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What is subsurface drainage – and why is it important?

Tiled balcony refurbished with subsurface drainage rectified. Waterstop Solutions

What is subsurface drainage – and why is it important?

What do we mean by subsurface drainage?

Why is it important and how do we fix it?


Advantages of reliable and durable subsurface drainage
  • Facilitates the evacuation of water.
  • Avoids pooling of water and saturation of tiling or topping screeds.
  • Reduces moisture-related distress.
  • Provides aeration to the building elements.
  • Aids in protecting the service life of the waterproofing membrane and after trades.


Subsurface drainage

Subsurface drainage aids the removal of water from the surface under a tiled rooftop, balcony floor, podium or deck. The purpose of subsurface drainage in buildings is to provide a permeable base layer that will serve as an escape route for excessive rain and stormwater that may find its way into the substrate. Non-porous sand and cement-based bedding, or premix screed/bedding cementitious products, are essential for efficient rain and stormwater removal, as well as aeration of the substrate.

Durable and reliable subsurface drainage systems are also crucial. If subsurface drainage systems are not completed or designed correctly, it will severely reduce the life expectancy of the waterproof membrane and after trades.

Specifying adequate and reliable subsurface drainage to complement the waterproofing as a complete system is critical during the building design. It is one of the most important parts of the building design to address for
balconies, flat roofs, green roofs, decks and terraced buildings.

Saturation of tiling and topping screeds can result in water finding its way into the substrate. The water becomes locked in place until it either evaporates back out through its path of ingress or leaks through the substrate to the level below. Constant water saturation under the tiles can also lead to debonding of tiles and deterioration of the waterproofing membrane. Salts and minerals that are present in the concrete can be drawn up from the tile bed through the grout by capillary action. When the carrier moisture evaporates, the salts and minerals crystallise creating a build-up of efflorescence (lime/calcium deposits) that appears as white coloured staining on the tiles, which can have a negative impact on slip resistance.


How do we rectify poor subsurface drainage?

Provide functional falls to reduce standing water. Use a waterproof screed to fill in the low points of the deck and grind down the high points.

Low points in the membrane will result in ‘water pooling’ in the substrate. As per the Australian Standard, the waterproofing applied to the deck must not have any low points that allow for late pooling and must allow the water to drain back to the drain points.

Modify drains by cleaning existing drains of calcium and debris build-up and drilling out the tops of drains to open weep holes.

Install effective sub-surface drainage with weep holes and puddle flanges to allow the cementitious tile bedding to drain freely into the drains.

Open up or remove any attachments that are closing up the edge of the balcony, which can trap water underneath the tiles.

Floor penetrations such as floor wastes and other fixtures need to be installed and maintained with extreme attention to detail so that the continuity of the waterproofing is not impaired and subsurface drainage is provided
above the membrane.

Select a cost-effective, long-life membrane that is capable of bridging cracks and joints.

Installing the correct waterproofing membrane under the tiles is essential because most membranes are not designed to hold water between the membrane and the tiles.

Minimise tiles contact with cement-laden materials to reduce the presence of efflorescence.

Rooftop balcony – Before repairs by Waterstop Solutions.


Rooftop balcony – Fully refurbished and subsurface drainage rectified by Waterstop Solutions.


Surface Sealant (PWR)

A budget-friendly way of addressing a leaking balcony is to reduce the amount of water entering the tile bed by installing a surface sealant, such as a Penetrating Water Repellent (PWR).
PWR is a breathable and non-colour changing surface sealer. Waterstop Solutions’ PWR is a clear white spirit-based repellent that will cure with a “dry look” offering minimal to no visual change to the substrate. This system penetrates into the substrate reducing the pore size so that liquid can no longer pass through the substrate while still allowing it to breathe. The PWR system also makes cleaning easy by reducing mould growth and dirt build-up, whilst protecting against salt attack and carbonation.


Epoxy grout

An alternative solution for repairing leaks on a balcony is to replace any existing cementitious grout with an epoxy grout. However, this requires re-cutting the grout and installing the epoxy-grout. The epoxy grout is not breathable. If there is considerable water trapped under the tiles after application and/or water can enter under the grouted tiles from elsewhere (i.e. a joint or pipe fitting), then water will do what water does – it will find other exit routes such as a floor to wall joint or the perimeter of the balcony. The results can be unsightly if this occurs.


With remedial waterproofing work and its proximity to a waterproof membrane we recommend that best – and safest – practice would be to engage a qualified and highly experienced remedial waterproofing tradesperson with current insurances and QBCC licence to perform this work.


Risks of engaging unskilled workers to rectify
subsurface drainage:

As with any remedial system, there are potential risks involved with having an unskilled person attempt this type of work.

  • Ineffective positioning of weep holes
  • Damaging other finishes
  • Blocking drains
  • Damaging pipework or puddle flange
  • Drilling through the membrane system
  • Insurance coverage


Waterstop Solutions’ team of specialist technicians are highly qualified and they perform subsurface
drainage remedial work on a regular basis. Most of our technicians hold a Certificate III Construction Waterproofing qualification, and these technicians either perform or supervise all works performed by Waterstop Solutions. The Certificate III qualification includes a considerable component on remedial waterproofing and this, coupled with Waterstop Solutions’ own extensive internal training, equips our technicians with the knowledge and skills to successfully troubleshoot as required when variations to the standard task occur on-site.


If you have any issues with leaking balcony, water ingress, cracking concrete slabs, structural cracks, concrete deterioration, moisture, seepage, efflorescence, rising damp and/or high-flow active leaks in concrete structures, we would be happy to assist you. You are welcome to contact us with any questions that you may have.

Brisbane office: (07) 3205 1899   


Sydney office: 02 9346 8308   

Email: [email protected]



Are you interested in finding out more about the various remedial building services we are providing?

Please click on the image below to download Waterstop Solutions Capability Statement

Waterstop Solutions Capability Statement

Waterstop Solutions Capability Statement is now available to download as PDF. (The image is clickable and opens up a link to our Capability Statement). 




Good maintenance begins with a proactive approach to maintain, keep, preserve and protect a building. 

Remember that seepage/water ingress into areas such as balconies, basements, concrete roofs, car parks and retaining walls, should not be ignored – call a remedial waterproofing professional today for an assessment to provide you with the best solution for your situation.

At Waterstop Solutions, we carry out a wide range of remedial works to concrete structures post-construction stage in Brisbane, Gold Coast, Sunshine Coast, Sydney and outlying areas in Queensland and New South Wales.

Equipped with major civil works experience, we can quickly determine the best approach to rectify all seepage and concrete deterioration issues.


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