Plumbing Balcony Stormwater Drainage

Overview

Inadequate balcony drainage can be a significant compliance and performance issue because even a relatively small balcony can generate substantial runoff during intense rainfall events. A blocked outlet combined with insufficient overflow capacity and poor balcony design, can result in water ponding and water entering a building.

The following is an extract from the Practice note: SD 02 Balcony Design and Stormwater Drainage.

Deemed to Satisfy Requirements (AS/NZS 3500.3)

AS/NZS 3500.3 Clause 3.8 applies to the design of balconies and terraces. The objective of clause 3.8 is to ensure that the drainage pipe systems for balcony outlets and overflows can accommodate flow rates appropriate to the catchment area and associated Annual Exceedance Probability (AEP) however, it does not prescribe a methodology to achieve this. The absence of a prescribed methodology allows any approach to be adopted, provided it demonstrates that the requirements have been satisfied.

In most cases to meet Deemed to Satisfy provisions of the NCC (AS/NZS 3500.3) a minimum 90mm pipe is sufficient, however it may need to be larger to suit the design flow rates.

In such cases, the following calculation methods may be relevant.

Calculations

It is worth noting that pipe sizing methods used throughout the standard are based on the Colebrook-White / Darcy-Weisbach equations. Although this is not presented in full within the standard, the equation can be used to demonstrate compliance with the Deemed to Satisfy (DtS) provisions of the National Construction Code (NCC), this is explained in Vic A5G4(6)(a) of the Plumbing Code of Australia (PCA) and A5G3 of Volumes One and Two of the Building Code of Australia (BCA).

The water-carrying capacity of a pipe depends on several factors, such as:

• Pipe diameter
• Pipe length
• Internal roughness (material type)
• Hydraulic head (height difference between the outlet and overflow)
• Pipe gradient (slope)
• Number and type of bends and fittings.

Standards Australia Handbook 39 (SA HB 39) includes an informative note stating that balcony drainage can be designed using the general design and sizing principles applied to roof drainage systems in AS/NZS 3500.3. This approach is appropriate because the roof drainage design methodology explicitly accounts for the relationship between outlet size, overflow height, sump and box gutter depth. As a result, the hydraulic head is inherently incorporated into the calculated flow capacity of the outlet and downpipe.

Caution: Grate designs may require a larger effective surface area to accommodate the flow rate which has been calculated for the balcony surface water. It is essential to check the manufacturer’s specified maximum design flow for the grate and confirm that it matches the calculated outlet flow rate.

Performance Solutions

A performance solution associated with a building permit must consider both the Building Code of Australia (BCA) and the  Plumbing Code of Australia (PCA). A performance solution prepared under the PCA should be presented to the Relevant Building Surveyor (RBS), to consider whether any relevant Performance Requirements under the BCA and PCA have been impacted.

The Performance Requirements, which aim to ensure the safest and controlled collection and disposal of stormwater, and the prevention of water ingress in buildings include:

• BCA: Vol 1 F1P1
• BCA: Vol 2 H2P1 and H2P2
• PCA: E3P1, E3P2, E3P4, E4P1, E4P2

Where a building permit is not required, a performance solution under the PCA must be in accordance with A2G2 of the PCA.

In all cases, performance solutions are required to be attached to the plumber’s compliance certificate.

Balcony overflow requirements

Clause 3.8 of AS/NZS 3500.3 requires an overflow to be designed for rainfall events with an Annual Exceedance Probability (AEP) of 1%. However, hydraulic capacity of the outlet pipe alone is insufficient, and the size, location, height, and configuration of the overflow outlets are critical in ensuring the effective performance of the balcony drainage system.

The overflow weir level must be set above the maximum hydraulic head of the primary outlet to ensure the primary outlet is operating at its full design capacity, before the overflow becomes active.

To ensure effective operation under surcharge conditions, overflow systems must incorporate a sufficient vertical margin (freeboard) between the top of the overflow outlet and the critical waterproofing system termination height. The freeboard should provide contingency for construction tolerances, surface turbulence and wind-driven inflow.

The invert level of the overflow designed to NCC Volume 1 and Volume 2 DtS requirements must be installed not less than 20 mm below any adjacent internal floor structural substrate. It is noted that the overflow must be above the hydraulic head of the primary outlet.

This sequence is essential, to maintain system integrity and to prevent internal water ingress during overflow conditions. The system must be designed holistically to ensure effective conveyance of overflow volumes, without compromising the internal building envelope.

The following is an extract from the Practice note: SD 02 Balcony Design and Stormwater Drainage.