Waterproofing Existing Basements and Cellars

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                                      DIY Basement Waterproofing System     

Waterproofing a basement or cellar? - Examine Tanking Cavity Drainage Chemical Grouting and de-watering with reference to types of structure and uses.  Explode myths surrounding the topic and read the unedited truth that people need to know about this subject.


Hello, please allow me to introduce myself,  My name is Raymond Foulkes, MSc DMS, Managing Director of Basement Systems UK Ltd, and Import Supplies Direct Ltd.  I have served several years as the elected Technical Officer for the British Structural Waterproofng Association - a part time unpaid vocational post - and during that time I was commissioned to write the Design Guide, 'Waterproofing Existing Basements' which was published by the BSWA in 2005 (cover shown above).

The guide took me two years to write and was initially 40 pages long.  Another three years of meetings with the technical committee to agree the exact text saw the guide whittled down to just 27 pages.

The result was a text agreed by representatives of every facet within the industry and the guide has become a benchmark text and has played a significant role in changing the way the industry works.

This (e-bay) guide is not an attempt to replicate the BSWA publication, for those who wish to read that, it is available from the BSWA website.  Although what I have to say here is, in a nutshell, the pertinent points for the typical homeowner plus a few 'untold truths' about basement waterproofing that were omitted from the BSWA guide during the editing phase.

What I say here is my own opinion, not that of a committee or trade organization and I am glad to have the opportunity to speak freely without the need to convince a 20 strong committee who by design have their own differing opinions and vested interests to protect.



Very simply, from a waterproofing perspective, basements fall into three categories:-

A. Non Waterproof Masonry Structures

B. Integrally Waterproof Structures

C. Drained Cavity Structures

Their uses are often classified into four grades:-

1.  Low grade (garages etc where some water on the floor really does not matter that much)

2.  Storage grade (the environment has to be reasonably dry but not to habitable standard so ventilation etc is not so important and some dampness may still be tolerated)

3.  Habitable grade (walls and floors have to be dry and humdity controlled to within a range suitable for living)

4.  Special needs (where total control of temperature and humility is essential for storing archives, fine wines, sensitive computing installations, valuable works of art etc).

Category 4 is not usually required for normal domestic purposes and will, here-on-in be ignored.

So put simply, your basement will be either A, B or C and you will want to use it for 1,2, or 3

What could be simpler than ABC 123!!!?

A little elaboration is need on the ABC I think...

Type A, the non waterproof masonry structures will include brick blockwork, stone.  The floors and walls ore more often than not separate elements of the structure, i.e. they are not tied together by steel reinforcement, and they can therefore move differentially to each other.  This is a crucial point when selecting a waterproofing system as may systems require a rigid structure for them to work effectively.


Type B, the (supposedly) integrally waterproof structure.  These are usually build of reinforced concrete where the walls and floor are tied together with reinforcement and the whole structure is designed to be suitable thick and strong and water tight without the need for additional waterproofing.  However, unfortunately, an engineers' or architects theoretical drawings and calculations are not always translated on site perfectly and a slight defect in a water-bar (the plastic strip that seals joints in the structure) a poorly compacted bit of concrete at the bottom of a pour, perhaps a little too much water in the mis resulting in shrinkage cracking can all lead to leaks where there should be none!

MANY MODERN BASEMENT STRUCTURES FALL INTO THE TYPE B CATEGORY  - we will only be concerned with the 'failed type B's as the successes obviously do not need waterproofing!

Type C.  Many civil engineering structures involving deep basements are constructed in the drained cavity format.  Next time you are n a basement car-park of a big shopping center, maybe two or storys down and you are looking at a nice neat DRY concrete block wall, perhaps you will wonder why it is so dry so far below ground - well perhaps not, - but if your single story domestic basement is flooded then perhaps you WILL wonder how they achieve this.

Very simply, the block-work wall that you are looking at is separated by a CAVITY between it and the earth retaining walls beyond.  The earth retaining wall is often very wet, maybe even running with water ingress but the inner wall is kept dry by virtue of the 'drained cavity' in-between.  The water from the cavity has to be drained somewhere and it is usually drained into a sump chamber from where it is pumped out.

Whilst is is rare for a domestic property to be constructed of a masonry or concrete drained cavity wall a 'miniature' drained cavity is often created by the application of a membrane to the earth retaining wall, thus creating a cavity between  the retaining wall and the membrane itself.  Thus a Type A masonry structure can often be converted to a Type C structure by the application of such a membrane. 

But this leads us on to the waterproofing.....


Towards the end of the last section I was describing how the 'structure' and the 'waterproofing' in a drained cavity situation  are integrally linked.  If the structure has a drained cavity then the drained cavity is part of the structure but is also an integral part of the waterproofing.  The same is true of a tanked Type A structure where the structure is just as important as the water-proofing as the former has to hold the latter in place.  This is crucially important point to understand, failure to appreciate how the structure and the waterproofing work together and depend on each other is a common cause of failure of waterproofing systems.

Waterproofing a Type A Structure

Basically there are TWO distinct approaches:-

1. Applying  a 'Tanking System'  This will be a coating of some sort bonded to the walls and floor to create an inside out 'tank'  A tank where the water is on the outside.

2.  Converting the structure to a Type C 'Drained Cavity' Structure by the installation of a drained cavity membrane together with drainage channels and sump and pump.

There are sub-divisions within these generic methods, but in principal you are either trying to 'hold water back' or you are 'draining it away.

Let us look in more detail at the each method.


Anything from bitumen paint; asphalt (a mixture of sand and tar) sand / cement render a nd screed with waterproof additives to specially formulated slurry coatings can and have been used.

The big drawback with this method is that you are fighting water pressure (potentially at any rate, not all basements are subject to water-pressure all the time but you should assume that any basement COULD be subject to water pressure at some time in the future).


Failure to appreciate this has led to many failures, unnecessary expense, heartache and tears.

Most type A structures are prone to differential movement - that is the walls and floor may move differentially to each other and form a small crack at the wall floor joint, or the walls and floor may flex inwards slightly, but enough to crack the tanking system.

Differently to differential movement but just as important is the 'integral strength of the substrate' particularly the tensile strength.

Now I do not want to get too technical here so let me explain what I mean in simple terms.

Imagine a waterproof coating adhering to the inside face of a brick wall.  Water on the outside of the wall is tending to 'push' on the back of the coating as if it were trying to push it off.  I say 'as if' because of course the water is not 'trying' to do anything it does not have a mind of its own, it is just responding to gravity and obeying the laws of physics.

So if you have that picture so far, imagine that the waterproof coating has been applied perfectly to that the bond between the brick and the coating is strong.  The brick is going to experience a 'stretching force as the face of the brick is pushed away from the wall.

And, yes it does not take too much imagination to visualise what happens next, the face of the brick comes away with the waterproof coating as bricks are not very good at resisting being stretched (the technical term is that they have a low tensile strength)

Such a failure would not happen in a swimming pool though.  Here the water pressure is on the other side and is tending to push the waterproof coating onto the wall, compressing the masonry behind it.  Now masonry is good in compression to the system does not fail.

Another serious issue is the build up of salts behind the tanking system.

Many people will be familiar with this every day phenomenon.

When water evaporates it leaves behind the small traces of substances that were dissolved in it.  We see this on the element of a kettle.  The same happens when water evaporates in the wall of a cellar or basement.  Salt crystals can often be seen on the wall surface (often mistaken for mold) sometimes these crystals form behind paintwork and push the paint off in blisters, which when popped, reveal the salt  crystals behind.

            Examples of Failed Tanking    

Many tanking systems are sold as 'breathable' renders.  They are designed to stop liquid water coming through but allow the wall to 'breathe' i.e. allow evaporation to take place and for moisture vapour to escape.

So..... if the water is going to evaporate, then it is going to leave behind a growing layer of salt crystals, the crucial question is, in a bonded system, where, exactly, is the space for these crystals?"

The simple answer is that there is none, so as the salt crystals grow they make their own space by pushing the render or other coating off the wall and we have another tanking failure.

Now, this is the point where I may be accused of being biased against tanking systems, but I am not!  I am biased against systems that fail, that is for sure, against systems that are applied to inappropriate substrates. 


And the vast majority of basement waterproofing projects are in older-style masonry structures.

Q.  Why does  tanking appear to work a lot of the time?

A.  BECAUSE THERE IS NO WATER PRESSURE A LOT OF THE TIME.  As soon as water pressure appears, i.e. from a burst water main, unusually heavy rain, change in ground drainage due to building works etc the tanking will 'fail'.

I have lost count of the number of times that people have told me that the tanking worked for years and then just happened to fail on the one day that there was water pressure. - "The day we left the hose on" or   "the day the pipe burst in the ground".

So, in conclusion, bonded tanking systems should only be used when it can be foreseen that there never will be any water pressure or when the structure is suitably rigid and the substrate has a suitably high tensile strength for the waterproofing system to be held in place by the structure without de-lamination or cracking.  These circumstances are relative rare and represent where tanking COULD be used not SHOULD be used.


It is explicitly recognized in the British Standard BS8102 “Protecting structures below Ground against Water” that this is the most reliable and trouble free method of waterproofing a basement.

Many specialist contractors (who used to be 'tanking specialists' have change to this method over recent years and it appears to be a one way street.  I have never heard of a specialist contractor turning from drained cavity to 'Tanking'.

In it's most basic form, this consists of fixing a plastic membrane (usually but not always) dimpled over the walls and floor.  The idea of the dimples is to create a 'space' for the water to flow - typically 8mm on walls and 20mm on the floor.

These membranes are not 'bonded' to the wall but mechanically fixed with plastic fixings and intervals, leaving the membrane un-bonded in-between.

Now in reality, if water IS running down the wall it usually in an immeasurably thin film, not 8mm thick, if that amount of water were ever to come down an internal wall surface no sump and pump would ever cope with it and the wall fabric would probably not last very long, so the need for the 8mm dimples is a basement waterproofing myth!

One of the big drawbacks with plastic membranes is that they prevent moisture vapour from moving through them but do noy retain heat so you can get a build up of moisture vapour on a cold surface - a recipe for condensation.  I have seen puddles on floors caused by this.

Using a thermally insulated membrane which keeps the vapour barrier warm and keeping some background heat on is the answer, possibly complimented with a dehumidifer.

Ventilation, whilst necessary for fresh oxygen and eliminating stale air IS NOT the answer to condensation as it can bring in more humid air from the outside.  If you heat and dehumidify your basement air - you want to keep it not ventilate it away!

A drained cavity system depends entirely upon the sump and pump system that ultimately evacuates the water from the basement.  This is an area that should not be skimped on as nothing will work without it.  We would always recommend the use of a bespoke sump and pump system which will include a pre-formed liner, pump stand and alarm.  Sump liners are often perforated to allow water directly from the ground below the floor slab.  This 'de-watering' of the earth under the floor often stops the walls and floor from leaking at all and so reduces the importance of the roll of the wall and floor membrane.  In fact in many cases the 'Cavity Drain Membrane' becomes no more than a vapour barrier rather than a conduit for leaking water.  Even less need for dimples then and more importance on the insulating qualities of the membrane (to prevent the membrane from becoming a condensation trap).

The most basic sump will have a single mains pump.  Battery back up pumps are available and should be seriously considered for habitable grade as the consequences of a failure can be huge, with carpets furniture, plasterboard, joinery all being at risk.

Battery pumps usually run off 12volt DC current directly from the battery, others however run off AC (current like y ou get from the mains) which is converted from DC (battery current) by a DC- AC converter.  Such converted current is not as efficient and much bigger batteries are required to give the same pumping capacity if you are using such a converter.

Battery pumps are somewhat limited as compared with the mains pump, they tend to be less powerful on the whole and depend upon a limited charge n the battery.  So it can be considered prudent to have a SECOND mains pump to act as the primary back up (protects against every cause of failure other than a power cut - and then a THIRD pump which is the battery pump which takes over in the event of a power cut.

Either way, careful consideration should be given to choosing a good quality sump and pump system that is appropriate for the project.  For larger basements more than one sump and pump may be needed.


                 Triplesafe                         Supersump              Waterguard                    Wall and floor membranes          

Whilst water may run down a wall easily  - as gravity works that way - it is not so easy for  water to run horizontally over a floor.  So the idea of having a 'dimpled floor membrane' that water has to somehow meander under and find the sump is rather outdated.  The modern way is to include a perimeter underfloor channeling system  that will direct the water directly under the floor to the sump (for example the Waterguard Chanel - see above).

Once the membranes are in place an internal finish of plasterboard supported on timber battens or metal stud is normally used for the walls and a floor finish of board or screed is laid over the floor membrane.

There is a neater floor treatment known as Thermal Dry Floor Tile which combines the water-profing together with the floor finish in the form of an interlocking plastic tile which requires no further overlay.

'Other methods'

To all intents and purposes what we have covered, relates to about 95% of existing basements.

However, there are a few 'other' things that are worthy of a brief mention as they are relatively rarely needed, but when they are they are important:-

1.  A variation on the underfloor channeling is an 'above floor channel, like a hollow skirting board that  bonded to the floor.  This is useful for structural floors that cannot be chased out to accept an underfloor channel.

2.  Resin Grouting.  This involves drilling and injecting the structure with water reactive grouts that use up the ingressing water in a chemical reaction and turn the resin and water mixture into an impervious substance deep within the 'leak pathway', thus preventing further leakages.  This methodology is only relevant on sound structures with defined clacks or joints, such as a cracked concrete retaining wall, not for generally porous substrates such as brick or block.

3.  'Dewatering System'  It is often possible to stop a basement from flooding and achieve a grade 1 or 2 environment by using a perforated sump liner and pumping system and underfloor channeling alone - no membranes to the wall or floor, so the system is not a 'drained cavity system as such but a 'de-watered system'.

4.  Combination Systems.  sometimes the best approach is to use a combination of systems rather than just one.  Resin grouting could be use to stem high water flows followed by a tanking or drained cavity system.  A de-watering system with additional waterproofing to the walls OR floor can sometimes be relevant.  Using perimeter underfloor drainage separates the walls and floor so that different systems can be applied to each, either different kinds of membrane or different generic systems, such as membrane to the wall and waterproof cement to the floor.  The latter is useful if bonded ceramic tiles and required as the final floor finish.

If you are considering a combination system you should really speak with a true expert in order to determine what will work and what will not - do not rely on friends or the local general builder if they are not  experts in basement waterproofing!

And one final point, If you want to know the 'cheapest' way to waterproof your basement, then I will tell you...

Do it right first time.

I have know so may people do a £500.00 bitumen paint job followed by a £3,000 'tanking slurry' followed by a £5,000.00 drained cavity system.

They thought that £500 was the cheapest but ended up paying £8,500.00.

If it isn't going to work, then it isn't the cheapest.

£5,000.00 is less than £8,500.00!!!

So what about DIY?

Well now with Basement Systems UK making professional systems available with full on site support you can achieve a totally professional, warrantied job with full after sales service and do most of the work yourself and save £'000's!!

So happy waterproofing and hope you found this guide useful!!!

Ray Foulkes MSc DMS
Managing Director Basement Systems UK Ltd - 0800 413 801

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