Live valley profile
True V, section normal to invert. Metal sits on the valley board in a notch of the valley rafter; the rafter stays on the wall plate. A small invert drop D does not change that stack — the board just sits slightly lower in the same pocket. D is capped at rafter depth − K − min remaining beam. Roof overhang OL along the sheet (SA HB 39 ≥ 100) and clear opening between sheet ends (SA HB 39 ≥ 150). we/he as NCC Housing Fig 7.4.4 / AS/NZS 3500.3 Fig 3.6.1 — we is not the air gap.
Results — issue these
Stock valley
Fold schedule (one sheet, invert on true valley)
A tall vertical return is not the usual house solution. The only upstand is the edge hem in J−K. Open gap >350 mm is wider than a 100+150+100 box opening, not a DTS width ban.
Baffle vs eaves purlin
The eaves purlin is structure (cladding screws into it). The valley baffle is a separate folded plate in the waterway, fixed to the valley tray and standing to within 3–5 mm of the underside of the sheet. Required when the valley changes angle or the pitches are uneven beyond 10° (NZ COP 5.5.6). Own drawing, after NZ COP 5.5.6A / BRDV01.
Hydraulic section
NCC Housing Fig 7.4.4 (explanatory). Same stack at D = 0 and D = 1 mm: jack rafters at the roof plane, valley board in a notch of the valley rafter, metal on the board, rafter on the wall plate. A larger NZ invert drop lowers the board in that pocket. Max D = rafter depth − K − min remaining beam (defaults 90 − 19 − 45 = 26 mm). Metal does not sit on bare rafters. Clear opening and OL stay SA HB 39 (≥150 / ≥100).
Plan
Drawn for a 90° plan angle (roof planes meet at right angles). Other plan angles are a Performance Solution and are not computed or drawn to scale here.
Section through eaves
Stramit Quad 115 on a metal fascia (a common example, not a DTS SKU). Invert clears the Quad back (do not cut the gutter) and hangs 50 mm into the open gutter. Housing 7.4.6 B+C: 10 mm spacer, back and front bead ≥10 mm below fascia top on the run — AS App G fascia-above-lip may want more (~12–25 mm); increase near a valley on zero-slope eaves only. Seal the fascia notch; if not sealed, set overflow from the notch sill. Dump is a point jet at the corner — do not smear Q along 12 m. DP at the mouth (5%); corner overflow is 1% with no DP credit (blocked-pipe). Issued valley depth assumes free discharge; a full eaves is an outlet check, not a higher k. A jump at the dump can locally overtop fascia / Quad wings — that is why the 1% weir is at the corner. Rainhead not required for a V into eaves (AS 4.5.5); 7.4.7d is an optional 3.5 L/s fascia box, not the default look. If you provide a rainhead, size its length from the valley exit velocity so the jet lands in the well.
Cited drawings
Compliance map — what this job is on and off
Things to check — especially on a Performance Solution
Documents cited
Short names used elsewhere in this tool, the document they refer to, and what is taken from each. Where a note cites a bare “AS”, “AS/NZS 3500.3”, “Housing”, “HB 39” or “COP”, it means the matching document below.
| Short name | Document | What is cited |
|---|---|---|
| AS/NZS 3500.3 | AS/NZS 3500.3 — Plumbing and drainage, Part 3: Stormwater drainage. 2021 edition is the NCC 2022 referenced edition; the 2025 edition and its Amendment 1 are noted where used. Adoption dates differ between states and territories — confirm the referenced edition for your project. | Appendix D 5-min intensities; Table 3.3.4; Cl 3.3.5.1; Fig 3.4.2 catchment; Table 3.6.2 / Fig 3.6.1 valley sizes; Cl 3.6.3 cladding in flow; Cl 3.7 box gutters (not used); Cl 4.5.4–4.5.5 rainheads; Appendix G eaves; Appendix H Figs H.2 / H.3 |
| NCC 2022 | National Construction Code 2022, Volume Two. | H2P1 (stormwater must not enter the building); H2D6 (deemed-to-satisfy); state adoption dates for the referenced AS/NZS 3500.3 edition |
| Housing | NCC 2022 Housing Provisions, Part 7.4 (eaves gutters and downpipes). | 7.4.3 overflow; 7.4.4 valley figure; 7.4.6 fascia overflow; 7.4.7d 3.5 L/s fascia box |
| SA HB 39 | SA HB 39 — Installation code for metal roof and wall cladding. | 5.4(f) ≥150 mm exposed waterway; 5.4(g) ≥100 mm cladding overlap; board ≥19 mm |
| NZ COP | NZ Metal Roofing Manufacturers Code of Practice, v26.09 (1 Sep 2026). | 5.3.3 / 5.5.4 valley outlet; 5.5.6 baffle and invert drop; 5.5.7 valley Manning n = 0.014 and 15 / 20 mm freeboard |
| Dambuster | Dambuster technical notes on non-regular (unequal-pitch) valleys. | 90° plan geometry: hydraulic side slopes from the two roof pitches (38°/14° → 36.7° / 4.4°) and the true-valley angle φ. Geometry only — not a valley water engine. |
| Martin & Tilley 1968 | CSIRO study behind the AS/NZS 3500.3 valley table (23° roof, 16.5° hydraulic sides; PDF in references/). |
Origin of the Table 3.6.2 sheet / he / we values |
| Covering CFD | This project's own CFD campaign — cfd/openfoam-gutter/COVERING.md (not a published Standard). |
Covering R = 1.55 (crest ÷ outlet Manning) on 10 L/s, L = 12 m, 10–35° covering corners |
Selections — why the defaults
| Item | This tool | Why |
|---|
Stock envelopes (not Standard SKUs)
| Code | mm | Use when | Do not use when |
|---|
Eaves / overflow (not valley sizing)
| Item | Note |
|---|
What we refused
| Item | Why |
|---|
What this tool does
Design storm
Compliance today
k and freeboard
Why this tool — and how it compares
Valley Gutter Design is an Australia-wide valley gutter sizer for the jobs that fall outside the AS/NZS 3500.3 look-up: unequal roof pitches, steep or shallow valleys, and catchments or intensities the table does not cover. It follows the Australian framework — AS/NZS 3500.3 and NCC 2022 H2P1 — and it tells you whether a job is inside the Deemed-to-Satisfy table or needs a Performance Solution.
What is different. Published valley methods size the mean water depth with Manning's equation and then add a flat allowance for waves. This tool keeps that mean but scales it by a crest factor R = 1.55 measured from CFD — the crest of the roll-wave envelope divided by the outlet Manning depth — so the issued metal depth is the crest, not the average. The crest factor, the covering band it was measured over (10 L/s, 12 m long, 10–35° valleys, 90° plan) and its limitations are all published on the Method and Sources tabs.
Unequal pitches. The invert is placed on the true roof-plane intersection, not the 45° plan bisector, and the section sides are solved for the real geometry. Most available tools either assume equal pitches or decline uneven roofs altogether.
Compared with the NZ Metal Roofing COP calculator (metalroofing.org.nz): that tool is the best published custom/asymmetric valley method, and this tool uses its mean-depth method (Manning n = 0.014, 15/20 mm freeboard). The differences are the design storm (NZ uses 2% AEP / 10-minute rain; this uses Australian 1% AEP / 5-minute) and the crest handling: NZ adds a flat freeboard with no crest factor; this tool adds a measured crest factor. NZ also does not size to the AS/NZS 3500.3 compliance framework.
Compared with roof-gutter-design.com.au (Valley Gutter 5): that tool is
CFD-based and capable, and its own runs are useful public evidence that a valley's
mean depth stays close to Manning up to about Q = 20 L/s (it drifts
above Manning beyond that — 1.36× at 100 L/s). What we do not adopt is its issued depth,
which applies a fixed 0.58 × mean factor to every case. That factor is not
measured from the CFD. It falls out of taking the AS/NZS 3500.3 Table 3.6.2 value
he = 43 mm as already including the 15 mm freeboard, subtracting 15 mm
to get 28 mm, and dividing the leftover by its own CFD mean (10.29 / 17.71 = 0.58). Fig 3.6.1
places freeboard above he, so 43 mm is water depth alone and the true
gap is 43 − 17.71 = 25.3 mm — table margin, not a wave the CFD produced.
The practical effect of applying 0.58 everywhere is that the issued depth is fitted back onto
the table instead of scaling with the flow, and it is also why the tool declines unequal
pitches to a box gutter. This tool takes the same mean-CFD/Manning agreement as its starting
point, but scales it by a published crest factor (R = 1.55) measured from the CFD
envelope, so the depth follows the flow rather than the table, and it handles unequal
pitches directly.
Jurisdiction. Australia-wide. AS/NZS 3500.3 and NCC H2P1 apply across every state and territory, and the engineering here does not depend on any one of them. The Rainfall tab lists a starter set of AS/NZS 3500.3 Appendix D towns, and the Custom intensity accepts a site IFD (BOM/ARR 5-minute 1% AEP) for anywhere in Australia — use it whenever your location is not in the list, or when a lot-specific IFD is available. Only the edition note (which AS/NZS 3500.3 edition your state references, and from when) is jurisdiction-specific; confirm it for your project. Not for New Zealand rainfall.
Things to check — especially on a Performance Solution
Same list as the Compliance tab. Walk it before you issue a PS; the outlet items still apply on DTS houses.
What is proven vs judgement
| Factor | In this tool? | Evidence | Confidence |
|---|
Rainfall intensities (5 min, mm/h)
Starter set of towns from AS/NZS 3500.3 Appendix D (all in the Illawarra/Sydney region). Valley uses the 1% column (Table 3.3.4(b) / Cl 3.3.5.1); the 5% column is eaves / downpipe information only (Table 3.3.4(a)). For any other Australian location, switch Intensity source to Custom and enter the site IFD (BOM/ARR 5-minute 1% AEP).
| Location | I 5% | I 1% |
|---|
AS/NZS 3500.3 Table 3.6.2 — valley size (mm)
Sheet / he / we by equivalent intensity, from AS/NZS 3500.3 Table 3.6.2 (Martin & Tilley 1968 geometry). Millimetres are identical in the 2018, 2021 and 2025 editions; the printed April 2025 Fig 3.6.2 is not used (reversed X-axis). Default is the 2021 table (the NCC 2022 referenced edition).
| Ieq mm/h | Sheet | he | we |
|---|