What is subsurface drainage – and why is it important?

21 Oct What is subsurface drainage – and why is it important?

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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.