Valley gutter calculator
Enter the Clause 3.4 catchment draining into the valley, or the plan areas of the two roof planes, with the route, the roof pitch and the 1% AEP intensity. The calculator gives the effective width and depth the valley needs and the sheet it is formed from, and checks what you propose.
The calculator runs both NCC 2025 routes: AS/NZS 3500.3 Clause 3.6 (Volume One F1D3 for Class 2 to 9, Volume Two H2D6(1)(a) for Class 1 and 10), or Housing Provisions 7.4.4(4) and Table 7.4.4c for Class 1 and 10 buildings outside Victoria (H2D6(1)(b)); a flatter roof takes a box gutter or a Performance Solution.
How it works
It reads from
- AS/NZS 3500.3:2025 Clause 3.6, Table 3.6.2 and Figure 3.6.2 (incl. Amd 1:2026)
- Housing Provisions 2025 7.4.4(4) and Table 7.4.4c
Up to 20 m²
Table 3.6.2 gives the minimum sheet width, effective depth and effective width by rainfall intensity, from 355 × 32 × 215 mm up to 200 mm/h to 435 × 43 × 292 mm up to 400 mm/h. It assumes a 23° roof, 15 mm freeboard and 25 mm of side rolls. The Housing Provisions repeat it as Table 7.4.4c.
Over 20 m²
Clause 3.6.2 uses Table 3.6.2 for catchments of 20 m² or less and Figure 3.6.2 for all others, up to the 40 m² limit of Clause 3.6.1(c). The calculator reads AS/NZS 3500.3:2025 (incl. Amd 1:2026) Figure 3.6.2, checked against the amended figure. The figure gives the effective width only; the depth and sheet shown are by analogy with Table 3.6.2.
Checks
AS/NZS 3500.3 Clause 3.6.1: roof slope not less than 1:4.5 (12.53°) and a nominal side angle of 1:3.4 (16.5°). Housing Provisions 7.4.4(4): pitch more than 12.5°, freeboard at least 15 mm, side angle at least 12.5°. Nothing — bedding, anti-vermin strips or cladding — may obstruct the effective area (Clause 3.6.3). Flatter roofs take a box gutter to AS/NZS 3500.3 or a Performance Solution under NCC 2025 A2G2.
Two planes, one pitch
For two roof planes the catchment is Ah1 + Ah2 + ½ |Av2 − Av1| (Equation 3.4.3.3): it equals their plan area only when their vertical projections are equal. The method is for two planes of the same pitch meeting at a square corner; unequal pitches or another plan angle take a box gutter to AS/NZS 3500.3, or computational methods (Clause 3.6.2 Note 1) in a Performance Solution under NCC 2025 A2G2. Valley sheets are cut from coil: order the next stocked width at or above the sheet width shown.
A worked example
The calculator's example inputs, worked through step by step.
Minimum effective width
254 mm
A valley draining 18.0 m² at 262 mm/h needs an effective width of 254 mm and depth of 38 mm, formed from a sheet at least 395 mm wide.
- Design rainfall intensity, 1% AEP, 5 minutes = 262 mm/hAS/NZS 3500.3:2025 Table D.1, Sydney City
- Flow in the valleyQ = Ac × I / 3600 = 18.0 × 262 / 3600 = 1.31 L/s
- Valley dimensionsTable 3.6.2 by intensity = 262 mm/h, 18.0 m² (≤ 20 m²) = sheet 395, depth 38, width 254 mmAS/NZS 3500.3:2025 Table 3.6.2; Housing Provisions 2025 Table 7.4.4c
Questions
What size valley gutter do I need?
For up to 20 m² of catchment, Table 3.6.2 of AS/NZS 3500.3:2025: at up to 200 mm/h an effective width of 215 mm and depth of 32 mm from a 355 mm sheet, rising to 292 × 43 mm from a 435 mm sheet at up to 400 mm/h.
What is the minimum roof pitch for a valley gutter?
Under AS/NZS 3500.3, not less than 1:4.5, which is 12.53° (Clause 3.6.1(a) prints 12.5°); under the Housing Provisions, more than 12.5° (7.4.4(4)(a)). Flatter than that the valley is designed as a box gutter to AS/NZS 3500.3 or a Performance Solution under NCC 2025 A2G2.
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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.