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Structural waterproofing and Ground Gas

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I’m on an interesting project at the moment involving structural waterproofing in conjunction with ground gas.

Construction has stopped and a timber frame structure is currently being stored in a warehouse in Germany (costing £000s per week).

Type B Concrete, ground gas

Why? Because Building Control recognised that no waterproofing specialist was involved in the project. I was then appointed to consult on the waterproofing on this project. Not ideal, but simple enough…

Er, no! This site was once a sewerage treatment site and soil investigation reports duly noted higher than acceptable levels of CO2. This is a Characteristic Situation 2 (CS2) meaning that some basic ground gas measures are required [BS8485:2015 Code of practice for the design of protective measures for methane and carbon dioxide ground gases for new buildings]. Simple enough?

Protective Measures for Ground Gas below ground

Er, no again! Basic measures are typically satisfied  by ventilation and/or a ground gas membrane. Ventilation in a basement structure is typically a non starter. And a ground gas membrane is also intrinsically waterproof which obviously relates to waterproofing a basement structure. Most ground gas membranes are not stuck to a substrate and not intended to oppose hydrostatic pressure.

Interestingly using a Type C cavity drain membrane system with ground gasses is typically unacceptable because any water ingress is likely to carry with it soluble gas which, once inside the structure is likely to release the gas. (The only possible exception might be Radon which would require a gas sump system).

I think I’ve come up with a solution, but I’ll wait to run it past the whole design team (architect, structural engineer, etc) tomorrow.

Read more here.

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CSSW Waterproofing Specialist

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For a good while now the British Standard when it comes to waterproofing a structure below ground recognises the need for a Waterproofing Specialist (typically stating this person should be CSSW qualified).

This still doesn’t seem to have caught on in practice – just yesterday I was on a site with a whole bunch of waterproofing in place which was not designed by appropriately qualified persons.

The following is some very brief footage of the site:

On this site the architect/structural engineer came up with some structural waterproofing designed (strongly influenced by a structural waterproofing manufacturer).

This led to use of ALL THREE structural waterproofing systems:

  1. Type A – (barrier protection; in this case a sodium bentonite sheet to the exterior of earth retaining walls)
  2. Type B – Reinforced Concrete designed to be completely waterproof (I’m yet to confirm compliance with BS EN 1992)
  3. Type C – A cavity drain membrane system

In addition to all three of the above there is a land drain installed just below the foot of the earth retaining walls.

A combination of two of the above systems is right (even necessary given the context); but issues are created in using all three. Firstly, this construction is FAR more expensive than it needs to be – I think if I’d been involved at design stage the construction cost would have been ~£100k lower! Secondly, there can be a tendency to assume that because there are so many ‘lines of defense’ that they don’t necessarily have to be done perfectly.

This brings me on the main issue I noted:

The Land Drain

It is absolutely necessary that a land drain is maintainable. This one is designed with two geofabric liners to keep silt out, but that’s not enough! A land drain must have roding points and it must be possible to jet wash the land drain in order to clear out debris which will inevitably collect over decades. Here’s a photo of one module of the land drain:

There is no way this system can be flushed or maintained (and there are no access points over a 60m run).

This means that in considering this structure’s structural waterproofing design I should completely disregard the land drain. What a waste, this land drain could have been so useful if it had just been designed correctly 🙁

The knock on effect of disregarding the land drain means that upward pressure on the underside of the floor slab (heave) must be given serious consideration. Unfortunately I don’t have time to go into that right now – I’ll save it for another post.


Getting an appropriately qualified structural waterproofing specialist in at design stage is a must.

Using a CSSW qualified structural waterproofing specialist is a requirement in order to be compliant (for NHBC, LABC and the relevant British Standards) and you will be caught out x months down the road when you try and get a warrantee on the structure.

Not only that, but using an independent specialist is likely to SAVE YOU A LOT OF MONEY rather than having manufacturers scare you into using all of their products at the same time!

Property Care Consultants provides structural waterproofing consultancy through CSSW.LONDON

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