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Box gutter calculator

Describe the roof and any walls above the gutter, choose the gutter's width and grade, and the calculator reads Figures H.1 to H.3 of AS/NZS 3500.3:2025: the minimum depth of the gutter and the rainhead that goes with it.

Under NCC 2025 the Deemed-to-Satisfy route for a box gutter is AS/NZS 3500.3: Volume One F1D3 calls it up for Class 2 to 9 buildings, and Volume Two H2D6(1)(a) for Class 1 and 10, because the Housing Provisions Part 7.4 route excludes box gutters (H2D6(3)(b)).

How it works

It reads from

  • AS/NZS 3500.3:2025 Clause 3.7 and Figure 3.7.4(A)
  • Figure H.1 (box gutter depth, with slope adjustment), Figure H.2 (rainhead), Figure H.3 (rainhead depth and length)
  • Clauses 3.4 (catchment, walls), 3.7.2 (30 mm freeboard), 3.7.3 (1:40 to 1:200, 16 L/s), 3.7.8 (downpipes)
  • Clause 3.7.8 (the downpipe on the rainhead floor), Table 4.3.2 and Clause 4.3.2 (expansion joints), Clause 4.9.4 (supports); SA HB 39 in Victoria (NCC 2025 Volume Three VIC E3D2)

Catchment and flow

Box gutters are sized for the 1% AEP, 5-minute storm. The catchment follows Clause 3.4, including half the face of any wall that wind-driven rain can reach (Equation 3.4.4.2 as printed by default, or as Appendix I works it), and Q = Ac × I / 3600. A rainhead takes at most 16 L/s.

Depth of the gutter

Figure H.1 gives the minimum depth including the 30 mm freeboard for the flow and the sole width, then adjusts it for the gradient: steeper gutters run shallower. The calculator never reads past a curve's printed end, and a gradient between the printed 1:40, 1:100, 1:150 and 1:200 lines is read on the next flatter line. For the Standard's Example 1 flow of 15 L/s in a 450 mm gutter at 1:200 the chart gives 140.3 mm, rounded up to 141 (the example writes 140).

The rainhead

Figure H.3 gives the depth of water in the rainhead for the flow and downpipe and the total depth as 1.67 times that; Figure H.2 Note 1 requires at least 1.25 times the downpipe. The length comes from the total depth and the gutter's depth at 1:200, and is never less than the downpipe's size along the rainhead plus 20 mm, our practical minimum: Clause 3.7.8 has the downpipe fitted to the rainhead's base but gives no length. A rectangular downpipe is set with its smaller side along the rainhead. The overflow weir sits 25 mm below the gutter sole and the rainhead is as wide as the gutter. Choose "Shallowest rainhead" to see the downpipe that needs the least depth; every downpipe that works is listed in the working.

Limits

Gradients from 1:40 to 1:200, flows to 16 L/s per rainhead (an upper roof discharging into it included), gutters 300 mm wide and over (200 mm for domestic work, up to the 600 mm the charts print), straight, of constant width, with vertical sides, sealed and discharging at their downstream end (Clause 3.7.6(g)). One outlet takes at most 16 × 3600 / I square metres of catchment (Figure 3.7.4(A) Step 4). Not for siphonic systems (Clause 10.1 Notes 2 and 3). Minimum dimensions are rounded up.

Installation and Victoria

Enter the gutter run, its material and how it is fixed to get the fall over the run, the expansion joints of Table 4.3.2 (both columns: one end fixed, and both ends free) and the support spacing of Clause 4.9.4 (continuous, or brackets at not more than 750 mm; continuous is advised for soles over 450 mm). In Victoria, SA HB 39 applies beside the Standard (NCC 2025 Volume Three VIC E3D2), and the result lists its box gutter items; where the two conflict, AS/NZS 3500.3 prevails.

A worked example

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

Minimum box gutter depth, ha

136 mm

A 450 mm box gutter at 1:200 needs at least 136 mm of depth for 13.6 L/s (140 mm rounded up to 5 mm). It discharges to a rainhead 450 mm wide, 185 mm long and 285 mm deep, with a Ø125 downpipe.

  1. Design rainfall intensity, 1% AEP, 5 minutes = 262 mm/hAS/NZS 3500.3:2025 Table D.1, Sydney City
  2. Vertical projection of the roofAv1 = Ah × tan θ = 150.0 × tan 5.0° = 13.1 m²AS/NZS 3500.3:2025 Figure 3.4.2(A)
  3. Wall area above the gutterAv2 = length × height (exposed height ≤ 10 m) = 60.0 m²AS/NZS 3500.3:2025 Clause 3.4.4.3 Note (single wall)
  4. Eq 3.4.4.2 as printedAc = Ah + 0.5 (Av1 + Av2) = 150.0 + 0.5 × (13.123 + 60.0) = 186.6 m²AS/NZS 3500.3:2025 Eq 3.4.4.2; for a parapet at the gutter, Eq 3.4.3.2(3) as printed gives the same (Figure 3.4.2(C)(b), Av3 = Av4 = 0)
  5. As Appendix I works it: the greater of wind onto the wall and onto the roof faceAc = Ah + 0.5 |Av2 − Av1| = 150.0 + 0.5 × |60.0 − 13.123| = 173.4 m²AS/NZS 3500.3:2025 Appendix I Examples 1 and 2 (informative); Figure 3.4.2(C)(b)
  6. Catchment area, the printed equationAc = Ah + 0.5 (Av1 + Av2) = 186.6 m²AS/NZS 3500.3:2025 Eq 3.4.4.2
  7. Design flowQ = Ac × I / 3600 = 186.562 × 262 / 3600 = 13.6 L/sAS/NZS 3500.3:2025 Figure 3.7.4(A) Step 6; Figure H.1
  8. 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(A) Step 4; Figure H.1
  9. Depth without slope adjustmentFigure H.1, lower right: design flow and sole width = 13.6 L/s, the 450 mm curve = 156.4 mmAS/NZS 3500.3:2025 Figure H.1
  10. Minimum depth including freeboard, with slope adjustment, haFigure H.1, lower left: across to the gradient line = 156.357 mm on the 1:200 line = 135.1 mmAS/NZS 3500.3:2025 Figure H.1
  11. Every downpipe that works, rainhead depth hr × length lrFigure H.3 and Figure H.2 Note 1 for each downpipe; mm = Ø125 285 × 185 · 150 × 100 200 × 165 · Ø140 185 × 160 · 125 × 125 185 × 160 · Ø150 190 × 170 · 150 × 150 215 × 170 = shallowest: Ø140, 185 mmAS/NZS 3500.3:2025 Figure H.3; Clause 3.7.5 Note 2
  12. Depth of water in the rainhead, hFigure H.3, lower: the Ø125 curve = 13.6 L/s = 170.3 mmAS/NZS 3500.3:2025 Figure H.3; Figure 3.7.4(A) Step 10
  13. Total depth of rainhead, hrhr = 1.67 h, and not less than 1.25 Di; rounded up to 5 mm = 1.67 × 170.314 = 284.424; 1.25 × 125 = 156.25 = 285 mmAS/NZS 3500.3:2025 Figure H.3 (top axis); Figure H.2 Note 1; Figure 3.7.4(A) Steps 10–11
  14. Length of rainhead from Figure H.3Figure H.3, upper: hr and ha at 1:200 = hr 285 mm, ha 135.1 mm, between the 125 and 150 mm curves, interpolated at the same hr = 183.1 mmAS/NZS 3500.3:2025 Figure H.3; Figure 3.7.4(A) Step 12 and Note 2
  15. Length the downpipe needs on the rainhead floorthe downpipe's size along the rainhead + 20 mm, our practical minimum: the Standard requires the downpipe fitted to the base (Clause 3.7.8) but gives no dimension = 125 + 20 = 145 mmAS/NZS 3500.3:2025 Clause 3.7.8; Figure H.2
  16. Length of rainhead, lrthe greater of the Figure H.3 length and the downpipe's length; rounded up to 5 mm = 183.129 and 145: Figure H.3 governs = 185 mmAS/NZS 3500.3:2025 Figure H.3; Clause 3.7.8; Figure 3.7.4(A) Steps 12–13
  17. Overflow weir height above the rainhead floorhr − 25 = 285 − 25 = 260 mmAS/NZS 3500.3:2025 Figure H.2

Questions

How deep does a box gutter need to be?

It depends on the flow, the sole width and the grade, from Figure H.1 of AS/NZS 3500.3:2025, and includes 30 mm of freeboard. In the Standard's example, 15 L/s in a 450 mm wide gutter at 1:200 reads 140.3 mm on the chart: the calculator gives at least 141 mm, and the example writes 140 mm.

What is the minimum width of a box gutter?

300 mm for commercial work. Figure H.1 is drawn for 300 to 600 mm, with 200 mm dashed for domestic construction only (Figure H.1 Note); a 200 mm gutter blocks more easily, and extra height is recommended (AS/NZS 3500.3:2025 Clause 3.7.3 Note 3).

What fall does a box gutter need?

Between 1:40 and 1:200, constant along its length (Clauses 3.7.3 and 3.7.6).

Does a rainhead need an overflow?

Yes: the front of the rainhead is left open above an overflow weir set 25 mm below the gutter sole, so a blocked downpipe overflows without backing water up the gutter (Figure H.2).

How many outlets does a box gutter need?

Each outlet takes at most 16 L/s (Clause 3.7.3(b)(i)), so one outlet serves at most 16 × 3600 / I square metres of catchment: 192 m² at 300 mm/h. Lay out the outlets, high points and expansion joints to suit (Figure 3.7.4(A) Step 4). More outlets along a gutter usually means sumps rather than rainheads.

Does the rainhead have to fit the downpipe?

Yes. Clause 3.7.8 has the downpipe fitted vertically to the rainhead's base, but the Standard gives no length. The calculator makes the rainhead at least the downpipe's size along it plus 20 mm, our practical minimum, and gives where the downpipe sits in it: SA HB 39 Cl 5.7.3, called up in Victoria by NCC 2025 Volume Three VIC E3D2 and good practice elsewhere.

Can a higher roof discharge into the rainhead?

Yes: Clause 3.4.5 sends a higher catchment to a rainhead or a sized sump. Enter the upper roof and its flow is added. Clause 3.4.5 Note 2 warns that such a rainhead may need to be larger, with a way to dissipate energy, which is beyond the Standard.

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