Skip to content

Long eaves gutter calculator

Enter the gutter, the run to its outlet at the end and its fall. The calculator traces the water surface along the run as the roof feeds it, finds the flow that fills the gutter, gives the Table 3.5.2 downpipe, and sets the capacity beside the deemed-to-satisfy value from Figure 3.5.5.

Under NCC 2025 the Deemed-to-Satisfy method for an eaves gutter is AS/NZS 3500.3 Figure 3.5.5 (Volume One F1D3, Volume Two H2D6(1)(a)); this analysis supports a Performance Solution under A2G2, assessed against Performance Requirement F1P1 for Class 2 to 9 buildings or H2P1 for Class 1 and 10.

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

  • Spatially varied flow with lateral inflow (Beij 1934; Chow, Open-Channel Hydraulics, 1959, ch. 12)
  • AS/NZS 3500.3:2025 Figure 3.5.5 for the deemed-to-satisfy comparison
  • AS/NZS 3500.3:2025 Table 3.5.2 for the downpipe (internal sizes)

What the charts assume

Figure 3.5.5 of AS/NZS 3500.3 assumes the least favourable position of the downpipe and bends. For a long straight gutter that discharges freely at its end, the real capacity depends on the length, the fall and the friction, which the charts do not separate.

The water surface along the gutter

Water enters along the whole length and leaves at the outlet. Momentum balance with that side inflow, Manning friction and the fall gives the depth at every point, starting from critical depth at the free outlet and working upstream. Level and frictionless, it reproduces the classical result that the upstream depth is √3 times the critical depth.

Capacity

The capacity is the flow at which the deepest water reaches the gutter's effective depth. The gutter is modelled as a rectangle twice as wide as deep with its effective area. That is the calculator's simplification; the Standard's charts assume a width about twice the depth for half-round, quad, ogee or square sections (Figure 3.5.5 Note 1).

The downpipe

Table 3.5.2 reads a downpipe by gutter area. The calculator takes the larger of the gutter's own area and the area Figure 3.5.5 needs for the design flow, so the downpipe suits both the gutter and the flow reaching it. Its sizes are internal. Beyond the table, or for a flow beyond the chart, it gives none and says so.

When to use it

This is an engineering check to support a Performance Solution under NCC 2025 A2G2, assessed against Performance Requirement F1P1 (Class 2 to 9) or H2P1 (Class 1 and 10), or to give a second opinion; it is not the deemed-to-satisfy method. A sole outlet part-way along the run, or a bend or corner in it, raises the water: the calculator refuses those layouts and points to Figure 3.5.5, which allows for them.

A worked example

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

Capacity of this run

2.94 L/s

A 18.0 m run at 1:500 of 7 300 mm² gutter carries 2.94 L/s before the water reaches its effective depth — up to 52.7 m² of roof at 201 mm/h. The deemed-to-satisfy chart allows 2.05 L/s.

  1. Design rainfall intensity, 5% AEP, 5 minutes = 201 mm/hAS/NZS 3500.3:2025 Table D.1, Sydney City
  2. Slope multiplierF from roof slope = 22.5° = 1.205AS/NZS 3500.3:2025 Table 3.4.3.2
  3. Catchment areaAc = Ah × F = 40.0 × 1.205 = 48.2 m²AS/NZS 3500.3:2025 Eq 3.4.3.2(2)
  4. Flow to the outletQ = Ac × I / 3600 = 48.2 × 201 / 3600 = 2.69 L/s
  5. Equivalent sectionwidth = 2 × depth, width × depth = Ae = 7 300 mm² = 121 mm wide × 60 mm deep
  6. Critical depth at the outletyc = (Q² / g b²)^(1/3) = 37.0 mm
  7. Deepest water along the runspatially varied flow, Manning n = 18.0 m, 1:500, n 0.011 = 56.6 mm
  8. Capacity of the runQ that fills the effective depth = 2.94 L/s
  9. Largest roof catchment for this runAc = 3600 Q / I = 3600 × 2.9445 / 201 = 52.7 m²
  10. Deemed-to-satisfy capacity for comparisonFigure 3.5.5(A), least favourable layout: Q = Acdp × I / 3600 = 7 300 mm² reads 36.9 m² on the 200 mm/h curve (2.050 L/s) and 32.7 m² on the 225 mm/h curve (2.046 L/s); flow interpolated linearly in intensity to 201 mm/h: 2.050 L/s, Ac = 3600 × Q / I = 2.05 L/sAS/NZS 3500.3:2025 Figure 3.5.5(A)
  11. Gutter area Figure 3.5.5 needs for the design flowAe on Figure 3.5.5(A) for Ac at I = 48.2 m² at 201 mm/h = 9 100 mm² (rounded up)AS/NZS 3500.3:2025 Figure 3.5.5(A)
  12. Downpipe sizeTable 3.5.2, first row with area ≥ Ae: the larger of the gutter's Ae and the area needed for the flow = 9 100 mm², 1:500 and steeper = 125 mm internal round or 100 × 75 mm internalAS/NZS 3500.3:2025 Table 3.5.2 (internal sizes)

Questions

Do long gutters carry less water?

Yes. Friction over a long run raises the water at the upstream end, so a level gutter's capacity falls as it lengthens. A fall towards the outlet recovers much of that.

Can I use this instead of the AS/NZS 3500.3 charts?

Not for a deemed-to-satisfy design: Figure 3.5.5 governs that. Use this to understand a particular straight run to a free outlet at its end, or to support a Performance Solution under NCC 2025 A2G2.

More roof drainage calculators

All calculators

The method behind every calculator, in one guide:roof and stormwater drainage to AS/NZS 3500.3:2025.

Ask about your own roof, drain or detention tank

The free plan answers from the National Construction Code, state building and planning legislation, ABCB handbooks, regulator practice notes and codes of practice, with the clause cited. Plus adds the Standards, including AS/NZS 3500.3:2025, from A$79 a month plus GST.

Try it freeSee plans and pricing