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Box gutter vertical overflow check

AS/NZS 3500.3:2025 does not cover a standpipe overflow inside a sump. This check estimates the water level when the normal downpipe blocks and the whole flow goes down the overflow pipe, so a Performance Solution starts from a number.

AS/NZS 3500.3, which NCC 2025 calls up for box gutters (Volume One F1D3, Volume Two H2D6(1)(a)), does not cover vertical piped overflows, so one needs a Performance Solution under A2G2, assessed against F1P1(3) for Class 2 to 9 buildings or H2P1(2) and (3)(b) for Class 1 and 10; this check gives that assessment a starting figure.

An engineering check, not the Standard's deemed-to-satisfy method. Use it to understand a design or to support a Performance Solution under NCC 2025 A2G2.

How it works

It reads from

  • AS/NZS 3500.3:2025 Clause 3.7.7.2 Note 3 (not covered), Clause 3.7.2 (30 mm freeboard)
  • Figure H.4 applied to the overflow pipe as a sump outlet; Figure H.1 for the free-flow depth

Why it is a check, not a design

The Standard's overflow devices are a rainhead, a sump with a side overflow and a sump with a high-capacity overflow. Vertical piped overflows, covered overflows and anything else are not covered (Clause 3.7.7.2 Note 3), so under NCC 2025 they need a Performance Solution (A2G2), assessed against F1P1(3) for Class 2 to 9 buildings or H2P1(2) and (3)(b) for Class 1 and 10.

The estimate

With the normal downpipe blocked, the whole flow must enter the overflow pipe over its rim. Figure H.4 relates the depth of water over a downpipe entry to the flow; applied to the overflow pipe, it gives the head over the rim. The gutter needs the rim height, that head and 30 mm of freeboard.

What a Performance Solution must add

A piped overflow gives no warning that the normal downpipe is blocked and can block itself. The high-capacity overflow device is the Deemed-to-Satisfy alternative for a sump that cannot use a side overflow.

A worked example

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

Gutter depth with a blocked downpipe

277 mm

With the normal downpipe blocked, 7.28 L/s must rise 227 mm over the rim of a Ø100 overflow pipe, so the gutter needs at least 277 mm (280 mm rounded up to 5 mm). This arrangement is outside AS/NZS 3500.3 and needs a Performance Solution; a sump with a high-capacity overflow is the Standard's own device.

  1. Design rainfall intensity, 1% AEP, 5 minutes = 262 mm/hAS/NZS 3500.3:2025 Table D.1, Sydney City
  2. Flow in gutter AQ = Ac × I / 3600 = 60.0 × 262 / 3600 = 4.37 L/s
  3. Flow in gutter BQ = Ac × I / 3600 = 40.0 × 262 / 3600 = 2.91 L/s
  4. Total design flow through the outletQ = QA + QB + Qsump = 7.28 L/sAS/NZS 3500.3:2025 Figure 3.7.4(C) Note 2 (by analogy)
  5. Largest catchment one outlet can take, Ac,maxAc,max = 16 × 3600 / I (16 L/s at the design intensity), rounded down = 16 × 3600 / 262 = 219.8 m²AS/NZS 3500.3:2025 Figure 3.7.4(C) Step 4; Figure H.1
  6. Depth without slope adjustment, gutter with the largest flowFigure H.1, lower right: design flow and sole width = 4.37 L/s, the 450 mm curve = 105.5 mmAS/NZS 3500.3:2025 Figure H.1
  7. Minimum depth including freeboard, with slope adjustment, ha, gutter with the largest flowFigure H.1, lower left: across to the gradient line = 105.464 mm on the 1:200 line = 95.6 mmAS/NZS 3500.3:2025 Figure H.1
  8. Depth of sump for the normal downpipe, hsFigure H.4, the Ø100 curve; rounded up to 5 mm = 7.28 L/s → 226.761 mm = 230 mmAS/NZS 3500.3:2025 Figure H.4
  9. Water over the overflow rim, whole flowFigure H.4 applied to the Ø100 overflow pipe = 7.28 L/s = 226.8 mmAS/NZS 3500.3:2025 Figure H.4 (by analogy)
  10. Gutter depth with the normal downpipe blockedrim + head + 30 mm freeboard = 20 + 226.761 + 30 = 276.8 mmAS/NZS 3500.3:2025 Clause 3.7.2

Questions

Is a vertical overflow pipe in a box gutter sump compliant?

Not under AS/NZS 3500.3:2025 as a Deemed-to-Satisfy solution: Clause 3.7.7.2 Note 3 excludes vertical piped overflows. Under NCC 2025 it can be used only as a Performance Solution (A2G2).

What should I use instead?

A rainhead where the gutter meets an external wall (the box gutter calculator), a sump with a side overflow (the side overflow calculator), or a sump with a high-capacity overflow device (the high-capacity overflow calculator), each designed by Appendix H. The high-capacity device is what many designers mean by a "vertical overflow".

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