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. It is a 5-minute burst listed for your locality. There is 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 and reproduces the tables. It 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 it at the roof catchment 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). It has 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. Clauses 7.4.6 and 7.4.7 give what each slot, gap, weir, nozzle or rainhead provides. Add them up per downpipe.

• Two routes under NCC 2025. Volume Two H2D6(1) accepts AS/NZS 3500.3 or these tables for a house or Class 10 building. Victoria accepts AS/NZS 3500.3 only; Western Australia keeps both.

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 NCC 2025 Volume Two. There are 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 (H2D6(1)). Part 7.4 is the table method this guide describes.

The table method can only be used where the roof drainage connects to a stormwater system that complies with H2D2. It also excludes box gutters (H2D6(3)). A Class 10 building is relieved of the 1% AEP requirement where, in the particular case, there is no necessity for compliance (H2D6(2)). A shed well clear of the house is an example, as opposed to a garage against it.

Two states vary this route. Victoria replaces H2D6(1) with AS/NZS 3500.3 alone and deletes H2D6(3). AS/NZS 3500.3 is the only Deemed-to-Satisfy route there, so the Housing Provisions tables are indicative, not a compliance pathway.

Western Australia keeps both routes and varies Part 7.4 itself (Housing Provisions 7.4.3 to 7.4.5, WA variations). It:

• writes AS/NZS 3500.3 into 7.4.3 as an alternative for the gutter itself, and into 7.4.5 for the downpipes that go with such a gutter;

• allows a gutter laid flatter than 1:500 where it is designed to AS/NZS 3500.3 (7.4.4(1)(a)).

For a Class 2 to 9 building there is no table method at all. Stormwater drainage must be designed and constructed in accordance with AS/NZS 3500.3 (NCC 2025 Volume One F1D3). 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. A hydraulic designer then sizes the system from the Standard, and our AS/NZS 3500.3 drainage calculators run that method step by step.

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. The 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. Heavier storms and blocked outlets both happen, so the gutter must also have overflow measures. In a storm up to 1% AEP, they 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. They are 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. That is the whole reason the Code sizes by locality rather than by rule of thumb. Where your site is not listed, the Housing Provisions give no other source (7.4.3(b) points to Table 7.4.3d): take the nearest listed locality with a similar climate, and confirm the choice with the building surveyor. Design rainfall from the Bureau of Meteorology for the exact site belongs to the AS/NZS 3500.3 route (Clause 3.3.5.1 and Appendix D).

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, and 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 catchment that drains to the downpipe. Part 7.4 does not say how to measure it; only South Australia's variation defines it, as the plan area with no allowance for slope. We take the plan area multiplied by the slope factor F of AS/NZS 3500.3 Table 3.4.3.2, the more conservative reading, and the calculator in section 4 asks for the catchment on that basis, as do our eaves gutter and downpipe calculator on its Housing Provisions route and our eaves gutter overflow calculator. On a 22° roof F is 1.20, so 50 m² in plan is a 60 m² catchment; on a flat roof the two readings agree.

Enter Table 7.4.3a at the catchment, rounded up to the next column (30, 40, 50, 60 or 70 m²). Read it at the 5% AEP intensity, rounded up to the next row.

For Wollongong, the 5% AEP intensity of 218 mm/h is read at the 225 mm/h row. The Code's own worked example uses the same rule on the overflow table, reading Wollongong's 1% AEP intensity of 311 mm/h at the 325 mm/h row of Table 7.4.4b. 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 catchment and an ordinary quad or medium rectangular gutter works anywhere in Australia. Down the 70 m² column 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)

There are three rules (7.4.5). A downpipe must:

• not serve more than 12 m of gutter;

• be located as close as possible to a valley gutter, because that is where the roof concentrates its flow;

• have a section that suits 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. The 75 mm round, 90 mm round, 100 × 50 mm and 100 × 75 mm downpipes are all suitable for gutter types A to D. 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:

1. Divide the total eaves gutter length by 12.

2. Divide the roof catchment by the largest catchment the table allows at your intensity.

3. Take the larger number.

Take a 15 × 10 m hipped roof at 22° in Sydney (201 mm/h, read at the 225 mm/h row), with 50 m of gutter. Its plan area is 150 m² and its catchment 180 m² (F = 1.20). The 12 m rule gives five downpipes. At the 225 mm/h row the largest column short of type F is 60 m², which gives three. Five it is, and at 36 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)

Every eaves gutter must be fitted with overflow measures capable of removing the overflow volume specified in Tables 7.4.4a and 7.4.4b (7.4.4(1)(c)). Table 7.4.4b gives it in litres per second per downpipe. Enter it at the catchment and the 1% AEP intensity, again rounded up to the next row. The table runs from 150 to 400 mm/h, mostly in 25 mm/h steps.

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 (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 365 mm/h row is printed that way in the Code; its values are those of 375 mm/h by the same formula, catchment times intensity over 3 600.

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, with 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. A Class 10 building may also be relieved of the 1% AEP requirement where, in the particular case, there is no necessity for compliance (H2D6(2)).

The Code's worked example

The note to 7.4.3 walks a Wollongong roof through the method. 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 catchment (plan area × F) 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.

For a calculation to hand to the building surveyor, the free eaves gutter and downpipe calculator runs these Housing Provisions tables too: choose the Housing Provisions route and it gives the gutter type and the downpipes from Tables 7.4.3a to 7.4.3c, or the AS/NZS 3500.3 route and it sizes the gutter and its downpipes from the Standard's charts. The eaves gutter overflow calculator runs this Housing Provisions overflow check, or AS/NZS 3500.3 Appendix F. Both show their working, and Get the report opens either in AEC Assistant, where the calculation report is exported with a free account.

5. Gutter fall: 1:500 (2 mm per metre) on the Housing Provisions route

On the Housing Provisions route an eaves gutter is installed with a fall of not less than 1:500 (7.4.4(1)(a)). That is 2 mm for every metre of gutter, or 24 mm over the 12 m that one downpipe may serve (7.4.5(a)). On a long fascia the fall is usually taken from a high point in the middle towards a downpipe at each end. That is also why the inverted nozzle is placed within 500 mm of a high point (7.4.7(3)).

A flatter gutter is outside Part 7.4. The national 7.4.4(1)(a) reads simply "not less than 1:500", so a gutter laid flatter is designed to AS/NZS 3500.3, the other Deemed-to-Satisfy route (H2D6(1)(a)). Western Australia's version of the clause says so in terms: "unless designed in accordance with AS/NZS 3500.3".

AS/NZS 3500.3 allows a flatter gutter and sizes it on its own chart. Figure 3.5.5(A) is for gutters at 1:500 and steeper and Figure 3.5.5(B) for flatter ones, and Table 3.5.2 has a column for each: a flatter gutter needs a larger effective area for the same downpipe, 4 700 mm² against 3 500 mm² for the smallest. Its continuous overflow measures also need a larger margin above the head they rely on, 6 mm against 3 mm (Clause F.4.1). The eaves gutter and downpipe calculator takes either fall.

Whatever the fall, deviations from it must be smooth and must not cause permanent ponding (Clause 4.5.3(a)). Where a building is likely to move because of reactive soils, a note to that clause advises a fall no flatter than 1:250, which is 4 mm per metre, so that the gutter keeps an effective fall of 1:500; or no flatter than 1:500, to avoid permanent ponding.

6. Installation rules that go with the sizing

• 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. The valley has 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. These run 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.

• Outlets, and a downpipe off the back of the gutter. AS/NZS 3500.3 requires gutter outlets to be fitted vertically to the sole of an eaves gutter (Clause 3.5.4), and its charts assume the outlet sits centrally in the sole (Figures 3.5.5(A) and (B), Note 1(e)). A Western Australian note to Housing Provisions 7.4.5 repeats that rule for a gutter laid flatter than 1:500. In Victoria, where SA HB 39 applies beside the Standard (NCC 2025 Volume Three VIC E3D2), an outlet at the back of the gutter is sized by HB 39 Clause 5.7.5: equal to the gutter's effective area where the gutter falls less than 1:25, and at least half of it at 1:25 or steeper.

• 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. The lower roof may also need sarking and flashing detailed for it.

• Bushfire-prone areas. AS 3959 sets the materials for gutters, box gutters and leaf guards by bushfire attack level (BAL). From BAL-12.5 up, a gutter or valley leaf guard, where one is fitted, must be non-combustible. Check it before choosing a profile or a guard.

7. 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. It is 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, which the eaves gutter and downpipe calculator runs: it gives the catchment one downpipe can serve, the downpipe that suits the gutter and how many the roof needs.

• A gutter flatter than 1:500. Part 7.4 asks for 1:500 or steeper (section 5); the Standard sizes a flatter gutter on its own chart.

• Box gutters, always. Part 7.4 excludes them (H2D6(3)(b)). The Standard sets box gutter sizes, sumps, overflow devices and the rainheads that go with them, and each has a calculator: the box gutter and rainhead, the side overflow sump and the high-capacity overflow sump.

• Any Class 2 to 9 building. The whole stormwater system goes to AS/NZS 3500.3, eaves gutters included (NCC 2025 Volume One F1D3). Our guide to the drainage calculators covers each part, from the roof catchment to the stormwater pipes.

• Victoria. NCC 2025 Volume Two H2D6(1), as Victoria varies it, names AS/NZS 3500.3 alone, so a Victorian house is designed to the Standard. Where the work is plumbing, Volume Three VIC E3D2 also calls up AS/NZS 3500.3, with SA HB 39 beside it; the Standard prevails where the two conflict. The eaves gutter and eaves gutter overflow calculators run the Standard's route.

How the Standard's method differs from the tables

The Housing Provisions tables are a simplification of the Standard. 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. It sets 1% AEP for eaves gutter overflow measures, valley gutters and box gutters. A note says 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 (Equation 3.4.3.2(2)). F is 1.00 for a flat roof, 1.20 at 22 degrees, 1.29 at 30 degrees and 1.50 at 45 degrees. Part 7.4 does not define its roof catchment area, except in South Australia, where it is the plan area. We apply F on the Housing Provisions route too, the more conservative reading, as step 2 above explains.

• The gutter is chosen by its effective cross-sectional area, not a type letter. With external brackets the effective area is the cross-section below a line at least 10 mm under the overflow: the front bead, the gutter back or the bottom of the slots (Clause 3.5.5(a)). Internal brackets take a further allowance off it (Clause 3.5.5(b)). Figures 3.5.5(A) and (B) give the catchment each area will drain at the design intensity. They cover effective areas from 3 500 to 20 000 mm² at 1:500 or steeper, and from 4 700 to 26 800 mm² flatter. Table 3.5.2 then gives the downpipe, as an internal size, for that area and fall. uPVC downpipes are marked by their outside diameter (AS 1273), so a pipe sold as "100" is smaller inside: check the maker's internal dimension. A Housing Provisions cross-section is measured up to the lowest overflow itself (the note to 7.4.3), so a gutter's effective area is smaller than its Table 7.4.3b figure: read Table 3.5.2 with the effective area from the gutter maker, never with a Table 7.4.3b area. Outlets are fitted vertically to the sole of the gutter (3.5.4).

• The overflow is designed with every downpipe blocked. This is the design case (Clause 3.5.3). Appendix F gives the volume as Q* = Ac × R / 3600, catchment in m² times the 1% AEP intensity in mm/h, for roof catchments up to 400 m² (F.1). Table 7.4.4b follows the same formula: 60 m² at 325 mm/h is 5.42 L/s, the table's 5.4. Appendix F's measures are continuous ones only: a gutter back gap (F.4.2, Table F.1), slots (F.4.3, Equation F.4.3), a front bead at least 10 mm below the fascia (F.4.4, 1.5 L/s per metre), or a design by computational methods (F.4.5). It sets a margin above the head each measure relies on of 3 mm on a gutter steeper than 1:500, and 6 mm on a flatter one (F.4.1). Dedicated measures at the outlets, the end-stop and front-face weirs, inverted nozzles and rainheads, are Housing Provisions 7.4.7 measures: deemed to satisfy on the Part 7.4 route (NCC 2025 Volume Two H2D6(1)(b)), and on the AS/NZS 3500.3 route a Performance Solution under NCC 2025 A2G2 unless a state variation accepts them, as Western Australia's does.

NCC 2025 references AS/NZS 3500.3:2025. Its preface lists the residential eaves gutter overflow and valley gutter requirements in Section 3 and Appendix F as the first of its major changes; what changed in AS/NZS 3500:2025 sets them out.

8. 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. The rule applies to every eaves gutter except one on a verandah or eave wider than 450 mm that is unlined or has its lining raked away from the building (7.4.4(1)(c) and (5)).

• 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. On a hipped roof with a high point mid-run, that is half the run's catchment on each side.

• A downpipe far from the valley. Valleys concentrate flow. The downpipe must be as close to the valley as possible (7.4.5(b)). 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.

9. 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 2025 Volume Two H2D6 for the Deemed-to-Satisfy routes and the Victorian variation, and Housing Provisions 7.4.3 to 7.4.5 for Western Australia's. NCC 2025 Volume One F1D3 for every other building class. AS/NZS 3500.3:2025 is purchased from Standards Australia. Inside AEC Assistant the Code is searchable by clause on the free plan, and the Standards, AS/NZS 3500.3:2025 among them, on Plus. Ask which gutter type a locality and catchment give, and the answer cites the table it was read from.