Engineering Tool

Roof Gutter & Downpipe Design Calculator

Size eaves gutters, downpipes and catchment per downpipe to AS/NZS 3500.3:2018 — from roof area, pitch and design rainfall intensity. 500+ Australian rainfall locations built in.

This calculator follows the general method of AS/NZS 3500.3:2018 — Plumbing and drainage, Part 3: Stormwater drainage (Section 3 and Appendix H). It computes the roof catchment area, the design stormwater flow, the maximum catchment per downpipe, the minimum number of downpipes, and the required downpipe size. Rainfall intensities are the 5 minute duration values from the Standard's Appendix E.

1 Design rainfall

Pick your location to load the AS/NZS 3500.3 Appendix E intensity, or enter a value from the BoM IFD data directly.

Eaves gutters: ≥20 yr (Table 3.3.4). Box/valley gutters: ≥100 yr.
mm/h

2 Roof catchment

Catchment area accounts for wind-driven rain on the sloping surface: Ac = Ah × (1 + 0.5·tanθ) (Cl 3.4.3.2).

m²
Horizontal plan area of the roof draining to the gutter (include the gutter width).
m
m
Length × width (plan).
24°
Multiplier F = 1.22  ·  Catchment area Ac = 175 m²

3 Eaves gutter

The gutter's effective cross-sectional area Ae (the waterway below the overflow line, per AS/NZS 2179.1) sets its flow capacity (Fig 3.5.4) and the downpipe size (Table 3.5.2).

Indicative typical values only — always confirm Ae from the manufacturer's data sheet.
mm²
L/s
If your manufacturer publishes a tested capacity, enter it here for an exact result.
Result · AS/NZS 3500.3:2018
4
Minimum downpipes
51 m²
Max catchment / downpipe
Required vertical downpipe — Table 3.5.2
100 mm Ø  or  100 × 75 mm
Roof slope multiplier, F1.22
Catchment area, Ac175 m²
Design rainfall intensity, I145 mm/h
Total design flow, Q7.05 L/s
Gutter flow capacity · Fig 3.5.42.21 L/s
Catchment / downpipe, Acdp54.8 m²
Flow per downpipe1.76 L/s
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Calculation basis & assumptions

This tool implements the AS/NZS 3500.3:2018 general method for eaves gutter systems (Clause 3.5, flow chart Fig 3.5.2; worked example Appendix H):

  • Catchment area — Ac = Ah × (1 + 0.5·tanθ), the closed form of the Table 3.4.3.2 multiplier F (Eq 3.4.3.2(2)). Wind direction need not be considered for a hipped/gable roof of constant slope with no contributing walls.
  • Design flow — Q = Ac × I / 3600 (L/s), with I the 5 min duration intensity.
  • Rainfall intensity — Appendix E (Australia), 5 min duration. 20 yr ARI for eaves gutters (Table 3.3.4); 500 yr ARI = 1.5 × 100 yr (Cl 3.3.5.1(b)).
  • Gutter flow capacity — linear digitisation of Figure 3.5.4 between its labelled axis endpoints (Ae 3000→18000 mm² ⇄ 0.68→6.0 L/s for 1:500 & steeper; 3000→24000 mm² for flatter). This reproduces the Appendix H example (Ae=7300 mm² → ≈2.1 L/s) within chart-reading tolerance. For critical work, enter the manufacturer's rated capacity instead.
  • Catchment per downpipe — Acdp = Qcap × 3600 / I; minimum number of downpipes = ⌈Ac / Acdp⌉.
  • Downpipe size — Table 3.5.2, selected on Ae and gradient.

The general method excludes localised wind effects, blockage by debris/hail, and reduced capacity from ground movement (Cl 3.2). It assumes regular inspection and cleaning.

⚠ Indicative design aid — verify before use

This calculator is provided as an engineering aid and reproduces the AS/NZS 3500.3:2018 method, but it is not a substitute for design by a competent person. Always confirm rainfall intensities against current Bureau of Meteorology IFD data and gutter properties against manufacturers' data. BOICA accepts no liability for use of this tool. For a checked, certifiable roof drainage design, get in touch.