Welsh Slate wins Westminster’s vote

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.”