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Leaking Concrete Slabs




We have seen a significant increase in enquires relating to leaking suspended concrete slabs. These are typically in the form of common spaces above carparks (such as pool decks or outdoor entertaining areas in apartment buildings), or private access balconies. These may be resulting in external or internal water leaks causing anything from the development of hairline cracks and efflouresence, to substantial areas of concrete cancer to overhead areas.


There are a wide variety of issues that may be the source of leaking slabs, and a number of these are provided below;


a. Tiled floor surfaces are often the main source of moisture penetration due to lack of / failure of existing waterproof membrane (WPM) below the tiled finishes. A lack of or failure of waterproof membrane (WPM) between the tiled surface and the concrete slab substrate will allow any water that is able to penetrate through the tiles and / or grout to come into direct contact with the concrete substrate. Without a functioning WPM, the moisture can then penetrate through the concrete and access the structural steel leading to concrete cancer and / or cracking in the concrete substrate. Furthermore, this moisture can then transfer to other areas below the tiled finishes and create leaks further away in the structure.


b. Outlet drainage ‘pops’, or overflow drains commonly in the junction of the balustrade and balcony / walkway floor surfaces can be too narrow and prevent adequate water drainage. This leads to areas where water is able to ‘pond’ and more readily soak into the concrete. In time this leads to delamination of concrete toppings to floor surfaces leading to tripping hazards, and eventual concrete cancer in these areas. We recommend the existing outlets are increased in size to minimum 40mm diameter to cope with the amount of water contained in these areas.

c. Dilapidated or failing expansion joints can provide areas for water penetration if they are not repaired or maintained on a regular basis.




The only way to provide long-term and reliable remediation results is to address the source of the issue being a lack of or failure of adequately specified and installed waterproof membrane 'WPM' below the tiles. The only way to address this is to replace the existing WPM with a new, durable and flexible WPM system. This involves:

  1. Remove floor and skirting tiles, and existing WPM to entire area.
  2. Prepare existing structural concrete substrate.

  3. Supply / install new screed substrate.

  4. Apply new waterproof membrane over new screed substrate – lap down into drains and lap up at wall junctions.

  5. Install new tile finishes. 

The above is a simplified methodology which is further refined with assistance of product material technical representatives to ensure all aspects of the work are integrated, inspected at key milestones during the works, and certified at the end of the project to achieve all required warranty conditions.

It is important to note that sealants simply applied to the surface of the tiled balcony floor do not provide a mid-long term solution. These coatings are non-flexible and crack following movement in the tiled or concrete substrate due to the natural process of thermal expansion and contraction. Any systems specified to address the issues at a particular site must be fit for purpose and take into account the specific dynamics of the structure and issues being experienced. 

Further, the method of removing the tiles and surface finishes directly above the damaged area, repairing the substrate, and then replacing the WPM in the localised area does not provide a long term solution. 

The flaw in this approach is that although the substrate can be repaired, there is no way of knowing whether water will continue to leak through tiles in other areas and penetrate through other areas – which would have otherwise be protected through applying waterproofing to the entire area of the slab under the tiles. Unfortunately this approach is quite common and leads to repeated repairs, and requirement to install new finishes in the future.

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