Stormwater Drainage

Stormwater drainage is not covered under the Plumbing and Drainage Act 2019 as it is considered building work. Plumbers are increasingly being asked by building certifiers to provide a Form 43 for work relating to the installation of stormwater drainage for a Class 1a building they have installed.



A Form 43 is used for the purposes of sections 68, 69 and 70(1)(a) of the Building Regulation 2021, to state the aspect work for a single detached class 1a building and class 10 building and structure is compliant with the building development approval.

The National Construction Code (NCC) references AS/NZS 3500.3:2021 as a means of satisfying the deemed-to-satisfy provisions for stormwater drainage. The Standard specifies a minimum gradient of 1 in 100 (1%) for pipe sizes of DN 90, 100 and 150.

The piping material to be used is U-PVC which has a Colebrook-White roughness coefficient, k (mm) of 0.003 to 0.015 as referenced in AS 2200 for thermosetting plastics. This relates to the “smoothness” of the inside of the pipe.

The lower value 0.003 represents the expected value for clean, new pipes laid straight. Where multiple pipes are to be joined they shall be connected via the use of a junction fitting or through a storm water pit. For in-ground stormwater the code recommends limiting the velocity to not more than 2m/s so as to limit frictional losses through fittings and other associated items.

AS/NZS 3500.3:2021 does provide a method of sizing graded stormwater pipes in Section 5. The full standard can be purchased on the Standards Store here.

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