Waterproofing Basements

Basements are used for a number of different purposes with the most common types as follows:

  • Car parks
  • Car parks with storage units
  • Plant rooms
  • Habitable room spaces

Depending on the basement usage there is a different demand on the waterproofing. The different requirements for the four types are discussed below.

  1. Carparks

Cars bring water into the basement when it is raining and drop much of their surface water onto the floor of the basement. This can be observed in shopping centre carparks where water is carried often hundreds of meters into the carpark as vehicles enter. With these types of basements it is important that there is a waterproof seal at the wall-floor junction otherwise water can pool on the floor of the carpark, as shown in Figure 1, which makes the basement unserviceable in the flooded area.

If there are many cars using the basement then it is important that the floor is sloped to a drainage system so excessive water does not build up on the floor of the carpark. Calcification on the wall of the carpark as shown in Figure 1 does not make the carpark unserviceable but is an acetic issue. If, however the carpark in built under an external exposed deck the area of the deck needs to be considered. If the deck has lengths in excess of 4 metres then there is the possibility of the deck cracking causing leakage into the concrete deck roof. This can result in water leaks into the roof of the car park with damage to the paint work of the vehicles that continually park in the same spot under the drip. Dripping from the roof of carpark with this type of cracking in a deck roof is shown in Figure 2.

With metal decking supporting the concrete roof it is very difficult to trace the source of the surface leak as the water flows along the metal  deck surface before it shows up on the under surface. In the event of such a leak occurring it is often easier to install a channel drain under the leak which will prevent dripping onto vehicles parked below.

  1. Carparks with storage units

Often basement carparks associated with units have storage cages. These are usually located in front of the carpark space and can have a spoon drain running along the wall-floor junction which makes the storage cage serviceability questionable as shown in Figure 3. In this case the user has lifted the valuables by placing them on a timber pallet. If a waterproof seal had been made at the wall-floor junction the water flow in this case would have most likely not occurred.

  1. Plant rooms

These usually have a floor sloped to a floor drain to cater for a plumbing leak that can occur as these rooms are usually only visited for maintenance. Calcification and slight water leakage with this type of basement is only an acetic issue as the drainage to the floor waste will handle any slight leakage into the basement through its structure. Waterproofing is not as critical for this type of usage.

  1. Habitable room spaces

These are becoming more frequent with the use of below ground theatre rooms to restrict noise pollution. They require a much greater detail into the waterproofing especially if they are a single skin wall construction. Also the floor requires complete waterproofing usually with two layers so a waterproofing membrane can be installed between the two layers along with a water vapour barrier. The waterproofing of the walls needs to have a lower water vapour transmission than the internal wall finish otherwise the vapour will end up trapped behind the internal finish and can condense. Once vapour condenses it can pick up salts from the wall resulting in flacking of the internal finish as shown in Figure 4.

These habitable rooms built below ground level will have remain to serviceable for the life of the building. With single skin walls this means the waterproofing usually will need to remain serviceable for the life of the building also, as often there is limited or no access to the external walls below ground level to replace any failing waterproofing. In some habitable room basements where the waterproofing has failed where access to the external wall was not possible the problem has been solved by installing a drainage system and a vented cavity by placing battens supporting a plasterboard wall lining away from the structural wall. Figure 5 shows the wall situation before the plasterboard wall was installed.

In the design of a waterproofing system for basements the major factor that must be considered is the use of the basement as different uses require different performance requirement for the waterproofing system.