The aim of this guide is to help the private homeowner to evaluate the conflicting advice that one is often presented with when several ‘experts’ within the industry are asked about the best solution to a basement waterproofing problem.


Basements fall into three categories, from the perspective of waterproofing:

  1. Non-Waterproof Masonry Structures
  2. Integrally Waterproof Structures
  3. Drained Cavity Structures

Their uses are often categorized 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 should be reasonably dry but not to habitable standard so ventilation etc., is not so important and some dampness can be managed)
  3. Habitable grade (walls and floors have to be dry and humidity controlled to within a range suitable for living)
  4. Special needs (where total control of temperature and humidity is important for storing archives, fine wines, sensitive computing installations, valuable works of art and others).

Normal domestic purposes are not used in category 4 and so, we will not discuss it.

In simple terms, it’s either your basement is A, B or C and 1, 2, or 3 may be what you want to use it for.

I think that the ABC needs little elaboration…

Brick block-work, stone are examples of type A, the non-waterproof masonry structures. The floors and walls are generally separate elements of the structure, which is to say that they are not tied together by steel reinforcement, and they can therefore move in different directions. As many systems require a rigid structure for them to work effectively, this is a crucial point when choosing a waterproofing system.


Type B, the (supposedly) integrally waterproof structure. These types is built of reinforced concrete with 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 further waterproofing. Nevertheless, it is quite unfortunate that the theoretical drawings and calculations of an engineer or architect are not always translated on site perfectly and a little defects in a water-bar (the plastic strip that seals joints in the structure), or 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 contribute to leaks where none should exist!

MANY MODERN BASEMENT STRUCTURES FALL INTO THE TYPE B CATEGORY – we will only consider the ‘failed type B’s as it is clear that the successes do not need waterproofing!

Type C. Civil engineering structures that involve deep basements are often constructed in the drained cavity format. Next time you are in a basement car-park of a big shopping center, and you are staring at a nice neat DRY concrete block wall, you may wonder why it is so dry so far below ground – or maybe not, – but if you have a flooded single story domestic basement, then perhaps you WILL wonder how they do this.

It’s not difficult; 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 ‘drained cavity’ in-between keeps the inner wall dry. The water from the cavity has to be drained somewhere and most times, it is drained into a sump chamber and pumped out from there.

Although, it is rare for a domestic property to be constructed of masonry or concrete drained cavity wall, a ‘miniature’ drained cavity is often created by applying a membrane to the earth retaining wall, so that a cavity is created between the retaining wall and the membrane itself. Thus a Type C structure can be made from a Type A masonry structure by using such a membrane.

But this leads us on to the waterproofing…..


I described the way the ‘structure’ and the ‘waterproofing’ in a drained cavity situation are integrally linked towards the end of the last section. If the structure has a drained cavity then the drained cavity is part of the structure although it 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 structure holds the waterproofing in place. It is very important to understand this; not appreciating the way 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, TWO distinct approaches exist:-

  1. Using a ‘Tanking System’: This will be a coating of some sort bonded to the walls and floor to make an inside out ‘tank’. The water is on the outside of the tank.
  2. By Converting the structure to a Type C ‘Drained Cavity’ structure by the installing a drained cavity membrane together with drainage channels and sump and pump.

These generic methods have sub-divisions within, but what you want to do is either ‘hold water back’ or you are ‘drain it away’.

Let us consider each method in detail.


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

The big setback with this method is that it is 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) that you are fighting.


Many failures like unnecessary expense, heartache and tears have resulted from not appreciating this.

Differential movement is something that most type A structures are prone to, i.e. it is possible for the walls and floor to 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 has a potential to crack the tanking system.

The ‘integral strength of the substrate’ is just as important as differential movement, particularly the tensile strength, although they are different.

I will explain what I mean in simple terms as I don’t want to be technical here.

Think of a waterproof coating sticking to the inside face of a brick wall. The water outside of the wall tends to ‘push’ behind the coating as though it were trying to push it off. I say ‘as though’ because the water is not ‘trying’ to do anything, it does not have its own mind, it is just responding to gravity and obeying the laws of physics.

Having that picture in mind, imagine that the waterproof coating has been applied perfectly and that the bond between the brick and the coating has good strength. There’s going to be a ‘stretching force that the brick will experience as the face of the brick is pushed away from the wall.

And, of course, you don’t need too much imagination to picture the next happening, the face of the brick breaks away with the waterproof coating as bricks are not very good at fighting against being stretched (technically put, they have a low tensile strength).

That kind of failure would not occur in a swimming pool though. Here the water pressure is on the other side and tends to push the waterproof coating to the wall, compressing the masonry that is behind it. The system does not fail because masonry is good in compression.

The build-up of salts behind the tanking system is another serious issue is.

Many people will be conversant with this every day phenomenon.

Small traces of the substances that were dissolved in water are left behind when it evaporates. We can observe this on the element of a kettle. It also happens when water evaporates in the wall of a cellar or basement. You can often see salt crystals on the wall surface (usually mistaken for mold), and these crystals sometimes form behind paintwork and push the paint off in blisters, which when removed, the salt crystals that are behind are revealed.

Many tanking systems are sold as ‘breathable’ renders. They are constructed to prevent liquid water from coming through but permit the wall to ‘breathe’ i.e. allow evaporation to take place and for moisture vapour to escape.

Then, if the water is going to evaporate, a growing layer of salt crystals will be left behind. The crucial question to ask is, “where exactly is the space for these crystals in a bonded system?”

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

At this point, I may be accused of being unfair against tanking systems, but I am not! I am unfair against systems that fail, systems that are applied to inappropriate substrates.


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

  1. Why does tanking appear to work a lot of the time?
  2. BECAUSE THERE IS NO WATER PRESSURE A LOT OF THE TIME. When water pressure appears, i.e. from a burst water main, unusually heavy rain, change in ground drainage due to building works etc failure is the result.

I can’t count 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 water pressure appeared. – “When we left the hose on” or “The day the pipe burst in the ground”.

In conclusion, bonded tanking systems should only be used when it can be foreseen that there will never 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 remain in place by the structure without de-lamination or cracking. These circumstances are relative rare and do not represent where tanking SHOULD be used, but where taking COULD be used.


“Protecting structures below Ground against Water” has been explicitly recognized in the British Standard BS8102 as 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 road. I have never learnt of a specialist contractor that turned from drained cavity to ‘Tanking’.

In the simplest form, this involves fixing a plastic membrane (usually but not always) dimpled over the walls and floor. The dimples’ concept is to make a ‘space’ for the water to flow – typically 8mm on walls and 20mm on the floor.

The membranes are not ‘bonded’ to the wall but fixed mechanically with plastic fixings and intervals, thus the membrane are left un-bonded in-between.

In the real sense, if water is running down the wall it usually does so in an immeasurably thin film, not 8mm thick. If that quantity of water happens to come down an internal wall surface, no sump and pump can cope with it and the wall fabric would probably not last very long. So the usage of the 8mm dimples is a basement waterproofing myth!

Plastic membranes prevent moisture vapour from moving through them but do not retain heat and this can cause a build-up of moisture vapour on a cold surface – and this is a major setback. I have witnessed this resulting in puddles on floors.

The answer is using a thermally insulated membrane that keeps the vapour barrier warm and some background heat. And you can possibly compliment it with a dehumidifier.

The answer to condensation is not ventilation, though it is necessary for fresh oxygen and eliminating stale air. The reason is that it can bring in more humid air from the outside. You will want to keep your basement air from being ventilated away if you heat and dehumidify it!

A drained cavity system depends completely upon the sump and pump system that removes the water from the basement ultimately. This area should be carefully examined as nothing will work without it. The use of a bespoke sump and pump system which will include a pre-formed liner, pump stand and alarm is what we always recommend. Sump liners are usually perforated to allow water directly from the ground below the floor slab. When the earth under the floor is ‘de-watered’, it stops the walls and floor from leaking at all and consequently reduces the importance of the floor membrane and roll of the wall. In fact, ‘Cavity Drain Membrane’ becomes no more than a vapour barrier rather than a conduit for leaking water, in many cases. There is even less need for dimples then and more importance should be placed on the insulating qualities of the membrane (to prevent the membrane from being a trap for condensation).

A single mains pump is what the most basic sump has. Pumps that are backed up with battery are available and should be considered for habitable grade as the consequences of a failure can be grave, having carpets furniture, plasterboard, joinery at risk.

Battery pumps use 12volt DC current directly from the battery, others use AC (current like you get from the mains) which is converted from DC (battery current) by a DC-AC converter. Converted current like that is not so efficient and much bigger batteries are needed to give the same pumping capacity for such a converter.

When compared with main pumps, battery pumps are somewhat limited. They seem to be less powerful on the whole and depend upon a limited charge in the battery. So it is wise to consider to have a SECOND mains pump for a primary back up (protects against every cause of failure other than a power cut) – and then a THIRD pump which as the battery pump which can be used when power is cut off.

Whatever the case may be, you should carefully consider when you want to choose a good quality sump and pump system that is fit for the project. You may need more than one sump and pump for larger basements.

As water run down a wall easily – because gravity works like that – it is difficult 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 out dated. Including a perimeter under the floor channelling system is the modern way. In this way, the water will be directed under the floor to the sump.

When you have the membranes in place, an internal finish of plasterboard supported on timber battens or metal stud is commonly used for the walls and the floor membrane is laid with a floor finish of board or screed.

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

‘Other Waterproofing Methods’

What we have covered amounts to about 95% of all purposes for existing basements.

But, there are a few ‘other’ things that are should be mentioned briefly as they are relatively rarely needed, but are important when they are needed:-

  1. A difference on the underfloor channeling is an ‘above floor channel’, just as a hollow skirting board that is bonded to the floor. This can be of use for structural floors that cannot be worked upon to accept an underfloor channel.
  2. Resin Grouting. This mainly has to do with 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 a substance that is impervious right within the ‘leak pathway’, and by that further leakages are prevented. This technique is not for generally porous substrates such as brick or block. It is only relevant on sound structures with properly defined clacks or joints, such as a cracked concrete retaining wall.
  3. ‘Dewatering System’. It is usually possible to stop a basement from flooding and achieve a grade 1 or 2 environment when you only use a perforated sump liner and pumping system and underfloor channelling – with 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. The best way sometimes can be using a combination of systems instead of only one. Resin grouting could be use to stop the flow of water followed by a tanking or drained cavity system. Using a de-watering system with waterproofing to the walls or floor can be relevant sometimes. The walls and floor can be separated when a perimeter underfloor drainage is used, so that different systems can be applied to each, either different generic systems or different kinds of membrane, like waterproof cement to the floor and membrane to the wall. The former is useful when bonded with ceramic tiles and required as the final floor finish.

You should contact a true expert in order to determine what will work and what will not if you are considering a combination system – do not trust your friends or the local general builder that are not experts in basement waterproofing!

Finally, I will tell you the ‘cheapest’ way to waterproof your basement, if you want to know…

Do it right first time!

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