Case Study / Waterproofing
Hillside Basement, North Devon
Specialist waterproofing and radon mitigation
Set against the dramatic backdrop of the North Devon coastline, this bespoke residential development presented a unique engineering challenge.
The property is built into a steep hillside near a cliff edge, requiring careful consideration of ground movement, hydrostatic pressure, and environmental risks.
At the heart of the design is a full basement level that houses a luxurious swimming pool spread across three slab levels, along with a plant room, changing facilities, and a dedicated yoga space.
We were instructed by Bristol-based Chartered building company, MAKE Group who engaged us to provide specialist input on the waterproofing strategy and oversee the installation process.
Our involvement was key in developing a robust solution that met both regulatory standards and the client’s expectations. As specialist waterproofing designers, we provided the technical leadership and design expertise necessary to address the complexities of the site and ensure a successful outcome.
The site is located in a designated high radon area and was shown to have high concentrations above the acceptable levels making radon mitigation a critical component of the basement design. Additionally, the coastal location and hillside construction demanded a robust waterproofing strategy to ensure long-term structural integrity and occupant safety.
Method
MacLennan was appointed to design and install a fully compliant waterproofing and radon protection system, in line with BS 8102 2022 standards. The approach combined both Type A and Type C waterproofing methods to provide comprehensive protection against water ingress and radon gas infiltration.
The first layer of protection involved the application of a crystalline slurry (Crystal Coat) directly onto the concrete substrate. This cementitious coating penetrates the surface and reacts with moisture to form insoluble crystals, sealing micro-cracks and pores within the concrete. This initial barrier was followed by the installation of a cavity drain membrane system, which not only manages residual water ingress but also serves as a radon barrier.
All membrane joints and service penetrations were meticulously double sealed to ensure airtightness and redundancy, particularly vital in high-risk radon zones.
To further address radon mitigation, a PAC (Positive Air Curtain) system was integrated into the basement design. This system utilised gravity drainage so there weren’t any sumps. Vent pipes were installed off of the gravity drainage to enable the radon to escape, the system is designed to accommodate fan-assisted extraction, actively drawing radon from beneath the slab and expelling it outside. The PAC system is fully maintainable, with access points for inspection and future upgrades, ensuring long-term performance and adaptability.
Drainage was another critical aspect of the design, given the multi-level slab configuration of the swimming pool. A gravity drainage system was implemented, directing water through internal channels to a large external pumping chamber. From there, water is pumped up to a designated soakaway, complying with local environmental regulations and ensuring efficient water management.
Results
MacLennan completed the waterproofing and radon protection works within the agreed timeframe and budget. The system was installed to the highest standards, with all works covered by MacLennan’s comprehensive warranty and backed by a £10 million Professional Indemnity insurance, providing full design liability and peace of mind for the client and project stakeholders.
The basement remains dry and gas-safe, with a fully serviceable and future-proofed system in place. The integration of waterproofing, radon mitigation, and drainage solutions has resulted in a high-performance space that meets the demands of both luxury living and technical resilience.
Importantly, MacLennan offers ongoing aftercare, including scheduled inspections of the system and remote monitoring technology. This proactive approach allows us to identify potential issues early, before they develop into costly problems, ensuring the integrity and performance of the waterproofing system for years to come.
Conclusion:
This project showcases best practice in below-ground waterproofing and radon mitigation for complex residential builds in challenging environments. By combining advanced materials, rigorous detailing, and compliance with BS8102:2022, MacLennan delivered a solution that ensures safety, durability, and functionality.
For architects and engineers working in radon-prone or coastal hillside locations, this case study highlights the importance of integrated design, early risk assessment, and collaboration with specialist contractors.
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