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Eaves gutter overflow calculator

Say what the building is and where, then enter the roof and the gutter run. The class and state decide the route: the AS/NZS 3500.3 Appendix F method for any building, or the Housing Provisions Part 7.4 tables for a Class 1 or 10 building outside Victoria. The calculator works out the volume the gutter must shed with its downpipe blocked in a 1% AEP storm, then checks your overflow measure or finds the least one that carries it.

The calculator runs both NCC 2025 routes: AS/NZS 3500.3 Clause 3.5.3 and Appendix F (Volume One F1D3 for Class 2 to 9, Volume Two H2D6(1)(a) for Class 1 and 10), or Housing Provisions 7.4.4 to 7.4.7 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

  • Housing Provisions 2025 7.4.4, Tables 7.4.4a and 7.4.4b, 7.4.6 and 7.4.7
  • AS/NZS 3500.3:2025 Clause 3.5.3 and Appendix F: Equation F.2, Table F.1, Equation F.4.3, Clause F.4.4

Two storms

The gutter is sized for the 5% AEP storm; its overflow measures handle the 1% AEP storm with every downpipe blocked (AS/NZS 3500.3 Clause 3.5.3). The volume is Q* = Ac × R / 3600.

The Housing Provisions route

For a house, Table 7.4.4b gives the litres per second per downpipe and Table 7.4.4a the litres per second per metre of gutter, read at the next intensity row up and interpolated between stated areas and lengths (7.4.4(3)). Continuous measures carry 0.5 or 1.5 L/s per metre; dedicated measures carry 0.5 to 3.5 L/s each (7.4.6, 7.4.7). The intensity is the one Table 7.4.3d lists (7.4.3(b)); a downpipe serves at most 12 m of gutter (7.4.5(a)); the route is for Class 1 and 10 buildings outside Victoria, and the roof drainage must connect to a stormwater system complying with H2D2 (NCC 2025 Volume Two H2D6(1)(b) and (3)(a)). Two continuous measures are not added: Part 7.4 gives no rule for combining them. The roof catchment is the plan area × the slope factor F of AS/NZS 3500.3 Table 3.4.3.2 (Part 7.4 does not define it, and F gives the larger figure), or a catchment area you enter.

The AS/NZS 3500.3 route

Appendix F (normative) covers roof catchments up to 400 m² (Clause F.1) with continuous measures: a back gap read from Table F.1 at the next lower printed head and gap, slots by Equation F.4.3, and a front bead at least 10 mm below the fascia at 1.5 L/s/m (a bead set higher earns nothing). A margin of 3 mm (6 mm on gutters at 1:500 or flatter) stays above the head each measure relies on. The calculator states the litres per second each metre of gutter must shed (Q* divided by the gutter length) and can find the minimum: how far below the fascia the gutter back must sit for a given gap, the slot area for a given slot level, or the front bead set-down. Above 400 m², Appendix F and its computational methods (Clause F.4.5) do not apply: split the catchment, or design the overflow as a Performance Solution under NCC 2025 A2G2.

Exempt gutters

A gutter on a verandah or eave wider than 450 mm needs no overflow measures if the eave is unlined, or raked with its lining sloping away from the building (AS/NZS 3500.3 Clause 3.5.3; Housing Provisions 7.4.4(5)). Choose that where the gutter is fixed, and the calculator says so instead of sizing measures.

An upper roof

Where an upper roof's gutter overflow or spreader lands on this roof, Clause 3.5.3 adds its catchment to Q*. Enter it as the upper roof; the downpipe spreader calculator carries it across for you.

A worked example

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

Design overflow volume, Q*

4.29 L/s

The gutter back gap carries 20.0 L/s over 10.0 m of gutter (2.00 L/s/m), at least the 4.29 L/s (0.43 L/s per metre) a 1% AEP storm sends with the downpipe blocked.

  1. Design rainfall intensity, 1% AEP, 5 minutes = 262 mm/hAS/NZS 3500.3:2025 Table D.1, Sydney City
  2. Slope multiplierF from roof slope = 20.0° = 1.18AS/NZS 3500.3:2025 Table 3.4.3.2
  3. Catchment areaAc = Ah × F = 50.0 × 1.18 = 59.0 m²AS/NZS 3500.3:2025 Eq 3.4.3.2(2)
  4. Design overflow volume, all downpipes blockedQ* = Ac × R / 3600 = 59.0 × 262 / 3600 = 4.29 L/sAS/NZS 3500.3:2025 Eq F.2, Clause 3.5.3
  5. Required per metre of gutterq = Q* / L = 4.2939 / 10.0 = 0.43 L/s/mAS/NZS 3500.3:2025 Clause F.4.1; Eq F.3
  6. Head the device may useh = headroom − margin = 13 − 3 = 10 mmAS/NZS 3500.3:2025 Clause F.4.1(a)
  7. Back gap capacityTable F.1 at the next lower printed head and gap = 10.0 mm head, 3.0 mm gap = 2.00 L/s/mAS/NZS 3500.3:2025 Table F.1
  8. Capacity over the gutter lengthQ = capacity × length = 2.00 × 10.0 = 20.0 L/sAS/NZS 3500.3:2025 Clause F.4.1

Questions

Do eaves gutters need overflow?

Yes, unless fixed to a verandah or eave over 450 mm wide that is unlined or raked away from the building. The overflow measures must shed the 1% AEP flow with the downpipes blocked (Housing Provisions 7.4.4; AS/NZS 3500.3:2025 Clause 3.5.3).

How much overflow does a gutter need?

Q* = catchment area × 1% AEP intensity / 3600, in litres per second. The Housing Provisions 7.4.3 worked example: a 60 m² roof catchment area in Wollongong (311 mm/h, read at the 325 mm/h row) with 10 m of gutter needs 5.4 L/s (Table 7.4.4b); one end-stop weir (0.5 L/s) and a front-face slotted gutter (0.5 L/s/m × 10 m) give 5.5 L/s, which meets it. To reproduce it here, choose the Housing Provisions route and enter the roof as a catchment area of 60 m²; entered as a plan area, the calculator multiplies it by the slope factor F, the conservative reading.

What overflow does a slotted gutter give?

0.5 L/s per metre under Housing Provisions 7.4.6 for slots of at least 1 200 mm² per metre with their lower edge 25 mm below the fascia. AS/NZS 3500.3 Equation F.4.3 computes it from the slot area and head, and gives 0.49 L/s/m for those slots on a gutter steeper than 1:500 (3 mm margin), 0.45 L/s/m flatter (6 mm), with the head taken from the lower edge of the slots.

How far below the fascia must the gutter overflow be?

Far enough that the overflow measure carries the 1% AEP flow with the downpipe blocked, plus a 3 mm margin (6 mm at 1:500 or flatter) (AS/NZS 3500.3 Clause F.4.1). For a back gap, Table F.1 gives the head a gap needs; for a front bead, at least 10 mm (Clause F.4.4). Choose "Find the minimum" and the calculator works out the set-down.

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