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."
