Gutter and Downpipe Sizing in Australia: The NCC Tables, AS/NZS 3500.3 and a Free Calculator (2025 Guide)

Gutters and downpipes are sized for rain, and the National Construction Code (NCC) tells you how much rain: a 5-minute burst listed for your locality, with one intensity for the gutter and a heavier one for the overflow. For a house the whole method is four table look-ups in Part 7.4 of the ABCB Housing Provisions; for anything larger, or a box gutter, or anywhere in Victoria, it is AS/NZS 3500.3. This guide sets out each step with its clause, reproduces the tables, walks through the Code's own worked example, and the calculator in section 4 runs the method for your roof.

In one minute

• Two storms, two jobs. The eaves gutter is sized to carry the 5% annual exceedance probability (AEP) 5-minute rainfall for the locality; the overflow measures are sized to shed the 1% AEP rainfall without water entering the building. Both intensities are in Housing Provisions Table 7.4.3d.

• The gutter type comes from Table 7.4.3a, read at the roof area draining to one downpipe (30 to 70 m²) and the next intensity row up. Type A to E is a standard profile; type F means design to AS/NZS 3500.3.

• One downpipe per 12 m of gutter, placed as close as possible to any valley gutter (7.4.5), of a section that suits the gutter type (Table 7.4.3c). Only the 75 mm round downpipe is ever excluded, and only with a type E gutter.

• Every eaves gutter needs an overflow measure (7.4.4(1)(c)). Table 7.4.4b gives the litres per second required; 7.4.6 and 7.4.7 give what each slot, gap, weir, nozzle or rainhead provides. Add them up per downpipe.

• The figures are the same in NCC 2022 and NCC 2025. Victoria uses AS/NZS 3500.3 only; Western Australia allows either route.

1. Where the rule lives, and which route you are on

For a house or a Class 10 building the Performance Requirement is H2P1 in Volume Two, and H2D6(1) gives two Deemed-to-Satisfy ways to meet it for gutters and downpipes: AS/NZS 3500.3, or Part 7.4 of the ABCB Housing Provisions. Part 7.4 is the table method this guide describes. It can only be used where the roof drainage connects to a stormwater system that complies with H2D2, and it excludes box gutters (H2D6(3)). H2D6(2) also relieves a detached Class 10 building from the 1% AEP overflow requirement where there is no need for it in the particular case: a shed well clear of the house, for instance, as opposed to a garage against it.

Two states vary this. Victoria deletes H2D6(1)(b) and (3): AS/NZS 3500.3 is the only Deemed-to-Satisfy route, so the Housing Provisions tables are indicative there, not a compliance pathway. Western Australia keeps both routes and writes AS/NZS 3500.3 into 7.4.3 as an alternative for the gutter itself, permits a 95 × 45 mm rectangular downpipe, and allows a gutter laid at less than 1:500 provided the whole system is to the Standard.

For a Class 2 to 9 building there is no table method at all. Volume One F1D3 says stormwater drainage must be designed and constructed in accordance with AS/NZS 3500.3, and anything else is a Performance Solution. That is why the AS/NZS 3500.3 route matters even to a house designer: the moment a project is an apartment building, a shop or a childcare centre, the tables below no longer apply and a hydraulic designer sizes the system from the Standard.

2. The two storms: what 5% AEP and 1% AEP mean

Annual exceedance probability is the chance that a given rainfall intensity is equalled or exceeded in any one year. The NCC uses the 5-minute duration, because that is the burst that fills a gutter. A 5% AEP intensity is the old 1-in-20-year storm; 1% AEP is the 1-in-100. The Housing Provisions explain the logic in the notes to 7.4.1: an eaves gutter system is sized to remove the 5% AEP rainfall provided it is not blocked, and because heavier storms and blocked outlets both happen, the gutter must also have overflow measures that, in a storm up to 1% AEP, direct the excess away so it does not pond against, enter or damage the building even with the stormwater system blocked.

Both intensities are tabulated per locality in Table 7.4.3d, based on the Bureau of Meteorology grid cell at the central post office of each town. The spread is large. Hobart is 86 mm/h at 5% AEP; Bundaberg is 266. A gutter profile that is generous in Tasmania is undersized in Queensland, which is the whole reason the Code sizes by locality rather than by rule of thumb. Where your site is not listed, use the nearest listed locality with a similar climate, or take the intensity from the Bureau of Meteorology's design rainfall data and read it against the same tables.

3. The method in four look-ups

Everything below is for one downpipe and the run of eaves gutter that drains to it. Repeat it for each downpipe on the roof.

Step 1: the rainfall intensity for the locality (Table 7.4.3d)

Read both figures. The 5% AEP intensity sizes the gutter; the 1% AEP intensity sizes the overflow. A selection of the table is reproduced here; the calculator carries every locality.

Locality	5% AEP (mm/h)	1% AEP (mm/h)	Locality	5% AEP (mm/h)	1% AEP (mm/h)
Sydney	201	262	Brisbane	236	306
Penrith, Sydney	178	240	Gold Coast (Tweed Heads)	252	332
Newcastle	225	316	Cairns	230	279
Wollongong	218	311	Townsville	235	300
Canberra	143	192	Darwin	233	274
Melbourne	132	187	Adelaide	120	174
Geelong	103	143	Perth	129	172
Ballarat	134	192	Bunbury	147	198
Hobart	86	120	Broome	232	287
Launceston	91	123	Alice Springs	165	239

Housing Provisions Table 7.4.3d, 5-minute duration rainfall intensities, selected rows. The full table lists 68 localities.

Step 2: the gutter type (Tables 7.4.3a and 7.4.3b)

Work out the roof area, in plan, that drains to the downpipe. Enter Table 7.4.3a at that area, rounded up to the next column (30, 40, 50, 60 or 70 m²), and at the 5% AEP intensity rounded up to the next row. The Code's worked example makes the rounding explicit: Wollongong's 311 mm/h is read at the 325 mm/h row. The cell gives the gutter type or types; Table 7.4.3b says what each type is.

5% AEP intensity	30 m²	40 m²	50 m²	60 m²	70 m²
90 mm/h	A or C	A or C	A or C	A or C	A or C
120 mm/h	A or C	A or C	A or C	A or C	A or D
140 mm/h	A or C	A or C	A or C	A or D	B or E
160 mm/h	A or C	A or C	A or C	A or E	B or E
175 mm/h	A or C	A or C	A or D	B or E	E
200 mm/h	A or C	A or C	A or D	B or E	F
225 mm/h	A or C	A or C	A or B	E	F
255 mm/h	A or C	A or D	B or E	E	F
275 mm/h	A or C	A or D	B or E	F	F
325 mm/h	A or C	B or E	F	F	F
425 mm/h	A or C	E	F	F	F

Housing Provisions Table 7.4.3a: gutter type by design rainfall intensity and roof catchment area per downpipe.

Type	Gutter	Minimum cross-section
A	Medium rectangular gutter	6 500 mm²
B	Large rectangular gutter	7 900 mm²
C	115 mm D gutter	5 200 mm²
D	125 mm D gutter	6 300 mm²
E	150 mm D gutter	9 000 mm²
F	Designed to AS/NZS 3500.3	—

Housing Provisions Table 7.4.3b. The cross-section is measured up to the lowest part of the overflow facility: the bottom of a slot, the back of the gutter, the weir edge, the nozzle or the rainhead opening (7.4.3 note).

The pattern in the table is the practical lesson. Down the 30 m² column every cell is "A or C": keep each downpipe to 30 m² of roof and an ordinary quad or medium rectangular gutter works anywhere in Australia. Across the 70 m² row the table runs out at 200 mm/h, which covers most of the east coast north of Wollongong. The cheapest fix for a type F result is almost always another downpipe, not a bigger gutter.

Step 3: the downpipes (7.4.5 and Table 7.4.3c)

Clause 7.4.5(1) sets three rules. A downpipe must not serve more than 12 m of gutter; it must be located as close as possible to a valley gutter, because that is where the roof concentrates its flow; and its section must suit the gutter type. The Code's note says the 12 m limit exists to ensure effective fall and adequate capacity in the 5% AEP storm. Table 7.4.3c is short: 75 mm round, 90 mm round, 100 × 50 mm and 100 × 75 mm are all suitable for gutter types A to D, and only the 75 mm round is ruled out for type E. Where the top of the downpipe carries a rainhead, its overflow should be directed away from the building.

So the downpipe count is set by two limits at once. Divide the total eaves gutter length by 12, divide the roof area by the largest catchment the table allows at your intensity, and take the larger number. On a 15 × 10 m hipped roof (150 m², 50 m of gutter) in Sydney at 201 mm/h, the 12 m rule gives five downpipes and the 70 m² column gives three; five it is, and at 30 m² each the gutter is type A or C everywhere.

Step 4: the overflow (7.4.4, Table 7.4.4b, 7.4.6 and 7.4.7)

Clause 7.4.4(1)(c) requires every eaves gutter to be fitted with overflow measures capable of removing the overflow volume in Table 7.4.4a or Table 7.4.4b. Table 7.4.4b gives it in litres per second per downpipe, entered at the catchment area and the 1% AEP intensity, again rounded up to the next row (the table runs in 25 mm/h steps from 150 to 400). Table 7.4.4a gives the same thing per metre of gutter for a continuous measure, entered at the ridge-to-gutter length; where both kinds of measure are combined, the Code's example uses Table 7.4.4b. Interpolation between rows is permitted by 7.4.4(3).

1% AEP intensity	30 m²	40 m²	50 m²	60 m²	70 m²
150 mm/h	1.3	1.7	2.1	2.5	2.9
175 mm/h	1.5	1.9	2.4	2.9	3.4
200 mm/h	1.7	2.2	2.8	3.3	3.9
225 mm/h	1.9	2.5	3.1	3.8	4.4
250 mm/h	2.1	2.8	3.5	4.2	4.9
275 mm/h	2.3	3.1	3.8	4.6	5.3
300 mm/h	2.5	3.3	4.2	5.0	5.8
325 mm/h	2.7	3.6	4.5	5.4	6.3
350 mm/h	2.9	3.9	4.9	5.8	6.8
365 mm/h	3.1	4.2	5.2	6.3	7.3
400 mm/h	3.3	4.4	5.6	6.7	7.8

Housing Provisions Table 7.4.4b: overflow volume in L/s per downpipe by 1% AEP intensity and roof catchment area.

The measures the Code accepts, and what each is deemed to shed, are in 7.4.6 (continuous, per metre of gutter) and 7.4.7 (dedicated, per downpipe). Each comes with construction conditions and a figure; meet the conditions and the capacity is deemed, no calculation needed (7.4.4(2)).

Measure	Capacity	Conditions	Clause
Front-face slotted gutter	0.5 L/s per m	Slot area at least 1 200 mm² per metre; lower edge of the slots at least 25 mm below the top of the fascia	7.4.6(1)
Controlled back gap	1.5 L/s per m	Permanent 10 mm spacer between gutter back and fascia, one per bracket, no wider than 50 mm; gutter back at least 10 mm below the top of the fascia	7.4.6(2)
Controlled front bead height	1.5 L/s per m	Front bead at least 10 mm below the top of the fascia	7.4.6(4)
End-stop weir	0.5 L/s	Clear width at least 100 mm; weir edge at least 25 mm below the top of the fascia; not where the end-stop abuts a wall	7.4.7(1), (2)
Inverted nozzle	1.2 L/s	At least 100 × 50 mm, lengthways in the gutter, within 500 mm of a high point; top at least 25 mm below the top of the fascia	7.4.7(3)
Front-face weir	1.0 L/s	Clear width at least 200 mm, clear height at least 20 mm; weir edge at least 25 mm below the top of the fascia	7.4.7(4)
Rainhead	3.5 L/s	75 mm hole in the outward face, centreline 100 mm below the top of the fascia; detailed to avoid nuisance discharge in ordinary rain	7.4.7(5), (6)

Housing Provisions 7.4.6 and 7.4.7, acceptable overflow measures and their deemed capacities. The Code’s Figures 7.4.6a to 7.4.7d show each detail; they are redrawn below.

Two exemptions. The overflow requirement does not apply to an eaves gutter on a verandah or an eave wider than 450 mm that has no lining, or whose lining is raked away from the building (7.4.4(5)): water spilling over the front of that gutter lands outside. And a detached Class 10 building may be relieved of the 1% AEP provision by H2D6(2) where it does not affect the dwelling.

The Code's worked example

The note to 7.4.3 walks a Wollongong roof through the method, and it is worth reading the numbers against the tables above.

Step	Reading	Result
Locality	Wollongong, Table 7.4.3d	218 mm/h at 5% AEP, 311 mm/h at 1% AEP
Catchment	60 m² to one downpipe, 10 m of eaves gutter	60 m² column
Gutter type	Table 7.4.3a, 218 read up to the 225 mm/h row	Type E: 150 mm D gutter, 9 000 mm²
Downpipe	Table 7.4.3c for type E	90 mm round, 100 × 50 or 100 × 75; not 75 mm round
Overflow required	Table 7.4.4b, 311 read up to the 325 mm/h row, 60 m²	5.4 L/s
End-stop weir alone	7.4.7(1)	0.5 L/s, not enough
Add a slotted front face	7.4.6(1), 0.5 L/s/m × 10 m	5.0 L/s
Provided	0.5 + 5.0	5.5 L/s, exceeds 5.4 L/s

The worked example in the note to Housing Provisions 7.4.3, with the gutter and downpipe steps added.

4. Size your gutter and downpipe

Choose the state and the nearest listed locality, set the roof area and the length of gutter draining to one downpipe, and tick the overflow measures you intend to use. The headline gives the gutter type; the cards give the intensities, the downpipe options, the 12 m check and the overflow sum with the clause for each. The same method with every locality, a printable receipt and a link to ask AEC Assistant about the result is in the free gutter and downpipe calculator.

5. Installation rules that go with the sizing

• Fall. An eaves gutter is laid with a fall of at least 1:500 unless designed to AS/NZS 3500.3 (7.4.4(1)(a)). Over a 12 m run that is 24 mm; on a long fascia the fall is usually taken from a high point in the middle towards a downpipe at each end, which is also why the inverted nozzle is placed within 500 mm of a high point.

• Brackets. Securely fixed at stop ends, corners and at not more than 1.2 m centres (7.4.4(1)(b)).

• Valley gutters. Only on a roof pitched above 12.5 degrees, with a side angle of at least 12.5 degrees, 15 mm of freeboard and the sheet width and effective depth from Table 7.4.4c for the rainfall intensity: from 355 mm wide and 32 mm deep at up to 200 mm/h to 435 mm and 43 mm above 350 mm/h (7.4.4(4)). Below 12.5 degrees a valley is designed as a box gutter to AS/NZS 3500.3 or as a Performance Solution.

• Materials. Metal gutters, downpipes and flashings to AS/NZS 2179.1, UPVC to AS 1273, compatible with everything upstream on the roof under 7.2.2(2), and lead-free on a drinking-water catchment (7.4.2).

• Box gutters and lower roofs. Box gutters are outside Part 7.4 entirely. The 7.4.1 notes also ask the designer to think about where a higher roof's overflow lands: if it discharges onto a lower roof, that roof's gutters and overflow must carry it, and the lower roof may need sarking and flashing detailed for it.

6. When you are in AS/NZS 3500.3 territory

AS/NZS 3500.3 Plumbing and drainage, Part 3: Stormwater drainage is the Standard behind the tables, and the route you take when the tables run out or do not apply.

• Type F, or more than 70 m² per downpipe. The table has no answer; the gutter is designed from the Standard's flow and capacity method. In practice a hydraulic consultant or a plumber working from the Standard's charts sizes it.

• Box gutters, always. H2D6(3)(b) excludes them from Part 7.4. The Standard sets box gutter sizes, sumps, overflow devices and the rainheads that go with them.

• Any Class 2 to 9 building. Volume One F1D3 sends the whole stormwater system to AS/NZS 3500.3, eaves gutters included.

• Victoria. The Part 7.4 route is deleted from H2D6, so a Victorian house is designed to the Standard, which the Building and Plumbing Commission also calls up under the Plumbing Regulations.

How the Standard's method differs from the tables

The Housing Provisions tables are a simplification of the Standard, and the Standard is worth knowing even when the tables apply, because it explains the numbers. Four differences matter.

• The same two storms. Table 3.3.4 of AS/NZS 3500.3:2025 sets 5% AEP for an external eaves gutter and 1% AEP for eaves gutter overflow measures, valley gutters and box gutters, with a note that in Australia the table is read with the NCC.

• The catchment is the plan area times a slope factor. For an eaves gutter the Standard's catchment is the plan area multiplied by F from Table 3.4.3.2, which runs from 1.00 at a flat roof through 1.20 at 22 degrees and 1.29 at 30 degrees to 1.50 at 45 degrees (Equation 3.4.3.2(2)). The Housing Provisions read the plan area straight into their tables, so a steep roof sits closer to the edge of a table cell than its plan area suggests.

• The gutter is chosen by its effective cross-sectional area, not a type letter. Clause 3.5.5 measures the area below a line 10 mm under the overflow (the front bead, the gutter back or the bottom of the slots), and Figures 3.5.5(A) and (B) give the catchment each area will drain at the design intensity, for gutters from 3 500 to 20 000 mm² at 1:500 or steeper. Table 3.5.2 then gives the downpipe for that area: 5 200 mm², the 115 mm D gutter of Table 7.4.3b, takes an 80 mm round or a 100 × 50 mm downpipe; 9 000 mm², the 150 mm D gutter, a 100 mm round or 100 × 75 mm. Outlets are fitted vertically to the sole of the gutter (3.5.4).

• The overflow is designed with every downpipe blocked. Clause 3.5.3 sets the design case, and Appendix F gives the volume as Q* = A × R / 3600, catchment in m² times the 1% AEP intensity in mm/h, on catchments up to 400 m². That formula is where Table 7.4.4b comes from: 60 m² at 325 mm/h is 5.42 L/s, the table's 5.4. Appendix F then lists the same continuous and dedicated measures, with a 3 mm margin above the design head on a gutter steeper than 1:500 and 6 mm on a flatter one (F.4.1).

Which edition of the Standard applies follows the NCC edition in force where you are. NCC 2022 references AS/NZS 3500.3:2021; NCC 2025 references AS/NZS 3500.3:2025, whose preface lists the residential eaves gutter overflow and valley gutter requirements in Clause 3 and Appendix F as the first of its major changes. NCC 2025 took effect on 1 May 2026 in the ACT, Tasmania, Victoria and Western Australia and applies from 1 May 2027 in New South Wales, Queensland and South Australia; the Northern Territory has not adopted it. What changed in the 2025 edition of each Part of the Standard is set out in AS/NZS 3500:2025: what changed and when it applies.

7. Common mistakes

• Sizing the gutter and forgetting the overflow. A gutter with no slots, no weir and a standard back is not Deemed-to-Satisfy, whatever its profile. 7.4.4(1)(c) applies to every eaves gutter that is not on an open verandah.

• Reading the intensity row down instead of up. Sydney's 201 mm/h is read at 225, not 200. At 60 m² that changes the answer from "B or E" to "E".

• Counting the whole roof against one downpipe. The tables are per downpipe. The area is the part of the roof that actually drains to that outlet, which on a hipped roof with a high point mid-run is half the run's catchment on each side.

• A downpipe far from the valley. Valleys concentrate flow; 7.4.5(1)(b) wants the downpipe as close to the valley as possible, and a gutter that overtops beside a valley is the commonest leak into an eave.

• Using the Housing Provisions tables in Victoria, or on a Class 2 to 9 building. They are not a pathway there. The numbers may be a useful sanity check, but the design is to AS/NZS 3500.3.

8. Where to read the source

ABCB Housing Provisions 2025, Part 7.4 Gutters and downpipes: 7.4.1 to 7.4.7 and Tables 7.4.3a to 7.4.4c, on the NCC website, free to read. NCC Volume Two H2D6 for the Deemed-to-Satisfy routes and the Victorian and Western Australian variations; Volume One F1D3 for every other building class. AS/NZS 3500.3 is purchased from Standards Australia. Inside AEC Assistant, the Code text and the Standard are searchable by clause; ask which gutter type a locality and catchment give and the answer comes back with the table row it was read from.