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Roof catchment area calculator

Enter the roof in plan, its slope and any wall that rises above the gutter, or build any layout from its roof planes and walls. The calculator applies Clause 3.4 of AS/NZS 3500.3:2025 in every wind direction, names the one that governs, and gives the catchment and the flow it sends to the gutter.

Under NCC 2025, stormwater drainage for Class 2 to 9 buildings is designed to AS/NZS 3500.3 (Volume One F1D3), and AS/NZS 3500.3 is a Deemed-to-Satisfy route for gutters and downpipes on Class 1 and 10 buildings (Volume Two H2D6(1)(a)); this calculator applies its Clause 3.4 catchment.

How it works

It reads from

  • AS/NZS 3500.3:2025 Clause 3.3.3 (rain at 2:1) and Clause 3.4.1 (greatest for any wind direction)
  • Table 3.4.3.2 slope multiplier F; Equations 3.4.3.2(1) to (3), 3.4.3.3, 3.4.4.1 to 3.4.4.3
  • Table 3.3.4 design AEPs; Table D.1 intensities for all 514 listed places (Clause 3.3.5.1; Appendix C.2.1(a)), the Bureau's IFD for any other site (C.2.1(b)), or, as an approximation, the listed place nearest the site

Rain does not fall straight down

AS/NZS 3500.3 takes wind-driven rain at 2 vertical to 1 horizontal (Clause 3.3.3). A sloping roof therefore catches its plan area plus half its vertical rise, and a wall above a gutter adds half its face. For a plain sloping roof that is the multiplier F in Table 3.4.3.2: Ac = Ah × F, where F is 1 + tan(slope)/2.

Every wind direction, the greatest result

Where a wall stands beside the gutter, rain driven towards the wall lands on its face while the roof face is in the lee; rain from the other way strikes the roof face while the wall throws a shadow. The catchment is the greatest for any wind direction (Clause 3.4.1). For a roof falling to a gutter against a wall, Equation 3.4.4.2 prints Ac = Ah + 0.5 (Av1 + Av2), while the Standard's Appendix I examples work Ah + 0.5 × the difference; the calculator applies the printed, larger equation, shows the Appendix I value beside it and offers it as an option. A roof rising to a wall takes Ah + 0.5 (Av1 + Av2).

Any layout, every wind direction

For roofs and walls that fit none of the Standard's figures, enter each roof plane (plan area, slope and the side it falls to) and each wall (width, height and the side it faces). The calculator tries the wind from the front, back, left and right in the form of Equation 3.4.3.2(3): whatever faces the wind adds half its vertical projection, a roof plane falling away from it subtracts, and a wall in the lee is taken as dry. Where walls stand at right angles it also tries the wind into that corner with both walls at full height, as Equation 3.4.4.3 does. The working shows every direction and names the one that governs (Clause 3.4.1). Where a roof plane falls towards a wall, the working also shows Equation 3.4.4.2 as printed, which adds the wall and the roof's rise together, for comparison; the Roof against a wall case applies that printed form.

High walls

A tall wall does not catch rain over its full height. For a single wall the Standard lets you count half its area up to 10 m of exposed height (Clause 3.4.4.3 Note); the calculator applies that cap. Two walls at right angles are counted at full height.

From area to flow

Flow is Q = Ac × I / 3600 in litres per second, with I the 5-minute rainfall intensity in mm/h. Eaves gutters and downpipes use the 5% AEP intensity; box gutters, valley gutters and overflow measures use 1% AEP (Table 3.3.4).

A worked example

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

Catchment area, Ac

72.3 m²

The roof drains as 72.3 m², 21% more than its plan area. It sends 4.04 L/s to the gutter in a 5% AEP storm and 5.26 L/s in a 1% AEP storm.

  1. Slope multiplierF from roof slope = 22.5° = 1.205AS/NZS 3500.3:2025 Table 3.4.3.2
  2. Catchment areaAc = Ah × F = 60.0 × 1.205 = 72.3 m²AS/NZS 3500.3:2025 Eq 3.4.3.2(2)
  3. Design rainfall intensity, 5% AEP, 5 minutes = 201 mm/hAS/NZS 3500.3:2025 Table D.1, Sydney City
  4. Flow at 5% AEPQ = Ac × I / 3600 = 72.3 × 201 / 3600 = 4.04 L/sAS/NZS 3500.3:2025 Table 3.3.4(a)
  5. Design rainfall intensity, 1% AEP, 5 minutes = 262 mm/hAS/NZS 3500.3:2025 Table D.1, Sydney City
  6. Flow at 1% AEPQ = Ac × I / 3600 = 72.3 × 262 / 3600 = 5.26 L/sAS/NZS 3500.3:2025 Table 3.3.4(b)

Questions

How do I calculate the catchment area of a roof?

Measure the roof in plan, including the gutter, and multiply by the slope factor F from Table 3.4.3.2 of AS/NZS 3500.3:2025 (1.09 at 10°, 1.18 at 20°, 1.29 at 30°, 1.50 at 45°). Where a wall rises above the gutter, add half the wall's face and take the greater result for any wind direction (Clause 3.4).

Why is the catchment larger than the plan area?

Wind drives rain at an angle. The Standard takes 2 vertical to 1 horizontal, so a sloping roof or a wall facing the rain collects more than its footprint. A 30° roof collects 29% more than its plan area.

Which rainfall intensity should I use?

The 5-minute intensity at 5% AEP for eaves gutters and downpipes, and at 1% AEP for box gutters, valley gutters and overflow measures (Table 3.3.4). AS/NZS 3500.3 takes them from Appendix D (Clause 3.3.5.1): Table D.1 lists 514 places, and the calculators offer every one. Give the site's latitude and longitude instead and the calculator reads the nearest listed place and says how far away it is. That is an approximation: for a site Table D.1 does not list, Appendix C.2.1(b) takes the Bureau of Meteorology's design rainfall (IFD) at the site's own latitude and longitude. Enter the IFD figure to follow it exactly; past 25 km the result asks you to. Housing Provisions Table 7.4.3d, which the Part 7.4 route requires, differs at eight localities (Yorketown, SA: 155 against 115 mm/h at 5% AEP).

How do I read the Bureau of Meteorology IFD for a gutter?

Open the Bureau's design rainfall page (https://www.bom.gov.au/water/designRainfalls/revised-ifd/) and enter the site's latitude and longitude. Read the 5-minute row, in the 5% AEP column for eaves gutters and downpipes or the 1% AEP column for box and valley gutters and overflow. If the table gives a depth in mm, multiply it by 12 for the intensity in mm/h: 22.8 mm in 5 minutes is 274 mm/h. A typed figure below the lowest in Table D.1 (78.8 mm/h at 5% AEP, 105 mm/h at 1%) is flagged as probably a depth.

My box gutter runs between two roofs and ends at a parapet. What is its catchment?

Choose Two roofs and enter each roof's plan area and slope, any wall above either roof, and the parapet across the gutter's end. The calculator tries every wind direction (Clause 3.4.1): wind from beyond one roof adds that roof's rise and any wall above it and subtracts the roof in its lee (Equation 3.4.3.3); wind along the gutter adds half the parapet; and where the parapet meets a wall at right angles, both are counted at once (Equation 3.4.4.3). The greatest governs.

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