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Box gutter sump and side overflow calculator

Enter the catchment or the flow on each gutter entering the sump, their width and grade, and the overflow channel. The calculator reads Figures H.4 and H.6 of AS/NZS 3500.3:2025 for the sump, the channel and the gutter depth when the downpipe blocks.

A box gutter sump and its side overflow are designed to AS/NZS 3500.3 under NCC 2025, called up by Volume One F1D3 for Class 2 to 9 buildings and by Volume Two H2D6(1)(a) for Class 1 and 10 (Part 7.4 excludes box gutters, H2D6(3)(b)).

How it works

It reads from

  • AS/NZS 3500.3:2025 Figure 3.7.4(B) (general method, Steps 1 to 15)
  • Figure H.4 (sump depth), Figure H.5 (the device), Figure H.6 (ℓoc and doc)
  • Clauses 3.7.2, 3.7.3, 3.7.7

Flows

Each gutter's flow is its catchment times the 1% AEP intensity over 3600; the outlet takes the gutters plus any roof falling straight into the sump, at most 16 L/s.

Sump and downpipe

Figure H.4 gives the depth of sump for the total flow and the downpipe size.

Overflow channel

Figure H.6(a) gives ℓoc from the largest flow in one gutter and its width; Figure H.6(b) the channel depth doc from the total flow and the channel width, drawn for channels 200, 300 and 450 mm wide (narrower channels are refused). The channel invert is at least 0.7 ℓoc above the gutter sole, and the gutter must be ℓoc + doc + 30 mm deep so the overflow runs without overtopping.

Sump size

The sump is the channel width plus 2 ℓoc long, and never less than 400 mm (Figure H.5). A channel longer than 450 mm has a minimum gradient of 1:10.

Where the Standard disagrees with itself

Example 2 quotes channel depths of 132 mm and 102 mm where its own Figure H.6(b) gives 126.1 and 99.8 mm. Appendix H is normative, so the calculator follows the figure.

A worked example

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

Minimum box gutter depth, dbg

161 mm

The box gutter needs at least 161 mm of depth (165 mm rounded up to 5 mm) so a blocked downpipe overflows through a 300 × 110 mm side channel. The sump is 400 mm long and 190 mm deep on a 150 × 100 downpipe.

  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 = 85.0 × 262 / 3600 = 6.19 L/s
  3. Flow in gutter BQ = Ac × I / 3600 = 85.0 × 262 / 3600 = 6.19 L/s
  4. Total design flow through the outletQ = QA + QB + Qsump = 12.4 L/sAS/NZS 3500.3:2025 Figure 3.7.4(B) Note 2
  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(B) Step 4; Figure H.1
  6. Depth without slope adjustment, gutter with the largest flowFigure H.1, lower right: design flow and sole width = 6.19 L/s, the 600 mm curve = 106.9 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 = 106.926 mm on the 1:200 line = 96.7 mmAS/NZS 3500.3:2025 Figure H.1
  8. Every downpipe that works, sump depth hsFigure H.4 at 12.4 L/s for each downpipe; mm = 150 × 100 190 · Ø140 175 · 125 × 125 175 · Ø150 130 · 150 × 150 85 = shallowest: 150 × 150, 85 mmAS/NZS 3500.3:2025 Figure H.4; Clause 3.7.5 Note 2
  9. Depth of sump, hsFigure H.4, the 150 × 100 curve, total flow; rounded up to 5 mm = 12.4 L/s → 186.367 mm = 190 mmAS/NZS 3500.3:2025 Figure H.4; Figure 3.7.4(B) Step 10
  10. Least distance between the sides of the channel and the sump, ℓocFigure H.6(a), largest flow in any one box gutter = 6.19 L/s, the 600 mm curve = 22.1 mmAS/NZS 3500.3:2025 Figure H.6(a); Figure 3.7.4(B) Step 11
  11. Minimum depth of overflow channel, docFigure H.6(b), total flow, 300 mm channel = 12.4 L/s, the 300 mm curve = 108.6 mmAS/NZS 3500.3:2025 Figure H.6(b); Figure 3.7.4(B) Step 12
  12. Minimum height of the gutter top above the channel invert, htht = [ℓoc + (doc + 30)] − 0.7 ℓoc = [22.147 + (108.561 + 30)] − 0.7 × 22.147 = 145.3 mmAS/NZS 3500.3:2025 Figure 3.7.4(B), Step 13
  13. Minimum gutter depth for the overflow conditionℓoc + (doc + 30) = 22.147 + 108.561 + 30 = 160.8 mmAS/NZS 3500.3:2025 Figure 3.7.4(B), Step 14; Figure H.5
  14. Minimum depth of box gutter, dbgthe greater of ℓoc + (doc + 30) and ha = 160.8; ha 96.7. Figure 3.7.4(B) sets the depth by the overflow condition; the free-flow depth is checked as well, as Figure 3.7.4(C) Step 13 does = 160.8 mmAS/NZS 3500.3:2025 Figure 3.7.4(B), Step 14; Figure H.1
  15. Minimum length of sumpwoc + 2 ℓoc, not less than 400 mm = 300 + 2 × 22.147 = 344.293 = 400 mmAS/NZS 3500.3:2025 Figure H.5; Figure 3.7.4(B) Step 15
  16. Channel invert above the gutter sole, minimum0.7 ℓoc min. = 0.7 × 22.147 = 15.5027 = 16 mmAS/NZS 3500.3:2025 Figure H.5

Questions

How big does a box gutter sump need to be?

At least 400 mm long, as wide as the gutter, and as deep as Figure H.4 of AS/NZS 3500.3:2025 gives for the flow and downpipe: about 220 mm for 16 L/s into a 150 mm downpipe.

Why does a side overflow make the gutter deeper?

When the downpipe blocks, water rises in the sump until it runs out through the side channel. The gutter has to hold that water plus freeboard, so its depth is set by the overflow condition (Clause 3.7.7.2).

Can the two gutters entering a sump differ?

Yes. Give gutter B its own sole width and gradient; each gutter is read at its own flow, width and gradient, and the larger reading of each dimension is adopted for the sump.

What catchment do I enter for each gutter?

Each gutter's own catchment for its worst wind direction (Clause 3.4.1). The Standard takes one wind direction for the whole outlet, so adding each gutter's worst case gives an equal or larger total, which is conservative. You can enter the flows in L/s instead.

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