Case Study / Waterproofing

Grade 1 Listed Sherbourne House

Guaranteeing Longevity with a Complete Waterproofing System

ref: Brett Charles Photography

MacLennan Waterproofing is honoured to have played a pivotal role in the restoration of a magnificent Grade I listed Georgian mansion located in the heart of Sherborne, Dorset.

This architectural gem, originally constructed in the Palladian style in 1720 for Henry Seymour Portman, had been closed to the public for nearly two decades and was in a state of significant disrepair.

However, following an extensive £3 million restoration project, the mansion has been reopened as ‘The Sherborne’, a vibrant, admission-free arts centre and visitor attraction.

The mansion has a rich history, having served various purposes over the centuries. It once functioned as Lord Digby's School, but when the school relocated in 1992 the building became redundant, leading to its gradual decline. Recognising the historical and cultural value of the mansion, a Trust was formed with the mission to restore the house, preserve it as a visual arts centre, and ensure public access to the renowned Thornhill mural.

Today, thanks to the dedicated efforts of the restoration team, The Sherborne stands as a testament to the successful preservation of historical architecture and serves as a cultural hub for the community.

Method

We were approached by our long-standing clients, with whom we have successfully collaborated on numerous projects over the years. Recognising the importance of integrating waterproofing considerations early in the design process, we joined the project design team at an early stage. This proactive involvement is crucial to ensure that the construction plan incorporates waterproofing measures from the outset. The British Standards recommend engaging a Waterproofing Design Specialist by RIBA Stage 3.

The project involved the excavation and construction of a significant basement extension at the rear of the property. This new extension was primarily designed to house plant rooms that would serve the main building and the kitchens above. Given the critical nature of the equipment to be housed in these plant rooms, it was imperative that the basement be kept dry and habitable.

Additionally, a separate outbuilding was being constructed with a ground-bearing retaining wall. This space also required our waterproofing, as both areas were considered Grade 3 habitable environments.

In accordance with BS8102:2022, which advises that two forms of waterproofing should be considered where the risk of failure is unacceptable, we implemented a dual-protection strategy. The first form of protection involved the application of our Crystalcoat system to the retaining and ground-bearing elements of the building. This system provides a robust initial barrier against water ingress.

The second, and most crucial, form of protection was the installation of a Type C Cavity Drain system. This system is maintainable and, therefore, can be guaranteed, providing a reliable long-term solution for keeping the basement dry.

Additionally, we provided expert advice on designing the service entry points into the basement. Pipe penetrations and service entries are critical areas where water ingress can occur if not properly sealed. To address this, we meticulously installed and sealed each penetration using high-quality gaskets designed to accommodate movement and ensure a watertight seal.

Results

MacLennan Waterproofing successfully installed a fully compliant waterproofing system in the basement of Sherbourne House in Dorset.

The project was completed on time and within budget, demonstrating MacLennan's commitment to efficiency and reliability.

The dual-layer protection, comprising the Crystalcoat system and the Type C Cavity Drain system, ensured the basement remained dry and habitable. MacLennan guaranteed the work, backed by their £10 million Professional Indemnity insurance, providing additional assurance to the clients.

This project not only preserved the structural integrity of the historic building but also contributed to its transformation into a vibrant arts centre and visitor attraction.