Refurbishment and retrofit extension in this Georgian home

23 March 2020


Basement type: Retrofit Building type: Residential
Type of waterproofing: Type B & C Member: Delta Membrane Systems
Other: Heritage  

The Project

Built around 1790, one of Alderneys most prestigious family homes is Val Des Portes. The house is said to have been the residence of the military commander of the island’s garrison. When purchased by its current owner, Val Des Portes required full refurbishment and updating the existing basement and extending it by 600m2. 

Val Des Portes’ owner chose to work closely with the Architect, Project Manager and Main Contractor and Structural Waterproofing Designers - Oaks Damp Proofing Limited. 


Delta Membranes were approached to give assistance on designing a robust waterproofing solution. The project was split into 2 phases. Phase 1, waterproofing the existing basement and Phase 2, retrofit extension to the existing basement to create a luxurious multiuse living area. Due to the partially retaining nature of the structure, the project required a combination of Type B and C waterproofing systems to ensure a Grade 3 habitable environment.

Phase 1 - Waterproofing existing basement

A ‘Type C’ Waterproofing System was selected as a maintainable waterproofing solution to the existing basement to ensure a Grade 3 habitable environment is achieved. In principle a cavity drain membrane system allows moisture or running water to travel behind the membrane within a controlled drainage system. A cavity drain system requires minimal preparation and disturbance to an existing substrate.

Phase 2 - Retrofit extension to existing basement

Type C waterproofing systems being the most reliable and maintainable approach to the project were again incorporated into the BS8102:2009 structural waterproof design. Prior to the installation of the Type C System,  all construction joints were sealed using Koster Repair Mortar Plus.

Oaks applied a layer of Koster NB 1 sealing slurry to stop migratory dampness rising from the columns. Koster NB 1 is a great product as it develops an intense bond to the substrate to which it is applied, sealing capillaries to ensure the structure is fully ‘locked down’.

Before installation of the Delta Type C membranes, all new concrete was treated with Koster Polysil TG 500 anti-lime inhibiter to seal the surfaces and reduce the leaching of ‘free lime’ which could cause blockages behind the membranes. Oaks added Delta PT as a suitable damp proofing base to the retaining wall of the garage which was to be rendered.

All floors and drainage elements were tested to ensure these would drain as the system was designed and BS 8102:2009 requirements were met.


Delta Registered Installers, Oaks Damp Proofing Limited delivered another great install in the agreed time frame and to budget. The property’s delighted owner is offered reassurance that her basement is fully protected against the long-term threat of water and damp ingress.

Technical Drawing 1

Technical Drawing 2

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