Box Gutter Design in Australia: Minimum Fall, Width and Depth (2025 Guide)

How much fall does a box gutter need, how wide should it be, and how deep? A box gutter sits within the roof, so water that backs up comes into the building rather than spilling over the edge. That is why AS/NZS 3500.3 treats box gutters more strictly than eaves gutters.

This guide sets out the rules in plain terms, and the calculator gives the minimum depth for a roof in any capital city.

In one minute

• The fall is constant, between 1:200 and 1:40, and the gutter runs straight to its outlet (AS/NZS 3500.3:2025 Clauses 3.7.3(a) and 3.7.6(g)). 1:200 is 5 mm per metre.

• The sole is at least 300 mm wide. In Australia 200 mm is allowed for domestic construction, but it blocks more easily (Clause 3.7.3 Note 3).

• The depth comes from Figure H.1 for the flow, the width and the fall, with 30 mm of freeboard included (Clause 3.7.2).

• It is sized for the 1% AEP, 5-minute storm, and one outlet takes no more than 16 L/s (Table 3.3.4; Clause 3.7.3(b)(i)).

• Every box gutter has an overflow: a rainhead, or a sump with a side or high-capacity overflow (Clauses 3.7.1 and 3.7.7.2).

Which rules apply to a box gutter?

AS/NZS 3500.3, Plumbing and drainage, Part 3: Stormwater drainage. NCC 2025 calls it up for Class 2 to 9 buildings (Volume One F1D3) and for houses and Class 10 buildings (Volume Two H2D6(1)). The Housing Provisions gutter tables are not an option: Part 7.4 excludes box gutters (Volume Two H2D6(3)). In Victoria, H2D6 requires AS/NZS 3500.3 for all gutters and downpipes, and Volume Three also calls up SA HB 39 for roof drainage, with AS/NZS 3500.3 prevailing where they conflict (VIC E3D2). Box gutters are designed by the general method in Clause 3.7 and Appendix H of the Standard, which also has three worked examples in Appendix I.

What is the minimum fall for a box gutter?

1:200, and no steeper than 1:40. The fall must be constant along the gutter (Clause 3.7.6(g)(iii)) and within that range (Clause 3.7.3(a)), and a box gutter up to 600 mm wide must not be installed flatter than 1:200. Any deviation must be smooth and must not cause permanent ponding (Clause 4.5.4(a)).

The gutter must also run straight, with no change of direction, and discharge at its downstream end rather than out of the side (Clause 3.7.6(g)(i) and (iv)). A box gutter that turns a corner is designed as two straight box gutters, each discharging at its downstream end.

Fall	Drop per metre	Over a 10 m run	Over a 20 m run
1:200, the flattest allowed	5 mm	50 mm	100 mm
1:100	10 mm	100 mm	200 mm
1:40, the steepest allowed	25 mm	250 mm	500 mm

How wide does a box gutter need to be?

300 mm for commercial construction. In Australia a 200 mm box gutter may be used for domestic construction, but the Standard notes that it is more prone to blockages and recommends extra height where you can give it (Clause 3.7.3 Note 3).

The cross-section has a flat sole of constant width and vertical sides (Clause 3.7.6(g)(ii)). Figures H.6(a) and H.8, which size a sump's overflow, are drawn for gutters 300 mm to 600 mm wide, so the general method gives no sump design for a 200 mm gutter: it discharges to a rainhead. Above 450 mm, the Standard recommends continuous support under the sole rather than brackets (Clause 4.9.4 Note 1).

How deep does a box gutter need to be?

As deep as Figure H.1 says for the flow, the width and the fall, and Figure H.1's depth already includes the 30 mm of freeboard the Standard requires (Clause 3.7.2). A wider gutter or a steeper fall needs less depth; the flattest fall, 1:200, needs the most.

The flow is the catchment area times the rainfall intensity, divided by 3,600, in litres per second. In Australia a box gutter takes the 1% AEP, 5-minute intensity for the site, where an external eaves gutter takes the 5% storm (Table 3.3.4 and Clause 3.3.5.1): 187 mm/h in Melbourne City, 306 mm/h in Brisbane City (Table D.1). The catchment follows Clause 3.4, for the worst wind direction: a flat roof's plan area or, for a freely exposed sloping roof, its plan area plus half the area of its vertical projection (rise × length). Walls and parapets change it.

The depth from Figure H.1 suits a gutter that discharges to a rainhead. With a sump, the gutter must also hold the water that rises when the downpipe blocks (Clause 3.7.7.2). A side overflow almost always needs a deeper gutter. A high-capacity overflow takes the larger of the two depths, which for a wide gutter at a low flow can be the Figure H.1 depth itself (Figure 3.7.4(C)).

How much roof can one box gutter outlet take?

16 L/s. The general method takes no more at one outlet: at a rainhead, the design flow of the gutter it serves (Clause 3.7.3(b)(i)); at a sump, the total flow through its outlet (Figures 3.7.4(B) and (C)). So the largest catchment one outlet can take is 16 × 3,600 divided by the intensity (Figure 3.7.4(A) Step 4). More roof than that needs more outlets, each taking less. A flow the general method cannot take needs a Performance Solution. Narrow gutters take less: Figure H.1 stops at about 10 L/s for a 200 mm gutter and just under 16 L/s for 300 mm, so a 200 mm gutter in Brisbane takes about 121 m².

City	1% AEP, 5-minute intensity	Largest catchment at one outlet
Brisbane	306 mm/h	188 m²
Darwin	274 mm/h	210 m²
Sydney	262 mm/h	219 m²
Canberra	192 mm/h	300 m²
Melbourne	187 mm/h	308 m²
Adelaide	174 mm/h	331 m²
Perth	172 mm/h	334 m²
Hobart	120 mm/h	480 m²

These are the city-centre values, and suburbs can be wetter: Table D.1 lists Port Adelaide at 184 mm/h against 174 for Adelaide City. Use the value listed for the site's own location in Table D.1 or, if it is not listed, the Bureau of Meteorology's design rainfall for its latitude and longitude (Appendix C, Clause C.2.1).

Rainhead or sump: which overflow?

Every box gutter system has an overflow device that carries at least the design flow at its outlet, from every gutter and roof that drains to it, and discharges to the open air (Clauses 3.7.1 and 3.7.7.1). The Standard recognises three:

• A rainhead, which overflows at its front. It needs no extra depth in the gutter (Clause 3.7.7.2 Note 2).

• A sump with a side overflow, a channel out through the side wall (Clause 3.7.7.2(a)).

• A sump with a high-capacity overflow, weirs into a second sump with its own downpipe (Clause 3.7.7.2(b)). The main sump is at least 150 mm deep (hs, Clause 3.7.6(e)).

A vertical pipe overflow in the sole is not one of them (Clause 3.7.7.2 Note 3). The roof and stormwater drainage guide draws all three, and each has its own calculator: rainhead, side overflow sump and high-capacity overflow sump.

Box gutter depth calculator

Pick the city, enter the roof that drains to the outlet and choose the sole width. The calculator gives the design flow and the minimum depth at 1:200 for a gutter that ends in a rainhead, and the full calculator then sizes the rainhead and downpipe with your figures filled in.

Quick examples

Every example is a box gutter at 1:200 ending in a rainhead, worked in the box gutter calculator.

Roof and city	Sole width	Design flow	Minimum depth	Rainhead and downpipe
60 m², Sydney (a house)	200 mm	4.37 L/s	121 mm, detail 125	125 mm long, 115 mm deep, Ø90
100 m², Melbourne	300 mm	5.19 L/s	112 mm, detail 115	125 mm long, 160 mm deep, Ø90
120 m², Adelaide	300 mm	5.80 L/s	116 mm, detail 120	140 mm long, 195 mm deep, Ø90
200 m², Perth	450 mm	9.56 L/s	120 mm	150 mm long, 225 mm deep, 100 × 100
150 m², Brisbane	300 mm	12.8 L/s	155 mm	205 mm long, 255 mm deep, Ø125
200 m², Brisbane	300 mm	17.0 L/s	Over 16 L/s: split the roof between two outlets

Common mistakes

• A fall flatter than 1:200, or a sag that holds water. The fall is constant and no flatter than 1:200 (Clauses 3.7.6(g)(iii) and 4.5.4(a)).

• Turning a corner, or taking the outlet out of the side. A box gutter runs straight and discharges at its downstream end (Clause 3.7.6(g)(i) and (iv)).

• A pipe through the sole as the overflow. It is not covered by the Standard (Clause 3.7.7.2 Note 3), so it needs a Performance Solution.

• No room to expand. Table 4.3.2 limits the length between expansion joints for each material, and a gutter fixed between two points more than 6 m apart, such as two sumps, needs a 25 mm expansion joint (Clause 4.3.2(a)).

• Sizing the drain below for the wrong storm. Downpipes and drains a box gutter connects to directly take the 1% AEP storm up to a surcharge outlet (Clause 5.2.3).

Where to read the source

The NCC is free at ncc.abcb.gov.au: Volume One F1D3 and Volume Two H2D6. AS/NZS 3500.3:2025 is sold by Standards Australia: Clause 3.7 and Appendix H for box gutters, Appendix I for the worked examples, Clause 3.4 for the catchment and Appendix D for rainfall. The gutter and downpipe sizing guide covers eaves gutters, and the AS/NZS 3500 2025 guide explains what changed in the series.

Inside AEC Assistant the Code text is searchable by clause, and on a paid plan the Standards are too. Ask how deep your box gutter must be for its roof and city, and the answer comes back with the clause it was read from.

This article is general guidance. The building surveyor or certifier for your project determines compliance.