Why Trace Cannot Install External Tanking Membranes (only) in Residential.

Let's start at the beginning. Waterproofing in the UK is in a sense governed by the British Standard 8102 (2009 revision) Code of practice for the protection of below ground structures against water from the ground.  There are a variety of other design guides in existence, however BS8102 is an 'approved code of practice', and therefore has an elevated legal status.  While it is not 'law' that you must comply with its guidance, if you do not, you had better have a good reason why. If we are responsible for the waterproofing design systems on a project; do not comply and do not have good reason why, and issues subsequently occur, then the guidance detailed within BS8102 is likely to be employed against us to prove liability. Phil Hewitt is the leading expert on such matters, and an example of the above that Phil was involved in, is detailed at the following link (click). Part of the guidance within BS8102 relates to environmental grades and how dry a space needs to be for a given usage.  I wrote a post on BS8102 environmental grades here (click), but the TLDR (too long didn't read) version is that for habitable space, no water penetration is acceptable.  For storage and electrical plant rooms, no water penetration is acceptable.  The only usage in which some seepage can be acceptable, is car parking and basic plant rooms/workshops, or equivalent spaces. In essence, for most spaces (particularly residential) water penetration is not an option and even minimal penetration constitutes failure of a waterproofing system.  Therefore, in the majority of circumstances it is extremely important that penetration does not occur. We promote the fact that we take total responsibility for all aspects relating to waterproofing on a project. What this means is that we provide the design (backed by £2.5m professional indemnity insurance), installation by in-house staff, maintenance, guarantees, and true FCA regulated third party insurance on those guarantees through GPI. In summary there is a single point of responsibility.... Trace. If there is a single point of responsibility and it is ours, and a grade 2 or 3 space leaks, then as per BS8102, obviously we have a problem.  If we did not resolve this under the terms of our guarantee (we guarantee the standard of environment), the likelihood is that we would be pursued legally and found liable. For this reason we are motivated to install systems which will not fail, and we will not consider the installation of systems which can fail in such a way that in the event of any issue, they cannot be viably repaired and so have to be replaced wholesale with a new system at considerable expense. In fact, by acting in this manner, we better comply with the aforementioned British Standard, because BS102 includes within it, a design flowchart which lists of factors to consider.  At the end of the process it asks: Has repairability been addressed? Yes - then you have a solution.  If it is a 'No' you are taken back to the start of the process. Despite this, waterproofing is commonly designed in such a way that there is no viable strategy for repair, and as a result, when systems do fail, the costs to remedy are substantial. This is because to achieve a reliable solution, it is typically a case of stripping out all internal fixtures/fittings/finishes/plasterboard/flooring etc., to get back to the structure, to then install internal cavity drainage waterproofing to all wall and floor areas, before putting everything back together and moving the homeowners back in again.  Essentially you are completely starting all over again with new waterproofing. The best illustration of this fact, is that NHBC, the largest provider of ten year structural insurance cover have published statistics relating to their basement claims.  NHBC have spent £21 million on basement claims since 2005 (click to read article). 62% of these basement claims were associated with failed tanking barrier systems, and this would reflect our findings in commonly dealing with waterproofing failure scenarios. It is these tanking systems which commonly provide no viable strategy for repair, and despite the design guidance within BS8102, are employed anyway. Why is this? In new construction there is a common desire to block water out and to keep the structure dry.  A popular way of seeking to achieve this, is to use a barrier tanking system, which is applied to the external face of the structure, thus keeping the structure and internal basement space dry. The 'structure' which receives this tanking, is often a reinforced concrete raft foundation, then with retaining walls formed over in reinforced masonry.  For those that aren't technical, a concrete raft is essentially a large slab of concrete containing steel reinforcement, that the walls are built upon.  Reinforced masonry walls are simply formed by constructing two leaves of block-work, and then filling the cavity in-between with concrete and steel reinforcement. A concrete raft typically provides reasonable resistance to the penetration of water in its own right, because concrete as a material is relatively impermeable.  It tends to be cracks and joints which allow penetration through it.  Therefore, a monolithically poured (no joints) raft of substantial depth with structural reinforcement (limits cracking), laid over an impermeable membrane, can provide an effective barrier to water. Reinforced masonry wall construction is popular because it is a simple way in which retaining walls can be constructed, this versus hiring/purchasing and employing the shuttering required to form cast in-situ reinforced concrete retaining walls. The limitation of reinforced masonry is that it is a permeable form of wall construction.  If joints form a typical weak point to allow penetration in any structure, every mortar bed in both leaves of block forms a joint, the depth of the concrete core is typically circa 150mm and often with only one layer of steel reinforcement. There is an unavoidable joint between the wall and the raft slab it sits upon and compaction of the concrete at that position may be poor, this resulting in honeycombed concrete and a permeable joint. Concrete has fluid properties at the time of pour, and in this form of shuttering has to be poured in 'lifts' or else the hydrostatic pressure caused by the 'liquid' will blow the shuttering apart and allow escape of the concrete.  Therefore, Once a lift has been poured, it is allowed to cure before adding more concrete over, and this creates more joints which can allow penetration of water. So yes, it is a permeable form of wall construction. The implication of this, is that where applying tanking externally, you are totally reliant on that tanking, because if water penetrates past that barrier, it is essentially going to come  through the structure by any available route that it can find. The use of external barrier tanking systems on such permeable wall constructions is part of the issue, in that firstly it is impractical to repair, and secondly, it can be a high risk approach. It is impractical to repair, because you cannot categorically know where the defect is that is allowing penetration.  Once water is past the membrane, it may track laterally within the depth of that wall construction, showing itself internally at a location away from the point of the defect externally. Furthermore, the permeability of the wall is such that you cannot effect a localised repair from the interior.  To illustrate, with cast in-situ concrete walls, if a crack leaks, then one option might be to inject a resin into that crack to seal locally.  This does not work with reinforced masonry, because the water will simply find the path of least resistance and penetrate elsewhere. We have also dealt with many examples where external excavation and repair has been attempted, only for this to fail in time, with Trace then providing an internal solution via cavity drainage. With regard to my statement that it is a high risk approach, in this there are a a variety of considerations. As stated, water in a standing column - which can occur when the ground around a structure becomes saturated - exerts hydrostatic pressure, which can be a substantial force, enough to force water through even the most minuscule of defects. It can be difficult to form a 100% perfect barrier to water when standing in a muddy excavation, and therefore, defects can and do occur.  Indeed another consideration listed within BS8102 is that designers should assume the risk that defects might be present in any form of waterproofing system. Because of this, it is common practice where external/inaccessible systems are concerned, to employ land drainage around the exterior of a basement, so that water is removed from the ground.  The principle is that by preventing water pressure from bearing upon the tanking, it won't be able to move through any defects present and therefore a dry internal environment will be created. The thing is, not ever site can be effectively dewatered using land drains, and in fact the water/utility companies are unlikely to allow substantial discharge of ground water into their systems long term anyway. This does not seem to stop such systems being installed, which when failing cause such substantial costs. Let's look at an example: External systems in theory are simple to install readily That's the theory, but as per the NHBC statistics, tanking (which will also include a smaller proportion of internal tanking systems), clearly fail, and again we have good experience of remedying this. Therefore, external barrier tanking systems are specified and installed There are those that supply and install waterproofing, who do not carry design insurances and will not therefore take responsibility for it.  This leads to split liabilities and potential difficulties for clients, main contractors and specifiers, particularly when problems occur with systems which cannot readily be repaired, because in the event that these have to be replaced wholesale, (as stated) the costs can be massive. 'Repairability' is an essay in itself, but suffice to say, this consideration is a part of the design process required within BS8102, that commonly seems to be ignored. Specifically, the use of external adhesive tanking membranes (liquid or sheet) applied to semi-permeable structures, such as ICF (insulated concrete formwork) or probably the most common form of retaining wall construction, which is reinforced masonry, where two leaves of block-work are constructed, and the cavity is filled with concrete, including a degree of reinforcement. We constantly inspect such structures (look at our twitter feed), they are sold on the basis that the tanking will simply block water out, but there is guidance detailed within BS8102 that as designers, we must assume the risk that defects might be present in any given waterproofing system. The reality is that water under pressure can move through even the most minuscule of defects, and the formation of a perfect (zero minuscule holes/defects) barrier tanking membrane system when you are standing in what is typically a muddy hole, just is not realistic. Example: The photos below are of a new-build basement in a clay site with reinforced masonry retaining walls.  It was tanked externally during construction with adhesive bitumen sheet membranes.  It failed before completion and so internal cementitious tanking was specified.  The space was fit out and finished, then it flooded a second time.  Trace then installed cavity drainage with no further issues. In any case, the photo directly below is a good example of penetration past a tiny defect.

Tanking failure, pen is shown for scale.

Tanking failure, pen is shown for scale.


Wider shot of water penetration past internal tanking.

Wider shot of water penetration past internal tanking.

If (as per BS8102) we have to assume that there will be defects in any system, then how can an external (inaccessible) barrier tanking system function? The correct design is to employ land drainage with a view to preventing pressure from bearing upon the tanking, but in many cases this is either too high, not maintained, not draining to a reliable outlet or just not included, so pressure comes to bear, penetration occurs and this is a problem. We are back to considering how such a system can be repaired, but first let's look at another example. On the majority of failed external tanking systems that we remedy, these have been installed by ground-workers or general building contractors i.e. non-specialists, and so it might be easy for suppliers or manufacturers to simply state that the system was not installed correctly, however it is harder to make this claim when installation is by a waterproofing specialist manufacturer approved contractors. This is a site that we were called to for a new one off bespoke house that had not yet been built, this is what the site looked like (yes we are designers/contractors, not photographers):

A very wet clay site.

A very wet clay site.

The drawings illustrated a (permeable) reinforced masonry retaining walls, with an external adhesive bitumen sheet membrane tanking system by one of the major manufacturers, and the drawing was well detailed, even showing how the membrane should be overlapped at joints:

External tanking membrane design

External tanking membrane design

We could not install this system because it would be inaccessible and could not therefore reliably be repaired.  We could see hat the site conditions (very wet) were less than ideal for such a system, because pressure upon probable defects, would result in penetration.  However; the contractor wanted a quote for what was on the drawing and so when we left site, we said (and I kid you not): when you have issues, give us a call. Six months later, the same contractor calls me and advises that external tanking has failed. I'm not surprised, but what is significant is that the waterproofing was installed by the manufacturer approved specialist contractor.  They are not a contractor that get's involved in a basement here and there, they are dedicated specialists, promoting themselves as such and presumably working on nothing but structural waterproofing projects. Therefore, I would assume that the installers are suitably experienced and trained, which to my mind should rule out the possibility of poor installation, yet this property suffered substantial water penetration. The reason for this is that such external tanking systems are just not that effective in excluding water under hydrostatic pressure, in such wet sites, and particularly when combined with a structure which provides no resistance to penetration in its own right. Following the contractors phone call, we were instructed to provide remedial proposals.  So what are the options? In some forms of structure, where the structure can be relatively impermeable in its own right (reinforced concrete) there may be an option to undertake localised repair if you can find and seal the specific pathway (such as a crack) through which water penetrates. You cannot do this in reinforced masonry. Joints in any structure are always a weak point, and every mortar bed is a joint.  There will inevitably be a substantial crack pattern because of this and the concrete core, typically includes insufficient steel to limit cracking  to a degree that water cannot pass through it.  This is in addition to the joint between the reinforced concrete raft and the wall placed upon it, which is another unprotected joint and typical weak point. For some reason, the original contractor tried to implement resin injection and unsurprisingly, the water just moved around it and penetrated elsewhere. External excavation and repair wasn't practical here since it leaked across a wide area, and because water can travel laterally within the depth of this form of wall construction, you cannot necessarily know what the exact point of the defect is.  It is certainly not an approach that could be guaranteed and we have addressed issues where this approach has been trialled, and has failed, with Trace providing the third and final remedy via cavity drainage. So, what was the only solution?  Wholesale replacement of the waterproofing using internal cavity drainage, allowing provision of worthwhile guarantee, which the homeowner needed to get NHBC cover. What was the cost of this?  The homeowner had to find an additional £24k for Trace to dry out the not inconsiderable basement space. I do not know how the homeowner fared with seeking recompense from the specialist tanking membrane installer, but I do know that they do not take design responsibility and carry no Professional Indemnity insurance, and so suspect that the split responsibilities will make this problematic. This week I've been to two properties with external tanking issues, and I'm advising on another problem property in very wet ground, waterproofed at the newbuild stage by the same specialist manufacturer approved contractor, which is a basement car park, leaks in various areas and again requires repair. The significant thing is that I don't doubt that this organisation does or can do good work, I do know however that this common form of waterproofing in this common form of structure, has a high failure rate in wet ground where pressure is allowed to bear, and that for anyone like Trace that takes total responsibility for the waterproofing on a project, we just could not consider it because if you do have an issue, it's either a massive cost to put right, or you are  in dispute, and we do not work like that. 

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