Welsh Slate | Case Studies | NBS BIM Library

Some 92,000 Cwt Y Bugail slates feature on a ground-breaking new/old roof. Formerly a seedy area worth avoiding, London’s Kings Cross has now been transformed into a go-to destination for office workers, students and tourists. Developer Argent’s regeneration of the area has sensitively re-purposed the early Victorian railway buildings into variously, a college campus, supermarket and offices, in which the interior changes are radical but the exteriors have been left largely unaltered. Not so for Coal Drops Yard, a pair of elongated Victorian coal warehouses originally built to distribute coal from the North of England across London. Here, the pitched roofs of two almost-parallel buildings, 150m and 100m in length and 39m apart, have been reconfigured to curve up at their northern ends and gently kiss each other. Designed by Heatherwick Studio, the roofs peel upwards and extend into the middle of the space between the two buildings. A new floorplate sits below the roof structure, with glazing filling the gap between the two, creating a third level of retail. The new roofs of the ornate cast-iron and brick structures have been slated with 92,000 new versions of the original Welsh Slates – 500mm x 250mm Cwt Y Bugails from the manufacturer’s Llan Ffestiniog quarry in North Wales – by Banbury-based specialist sub-contractor Attleys Roofing. Attleys already had some Kings Cross experience, using 6,400 500mm x 300mm Heather Blues from Welsh Slate’s main Penrhyn quarry for the re-roof of the nearby German Gymnasium - the first purpose-built gym in England - which is now enjoying a new lease of life as a designer restaurant. Coal Drops Yard was a whole new ball game, testing Attleys to the limit, but the result is an extraordinary reinterpretation of the canal-side site. Main contractor BAM Construction was responsible for the structure of the new roofline. More than 50 new steel columns were carefully threaded through the existing structure to support the roof independently of the building, a point cloud survey being carried out to help calculate where to put the columns without clashing with the existing building fabric. Primary support is provided by a set of large cranked beams supported on cores in each building which join in the middle over the yard. There are four primary beams, two on each building. These dip down towards the centre to create the valley between the two roof structures and have been nicknamed “giraffe beams” in reference to their angled neck and head - the structure looks like two giraffes rubbing noses together. Tie beams at floor level take the tensile loads generated by the weight of the roof. The giraffe beams support two ribbon trusses that define the upper and lower edge of each roof structure and connect in the middle over the yard at the lower edge. These are 7m deep in the middle and taper to 5m at the ends where these join the existing roof. Tubular steel sections were used to create the trusses because of the complex geometry. The new floorplate below the roof is suspended from above using macalloy bars and is tapered towards the perimeter to minimise the impact on the view. More than 60 panels of full-height structural glazing between the floor and new roof are stepped rather than curved or faceted for aesthetic reasons and has the added benefit of hiding the macalloy bars. Temporary trusses were erected to support the giraffe beam assembly and ribbon trusses during construction. The giraffe beam assembly was erected first. The ribbon trusses were brought to site as components, bolted together on the ground into fully-assembled sections complete with rafters, craned into position and bolted together where they meet in the middle. The whole roof structure was then de-propped. Visually, it was important to maintain a seamless transition from the existing roof to the new section so the original timber roof trusses were retained where possible, with some localised strengthening required where the timber had been damaged. The gap between the existing and new roof structure was then boarded over ready for the new Welsh slates.The two roofs are bolted together where they meet in the middle. Heatherwick Studio chose to position the new roof element at the northern ends of the two buildings as the eastern building had been devastated by a fire in 1985. Used more recently for warehousing and nightclubs, they were largely abandoned in the 1990s. Group leader Lisa Finlay said: “Our challenge was to radically remodel this Victorian infrastructure to meet the needs of a modern urban development without losing what made them special. To do this, we focused on understanding their original function and how they were adapted over time so we could appreciate how best to preserve and reuse the existing fabric, whilst also introducing new elements. One of which is an entirely free-standing new structure threaded through the historic buildings, from which a spectacular new third level is suspended.” Attleys were on site for a total of almost 18 months. The roof pitch of the original sections of roof was 28° but as the curves swept around and met in the middle this changed to 47°. Thereafter the length also decreased from 8.4m to 6.9m at the kissing point where the two curved roofs met but the same number of courses of slates (44) had to be maintained. This meant Attleys had to decrease the gauges and not only cut the sides of the slates to take them around the curve but also the tops of the slates to maintain the same number of courses. No clever nailing or hidden bibbing was required to stop rainwater flowing diagonally at this point as the pitch was so steep. Attleys’ managing director Shaun Attley met with Heatherwick Studio and BAM’s design team up to two years prior to commencement on site to discuss the design issues and how to achieve the aesthetics requested while using the products selected. Shaun advised the project team that the Cwt Y Bugail slates could not be used for their initial roof design which exceeded a 90° pitch so it was re-designed to ensure that where the eaves met it was at a pitch commensurate with Welsh Slate’s fixing recommendations and warranties. Due to space being at a premium, all the Cwt Y Bugail slates were cut off-site at Attleys’ depot in Banbury. To do this, Attleys had to calculate how much of the slates needed cutting off. This was done by taking measurements from the steel work underneath - from steel to steel at the eaves and steel to steel at the ridge which was generally over 10-15m depending where you were on the curve (on the internal curve or external curve). The kissing point determined how the slates were cut to ensure the perp lines and side laps were maintained. And because the roof pitch was increasing while the rafter length was decreasing, Attleys also had to cut the tops of the slates down so they suited the decreasing gauges. A total of 32,000 of the 90,000 slates used on the project had to be cut with hand guillotines so the dressed edge could be maintained around the curve which comprised 1,600m2 of the total 4,600m2. Shaun Attley said: “Our timescale for the sections of roof was provided to BAM Construction and this was put into their overall programme with all other sub-contractors but unfortunately our commencement on site was delayed due to previous issues in the construction of the steel frame prior to us starting. “This meant we were up against the clock in trying to pull back time to ensure the overall programme was still met and we were asked by BAM to find ways of doing as much as possible off-site and increasing labour and production on-site to pull back the time lost. By working together with other sub-contractors and the main contractor we managed to finish a week earlier than our anticipated 43-week overall programme.” He added: “The project was challenging at the beginning to ensure we set the roof out properly but as the contract progressed it became easier. Welsh slates are easy to work, and we work with them all the time, but in this case it was challenging getting the slates to course all the way round. Attleys’ SMSTS-trained supervisor ran the day-to-day safety, conducting Toolbox talks every morning and attending daily safety briefings by BAM Construction staff and supervisors from all trades on site to brief each other of any risks and ensure trades were not disrupting one another. The team of 10 Attleys operatives, which included two NVQ Level 2 apprentices, carried out daily visual inspections of tools and all power tools were PAT tested every three months. Shaun Attley visited weekly to inspect the job for workmanship and production and also to gather feedback on any safety issues or to see if any safety measures could be instigated to improve the overall safety for their and other operatives on site. “We found some of these measures not only improved safety but also helped improve production,” said Shaun. Attleys also had an independent safety inspector (from The Health & Safety People) visit site monthly to see if there were any safety improvements they could recommend. Far from handling eight million tonnes of coal a year, Coal Drops Yard is now forecast to pull in 12 million visitors a year.

Two types of Welsh Slate feature on the new roof of London’s Irish Embassy. The £2.5 million re-roof of the Grade II listed Irish Embassy in London, with more than 10,000 Penrhyn Heather Blue slates from Welsh Slate, has required its own exceptional levels of diplomacy. DarntonB3 Architecture had multiple challenges to factor in when it came to specifying the replacement slates, including the City of Westminster’s planning department, which was keen to see as many of the existing slates re-used as possible and required convincing to embrace the “new” metric sizes. Then there were landlords Grosvenor Estates, leaseholders the Department of Foreign Affairs and Trade, and Historic England, not to mention the site being opposite Buckingham Palace, on a “Red route” with diplomatic bays nearby, and between two of the most important Conservation Areas of Belgravia. Work began on the landlocked Grosvenor Place site in March 2018, a year after DarntonB3, who are specialist conservation architects, were appointed to oversee the project, and has just completed (January 2019). Two types of Welsh slates have been used on the former terraced town mansion with decorative metal roof crest that was designed by architect Thomas Cundy, who was surveyor to the Grosvenor Estate at the time, and built in 1868 in a French Renaissance style. Roofing contractor Mundy Roofing was involved at an early stage in the project and were ultimately appointed as principal contractor. Specialising in leadwork and natural slate roofing, they were pivotal to the project’s success. A total of 600m2 of County-grade 500mm x 300mm slates have been used on the numerous standard 30° to 35° pitched roofs while Capital-grade bespoke arrow-head slates of the same colour but sized at 400mm x 250mm were used around dormer windows on the 80° mansard roof elements that are reminiscent of Paris and feature lead secret gutter detailing around the perimeter. There is also some vertical slating to the rear elevation. The old 5mm to 7mm thick slates had been on the roof since it was first built 150 years ago. The rectangular ones had been slightly longer and narrower, at 510mm x 255mm, while the arrow-head slates had been smaller all round, at 350mm x 200mm. But a roof survey showed they were not laid to the correct bond or headlaps and fixing points were far from ideal and non-existent in places (the headlaps between 0 and 50mm), leading to the hardened sarking underneath becoming sodden in places. In fact, most of the roof had been repaired over the years with different types of slates using temporary lead tags or painted with a bitumen solution in an attempt to prolong its life. DarntonB3 argued that if the roof was re-installed as existing, its appearance would alter as they would have no option but to lay the slates at the correct headlap which would create additional courses. In addition, using the slightly larger arrow-head slates for the mansard roof would enable them to form a more robust detail at the abutments to the dormers and party wall. Home to the Irish Embassy for the past 70 years, the building comprises office and entertainment space. The traditional timber truss roof featured timber sarking boards with penny gaps, a form of construction usually found in Scotland. The slates were then fixed with copper nails directly to the boarding without any timber battens. Once city planners had agreed to 100% replacement of the Welsh slates, at the new metric sizes (a process that took a year), the addition of timber counter battens, to improve ventilation of the roof and prolong the life of the new slates, was also proposed by the Architects. Mundy Roofing produced sample comparison mock-ups to demonstrate to the conservation officer this change would not be detrimental to the building’s character. As it is, the interface details where slates have been lifted has not altered the character of the building and the introduction of battens would not be known by the general observer. DarntonB3 senior associate Matthew Jones, who was project manager and lead architect throughout, said: “Westminster City Council are regarded as one of the leading conservation-led councils in the UK, with some of the highest standards and criteria to meet, and dialogue with the conservation officer was detailed and robust. The need for wholesale replacement of the existing slate due to them being at the end of their life was a delicate decision and sufficient evidence of this necessity was proven. The replacement of the slates with metric sizes was also an extensively discussed item but the principal contractor and Welsh Slate worked with us to develop the narrative.” Keith Hamilton, an architect accredited in building conservation, acting for DarntonB3 alongside Matthew was reasonably sure Welsh slates had been used previously but was keen to ensure the correct thickness and grading were eventually used throughout the renewal process. He said “We have specified Welsh Slate on numerous other projects and their reputation for the highest quality precedes them. We were able to argue the merits of increased ventilation behind the slates using cross battening in lieu of direct nailing to the existing sarking board, which in the majority of areas had survived over 150 years’ performance. “The risk of lack of ventilation on the lower roof pitches behind the slates was particularly relevant at the eaves and head. We had previously considered introducing slate vents and felt underlay to augment any need for ventilation but the existence of the ‘penny gaps’ in the sarking boards encouraged us, to believe that this was not required. “This was another point of continued discussion with the conservation officer who was against an underlay in this instance. The timber sarking was found to be in remarkable condition considering the lack of existing underlay and the direct fix of the slates, highlighting the quality of the original slates. Hence, there is no secondary layer apart from the slating itself and we trust the quality of the new Welsh slate will replicate the existing quality and last another 100 years.” Due to tight access on the roof, they were also able to widen the lead gutters and set back the lower courses of slates to avoid getting them broken. All the new Penrhyn slates were holed and traditionally fixed with 38mm copper nails as opposed to clipped or other methods. The standard-size slates were able to cope with the wide variation of roof pitches and new rooflights encountered by varying the lap and gauge slightly. In virtually every case, the slate junctions are with lead or copper flashings and as it was appreciated there is some risk of staining, all lead was treated with patination oil. Using new treated timber battens for fixing the new slates proved a great success as direct fixing into the old hardened sarking boarding would have been a major problem and time consuming. Matthew said the support they had received from Welsh Slate had been “fantastic” and included a site visit to match the type of slate, a letter explaining the need to change from imperial to metric sizing, technical drawings of the arrow-head slates, technical information on the end life of slates and their unsuitability for re-use, on-time deliveries with little if no wastage, and recommending experienced slating contractors. Mundy Roofing were on site for a total of 10 months. Work included the rebuilding of three chimney stacks, involving 30 tonnes of stone and brickwork, and restoration of traditionally-forged wrought ironwork to the pavilion roof crest, all underneath a temporary roof. Russell Mundy said: “This project was extremely challenging due to it being a working embassy but Welsh Slate were excellent with their support in achieving planning consent and the product has received widespread praise from the client.” Matthew said: “Due to the extremely difficult access to this roof, we were conscious to use a slate that will require little maintenance (if any) and satisfy appearance for an extended period of time. Welsh Slate were able to provide technical studies comparing different types of slate and their longevity. This enabled the landlord, Grosvenor Estates, that the new roof should outlast the previous roof and match it entirely with other buildings nearby. Welsh Slate’s evidence on the existing slates being at the end of their useful life, and the lifecycle information of the new slates, helped give the conservation officer comfort that the right approach was being taken for the building.” Keith added: “Essentially, Welsh Slate ensured we got the right slate, quality, consistency and sizes for the varying roof pitches and conditions. There have been no problems with mixing batches or colour variations which can occur. They also met the stringent programme requirements, reducing risks of delay from the main contractor. To our knowledge, there have been no rejects on quality of slates. “The final result is the new slating looks exactly like it was envisaged in 1868, except with the knowledge it is better-fixed and easier to reach for maintenance, with a discreet fall arrest system fitted. The client is extremely happy with the quality of the final works.” Andrea Fox, senior architect with the property management unit of the Department of Foreign Affairs and Trade, said: “The team have provided us with an exceptional and beautiful project that we know will stand the test of time and one we are extremely proud to have commissioned.” “I want to thank the Welsh Slate team for the support they provided during our project and especially in relation to protracted issues relating to the listed building consent approval.”