Planning for waterproofing of concrete balconies

The success of any water management strategy will rely on ensuring the area is carefully planned and prepared and the following questions can guide your choices when planning or managing construction.

1. Are the area and method covered by the DTS provisions?  
2. How are falls provided with a minimum gradient of 1:80 to a stormwater drainage system?   
3. Has the surface been prepared to receive the membrane?  
4. What surface finish will be used?  
5. Does the design use floor wastes or external gutters?

Are the area and method covered by the DTS provisions?

The changes to Part F apply to concrete balconies, waterproofing membranes, drainage and surface finishes. They do not apply to roofs and covered roof tiles (AS 2049), metal roof sheeting (AS 1562.1), plastic roof sheeting (1562.3), slates and shingles (AS 4597), or surfaces directly over the ground.

When constructed of concrete the substrate in concrete roofs, balconies, podiums, or similar parts of a building will need to meet the relevant standard (AS 3600) and the other obligations of Part F1, including: 

• Grades, or falls to structural substrates to drainage 
• Minimum step-downs from internal areas to external areas 
• Monolithic integral hobs in some cases.   

Remediation

It will be impractical for many of these changes to be applied in existing buildings undergoing alteration. In these cases, state and territories’ regulatory systems generally have provision to allow the application of more relevant standards. In the ACT and NSW, the minimum standards of BCA 2022 can continue to be applied under the relevant legislation.

How are falls provided with a minimum gradient of 1:80 to a stormwater drainage system? 

Falls are required to be provided in the structural substrate. This prevents the use of screeds or toppings from being used as a means of achieving falls.   

A structural substrate is the surface of a structural member to be waterproofed as required by Part F1. Part B1 of NCC Volume One therefore requires the structural substrate be designed for the actions to accommodate an expected 10-year deflection.

The 10-year deflection design requirement should be undertaken by the structural engineer in conjunction with the hydraulic engineer to ensure that the minimum 1:80 fall of the concrete structural substrate to the required stormwater drainage system required by Clause F1D4(b) is maintained. 

Depending on the size of the balcony, to achieve the required defection and falls, the presence of sanitary drainage within the slab may require a thickening of the slab. Careful design and placement of drainage and reinforcement is required to maintain adequate concrete cover for crack control under AS 3600.  

This can be achieved by ensuring the predominant plane of the surface sheds away from the habitable spaces, avoiding the use of cold joints, construction joints and post tensioned pockets. Where joints do occur, they must be located at the high point and protected in accordance with AS 4654.2. 

These provisions will ensure that the minimum 1:80 fall of the concrete structural substrate to the required stormwater drainage system is maintained.  

Has the surface been prepared to receive a membrane? 

Membrane selection must be compatible with the receiving surface. Steps taken to ensure the surface is adequately cured, smooth and dust free will ensure proper adhesion and avoid impurities penetrating the membrane in construction and service.  

Substrates often require primers to ensure soundness and surface adhesion.  

Clause F1D5(2) states that the surface of structural substrates must be prepared in accordance with F1D7(1)(b) (and AS 4645.2) are: 

• Free of any coatings or sealers which may affect the ability of the waterproofing system to bond to the substrate 
• Without protrusions  
• Free from voids 
• Clean. 

The order of works should be carefully planned and special attention paid to the interface of membranes with upstands, doors, façade elements, movement joints and drainage points.  

Further preparation of metal elements could be required which requires sealing and priming prior to additional membrane applications. 

What surface finish will be used?

NCC 2025 F1D10 requires that the flooring or surface finish of a roof, balcony, terrace, podium, or similar part of a building be either: 

• Self-draining; or  
• Directly fixed to the membrane that is laid directly on to the structural substrate (see F1D7). 

This prevents the use of screeds being used to achieve level surfaces between internal and external spaces, and to create the required fall of 1:80 to the drainage system. 

The NCC does not prescribe methods for bonding tiles to the membrane, but guidance is available in AS 3958 Installation of ceramic and stone tiles. 

F1D4 specifies that a concrete roof, balcony, podium or similar part of a building must have a minimum 70 mm step-down from the internal structural substrate to the external structural substrate.  

The minimum 70 mm step-down should not be confused with the minimum waterproofing termination height specified in AS 4654.2, or the height between finished internal and external surfaces.   

Where the design requires a level entry, the use of a suspended overlay will need to be provided. These overlays are typically supported on pedestals, joists or other supports designed to carry, support and distribute the load of the overlay finish.  

Waterproofing termination height

The vertical upward termination height for a membrane refers to the finished height of the membrane above the finished surface level to prevent water, including wind driven water, flowing over the top of the membrane.

Appendix A of AS 4654.2 specifies minimum termination heights of the waterproofing membrane depending on wind classification. This can vary from 40mm for N1 wind class to 180mm for N6.

Figure 2 shows the minimum step-down height required by NCC 2025 and the minimum membrane termination height required by AS 4654.2.

Does the design use floor wastes or external gutters?

Draining of concrete roof, balcony, podium or similar part of a building

The Performance Requirements require water to be disposed of in a way that avoids:

• Unhealthy or unsafe conditions, or loss of amenity for occupants within the building; and 
• Undue damage to internal surfaces and other building elements. 

Water required to be drained must be disposed of in a way that: 

• Avoids the likelihood of damage to the building 
• Is conveyed through a drainage system to an appropriate outfall.

Clause F1D3 requires stormwater drainage to be designed and constructed in accordance with AS/NZS 3500.3. 

Types of drainage 

There are various methods of connecting into the drainage system. The number and type of connections will be dependent on the calculation in accordance with AS/NZS 3500.3 of the rainfall intensity and the size of the area collecting the rainfall from the rain event.

Drainage could include a floor waste connected to a drainage system or an external gutter.

Monolithic hobs

Where a drainage outlet will be installed inside the roof, balcony area either at the perimeter or a central outlet, a monolithic hob of at least 70 mm high poured integral with the structural substrate is required.

Clause F1D4 requires the drainage system of a concrete balcony incorporates overflow provisions. The invert level (bottom of any overflow) must be not less than 20 mm below any adjacent internal floor structural substrate.

Figure 3 shows a central draining balcony with a perimeter monolithic hob with an overflow provided. For connection details of membranes to the drainage outlets and overflows refer to AS 4654.2. Figure 4 shows a single waste near the hob.

Using external gutters for drainage

Draining into an external gutter can require careful detailing to maintain the required gradient, incorporate overflow requirements and ensure the membrane will adequately terminate into the gutter. 

When do the new requirements take effect?

The timeline for adoption of NCC 2025 is determined by the states and territories. Visit our NCC 2025 hub for the latest updates as they become available.  

The above is intended to provide general information in summary form. The content does not constitute specific advice and should not be relied upon as such. Formal advice should be sought by members and customers with respect to particular matters before taking action.