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).
This guide is not an attempt to replicate the BSWA publication, for those who wish to read that, it is available from the BSWA website.
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.  I am all too aware that even if someone does have home insurance they will still want to ensure nothing goes wrong with their home. Having a flooded basement is possibly the last thing someone could want, so I want to help them prevent that from happening. 

Why does my basement flood?

Very basically, because water underground is applying pressure to the floor or walls of your basement and trying to get in! SEE VIDEO
However, it depends on the structure of your property!
Very simply, from a waterproofing perspective, basements fall into three categories:-
A. Non Waterproof Masonry Structures
B. Integrally Waterproof Structures
C. Drained Cavity Structures

Type A
The non-waterproof masonry structures will include brick blockwork and stone.  The floors and walls are 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 many systems require a rigid structure for them to work effectively.
Type B
The (supposedly) integrally waterproof structure.  These are usually built of reinforced concrete where the walls and floor are tied together with reinforcement and the whole structure is designed to be suitably thick and strong and water tight without the need for additional waterproofing.  However, unfortunately, an engineer's or architect's 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 mix - resulting in shrinkage cracking - can all lead to leaks where there should be none!
Type C
Many civil engineering structures involving deep basements are constructed in the drained cavity format.  Next time you are in a basement car-park of a big shopping center, maybe two or more floors 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 walls are often very wet, maybe even running with water, 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. 
And this leads us on to the waterproofing.....
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.

Why doesn’t Tanking Work?

Of the 2 distinct approaches, the “miniature drained cavity” approach is a clear winner. Here we look at why tanking fails so often.
Anything from bitumen paint; asphalt (a mixture of sand and tar); sand / cement render and screed with waterproof additives; to specially formulated slurry coatings can, and have, been used.
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
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'.
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).

So why would it fail?

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.
To be distinguished from  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 so the system does not fail.

Additional Problems with Tanking!

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 mould) sometimes these crystals form behind paintwork and push the paint off in blisters, other times they can form behind the render and push the entire render off in huge slabs - see pictures below:-
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 "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.

What should I do about it?

It is explicitly recognised in the British Standard BS8102 (1990) '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 'Drained Cavity Systems' 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 its 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 is 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!
So what sort of membrane is needed?
One of the big drawbacks with plastic membranes is that they prevent moisture vapour from moving through them but do not 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.
Basement Systems UK in conjunction with Astrofoil Internatonal have developed specially insulated membranes based on radiant heat reflection and drained cavity technology.  These membranes are marketed under the brand name 'Thermal Dry' and include both dimpled and non-dimpled membranes for walls and a high compression strength membrane 'Thermal Dry Floor Membrane' for floors.  
These membranes were specifically designed to overcome the condensation problem that plagues the 'plastic only' membranes.  They have the additional benefit of keeping the whole room warm and even the rooms above.
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!

Choosing the Right Sump and Pump

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 wall and floor membrane and demotes their role to that of a vapour barrier.  Even less need for dimples then, and a greater need for 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 you 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.

Why use a Perimeter Drainage Channel?

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 conduct the water directly under the floor to the sump (for example the Waterguard Chanel - see above).

Finishing Touches

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.
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!  

What is the Cheapest Way?

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 known so many people do a £500.00 bitumen paint job followed by £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 happy waterproofing and hope you found this guide useful!!!

If you wish to read more about Cellar Conversions in the UK - see this site Cellar conversion Expert UK

Ray Foulkes MSc DMS

Managing Director Basement Systems UK Ltd - 0800 413 801                                  

Guide written by Raymond Foulkes M.Sc.DMS, MD of Import Supplies Direct Ltd and Basement Systems UK ltd.

Trade and specification enquiries should be directed to raybsuk@googlemail.com

Use of this guide is subject to our standard terms and conditions.

Copyright 2009 Import Supplies Direct Ltd. 

Guide to Basement Waterproofing
Guide to Choosing the Right Insulation
Secure Online Shopping
Not sure how to install insulation? Read our DIY guide here