Installing a Drained Cavity System For Basement Waterproofing

Nancy B. Alston

Whether you are dealing with a new build or refurbishment project, commercial or domestic the drained cavity approach of basement waterproofing is extremely popular and has become a professional specifier’s and waterproofing contractor’s weapon of choice when protecting below ground structures.  
 
The cavity drain system is an internally applied basement waterproofing system which consists of a series of high density polyethylene membranes (dimpled sheet profiles) which are mechanically fixed to the basement walls creating an air gap (usually 8mm) on the vertical plane.  The air void created by the membrane terminates at the wall/floor junction into a box conduit channel, a Basedrain, which then guides any water entering the basement (usually entering at the wall/floor junction or the joints in the concrete) to an open elevation or to a sump pump.)  This basement waterproofing technique in summary is creating a maintainable air void within the basement and thus moving water which depressurises into the gap away before damaging internal fixtures.  

The advantages of this approach are that you can apply the system in all weather conditions which can be a real problem when using external spray applied or bonded membrane products which have limitations on when you can apply them i.e structure would need to be dry or the temperature is too low for a 100% bond to be achieved.  

Other significant advantages are: minimal surface preparation – so no sandblasting or bush hammering, which is messy and can damage structures; the cavity drain system is reversible so can be used on listed and historic buildings (tanking cannot!); and most important of all, it’s easily maintainable and repairable if required.  The same cannot be said for any form of external waterproofing which once installed, if there is a problem, is very difficult to remedy; and in many cases failed external basement waterproofing is fixed by retro fitting an internal drained cavity system.
 
When utilising the cavity drain system for your basement waterproofing project the key is removing water that does enter the system.  As described above the wall floor junction drain, or Basedrain, is a highly effective way of achieving this.  The Basedrain should either discharge out to an open area, if you have one in your development (like a property built into a hill side), where water removal can be achieved by gravity or if this is not possible a sump and pump unit.  

The Sump and pump unit’s specification for cavity drain systems should be done by a specialist waterproofing contractor as a part of the full basement waterproofing design and install.  This way the basement waterproofing contractor can guarantee the installation and offer maintenance contracts with the pumps themselves.  As people are using their basement areas for living accommodation the sump and pump systems can also be designed to accommodate sinks, condensation overflow, showers and even toilets.  In many cases the sump and pump can also be housed in the light well (if you have one) and can deal with surface water drainage. 

Without a doubt the key to using a cavity drain membrane system is the drainage aspect, as the system is not like a traditional tanking system, where you are seeking to hold water back. It is more like a water management system so the principle is to make your structure as watertight as possible yet accept that it may leak. This is where the cavity drain membrane “mops up” the ingression and removes it.  

If you are using a sump and pump to remove water from the cavity drain system then you must also consider what will happen in a power cut?  Answer: Your pump will not work!  So when you are getting advice on the right kind of pump system for your basement waterproofing project you will need to have a battery backup facility and two pumps as a minimum.  For minimal extra cost you can get protection against power outage. One tip when looking for such systems is to make sure that the back-up pump has the same duty and capability as the primary pump or you will just be wasting your money. The cavity drain systems recognition is unquestioned now as the most reliable and maintainable form of structural waterproofing and has been embraced by professional contractors and specifiers alike.  

Even insurance companies now favour the use of cavity drain systems and offer longer periods of protection when compared to conventional tanking in the United Kingdom.

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