Basement waterproofing

cavity membrane and drainage


A common question in respect of waterproofing a basement conversion is, do you have to install drainage materials especially if the basement is dry.

The answer, in its simplest, unthought through reply, is; no, you don’t HAVE to install drainage materials.

However, despite the ground water conditions, if the risk of water entry is high or low, if you install a waterproofing system using cavity drain membrane as show in the image with this article, this is referred to as Type C in accordance with the british standard for structural waterproofing. Type C is a reference to the method of waterproofing and in this instance, Type C is the installation of cavity drian membrane to the internal face of a below ground structure and which its recommended, is installed complete with drainage which is designed to collect ground water ingress from the floor slab and channel it toward a sump system which would contain, one, two or even three submersible pumps.

A Type C, cavity membrane installation complete with drainage and a sump system would be fully compliant with the british standard BS8102: 2009. It would be building reg compliant, would be signed of by building control and NHBC. It would be a guaranteed system and can be supported by a 10 years insurance guarantee.

In a nutshell, incorporating drainage with a cavity membrane basement conversion installation is the only way to proceed in order it is fully compliant, would be signed off and can be guaranteed.

When wouldn’t you use drainage?

I’m often told by a customer the basement they intend to convert has never flooded and is always bone dry. The ground conditions are free draining and the soild is very permeable. The water table is well below ground level. With all these conditions it is difficult to justify the inclusion and expense of drainage to a membrane installation. My way forward to this situation is to point out the consequences of not including drainage.

1) If the consequence of a failure and water ingress is not catastrophic then the decision not to include drainage is low risk. i.e. If you do not convert to a fully fitted out, slef contained Living area but say, a storage area to keep some tools, etc.

2) If your job does not need to be signed off by local authority.

3) That you do not need to have a workmanship guarantee support the installation for a period of 10 years.

4) That you are prepared to accept you do not have a water proofing system but that you have a dry lining system with cavity membrane.

If you need any design assistance with your basement conversion project our qualified staff can provide you with technical assistance. Call 01329 235252 for technical support. Oh, and we avoid using jargon and speak in plain English!

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