Castle Drogo

Bauder Total Roof System (BTRS)

Synopsis

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 the 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 its 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.

Bauder Building Board

Project:	Castle Drogo
Location:	Devon
Project type:	Refurbishment
Specifier:	Inskip Gee
Approved contractor:	Clegg and Shortman

The specification

The National Trust appointed conservation architects Inskip & Jenkins (now Inskip Gee) to put forward a proposal to restore the building 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, the defects present, and their cause.

The original asphalt waterproofing had already failed before the castle was completed in the 1930s. This was due to the material’s inability to cope with thermal and structural movement imposed by the building. The vertical tanking within the walls had become degraded and porous due to a reaction with the Hydraulic lime mortar hearting in the wall core. The castle was re-waterproofed with mastic asphalt in the 1960s and again in the 1980s, but the leaks continued. It became apparent that this material couldn’t cope with the stresses imposed by the building, after the failed attempts, an alternative waterproofing solution was required at Castle Drogo.

Mehmet Berker, a Project Architect had previously specified 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. This system is suitable for flat roof areas and inner wall vertical tanking, bituminous waterproofing membranes are not designed or intended for use beneath heavy wall structures, as the bitumen cap sheet would extrude if high loads were imposed upon it. However, there needed to be full waterproofing continuity between all horizontal and vertical surfaces forming the roof structure.

 

System installed

Doug Ross, former Technical Director at Bauder, submitted a unique proposal to Inskip & Jenkins. His solution involved combining the Bauder Total Roof System with a compatible high-performance damp-proof course (DPC) and cloak system. Having previous experience working with a high-performance bitumen polymer 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. This posed a challenge of, how to connect two different waterproofing systems since Permabit DPC is unable to withstand the high temperatures associated with the use of a gas torch when installing the Bauder bitumen membranes.

Bauder became first involved with the Castle Drogo project in 1997. During this time, Doug was developing the application of Bauder self-adhesive bituminous membrane products, to be installed using hot-air technology. It soon came to the realisation, that this type of membrane could adhere to the Permabit DPC and its associated pre-formed cloaks using hot air to provide secure adhesion without causing heat damage to the products. Doug investigated further and found that the torch-applied roof membranes could be directly bonded to the self-adhesive membranes. This meant that all of the key components had the potential to be bonded together to provide a functional waterproofing solution.

Other challenges arose, including improvements to the drainage performance and surface run-off. As some of the outlets had been sealed off, the functioning ones were forced to handle more water than they could cope with. This led to high volumes of water being discharged from the gargoyle outlet spouts, resulting in them blowing back and saturating the external walls below. To resolve this, finding a way to control, balance, and reduce the water flow to the outlets for each roof area was essential.

The original granite paving was bedded in mortar, with most of the slabs being removed in the past to allow access to carry out repairs, but never replaced. Our solution involved laying the paving slabs on crushed gravel bedding above a drainage layer. This made the drainage permeable and reduced the surface run-off during periods of intense rainfall. The slabs were set to the correct levels to restore the original surface. To facilitate the installation of the upstand detailing, the abutment walls had to be opened up in short sections, excavating back to the internal wall and removing the rubble core to expose the original asphalt tanking to the inner wall surface. These opened sections were usually only 1-1.5 meters in width and less than a metre in height to provide the necessary working access.
 
 

The installation

Three stages of restoration 

Castle Drogo’s roof refurbishment was carried out in three phases. The first stage began in 2006, as a trial run on the Chapel roof. Several detailing challenges were found on the chapel roof which existed elsewhere on the entire castle roof including parapet walls, doorways, and abutment wall tanking alongside the complexity of the custom-made DPC cloaks within the wall structures. This pilot phase was important for helping all parties involved in the project to work out and understand the best methods for sequencing and efficiency. Proving successful, the Chapel became the first roof in Castle Drogo to be watertight.

Phase two, in 2013, involved replacing more than 1,100m² of waterproofing on multiple roof areas set at differing heights that were all linked together. Some of these roof areas were at ground level, meaning that different detailing was required.

Phase three, in 2017, saw the completion of the roof’s remaining areas. This involved complex detailing, plus 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.

Installation of the new waterproofing proved to be challenging throughout, starting with the structural concrete roof deck needing alteration to receive the sheet membrane system, through to the granite blocks that concealed the waterproofing. 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. This was a task beyond just manpower, so a gantry crane was required. Over the three phases of the refurbishment, the scale of the replacement waterproofing was equivalent in size to two football pitches.

 

The result

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.
 

Advocacy 

Maurizio Coglia of Inskip + Jenkins Architects:
“Following evaluation, we decided to develop a strategy with Bauder for the design of the waterproofing system to ensure sound performance and longevity of the construction and to minimise future maintenance requirements. Above all, the choice of the Bauder system was determined by its resistance to differential movement, this being the main cause of the failure of the original asphalt waterproofing installed at Castle Drogo. The other advantages of the Bauder system have been its easy application and compatibility with the IKO damp-proof courses and cloaks used within the wall structures. The architectural benefits are significant: The Bauder hard landscape proposal included inverted insulation to eliminate condensation issues that affected bedrooms immediately below the roof.
 
The Bauder system allowed the reinstatement of the granite paving finish and has recovered a key element of Lutyens’ original design. Most of the original roof paving was lost following years of waterproofing repairs. The continuity of the new waterproofing from the roof into the internal wall structures and parapets has also allowed the removal of the lead DPCs installed by the National Trust during reroofing works in the 1980s and this has dramatically improved the castle’s appearance and returned it to Lutyens’ design of sheer uninterrupted walls of granite. During both design development and construction, Bauder’s service has proved to be excellent. Bauder has provided continuous quality control of the application and design consultation in tight coordination with us to resolve all the technical and detailing challenges that such a complex building has revealed to us.”