Castle Drogo was the last castle to be built in England in 1930, located on the edge of Dartmoor National Park. This Grade I listed building is now under ownership of the National Trust, designed by Architect Edwin Lutyens for Julius Drewe. Built from local granite, this building featured one of the earliest serviceable flat roofs that suffered from water ingress since it’s construction. Urgent remedial work was conducted over three phases to save the building from further damage. The refurbishment project began in 2006 and was finally completed in 2021.
Understanding Castle Drogo
The National Trust appointed conservation architects Inskip & Jenkins (now Inskip Gee) to put forward a proposal to restore the building back to Lutyens’s original design. Repairing the defects and preserving the original materials where possible.
To restore the waterproofing integrity at Castle Drogo, carrying out research through non-destructive, investigation methods was valuable. The research enabled an understanding of how the building had been constructed, and defects present and their cause.
The original asphalt waterproofing had already failed before the castle was completed in the 1930’s. This was due to the materials inability to cope with thermal and structural movement imposed by the building as the vertical tanking within the walls had become degraded and porous.
Mehmet Berker, a Project Architect had previously specified Bauder Total Roof System (BTRS) on other historic building projects and was curious to establish if this bituminous waterproofing system had the potential to solve the complex detailing issues at Castle Drogo.
Bauder became first involved with the Castle Drogo project in 1997 when Doug Ross, Technical Director submitted a unique proposal. This involved combining BTRS with a compatible high performance DPC’s and cloak system.
Having previous experience working with a DPC product called Permabit (manufactured by IKO), Doug identified that this product could withstand the structural loads of the walls, whilst being chemically compatible with the bituminous Bauder waterproofing products.
Three stages of restoration
The roof refurbishment at Castle Drogo was carried out during three phases. The first stage began in 2006, as trial run on the Chapel roof as several detailing challenges were found which existed elsewhere. Detailing included: parapet walls, doorways, abutment wall tanking alongside the complexity of the custom made DPC cloaks within the wall structures. Proving successful, the Chapel became the first roof in Castle Drogo to be watertight.
Phase two in 2015 involved replacing more than 1,100m2 of waterproofing on multiple roof areas set at differing height levels that were all linked together. Some of these roof areas were at ground level which presented some different detailing challenges.
Phase three in 2019 consisted of the completion of the roof’s remaining areas which required complex detailing and linking the waterproofing to the stepped DPC cloaks to the abutment walls of the spiral steps leading up to the roof of the north tower.
Throughout the roof’s refurbishment, a total of 2,355 granite blocks weighing 680 tonnes had to be individually numbered, removed and being unique, reinstated back in their original location. Over the three phases, the scale of the replacement waterproofing was equivalent in size to two football pitches.
A successful outcome
The dedication and collaboration between all parties involved in this project was the key to its successful completion. Turning a concept, into a robust, functional waterproofing system required expertise from many contributors. Approved contractors Clegg and Shortman installed the waterproofing on all three phases, thus providing continuity throughout the project. The roofers worked in tandem with skilled masons to ensure all the finished work was sequenced, completed correctly, inspected, and recorded photographically before replacing stonework. This complex waterproofing installation with its intricate interface detailing will never be seen by the many visitors to Castle Drogo. It remains completely hidden beneath the granite stonework, keeping this unique building watertight for many years to come.
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