There are many misconceptions relating to the subject of

'Basement Waterproofing' or as it sometimes called 'Basement Tanking or Cellar Tanking'

Strictly speaking, 'Tanking' is only one generic method of Waterproofing a basement. The other main method is known as 'Cavity Drainage' although the layman often uses the term 'tanking' to describe both.

Although there are many variations on these two themes they are differentiated by the following important distinctions : -

Tanking is defined by the fact that it is bonded to the substrate and tries to hold water back literally forming a three dimensional 'tank', but with the water on the outside.

Cavity Drainage employs a mechanically anchored membrane (fixed with anchors deep into the substrate) but is not bonded to it, there is a space behind the membrane alllowing the wall to breathe, move and even leak water without causing any problems to the waterproofing system.

- A drained cavity system DOES NOT attempt to hold water back.

This is huge distinction which makes drained cavity systems inherrently more reliable than bonded tanking systems See British Standard BS8102) for the following reasons: -

- A drained cavity system does not rely on the properties of the structure to work whereas structural movement is often the root cause of the failure of tanking systems.

As a drained cavity system does not have to succeed at this difficult task it cannot 'fail' at it!

A drained cavity system invariably involves the installation of a sump and pump system and it is important to get the specification of this right. In fact many drained cavity systems have failed due to inadequacy of the sump and pump system which is the heart of any such system.

In fact, the applicaiton of a cleverly designed sump and pump system can change a drained cavity system into a 'dewartering system'. This is one of the best kept 'secrets' in the industry, despite the fact that much has been published on the subject. The key is to have a perforated sump liner and a perforated underfloor drainage system that collects water BEFORE it reaches the basement structure. Rather than using a sump as the receptacle for water which has percolated through the structure and has drained through the 'drained cavity' system, the water is captured at a much deeper level.

A basement waterproofing system based on perforated drainage and sump liner is the only system that changes the external envirfonment of the basement to a drier one and basically 'demotes' the importance of a drained cavity membrane to that of a vapour barrier.

So if a Drained Cavity membrane has an easier time than a bonded tanking system it is made easier still by the use of a perforated drainage system under the floor and in this sense EASY = MORE RELIABLE.

Perforated underfloor drainage systems were used by the Victorian British and perfected in the 1850's by Henry French from Boston USA, a system which became know as the French Drain. This is a facinating piece of history which will be the subject of a forthconing article - watch this space!

Even today, many 'specialists' who install these perforated systems think of them as 'recepticles' for the water that has drained down through the cavity drainage system, simply because they were 'doing' drained cavity systems for years before the perforated 'Decendent of the French Drain' was re-introduced to the UK in the 1990's by Ray Foulkes.

In a nutshell, it is a common perception that the sump is the LAST place that the water gets to whereas in reality, using a correctly designed system, installed in the right way, it is the FIRST place that the water gets to, maing the basement safer and drier in the process.

One of the biggest technical innovations in the field of drained cavity mebranes was the develoment of the thermally insulated membrane which overcomes the condensation problem that can affect all plastic membranes - The patented Thermal Dry System.

Ray Foulkes has written extensively on this subject - see the links on the left to read more of his articles.

See the illustrated Guide to Waterproofing Basements

(coming soon)

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