Case Study / Waterproofing 

Box Hill House, Dorking

External and Internal waterproofing for a new build extension

Box Hill House, Dorking

MacLennan were approached by Structural Engineering company AB34 Ltd who specialise in designs for small residential projects in Surrey and Sussex.

This project, for a family home in Dorking, was for a new build lower ground floor extension with a reinforced masonry retaining wall. The retaining walls wrap around on three sides to varying heights.

The site is at the top of a hill, with sandy ground conditions underlain with Folkestone Formation geology - sand. The ground slopes away relatively steeply from the proposed build location.

The structure would be concrete build block work and requires 2 forms of waterproofing, external and an internal cavity drain system.

Box Hill House, Dorking


Drawings of the proposed new building were sent with the initial enquiry, so our experienced surveyor could determine the most suitable waterproofing solution. The client wanted external waterproofing and it was suggested that where an externally applied membrane system is employed with this form of construction, Maclennan would always recommend the installation of a land drain, this would mainly apply to new build projects. The land drain would need to drain to a suitable soak away or a pumped system designed by a geotechnical engineer.

To the outside of the structure an angle fillet was fitted around base of walls and then PostBond Waterproofing Membrane was installed over all internal Angles both Horizontal and Vertical. Lastly, 50mm thick Insudrain would be installed followed by Geotextile filter layer prior to back fill.

For the internal waterproofing solution, it was suggested that the best form of waterproofing is an internal cavity drain system. However, as gravity drainage is to be employed for this project, there was no need to have any sumps and pumps installed to discharge water from the basement. With a gravity feed approach, there is a requirement for the cavity drain system to drain into the surface water drainage system. This can be achieved by the installation of a 110mm UG pipe through the slab connected into the surface drainage system.

With this type of draining there is the possibility of reverse flooding of the cavity drain system, this option can only be employed where there is no possibility of the drainage surcharging and backing up. Should there be any doubt regarding the gravity fed drainage system, then employment of sumps and pumps is recommended.


Our experienced surveyor designed and supplied the most suitable waterproofing solutions for this project, which was then installed by our fully qualified technicians. The new rear extension to this family home is now a grade 3 structure which is suitable for  habitable living conditions where any water ingress would be unacceptable and so drained away from the property.

The works carried out were completed within the agreed time scales, within budget and provided the company guarantees.

3d Details

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