Skip to content

Eaves gutter and downpipe calculator

Choose the route, then enter the whole roof, the part draining to the busiest downpipe, or each downpipe's part. On the AS/NZS 3500.3:2025 route the calculator reads Figure 3.5.5 for the catchment one downpipe can take and Table 3.5.2 for the downpipe; on the Housing Provisions route it reads Tables 7.4.3a to 7.4.3c for the gutter type and downpipe. Leave the gutter blank and it gives the gutter each number of downpipes needs, or pick the gutter from a maker's range and the maker's published effective area is used, with its document cited.

The calculator runs both NCC 2025 routes: AS/NZS 3500.3 Clause 3.5 (Volume One F1D3 for Class 2 to 9, Volume Two H2D6(1)(a) for Class 1 and 10), or the Housing Provisions Part 7.4 tables for Class 1 and 10 buildings outside Victoria whose roof drains to a system complying with H2D2 (H2D6(1)(b) and (3)).

How it works

It reads from

  • AS/NZS 3500.3:2025 Clause 3.5 and Figure 3.5.2 (general method, Notes 3 and 5 to 8)
  • Figures 3.5.5(A) and (B), gutter area against catchment per downpipe; Table 3.5.2 downpipe sizes (internal)
  • Clause 3.5.5 effective area; Table 3.4.3.2
  • NCC 2025 Housing Provisions 7.4.3 to 7.4.5 and Tables 7.4.3a to 7.4.3d (the Part 7.4 route); Volume Two H2D6
  • The gutter makers' published effective areas and downpipe internal sizes, each cited to the maker's document and page

Two routes

NCC 2025 gives two Deemed-to-Satisfy ways to size an eaves gutter. AS/NZS 3500.3 applies to any building (Volume One F1D3; Volume Two H2D6(1)(a)). For a Class 1 or 10 building outside Victoria, the Housing Provisions Part 7.4 tables are the alternative (H2D6(1)(b)), where the roof drains to a stormwater system complying with H2D2 and not for box gutters (H2D6(3)). The calculator asks for the building class first, and offers Part 7.4 only for a Class 1 or 10 building outside Victoria: for any other building, and in Victoria, it does not ask for the route.

The roof: whole, busiest or each downpipe

Enter the whole roof alone and the calculator counts the downpipes it needs; with no gutter entered it lists, for each number of downpipes, the gutter area and the downpipe (assuming the roof divides evenly). Enter the part draining to the busiest downpipe to size or check that one (Figure 3.5.2 Steps 9 and 10); add the whole roof for the number of downpipes. Or list each downpipe's part and gutter length: the busiest governs, every downpipe is checked, and the one with the most roof per metre of gutter goes to the overflow calculator. Plan area × slope factor F holds for a dwelling with no wall or flat roof draining to the gutter (Figure 3.5.2 Note 5); otherwise enter Ac from the roof catchment calculator.

The gutter's capacity

Figures 3.5.5(A) and (B) plot catchment per downpipe against the gutter's effective area for lines of rainfall intensity. Between two intensities the calculator interpolates the flow (Ac × I / 3600), which is nearly the same on every curve at a given gutter area; past the end of a curve it takes the end point, and outside the gutter areas Figure 3.5.2 Note 3 covers it refuses. A flatter gutter needs roughly a third more area for the same flow.

Effective area

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). The maker publishes it for each gutter (AS/NZS 2179.1 Clause 3.7). Choose "From a maker's range" to pick the gutter from those each maker sells in the site's state (or in any state): the calculator uses the maker's figure, cites its document and page, says what the maker states the area is measured to, and flags any figure to confirm with the maker. Or enter your own figure. Without one, choose "Work it out from the profile" and enter the profile: a square gutter is taken as width × depth, a quad as a trapezium (its rounded front holds more), a half-round as a circular segment, each below the 10 mm line and rounded down to 100 mm². Internal brackets reduce it; without the maker's figure the bracket's edge may be deducted, up to 15%. Leave the gutter blank to size it: the result gives an indicative section twice as wide as deep and an indicative half-round.

Downpipe and number of downpipes

Table 3.5.2 gives the downpipe for the gutter's effective area and fall: the first row not smaller than the gutter. Its sizes are internal; a uPVC downpipe is marked by its outside diameter (AS 1273), so check the maker's internal dimension. A downpipe you propose is checked against the table: a size, or a maker's downpipe where the maker publishes its internal size. The minimum number of downpipes is the whole roof's catchment divided by the largest catchment one downpipe can take, rounded up (Figure 3.5.2, Step 8).

The Housing Provisions tables

Table 7.4.3a gives the gutter type, A to F, by the 5% AEP intensity of Table 7.4.3d (7.4.3(b)) and the roof catchment per downpipe, read at the next listed row and column up. Table 7.4.3b describes the types (type F is designed to AS/NZS 3500.3) and Table 7.4.3c the downpipes that suit them. The gutter falls at least 1:500 (7.4.4(1)(a)), its brackets sit at stop ends and corners and not more than 1.2 m apart (7.4.4(1)(b)), and a downpipe serves at most 12 m of gutter (7.4.5(a)), as close as possible to valley gutters (7.4.5(b)). Table 7.4.3b's areas are measured to the lowest overflow, not 10 mm below it, so they are not effective areas for the AS/NZS 3500.3 route. In Western Australia, Part 7.4 also lets a house's eaves gutters be sized to AS/NZS 3500.3 (WA 7.4.3(a)(i)); designed that way under Part 7.4, each downpipe still serves at most 12 m of gutter (WA 7.4.5(a)).

Limits

The methods allow for no blockage by snow, hail or debris and assume regular inspection and cleaning (Clause 3.2 Notes 1 and 3). Snow does not change the size (Clause 3.3.2); a green roof is taken at its full run-off (Clause 3.4.6); the system must not let water flow back into the building (Clause 3.5.1). Australian design storms only; not for siphonic systems (Clause 10.1 Notes 2 and 3). Overflow measures are sized separately, with the gutter overflow calculator.

A worked example

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

Largest catchment per downpipe

36.7 m²

A gutter of 7 300 mm² drains up to 36.7 m² per downpipe here, so it carries this 36.2 m² catchment into a 100 mm internal round or 100 × 75 mm internal downpipe.

  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 = 30.0 × 1.205 = 36.2 m²AS/NZS 3500.3:2025 Eq 3.4.3.2(2)
  4. Flow to the downpipeQ = Ac × I / 3600 = 36.15 × 201 / 3600 = 2.02 L/s
  5. Effective gutter area neededAe on Figure 3.5.5(A) for Ac at I = 36.2 m² at 201 mm/h meets the curve at 7 211 mm², rounded up to the next 100 mm² = 7 300 mm² (rounded up)AS/NZS 3500.3:2025 Figure 3.5.5(A); Table 3.5.2 Note a (to the nearest 100 mm² — rounded up here, the safe side)
  6. Largest catchment per downpipeAcdp from Figure 3.5.5(A) at Ae and I = 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 = 36.7 m²AS/NZS 3500.3:2025 Figure 3.5.5(A); Figure 3.5.2, Step 7
  7. Capacity of the chosen gutterQ = Acdp × I / 3600 = 36.7124 × 201 / 3600 = 2.05 L/sAS/NZS 3500.3:2025 Figure 3.5.5(A) (from the curves, not the printed flow axis)
  8. Largest plan area per downpipeAh = Acdp / F = 36.7124 / 1.205 = 30.5 m²AS/NZS 3500.3:2025 Eq 3.4.3.2(2)
  9. Catchment of the whole roofAc = Ah × F = 150.0 m² plan = 180.8 m²AS/NZS 3500.3:2025 Figure 3.5.2, Step 4
  10. Minimum number of downpipesn = Ac / Acdp, rounded up = 180.75 / 36.712 = 5AS/NZS 3500.3:2025 Figure 3.5.2, Step 8
  11. Downpipe sizeTable 3.5.2, first row with area ≥ Ae = 7 300 mm², 1:500 and steeper = 100 mm internal round or 100 × 75 mm internalAS/NZS 3500.3:2025 Table 3.5.2 (internal sizes)

Questions

How many downpipes does my roof need?

Enter the whole roof and the gutter: the calculator divides the roof's catchment (plan area times the slope factor) by the largest catchment one downpipe can serve with that gutter, from Figure 3.5.5 of AS/NZS 3500.3:2025, and rounds up. Leave the gutter blank and it lists the gutter each number of downpipes needs. In the Standard's own example a 144 m² house at 24° in a 145 mm/h area needs four downpipes with a 7 300 mm² gutter laid at 1:500 or steeper, and five laid flatter.

What size downpipe do I need for my gutter?

Table 3.5.2 of AS/NZS 3500.3:2025 sets the downpipe by the gutter's effective area and fall, as an internal size. A 7 300 mm² gutter at 1:500 or steeper takes 100 mm internal round or 100 × 75 mm internal; laid flatter than 1:500 it takes 85 mm round or 100 × 50 mm. A uPVC downpipe is marked by its outside diameter, so a pipe sold as "100" is smaller inside: check the maker's internal dimension.

Is this different from the NCC Housing Provisions tables?

The calculator runs both. AS/NZS 3500.3 sizes the gutter's effective area from Figure 3.5.5 and the downpipe from Table 3.5.2. The Housing Provisions Part 7.4 tables pick a gutter type A to F by rainfall and roof catchment per downpipe (Table 7.4.3a), with the downpipes that suit it (Table 7.4.3c), a fall of at least 1:500 and at most 12 m of gutter per downpipe. Under NCC 2025, Part 7.4 is a Deemed-to-Satisfy route for Class 1 and 10 buildings outside Victoria (Volume Two H2D6(1)(b)); AS/NZS 3500.3 is the other route for them (H2D6(1)(a)), the route for type F gutters, and the only one for Class 2 to 9 buildings (Volume One F1D3) and in Victoria.

What if I don't know my gutter's effective area?

The maker publishes it for each gutter (AS/NZS 2179.1 Clause 3.7): pick the gutter from its maker's range and the calculator uses that figure. For a gutter not listed, choose "Work it out from the profile" and enter the profile: quad, half-round or square, its width across the top (at the level of the lowest overflow point, if the front stands higher than that point), the sole width for a quad, and the depth from the sole to the lowest overflow point (the front bead, the top of the back or the bottom of any slots). The calculator takes the area below a line 10 mm under that point (Clause 3.5.5(a)), on the safe side of the real profile, and shows the working.

Can the downpipe come 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 the charts assume the outlet central in the sole. In Victoria SA HB 39 also applies (NCC 2025 Volume Three VIC E3D2) and sizes an outlet at the back of the gutter: 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 (HB 39 Cl 5.7.5).

Which gutter profiles are listed?

The gutters whose effective area Lysaght, Stramit, Metroll, Stratco, Steeline, Pantex, Ace Gutters, Rollsec, Queensland Sheet Metal, Commercial Sheet Metal and No.1 Roofing publish, listed by the states each maker sells them in; choose "All states" to see the rest. Each result cites the maker's document and page. Where a maker prints more than one figure for a gutter, the lowest is used and the others are listed; where the maker's own documents cast doubt on a figure, the result says what to confirm. Fielders, EDGE Building Products and ABC Seamless publish gross cross-sections rather than effective areas, which would overstate a gutter, so theirs are not listed: enter the effective area yourself or work it out from the profile. A maker's downpipe is listed where the maker publishes its internal size.

How far apart can downpipes be?

AS/NZS 3500.3 sets no spacing: each downpipe's catchment must be within what the gutter carries (Figure 3.5.2), and a downpipe or bend anywhere along a catchment takes the whole of it (Note 6). On the Housing Provisions route a downpipe serves at most 12 m of gutter and sits as close as possible to valley gutters (7.4.5(a) and (b)). List each downpipe's part and gutter length to check both.

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