Cavity drainage

The alternative to tanking is to utilise drainage to remove water and therefore prevent it from causing a problem.  This method is commonly referred to as cavity drainage and typically utilises three components:
1.    a perimeter drainage channel (typically referred to as Aquachannels),
2.    a sump basin and pump, and
3.    the cavity drain membrane.

The typical components of a cavity drainage system. At the back is the sump basin, with Aquachannels shown in the foreground alongside a high water alarm.  To the rear is a single submersible pump. Sumps can contain multiple pumps to ensure continuous operation in the event of mechanical failure. (Image courtesy of Triton Chemicals Ltd www.triton-chemicals.com).

Aquachannel installed in a trough formed at the perimeter of a concrete floor.  The channel is surrounded by 20mm clean gravel.  The wall behind the Aquachannel is clad with a cavity drain membrane. (Image courtesy of Triton Chemicals Ltd www.triton-chemicals.com).

Cavity drainage systems have become increasingly popular in recent years, because they offer a number of advantages over traditional tanking methods.  Some of the reasons for this are:
•    cavity drainage requires far less preparation prior to installation, and
•    the components are relatively easy to repair or modify.

Cavity drainage systems are not completely free from issues.  The installation of Aquachannels requires the perimeter of the floor to be broken out and this may affect the stability of the building.  Furthermore, some types of construction such as reinforced concrete rafts (concrete floors that also serve as foundations) cannot usually be cut into, and alternative methods of drainage have to be implemented.  Therefore, the design of cavity drainage systems is not a trivial issue and a thorough understanding of building construction is a necessity.

The studded cavity drain membranes that are applied to the surface of walls and laid over floors are really just there to control vapour.  The primary waterproofing protection is provided by the drainage system that is typically served by one or a combination of pumps.  This means that the greatest potential issue of a cavity drainage system is the failure of a pump to get rid of ingress water.  This is a particular concern if a system incorporates just a single pump.  For this reason, it is essential that a thorough risk assessment is undertaken at design stage and suitable back-up measures are incorporated in the system, particularly if the electricity supply is unreliable.