21 November 2023

Welsh Slate had been on the roof of Wilkins Hall, King’s College, for almost two centuries!


One of the most important buildings at King’s College, Cambridge, is now enjoying a new lease of life, thanks to Welsh Slate.


The Grade I listed Wilkins Hall, facing on to the college’s Front Court, forms part of a world-famous arrangement of buildings that includes King’s College Chapel, the Gibbs Building and the East Front to King’s Parade.


Built between 1824 and 1828 by William Wilkins, the building is a focal point for college life, providing its main dining space, as well as the venue for numerous formal dinners and celebrations attended by college members and visitors alike. Beyond this, the hall provides a key part of Cambridge’s architectural landscape and has contributed to its worldwide reputation.


But after almost two centuries, the original Welsh slates on the roof were failing, along with the stonework, guttering, windows and ceiling inside. A feasibility survey by specialist heritage architects Purcell identified that the south-facing roof slope, which is visible only from a handful of locations within the college, provided a rare opportunity to install energy-generating photovoltaic (PV) panels over the new slates.


Wilkins Hall features a diminishing course of Welsh slates which accentuates the sense of perspective to the original roof.  But the roofing had started to fail, with slate damage on the roof directly correlating with water damage to the ceiling below. Removed slates provided evidence of at least a partial re-roofing in 1897, although many appeared to date from the original construction, far exceeding the expected lifespan of roofing slates.


The plan initially was to replace 25% of the roof with new large-format 40” x 28” Penrhyn Heather Blue slates from the same Penrhyn quarry as the originals, but after a closer inspection, it was advised that the remaining 75% of the roofing would only have approximately a 20-year lifespan so it was agreed to replace them all, but still laid with a diminishing course.


Careful recording of the existing diminishing course arrangement allowed it to be re-established, with discreet additional ventilation provided at the eaves and ridge to improve airflow across the roof void. In addition, the PV panels were laid on top of the new Welsh slates on the south-facing pitch.


Alasdair Jones, architect and associate with Purcell, said: “The roof pitch was approximately 30° but given the lifespan of the original slates, we knew this wouldn’t be a problem with the replacement Welsh Slate.


“More importantly, the roof had a diminishing course arrangement to the slates, which added to the character of the building. It was essential that this arrangement was reinstated during the re-roofing works.”


He added: “The Welsh Slate is a key element of the project as making the hall watertight after increasing leaks was the project’s main driver. Using Welsh Slate allowed us to achieve this objective as part of a wider set of repair works to the hall as a whole.


“From a conservation and appropriateness point of view, it was very important that Welsh Slate was used, given that the original slates were sourced from Wales. And while the conservation approach was the main driver for slate specification, the use of a highly durable product which did not require transportation by air was very appealing.


“The client has been very pleased with the overall result. The hall is one of the most important buildings in the college and holds a large number of events. It is now watertight and much more maintainable than previously. We are confident the Welsh slates will continue to perform well for a long time into the future.”

The scaffolding and temporary roof had a three-month build time before main contractor RG Carter could complete a full inspection of the north side of the roof. The initial survey was via a drone whilst the scaffolding was being erected, but it was only once the full inspection had been completed that the true extent of the works required became apparent. This led to extra work needing to be carried out and also highlighted the extent of the gutter repair work.


As Wilkins Hall is used as the main dining hall for the students and community, it remained open at all times so the works being carried out on the roof with an open room below was testing from a Health and Safety perspective. RG Carter designed a ‘birdcage’ scaffold to span the room and boarded out all of the roof space with plywood to prevent any fall through.


The college’s decision to install PV panels on the rear-facing roof slope also proved a challenge as PV panel manufacturers do not manufacture their arrays to fit on a roof with diminishing courses, their fixings down legs being set to fit a standard tile batten centre. This led to RG Carter adding extra support within the roofing structure and the roofing contractors, White Roofing Services, cutting slates to fit around these fixed positions.


A spokesperson for RG Carter said: “Welsh Slate were extremely accommodating, taking on the order and incorporating it with another slate order for Trinity College Belfast which also had a diminishing roof, meaning that the off-cuts from Wilkins Hall would work out for Trinity, ensuring there was no waste.


“This project was logistically challenging due to various road closures around the centre of Cambridge and all deliveries needed to be on site before 9.30am. In addition, all materials had to be transported from the road outside Kings College, hoisted up a level, moved along the platform to another hoist, that lifted to various levels, with the reverse for taking down waste material, including the old slates.”

White Roofing Services spent 10 weeks installing the 500m2 of Welsh roofing slates using copper nails around dormer windows and lantern lights through the ridge.


Director Michael White said: “Whilst the roof itself was a fairly straightforward area, the project was challenging due to the site constraints and general high specification requirements, meaning quality control was of the utmost importance. But the Welsh Slate performed perfectly. We had no issues with their slates at all.”

Despite being held in such high standing as to have the building named after him, William Wilkins’ design was not without its problems. Most notably, the lead-lined parapet gutters suffered from insufficient falls, excessive bay lengths (causing increased stress to the lead from thermal contraction and expansion) and minimal ventilation to the underside of the lead. A measured site survey revealed it would not be possible to re-establish the lead-lined gutters at the necessary fall and bay lengths on account of the insufficient depth available behind the parapet.


As such, Purcell proposed the replacement of the gutters with terne-coated stainless steel gutters, laid to a constant fall. The benefit of stainless steel is that it can be formed in singular lengths over a constant fall, removing the need for junctions and the dimensional constraints caused by steps. The flat front and rear elevations of Wilkins Hall naturally lent themselves to such gutters, and this approach was continued around the central bay to the front elevation.


Purcell’s work culminated with the interior of the hall. Repairs to the ceiling and its decorations, along with specialist cleaning, allowed the magnificent ceiling to be returned to its former glory. Alongside this, specialist repair works were carried out to the stained-glass windows which dated back to the 1830s. The reinstalled conserved glazing provides a magnificent illumination to the hall and will allow it to readopt its prominent position as a focal point for collegiate and civic life within the city.


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