What is Tanking?

Tanking is a generic term used to describe various mediums which are used to deal with water entering a below ground building. Tanking membranes are really barriers which are applied to the structure to physically hold back water ingress and are the more traditional method of dealing with water from the ground.

We feel that there is a more modern, reliable and risk free alternative to tanking membranes when we have actual or the potential for water ingress in to a structure. The alternative is Cavity Drain Membrane

Systems, and this article explains why it is the best approach

Tanking membranes are applied to a building in order to provide a physical barrier to prevent to ingress of water. They can be applied either internally or externally. Examples of tanking membranes include:

• Bituminous bonded sheet tanking membranes

• Bituminous paint tanking membranes

• Polyurea tanking membranes

• Epoxy coatings

• Cementitious tanking membranes

• Expanding clay sheet tanking membranes

Tanking Membranes Require Extensive Preparation Before Application

Tanking membranes will only be effective if the preparation to the substrate they are applied to is good and if they are applied 100% defect free. Often some pretty exhaustive and sometimes damaging and expensive preparation techniques are required, like needle gunning, bush hammering, hacking off wall finishes, shot blasting, raking out mortar joints, re-pointing, applying salt neutralising solutions etc.

Once the substrate has been prepared the tanking membranes needs to be applied. This is often where the problem occurs with tanking membranes. By their very nature tanking membranes need to be 100% defect free to achieve their design function of holding back water. You know that if you add water to a bucket with a hole in it the water will leak out and its the same with a basement tanking system except that the water will leak in if there is a defect.

Cavity Drain Membranes – The Reliable Alternative

Newton 508 mesh used in Royal Courts of Justice

We realise that water will try to follow the path of least resistance when it is under pressure. This is what happens when water in the ground reaches a below ground structure and it will seek out weak points i.e. defects in tanking membranes. With our years of experience in the industry we know the responsibility for the application of tanking membranes is all too often given to unqualified and inexperienced site hands and labourers.

In the same way it is all too common for a plasterer on a job to be handed a container of a waterproofing agent and asked to add it in to his plaster mix and then once applied this constitutes tanking. Both of these scenarios present difficulties if there is a problem in the future in respect of identifying and retrospectively rectifying the problem – something which is very difficult when your tanking membranes are buried under tonnes of earth on the outside of a building. No disrespect is meant but this its just that in our opinion the waterproofing is such an important element of the building.

Can you imagine the devastating consequences of tanking membranes failing and flooding occurring once you have your furniture fittings and floor coverings in place?

Waterproofing below the ground should be given high priority and our my opinion should be applied by a professional specialist waterproofing contractor. Workmanship is not the only problem associated with tanking membranes. In particular it is common with externally applied tanking membranes that a land drain is introduced externally to help dewater the surrounding ground area and stem the flow of water that comes to bare against the structure and tanking membranes.

However it is almost impossible to accurately calculate the volume of water which could present itself in the future. Therefore how do you calculate what size land drain you will need to do the job? Also can you guarantee that the land drain will remain unblocked and free from defects for the life of the building?

Implications of BS 8102 – Newton System 500 Meets Requirements

As professional waterproofers we are governed by British Standard 8102 – the code of practise for the protection of structures from water from the ground and this recommends a maintainable waterproofing system. There are not many designs which include externally applied tanking membranes that are maintainable.

To conclude we feel that tanking membranes whilst more traditional in their approach to keeping water out of a structure are more open to risks of failure most often because they rely on 100% defect free workmanship and in the high court case Outwing vs Weatherald it was deemed that it is not reasonable to expect a tanking or waterproofing system to be applied 100% defect free. Also they are not often designed to be maintainable in accordance with the recommendation put forward by the BS8102.

Cavity Drain Waterproofing Systems – The Recommended Alternative

The way that our system works is firstly to accept the following principles :-

• Earth against a structure is a conduit and direct route for water in the ground to reach a building.

• If the earth cannot be removed then you must expect that water will come to bare against the structure to the full height of the areas below the ground unless comprehensive geological inspections can prove otherwise.

• Even when the site investigation indicates dry conditions, the risk of some water logging in the future should be assumed.

• Even in a permeable subsoil, ground water requires time to drain away and this can result in limited pressure periodically coming to bear against the structure

• Even when a structure is well built, if it is below the ground you must expect that if water comes to bare against it, it will enter the fabric of the building at some time in its life.

• When water enters a structure all it wants to do is find an air gap and depressurise.

Drainage channel

With the above in mind our Cavity Drain Membrane System comprises High Density Polyethylene and Polypropylene membranes which are moulded into a stud formation and applied internally to the wall and floor. Our membranes are impermeable to moisture and moisture vapour. The studs create an air gap which allows the water entering through any defects or construction joints in the building to depressurise. Once depressurised it looses its potency and falls to the wall floor junction where our Drainage channel perforated drainage conduit is situated either recessed into the floor slab or sat within an insulation layer on top of it.

The Basedrain conduit also acts as a depressurisation zone at the wall/floor junction which is the critical zone where water often enters or ends up and from there it collects and manages water to a suitable point where it can be discharged from the building. The water is either discharged freely via passive drainage including open elevations, existing or designed in drainage or it is discharged via powered methods which usually means the insertion and use of a sump and pump system.

The run of Basedrain also has inspection ports strategically placed in it so that inspection and maintenance can be achieve which conforms to the requirement of BS8102. John Newton and Co. have many different pumping systems ranging from very basic to very high levels of performance and protection.

We can also specify and supply whole house sewerage systems and rain water harvesting units depending on requirements.

Some of the advantages of a Cavity Drain Membrane System are:-

• The system depressurises water so it is never subject to aggressive water pressure

• They are designed so that defects in the membrane will not affect the efficacy of the system. Damage is easily repairable

• They are maintainable via inspection ports in the drainage conduit and conform to the British Standard requirement

• They can be tested to assess performance before fixtures, fittings and finishes are applied

• Very suitable for heritage projects as they are sympathetic to the structure and easily removable

When fitted by a Specialist Basement Contractor, insured guarantees can be obtained and design liability will often be taken by the contractor.

To conclude, in our opinion it is safer to assume that water will enter the fabric of a below the building at some time in its existence and it is very difficult to physically stop aggressive ground water. We should also assume change in underlying conditions so build the structure strong but factor in inevitable human error or difficult site conditions and have an effective contingency plan which will negate the need for any other system and that is a Cavity Drain Membrane system .