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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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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).

Download a CPD presentation here:

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.

You can find out more about the services I provide here.

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.


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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Three main pitfalls in getting a damp survey

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Damp is a common problem, but many of us don’t know where to turn to get the right advice. Here are what I believe to be the three main pitfalls to avoid when obtaining a damp survey:

#3 – Avoid trying to come to a consensus view.

What do I mean? The thinking goes like this: “If I ask three people who know about buildings to tell me what the problem is I’ll get a majority point of view and then I’ll know what to do.

Sounds reasonable, and could work, but in reality it takes a lot of time and often you just don’t get a consensus – you get confused.

#2 – Don’t pay for a contractor to survey

I’m not having a go at contractors – most of their time is worth a lot more than you have to pay to get a survey from them (the PCA members anyway). So why not pay their fee of £90 rather than the £300 odd an independent surveyor wants?

Mostly because you’re going to want a second, and maybe a third quote and you don’t want to have to pay for those as well.

Also because, as honest and fair as the contractors I know are, there is an urge to diagnose/recommend in order to win work. Ultimately, a lot of diagnosis and recommendations when it comes to damp is subjective. When there’s a blurred line we must expect a contractor to want to fall on the side which wins work. Unfortunately this works both ways: sometimes the client doesn’t seem to want to spend money so the recommendation is only a half measure; other times the work isn’t necessarily required but the client appears willing to spend in order to have peace of mind.


Don’t buy into a ‘free survey’. I only know of a few people who provide ‘free surveys’ and these are either not qualified and therefore prone to tell you to do work which may or may not rectify the problem; or commission based surveyors who have a blatant need to sell you work.

So what is the best thing to do? I recommend instructing an independent surveyor to inspect the property, diagnose the problem and provide a clear recommendation of what to do about it WITH SUFFICIENT INFORMATION TO ENABLE A CONTRACTOR TO QUOTE WITHOUT HAVING TO CONDUCT ANOTHER SURVEY.

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Rising damp and condensation in London

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On a recent survey there was a particularly problematic wall which had received at least two chemical damp proof courses. The visual evidence of damp was very evident:

2015-02-19 11.13.13

The skirting boards have a moisture content of over 30%.

2015-02-19 11.03.42

The wall’s surface temperature is 11.7 degrees Celsius.

2015-02-19 11.12.27

The dew point is 11.4 degrees Celsius.

What does all this mean? Well if a surface is colder than the dew point you get condensation. Although this is not the case at the time of the inspection you can be sure that as outside temperatures plummet so will the internal surface temperature of the solid brick wall.

Is there rising damp? On this survey there wasn’t scope to perform a salt test etc. and  because I know there’s condensation it is difficult to state with certainty that there is rising damp. What I would say in a blogging context is that I think the treatment for rising damp hasn’t helped with the condensation problem.

Interestingly this room has a heat exchange extractor fitted which is performing perfectly well. Yes the property needs more heat; but, my question is: Why didn’t they use a different salt protection system following installing a new chemical damp proof course. An air-gap membrane would leave warmer internal surfaces; even a renovating plaster rather than dense sand/cement render would be a vast improvement.

This was the feedback I received from the client:

“Ben was available for a damp survey very promptly. I received a quick turn around of the report, and I know this was money well spent. I now have a clear plan as to what walls / issues need attention. Ben was extremely polite and helpful and I would highly recommend him to anyone / or single mothers out their struggling to know where to start with any damp they may have in their home. After being targeted by expensive external wall treatment companies and told lies to sell me their product, I now have been given the peace of mind, and a clear plan to work with, provided by Ben. He has been in contact, and I know I have him to refer too if there is anything that I don’t understand on the report or I need further advice.” – Sirita in Dartford

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Dry Rot in London

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damp corner

Condensation damp in a built in wardrobe

Last week I was instructed to inspect some damp in an apartment within a converted Victorian building. The damp was in fact condensation within a built in wardrobe.

Just 2 meters away I noticed tell-tale cracking to a skirting board. The occupants informed me that this was related to a leak from the adjacent shower cubicle some time ago and was now resolved. I removed a small section of decayed timber and found the thriving mycelial growth of Dry Rot in London.

serpula lacrymans

dry rot mycelial growth


Although the extent of growth didn’t allow definite identification I believe this is dry rot (serpula lacrymans) as a result of cracks across the grain and white fluffy mycelial growth with a pink/grey tinge.

Traditional treatment of dry rot involves stripping all timber within a 1 meter radius of any evidence of fungal growth. This is clearly very disruptive and in this situation would result in the expense of extensive replacement of flooring as well as a new bathroom suite.

There are alternative approaches. In this case a repair to the tile grout in the shower cubicle would prevent the moist environment needed for the fungus to grow; however, the fungus can remain dormant for as long as twenty years.


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When rising damp is caused by condensation

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I recently completed a very interesting survey – I just wish my photography skills could do this justice:

2014-12-08 14.30.21

My client contacted me because of a long term problem with damp in a bay window. Visible dampness to the wall under the window was noted and some defects were apparent; the most notable of which was: ground levels to the exterior have been raised at some point so that the original slate Damp Proof Course was bridged (this also caused some serious risks to do with sub-floor ventilation, but that’s not for this post).

This change in ground level would explain the injection of a chemical damp proof course; but not the current problem.

Measurement of the dew point and surface temperatures made very clear that condensation on the wall was ongoing. What was interesting about this situation is the above photo from which we can tell:

  • a chemical damp proof course was injected into the brickwork
  • beads of condensation are present on the brickwork
  • condensation isn’t being absorbed into the bricks – presumably because the chemical damp proof course has done its job!

The condensation in this area is the result of low surface temperatures which is at least partly due to the window seat which prevents hot air from the radiator (on the other side of the room) from warming this wall. (You could sort of describe this as interstitial condensation).

One solution might be simply to remove the window seat – we’ll start there and monitor the situation. This is one of the benefits of using a freelance surveyor; I’m not going out of my way to try and sell unnecessary work such as another chemical damp proof course.

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Timber Identification

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I’ve just returned from a brilliant training day provided by the Property Care Association entitled “Wood Identification for the Infestation Surveyor”.

Gervais Sawyer was outstanding. He knows the subject matter inside out and is sincerely passionate. Not only that, he was able to simplify the content so that everyone could keep up and he gave each of us sharpened chisels and eye glasses!

What’s the benefit? Well firstly I’m genuinely interested and its a fascinating subject. My first degree was in Materials Science and I love wood (my Christmas list now contains various items relating to timber identification; including a microscope). Secondly, my every day job leads to encounters with timber which is at risk of decay. The better I can understand the species and the cellular structure of the timber in question the better I can understand the situation and specify repairs.

My hope is that this detailed identification of timber will be of use in scenarios with Listed Buildings where conservation officers often require like-for-like materials to be used in repairs. I’ve previously sent samples off to Gervais for identification when working on an iconic building (Cardington Hangars) and as a result the same timber species was used for replacement of decayed timber. Here’s a sample of one side:

ts scotts pine

Scotts Pine

ts oak




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Home buyers report for properties with a converted basement

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I’ve just come back from a survey looking at a lovely basement, but with some damp in one corner.

basement damp

My investigation almost immediately revealed that the basement has type ‘A’ structural waterproofing some some sort of minor defect. For the uninitiated hear alarm bells and bad news!

I did some further investigation before informing my client that the waterproof barrier has a small defect on the other side of this wall somewhere within a 3m2 area. The lovely couple asked for the solution and I duly explained that the only real solution is to install a sump and pump and cavity drain membrane to the entire basement. This seemingly small defect will cost over £40,000 to put right!

At the same moment my client comprehended the possible cost they began bad mouthing the chartered surveyor to whom they paid £900 when they first purchased the property.

Here are my top tips for building surveyors when looking at a property with a converted basement:

1. Inform the client of what you notice.

The nature of the building survey is that you’re obliged to inform the client of any defect you notice; but that doesn’t necessarily mean diagnose.

2. Refer to a specialist.

I know you’ve been annoyed by the ‘report’ that some “damp specialists” have provided in the past but there really are some genuine specialists out there who know what they’re doing when it comes to structural waterproofing. Point your client to the Property Care Association and encourage them to find an independent surveyor who holds the following qualification: Certificated Surveyor in Structural Waterproofing.

3. Refer your client to their legal advisers

Your client needs to know whether the work carried out was done correctly. Records from building control along with reports and guarantees from structural waterproofing specialists should be collected by solicitors as part of due diligence during the conveyance. However, does your client or the solicitor know what to look for? Surely not. I think that the outstanding building surveyor would both notify the solicitor of the need for documents and inform them what to be looking for. Going further still the solicitor should supply the independent specialist surveyor with this information and ask for an opinion.

Even if the structural waterproofing was carried out perfectly, has the system been maintained. Will the guarantee company easily assign guarantees to your client when they are the owner; or will it only then come to light that the vendor hasn’t paid for proper maintenance and the guarantees are void.

Here’s my shortlist for what a solicitor should collect from the vendor:

  • copies of structural waterproofing company’s
    • report
    • amendments
    • guarantees
    • receipt confirming payment was made in full
    • maintenance schedule (this may be provided through a third party)
  • copies of sign off from building control
  • copies of maintenance records
  • a formal letter from guarantee issuing company that they will willingly assign the guarantee upon completion

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