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Charged downpipe calculator

Enter each downpipe's roof and its height above the tank inlet, and each length of line with its fittings. The calculator works out the head every downpipe needs and whether any gutter will surcharge before the tank takes the flow.

AS/NZS 3500.3, which NCC 2025 calls up for roof drainage (Volume One F1D3, Volume Two H2D6(1)(a) and, for plumbing work in Tasmania and Victoria, Volume Three E3D2), sets charged-line rules for New Zealand only, so in Australia this check supports a Performance Solution under A2G2 against F1P1 (Class 2 to 9) or H2P1 (Class 1 and 10); in Tasmania, Volume Three TAS B7D3(2) calls up the Director's Guidelines for charged downpipes to rainwater tanks supplying drinking water on Class 1 and 10 buildings.

An engineering check, not the Standard's deemed-to-satisfy method. Use it to understand a design or to support a Performance Solution under NCC 2025 A2G2.

How it works

It reads from

  • AS/NZS 3500.3:2025 Clause 11.5.1 Note (grade line check) and Section 11 (rules for New Zealand)
  • Table 3.3.4 (design storm); Table 3.4.3.2 and Eq 3.4.3.2(1) (catchment); AS/NZS 1260 Table 4.1 (PVC-U DWV bores)
  • Colebrook–White friction; typical fitting coefficients

How a charged line works

Downpipes drop below ground, run to the tank and rise into it; the line stays full to the tank inlet. Rain pushes water through only if the lowest gutter outlet sits high enough above the inlet to overcome the friction and fitting losses.

The check

One row per downpipe, from the furthest to the tank. Each downpipe's flow at the gutter design storm of Table 3.3.4 (5% AEP for eaves gutters, 1% for box and valley gutters) joins the line, and every section carries the flow of the downpipes above it. Friction is Darcy–Weisbach with Colebrook–White; fittings take typical coefficients for plastic fittings (the Standard gives none). The head a downpipe needs is its own entry loss plus every loss from it to the tank, and the exit; it must not exceed that downpipe's height above the tank inlet. Bores are AS/NZS 1260's minimums for PVC-U DWV, or enter another pipe's.

The published rules

AS/NZS 3500.3:2025 Section 11 applies in New Zealand: DN 100 minimum, not reducing downstream; a 1:100 minimum grade toward a flush point; and the tank inlet soffit at least 300 mm below the lowest gutter outlet or debris screen, more for long or busy lines. The tank overflow is at least DN 90 and no smaller than the inlet; debris screens have openings of 1 mm or less; a first-flush diverter holds 0.3 L per m² of roof. In Australia AS/NZS 3500.3:2025 sets no charged-line method, so a design that rests on this check is a Performance Solution under NCC 2025 A2G2, assessed against F1P1 (Class 2 to 9) or H2P1 (Class 1 and 10). In Tasmania, Volume Three TAS B7D3(2) calls up the Director's Guidelines for charged downpipes to rainwater tanks supplying drinking water on Class 1 and 10 buildings.

A worked example

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

Head needed

0.35 m

Every downpipe has head to spare for 8.07 L/s to the tank: the least margin is 0.85 m, at downpipe A (0.35 m needed, 1.20 m available).

  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 of each downpipeAc = Ah × F = A 40.0 → 48.2 · B 40.0 → 48.2 · C 40.0 → 48.2 = 144.6 m²AS/NZS 3500.3:2025 Table 3.4.3.2; Eq 3.4.3.2(2)
  4. Flow from each downpipeQ = Ac × I / 3600 = A 2.69 L/s · B 2.69 L/s · C 2.69 L/s = 8.07 L/sAS/NZS 3500.3:2025 Clause 11.5.1 Note (the gutter design storm, 5% AEP for eaves gutters, Table 3.3.4)
  5. Section A: DN 100, 12.0 m, 2.69 L/shf = f (L/D) v²/2g; hm = ΣK v²/2g = bore 98.0 mm (AS/NZS 1260 Table 4.1), v 0.36 m/s, Re 34 824.98, f 0.0231 (Colebrook–White, k 0.015 mm); ΣK 1.85 = hf 0.018 m + hm 0.012 m
  6. Section B: DN 100, 8.0 m, 5.38 L/shf = f (L/D) v²/2g; hm = ΣK v²/2g = bore 98.0 mm (AS/NZS 1260 Table 4.1), v 0.71 m/s, Re 69 649.97, f 0.0201 (Colebrook–White, k 0.015 mm); ΣK 2.10 = hf 0.043 m + hm 0.055 m
  7. Section C: DN 100, 5.0 m, 8.07 L/shf = f (L/D) v²/2g; hm = ΣK v²/2g = bore 98.0 mm (AS/NZS 1260 Table 4.1), v 1.07 m/s, Re 104 474.95, f 0.0186 (Colebrook–White, k 0.015 mm); ΣK 1.75 = hf 0.056 m + hm 0.102 m
  8. Entry and exit lossesK 0.5 v²/2g at each downpipe; K 1.0 v²/2g into the tank = exit at 1.07 m/s = exit 0.058 m
  9. Head needed at each downpipeentry + Σ (hf + hm) from the downpipe to the tank + exit = A 0.347 m · B 0.316 m · C 0.219 m = A governs: 0.347 mAS/NZS 3500.3:2025 Clause 11.5.1 Note
  10. Water held in the lineΣ π D² / 4 × L = 0.19 m³

Questions

How much height do I need for a charged downpipe system?

Enough to overcome the friction and fitting losses at the design flow, and in New Zealand at least 300 mm between the lowest gutter outlet and the tank inlet soffit (AS/NZS 3500.3:2025 Clause 11.5.1). Long or busy lines need more.

Can a charged line be larger than DN 150?

Yes. The calculator takes DN 65 to DN 300: the bore of a PVC-U DWV pipe comes from AS/NZS 1260, and any other pipe's bore is entered from its maker's data. Each section may be a different size; Section 11 (New Zealand) asks that the line not reduce in the direction of flow.

Does AS/NZS 3500.3 cover charged downpipes in Australia?

No. Its charged-line rules (Section 11) are for New Zealand. In Australia a charged line designed by this check is a Performance Solution under NCC 2025 A2G2; in Tasmania, NCC 2025 Volume Three TAS B7D3(2) calls up the Director's Guidelines for charged downpipes to rainwater tanks supplying drinking water on Class 1 and 10 buildings.

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