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Balcony, terrace and flat roof drainage calculator

Enter the surface, any wall above it, the site's rainfall, the outlets and the overflow. The calculator gives the 5% AEP flow the outlets must take and the 1% AEP flow the overflow must take with every outlet blocked, and for a Class 2 to 9 building checks the NCC 2025 F1D4 levels.

NCC 2025 calls up AS/NZS 3500.3 for this drainage (Volume One F1D3 for Class 2 to 9 buildings, Volume Two H2D2(1)(a) for Class 1 and 10), and Volume One F1D4 adds the fall, step down, hob and overflow level for a concrete roof, balcony or podium of a Class 2 to 9 building.

How it works

It reads from

  • AS/NZS 3500.3:2025 Clause 3.8 (design storms); Clause 3.4 (catchment, walls at 2:1); Clause 3.3.5.1 and Table D.1, or the Bureau's IFD for the site
  • Figure H.4 (a sump over a vertical downpipe); Clauses 3.5.3(a) and 3.7.7.1 (overflows with the outlets blocked)
  • NCC 2025 Volume One F1D2, F1D3 and F1D4 with its Notes; Volume Two H2D2 and H2D8

Design storms

In Australia, Clause 3.8 designs the drainage of a balcony or terrace for the 5% AEP five-minute intensity and its overflow for the 1% AEP intensity. The catchment is the plan area plus half of any wall that wind-driven rain strikes (Clause 3.4.4). The overflow carries the whole 1% AEP flow with every outlet blocked, as the Standard designs its other overflows.

Outlet capacity

For a sump over a vertical downpipe the calculator reads Figure H.4 for the depth the flow needs, and gives the HB 39 limit on where the downpipe sits in the sump. AS/NZS 3500.3 gives no capacity method for a floor outlet, a puddle flange or an overflow opening through a hob: enter the maker's rated capacity, or leave it blank for the capacity each must have. Where the surface drains to a channel and a sump, the channel is a box gutter: use the box gutter calculators.

NCC 2025 F1D4

A concrete roof, balcony, podium or similar part of a Class 2 to 9 building needs a drainage system to AS/NZS 3500.3 with sufficient overflow; its structural substrate graded at 1:80 or steeper; a 70 mm step down from the internal floor's structural substrate where one adjoins; a monolithic hob at least 70 mm high at the outer edge unless it drains straight to a gutter; and the overflow invert at least 20 mm below the internal floor's structural substrate. A tile bed, screed or topping is not the structural substrate. F1D4 does not apply to timber decking or other perforated flooring, or to a surface directly above ground (F1D2).

Class 1 and 10

Volume Two calls up AS/NZS 3500.3 for drainage (H2D2(1)(a)), so the Clause 3.8 storms apply; F1D4 is a Volume One provision. External waterproofing of a balcony or terrace over an internal space is H2D8, to AS 4654.1 and AS 4654.2.

A worked example

The calculator's example inputs, worked through step by step.

Design flow, 5% AEP

0.97 L/s

The 17.4 m² catchment sends 0.971 L/s at 5% AEP to the outlet and 1.27 L/s at 1% AEP to the overflow with the outlets blocked. Still to choose: an outlet rated for at least 0.971 L/s; an overflow rated for at least 1.27 L/s. The levels entered meet NCC 2025 F1D4.

  1. Vertical wall areaAv = length × height (exposed height ≤ 10 m) = 10.8 m²AS/NZS 3500.3:2025 Clause 3.4.4.3 Note (single wall)
  2. Catchment areaAc = Ah + 0.5 Av = 12.0 + 0.5 × 10.8 = 17.4 m²AS/NZS 3500.3:2025 Eq 3.4.4.1
  3. Design rainfall intensity, 5% AEP, 5 minutes = 201 mm/hAS/NZS 3500.3:2025 Table D.1, Sydney City
  4. Design rainfall intensity, 1% AEP, 5 minutes = 262 mm/hAS/NZS 3500.3:2025 Table D.1, Sydney City
  5. Design flow, 5% AEPQ = Ac × I / 3600 = 17.4 × 201 / 3600 = 0.971 L/sAS/NZS 3500.3:2025 Clause 3.8(a)(i)
  6. Overflow flow, 1% AEPQ = Ac × I / 3600 = 17.4 × 262 / 3600 = 1.27 L/sAS/NZS 3500.3:2025 Clause 3.8(a)(ii)

Questions

What are the NCC requirements for balcony drainage?

For a concrete balcony, roof or podium of a Class 2 to 9 building, NCC 2025 Volume One F1D4 requires drainage to AS/NZS 3500.3 with sufficient overflow, a fall of at least 1:80 in the structural substrate, a 70 mm step down from the internal floor's structural substrate where applicable, a monolithic hob at least 70 mm high at the outer edge unless it drains straight to a gutter, and the overflow invert at least 20 mm below the internal floor's structural substrate.

What storm is a balcony overflow sized for?

In Australia, AS/NZS 3500.3:2025 Clause 3.8 sets the 1% AEP five-minute intensity for the overflow and the 5% AEP intensity for the outlets. The overflow has to carry the whole 1% AEP flow with the outlets blocked.

How much water does a balcony floor outlet take?

AS/NZS 3500.3 gives no capacity method for a floor outlet or puddle flange, so the figure comes from the outlet maker, at the ponding depth you allow. The calculator gives the flow each outlet must take; a sump over a downpipe is sized from the Standard's Figure H.4.

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The method behind every calculator, in one guide:roof and stormwater drainage to AS/NZS 3500.3:2025.

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The free plan answers from the National Construction Code, state building and planning legislation, ABCB handbooks, regulator practice notes and codes of practice, with the clause cited. Plus adds the Standards, including AS/NZS 3500.3:2025, from A$79 a month plus GST.

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