This page discusses what cementitious ‘damp proof tanking’ is, and what you need to consider in using it.
Internal Systems in Existing Structures:
‘Damp proof tanking’ is a specific type of waterproofing which functions by attempting to block water out of a structure, by including a barrier product on or within that structure. If water is totally blocked out, then the internal basement/cellar environment is protected and remains dry.
The term is commonly used to describe various types of waterproofing systems generally, but in its true sense it is specific to ‘barrier protection’, (or ‘Type A (barrier) protection’ within British Standard 8102).
If looking at the use of tanking, what do we need to consider?
Continuity of a Damp proof tanking system is critical where water pressure will come to bear, because if it is not continuous (i.e. if there are holes, or if it is not applied to all areas of a basement or cellar), then water will move through those holes or unprotected areas when water pressures upon them. Just as a swimming pool with a hole in it will not hold water, a tanking system with a hole will not block it out.
While it may seem obvious that tanking all areas of a structure is a prerequisite to fully blocking out water, you would be surprised how many failures occur because (for example) tanking is included to the walls but not the floor. Where such specifications are employed, this does not form a ‘tank’ or ‘tanking system’ in the true sense of the word.
In this photo, we were installing an internal drainage based waterproofing system to address failures in the existing tanking in a Manchester property. We formed weep holes at low level and the hollow core block wall which was full of water poured out for over half an hour. In essence we had removed the ‘continuity’ of the tanking system:
Forming a bond with the substrate:
In existing properties, tanking is applied to the internal face of the structure, with a variety of products potentially being employed, such as multi-layer cement based products, or cement slurries which are applied over a base coat render (which provides a flat/level surface to accept the slurry).
Note that in new construction, tanking is often applied externally; however the following principally concerns itself with internal tanking systems.
Although far less commonly employed in existing properties, we do also see liquid applied bitumen based systems or mastic asphalt. In all cases, the tanking is designed to block out water.
Because Damp proof tanking in existing structures is applied to the internal face of the structure, it is particularly reliant on the formation of a bond between the tanking and the surface or substrate on which it is applied. The reason for this is that water under pressure moving through the structure, will bear upon the tanking barrier when it reaches the internal face, effectively trying to force the material off of the substrate, and hence the requirement for a good bond.
One possible way to address this is to properly prepare the surface of the substrate, so that the tanking can ‘key’ to the surface on which it is applied. This means raking out mortar joints in masonry or scabbling concrete to remove surface laitance.
Photo right shows a property that we inspected in the North West where some preparation works had already been undertaken, with parties unknown ‘scoring’ the brick-work to provide a key. This property was not tanked, which is somewhat fortuitous in that the floor and wall construction would not stand any great loading applied by hydrostatic pressure, as the floor was brick set, with walls deflecting inwards under soil pressure alone, possibly exacerbated by raked out and therefore weakened mortar joints.
Where we encounter failures in internally applied tanking systems, in some cases it is because they have not undertaken any preparation, and the product de-bonds, this then allowing water penetration.
The key aspect to remember here, is that if you get a Damp proof tanking quote from your builder in the hundreds of pounds, they may not have included for preparation works, which is definitely ‘half a job’.
Recently we installed a remedial waterproofing system in a Manchester basement car park (see photos below), where the waterproofing installed during construction failed, with a remedial internal tanking system then being installed by a ‘specialist’ contractor. They did not prepare the substrates, and then employed a product which was not suitable as a wearing finish (i.e. cars cannot drive on it), and it also appears that too much water was added to the product to make it workable, this also making it weak. Left & middle photos shows tanking delamination, with efflorescent salts being visible in the right hand photo which is demonstrative of moisture penetration.
As a result the system failed, and we were employed to install a third system, when one would have sufficed, had it been correctly designed and installed. This is all too common.
If anything, more modern construction such as reinforced concrete, can lend itself to the application of internal tanking systems because substrates (if properly prepared) can be extremely strong, and are more likely to absorb imposed loads versus older structures- see below.
While preparation is important, in some structures (typically older properties), the substrates may become weak with age, making it less suitable to receive tanking products where formation of a good bond is key. In cases where tanking is employed regardless, de-lamination of the substrate can occur, whereby it is the surface of the substrate which fails (not the tanking), albeit with the same result.
The photo’s below show a piece of cementitious damp proof tanking from a property in the North West, with wall finishes (wallpaper) being visible on one side, and the other you can see where the face of the brick substrate has come away, remaining attached to the section of render.
Where slurry tanking systems are applied to existing structures, as stated this may involve the application of a strong render (i.e. 3 parts sand to 1 part cement), to form a solid base on which the slurry is applied.
To achieve a strong render, correctly graded sand must be employed, meaning that the particle sizing of the sand is controlled. Furthermore the mixing must be accurate to ensure that a sufficiently strong mix is produced.
These photo’s (below) show a piece of render with slurry tanking applied to it (slurry visible in the right hand photo). This was taken from a Manchester property where the tanking had failed with this resulting in ground water penetration. In looking at the samples collected, we determined that ‘builders sand’ had been employed (i.e. it was not properly graded) which is evident from the large ‘grit’ particles visually identifiable. The system was installed by a general builder who signed a guarantee on the system, as did the product manufacturer. When the system failed, the contractor was no longer in business and the product manufacturers guarantee only covers provision of replacement materials, which they could not honour because of faulty installation, which they assumed was the case without visiting site.. While this illustrates the impact of using the wrong sand, it is also an example of a homeowner being left in the lurch when problems occur. This is why client protection is so important, and you can see our thoughts on that topic HERE.
We have seen failures in systems where the render was designed as 3:1 or 4:1 but when tested, was found to be 6:1, thus providing a weak bond. This can happen when it is mixed by volume (i.e. 1 shovel of cement, 3 of sand). Cement can bulk when air becomes entrained and hence this can result in inaccurate mixes. Ideally mixes should be by weight, not by volume.
The other consideration where employing tanking, is the potential loading which can be applied to the structure where water pressure comes to bear. The weight of a standing body of water exerts a pressure, which is called ‘hydrostatic pressure’, and the deeper the body of water, the greater the pressure.
If a basement or cellar structure is tanked, then the load applied by a weight of water (in the ground around the structure) is exerted onto the tanking, and the structure on which it is applied.
While this may be a non-issue in (for example) a modern steel reinforced concrete basement structure; if you have (for example) a Victorian – or older – property, with brick/stone walls and a floor construction in brick-set/stone flags with a nominal layer of concrete above it (as is often the case), then when that loading occurs, if it is too great for the structure to absorb, a minimal degree of movement can occur (typically at the wall floor junction), which cracks a rigid (i.e. cementitious) tanking system with this resulting in failure.
As example of this occurring is shown below. This is a 100 plus year old cottage in North West England which was cementitiously tanked with the objective of addressing periodic problems of severe flooding in the cellar space. The designer was negligent or ill-informed and did not consider the nature of the floor construction and its ability to absorb the loads applied by hydrostatic pressure. As a result, when water pressured, the tanking cracked at the wall floor junction resulting in a flooded cellar. Repairs were attempted (this time calling in the tanking manufacturer), and they installed a ‘putty’ like flexible material at the wall floor junction, which you can see on the left, but it just flooded again with cracking also occurring in the door threshold visible in the right hand photo. Trace were then called in and we remedied using an internal drainage system, providing worthwhile guarantee.
It is worth noting that in virtually all cases this is a very small degree of movement i.e. limited cracking which can reduce once water pressure is relieved, and in older structures (Victorian etc.) should represent little implication other than failure of the waterproofing.
Therefore, the structure should always be considered in addition to the strength of the substrates. This is another reason why we see failures of cementitious tanking systems, because they are installed without investigating the structure to check that it is adequate for the potential loads which may be applied.
In respect of these loads, BS8102 advises that one should undertake a site investigation to determine the nature of the ground and the likely highest level of the water table, or if you do not do this, assume that water may bear to the full height of the below ground structure.
Therefore, while you could ideally identify that it is unlikely that pressure would come to bear, we would also consider the advice within BS8102 which states ‘Even when site investigation indicates dry conditions, the risk of some waterlogging in future should be assumed’. Furthermore, particularly in habitable space where the implications of failure are typically dire, we would advise designing such situations out of the equation in any case.
Defects, finding them, and system testing:
As stated, a ‘tank’ must be perfect to block out water, where this comes to bear. Therefore, ideally any tanking system should be installed ‘defect free’ (or perfectly), however for whatever reason (workmanship or product failure) defects can and do occur. Therefore what are the consequences, assuming that a space is habitable and therefore water penetration is unacceptable?
Well, if water pressure never comes to bear, then such defects should not be of any consequence, however in the event that waterlogging occurs, the effect of such defects (assuming that the structure is not type B / integrally waterproof) would be that water penetration occurs, which is obviously a problem in a habitable space.
Testing for defects is an important consideration because ideally you want to know whether the system is working or not at the earliest stage (during construction) so that it can be repaired if necessary. You do not want failure to occur a year later when a space is fit out, finished and occupied because the consequences of failure become much greater.
A limitation with tanking is that to properly test the system, you ideally need to saturate the ground external of the structure, which may not be practically achievable i.e. this may only occur during extreme weather which one cannot predict.
|In respect of finding the point of any defect, Ideally where internal tanking is employed, it is left accessible for inspection, which means that when water penetrates, it is a simple matter of being able to see where the water is penetrating and then remedying this.Photo right shows cementitious and resin tanking by Trace Basement Systems, installed at the Portland Canal Museum in Manchester where it will remain on permanent display.|
In modern habitable space, there is a requirement to comply with the Building Regulations which means the inclusion of insulation to the walls and floors within a space. This often then involves installation of plasterboard over the walls and screed or floating timber over the floors. The implication of this is that the tanked substrates are hidden, which may make it difficult (and potentially costly) to then find and address the actual defect.
Photo’s below are of a property in the North West that was tanked externally during construction, with this failing at an early stage. The space was then tanked internally and fit out. Note that plasterboard was fixed on dabs (you can see the adhesive dabs on the walls in the left hand photo) so that the tanking would not be pierced. Water penetrated through defects in the wall tanking, some of which were tiny (see right hand photo with ballpoint pen lid shown for scale), but because these were hidden behind plasterboard, they could not determine the point of penetration and so stripped out fully.
Movement and Vibration:
Where rigid cementitious tanking systems are employed, these can crack as a result of movement or settlement in a structure, or through vibration, so for example such a system may not be appropriate where property is directly next to roads with heavy vehicular traffic.
The photo below was taken in the same North West basement as shown above, where settlement/movement also induced cracking in the internal tanking.
Formation of fixings into internal cementitious systems must be carefully designed and installed, since simply penetrating a tanking system with a nail for example, may simply provide an opening which may allow water under pressure to penetrate.
Benefits of damp proof tanking:
Functional tanking systems require no maintenance (unless reliant on associated drainage), so in theory imply no long term maintenance costs.
Conclusions about damp proof tanking:
This article is by no means exhaustive (in particular it does not cover external tanking) and simply details what tanking is, and some of the considerations applicable to the use of such systems primarily where included internally in existing structures.
Tanking works, but must be carefully considered at the design stage, and very carefully installed. It is not in our opinion a job for a non-specialist contractors. At the time of writing we are dealing with three failed cementitious tanking systems in Manchester, North West England and further afield. Where it doesn’t work we are often asked to advise and so have great experience of what causes failures. As a company we have employed tanking systems in varied applications, but generally choose not to rely on this method of waterproofing as the sole form of protection, because there are other lower risk solutions.
Our intention is to add additional information to this page when time allows, but if you should have any questions please feel free to contact us or ask questions through our ask the expert page.
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