Tanking Membranes

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 a barrier which is applied to the structure to physically hold back water ingress. But when we have actual or the potential for water ingress there is an alternative to tanking membranes. The alternative is Waterproofing Membrane Systems. I will attempt to explain the difference.

Tanking membranes as I have said are applied to a structure in order to present a physical barrier to prevent to ingress of water. They can be applied either internally or externally. Different types include:

Cementitious tanking membranes 
Bitumous paint tanking membranes 
Bitumous Bonded sheet tanking membranes 
Expanding clay sheet tanking membranes 
Polyurea tanking membranes 
Epoxy coatings

The key to any type of tanking membrane system working is its key to the substrate to which it is applied and this often involves some pretty exhaustive and sometimes damaging and expensive preparation techniques like needle gunning,bush hammering,hacking off wall finishes, shot blasting, raking out mortar joints, re-pointing, applying salt neutralising solutions etc.

Once the preparation has been achieved the tanking membranes needs to be applied as defect free as possible. This is often where the problem occurs with this type of product. By their very nature tanking membranes need to be 100% defect free to achieve their design function. Water will follow the path of least resistance and when it is under pressure,as it can be when it reaches a below ground structure, it will seek out weak points I.e defects in tanking membranes. Too many times I have seen the responsibility for the application of tanking membranes simply given to unqualified and inexperienced site hands and labourers.

No disrespect is meant but this its just that such an important element of the build in a structure below the ground should be given high priority and in my opinion should be applied by a professional waterproofing contractor. 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 their 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.

In addition to the tanking membranes themselves it is common, especially 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 so how do you know what size land drain will be suitable? Also how can you guarantee that the land drain will remain unblocked and free from defects for the life of the building? British Standard 8102 which governs the waterproofing industry in the UK recommends a maintainable waterproofing system. Not many externally applied tanking membranes that I have seen are maintainable.

To conclude I 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 are difficult to maintain in accordance with BS8102.

Waterproofing membrane systems.

Most commonly termed as Cavity Drain Membrane Systems. The way that this type of system works is firstly to accept the following principles that:-

• Earth against a structure is a conduit and direct route for water in the ground to reach a building. 
• If the earth can not 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 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 de-pressurise. 
Therefore a cavity drain membrane comprises dimpled membranes applied internally to the wall and floor which are impermeable to moisture and moisture vapour. The dimples create an air gap which allows the water entering through any defects to de-pressurise. It then looses its potency and falls to the wall floor junction where a box drainage conduit is situated either recessed into the floor slab or sat within an insulation layer on top of it. The drainage conduit also acts as a de-pressurisation zone at the wall/floor junction and also to collect and manage 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 include the insertion of a sump and pump system.

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

The system de-pressurises 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.

Decent manufacturers/suppliers will recommend installation by an approved specialist. When fitted by a specialist insured guarantees can be obtained and design liability will often be taken.

To conclude. In my 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.

Read an interesting article about waterproofing membranes