roberts installers who worked on the project. The slate installers alone had over 100 years’ experience between them (ph: EJ Roberts Roofing).

How long did the roof take from design to completion?

Rob Edwards: From an initial approach in 2019 through to start on site in 2021, design and construction of the roof at Lucent W1 has taken more than four years to complete. Construction alone required monthly and sometimes weekly visits, dozens of operatives, and hundreds of meetings online and in person. Now that it’s completed, the large collaborative team that made it happen can look back on a landmark project executed with unprecedented levels of commitment and a huge amount of pride.

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