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Tag Archives: NHBC

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Structural Waterproofing Design – Underwriters Crack Down

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Insurance underwriters such as NHBC, Premier Guarantee and LABC are all becoming more stringent on structural waterproofing design, and its delaying projects.

waterproofing below ground structure

I’m seeing an increase in the number of basement projects that insurance underwriters are unwilling to accept. In nearly every case the reason is that waterproofing design hasn’t been carried out by a structural waterproofing specialist.

Waterproofing a structure below ground is a specialist field which calls for experience, training and examination. This is typically represented in the CSSW qualification (Certificated Surveyor in Structural Waterproofing).

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BS 8102:2009 (Code of practice for protection of below ground structures against water from the ground) calls for a suitably qualified and experienced professional within the design team on any project involving below ground structures. The technical manuals from NHBC, Premier Guarantee and LABC all do the same and this is becoming more widely recognised and enforced.

Why the crackdown? Because when there’s poor structural waterproofing design and defects occur the costs are dramatic. Between 2005 and 2013, claims related to waterproofing below ground cost NHBC in the region of £21 million and affected around 890 homes [ref]. This lead to the introduction of a new chapter in their technical manual (Chapt 5.4).

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Here’s a photo of the outside wall of a multi-million pound basement in London that Property Care Consultants were called in to investigate:

poor structural waterproofing design resulting in water ingress

Water is coming in through a construction joint above a basement swimming pool. Perhaps this problem doesn’t look too dramatic, but it wasn’t limited to just one location and the remedy lost the end user at least 75mm off all walls and floors. Not to mention adding several months and about £80,000 to the construction. Fortunately this issue was picked up during construction and before fit out. What would the costs have been if the entire swimming pool/sauna/steam room complex had to be stripped out (together with the high spec ceramic tiles and interior).

On other projects Property Care Consultants have stepped into entire terrace structures had to be re-built and earth retaining walls moved because of poor waterproofing design.

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Don’t let your project become like the one above, instruct a suitably qualified waterproofing specialist early in the design stage.

Adding Property Care Consultants to a design team when dealing with below ground structures will prevent horrible waterproofing problems and produce tender documents which save far more than consultancy fees.

Property Care Consultants provides structural waterproofing consultancy through CSSW.LONDON


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Structural Waterproofing Design

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When building a new structure below ground level it’s vital that there be a structural waterproofing design which prevents ground water entering the building. This is a requirement of BS 8102:2009 Code of practice for protection of below ground structures against water from the ground.

Unfortunately, there has been a history of getting this aspect of construction design wrong and those who insure new build property have paid the price. NHBC (National House Building Council) who provide ten year guarantees on many new build properties said in 2013 that claims on basements have cost the industry over £21 million since 2005 (you can read more here).

As a result NHBC, LABC and others including Premier Guarantee have all become much more stringent on the requirements for structural waterproofing design – and rightly so!

Unfortunately there still seems to be a lag between new requirements and contractors (and architects etc.) becoming aware. This seems to result in very late invitations to the party for structural waterproofing specialists. What I mean by that is that in the past week I’ve had three calls to construction sites which are half (or nearly fully) finished residential properties where my remit has been to inspect the structural waterproofing design and confirm that it meets the requirements of BS 8102. Two of these developments don’t have appropriate structural waterproofing and putting that right at this stage is a nightmare.

What would be far better, and is in fact the requirement of BS 8102, would be to have a structural waterproofing specialist meet with the architect etc. while plans are still coming together.

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Property Care Consultants provides structural waterproofing consultancy through CSSW.LONDON


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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.

Conclusion

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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