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Box gutter high-capacity overflow calculator

Enter the catchments on the gutters meeting at the sump, their width and grade. The calculator reads Figures H.1, H.4, H.6(a) and H.8 of AS/NZS 3500.3:2025 for the weir height, the gutter depth and the sump.

A high-capacity overflow sump is one of the AS/NZS 3500.3 box gutter overflow devices that NCC 2025 calls up through Volume One F1D3 for Class 2 to 9 buildings and 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(C) (Steps 1 to 17), Figure H.7 (the device)
  • Figures H.1, H.4, H.6(a) and H.8; Clause 3.7.6(e) (sump at least 150 mm)

How the device works

The primary sump feeds the normal downpipe. If it blocks, water rises over two weirs into an overflow channel, then to a secondary sump and its own overflow downpipe (Clause 3.7.7.2(b)). Each weir is as long as the adjacent gutter is wide (Note 1).

Readings

Figure H.4 gives the sump depth for the total flow, never less than 150 mm. Figure H.6(a) gives the weir height ℓoc and Figure H.8 the height ht of the gutter above the weir crests, both for the largest flow in any one gutter.

Gutter depth

The gutter is the deeper of its free-flow depth from Figure H.1 and ht + ℓoc. In the Standard's Example 3 (600 mm gutters, 16 L/s, 8.5 L/s in the busier gutter) that is 110.2 mm, rounded up to 111, just over the 106 mm free-flow depth.

Depth of the sump

Figure 3.7.4(C) and Figure H.7 Note 1 disagree on where the sump depth is measured from when ℓoc is under 60 mm. The calculator takes the deeper reading, hs + (60 − ℓoc) below the gutter sole, and shows both: 253 mm for Example 3, where the example prints 220.

A worked example

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

Minimum box gutter depth, dbg

102 mm

Each box gutter needs at least 102 mm of depth (105 mm rounded up to 5 mm). The overflow weir crests are 23.8 mm above the gutter sole, and the 600 mm sump's floor is 237 mm below the gutter sole (hs 200 mm from its datum) 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 = 95.0 × 262 / 3600 = 6.91 L/s
  3. Flow in gutter BQ = Ac × I / 3600 = 70.0 × 262 / 3600 = 5.09 L/s
  4. Roof straight into the sumpQ = Ac × I / 3600 = 10.0 × 262 / 3600 = 0.728 L/s
  5. Total design flow through the outletQ = QA + QB + Qsump = 12.7 L/sAS/NZS 3500.3:2025 Figure 3.7.4(C) Note 2
  6. 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
  7. Depth without slope adjustment, gutter with the largest flowFigure H.1, lower right: design flow and sole width = 6.91 L/s, the 600 mm curve = 110.6 mmAS/NZS 3500.3:2025 Figure H.1
  8. Minimum depth including freeboard, with slope adjustment, ha, gutter with the largest flowFigure H.1, lower left: across to the gradient line = 110.567 mm on the 1:200 line = 99.6 mmAS/NZS 3500.3:2025 Figure H.1
  9. Every downpipe that works, sump floor below the gutter soleFigure H.4 at 12.7 L/s for each downpipe; mm = 150 × 100 237 · Ø140 222 · 125 × 125 222 · Ø150 187 · 150 × 150 187 = shallowest: Ø150, 187 mmAS/NZS 3500.3:2025 Figure H.4; Clause 3.7.5 Note 2
  10. Depth of sump, hs, not less than 150 mmFigure H.4, the 150 × 100 curve, total flow; rounded up to 5 mm = 12.7 L/s → 197.451 mm = 200 mmAS/NZS 3500.3:2025 Figure H.4; Clause 3.7.6(e); Figure 3.7.4(C) Step 10
  11. Height of the overflow weirs above the sole, ℓocFigure H.6(a), largest flow in any one box gutter = 6.91 L/s, the 600 mm curve = 23.8 mmAS/NZS 3500.3:2025 Figure H.6(a); Figure 3.7.4(C) Step 11
  12. Minimum height of the gutter above the weir crests, htFigure H.8, largest flow in any one box gutter = 6.91 L/s, the 600 mm curve = 78.1 mmAS/NZS 3500.3:2025 Figure H.8; Figure 3.7.4(C) Step 12
  13. Minimum depth of box gutter, dbgIs ha < ht + ℓoc? Yes: dbg = ht + ℓoc. No: dbg = ha = ha 99.6; ht + ℓoc = 78.024 + 23.811 = 101.9 = 101.9 mmAS/NZS 3500.3:2025 Figure 3.7.4(C), Steps 13–14
  14. Sump datum by Figure 3.7.4(C), Steps 15–16Is ℓoc > 60? Yes: the sole of the box gutter. No: the downstream sole of the overflow channel, (60 − ℓoc) below the sole of the box gutter = ℓoc = 23.8, not over 60: hs from the channel's downstream sole, 60 − 23.8 = 36.2 mm below the gutter sole; 200 + 36.2 = floor 237 mm below the gutter soleAS/NZS 3500.3:2025 Figure 3.7.4(C), Steps 15–16; Figure H.7 Section B-B
  15. Sump datum by Figure H.7 Note 1(a) ℓoc < 60: from the sole of the box gutter; (b) ℓoc > 60: from the downstream sole of the overflow channel ("60 − ℓoc below the sole") = ℓoc = 23.8 < 60: hs from the gutter sole = floor 200 mm below the gutter soleAS/NZS 3500.3:2025 Figure H.7 Note 1; Appendix I Example 3, Step 16
  16. Sump floor below the gutter sole, adoptedhs + max(0, 60 − ℓoc): the deeper of the two readings; rounded up = 200 + (60 − 23.8106) = 236.1894 = 237 mmAS/NZS 3500.3:2025 Figure 3.7.4(C), Step 16 (conservative reading); Figure H.7 Note 1

Questions

When should I use a high-capacity overflow instead of a side overflow?

The Standard lets the designer choose (Clause 3.7.7.2). A side overflow needs its channel to discharge through the side of the building; a high-capacity overflow sends the water over weirs, along an overflow channel to a secondary sump and down its own overflow downpipe (Figure 3.7.3(c)), so it suits a gutter that has no wall to discharge through.

How deep is the sump?

As Figure H.4 gives for the total flow and downpipe, and at least 150 mm regardless of the position of the normal outlet (Clause 3.7.6(e)). Measured from the deeper datum where the Standard disagrees with itself (see above).

Is a 'vertical overflow' sump the same as a high-capacity overflow?

Many designers call the Standard's high-capacity overflow device a vertical overflow: water rises over weirs into an overflow channel and down its own overflow downpipe (Figure 3.7.3(c)). A plain standpipe overflow in a sump is a different thing and is not covered by the Standard (Clause 3.7.7.2 Note 3).

Do both downpipes fit in the sump?

Figure H.7 puts the normal and overflow downpipes side by side in the sump's middle 200 mm bay, each in half the gutter's width. The calculator takes each half to need the downpipe's size plus 20 mm each way (our practical minimum) and passes over downpipes that do not fit: on a 300 mm gutter, that rules out Ø140, Ø150 and 150 × 150.

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