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Downpipe spreader calculator

Enter the upper roof, the lower roof it lands on, any wall between them and the lower gutter. The calculator adds the catchments as Clause 3.4.5 requires, sizes the lower gutter and downpipe for both roofs, lists the protection the lower roof needs, and hands both roofs to the overflow calculator for the lower gutter. It can also check the lower roof sheets against their maker's drainage table.

Spreaders onto a lower roof are an AS/NZS 3500.3 provision (Clause 3.4.5) with no Housing Provisions equivalent, so under NCC 2025 this is the Volume One F1D3 method for Class 2 to 9 buildings and the Volume Two H2D6(1)(a) route for Class 1 and 10.

How it works

It reads from

  • AS/NZS 3500.3:2025 Clause 3.4.5 and its Notes
  • Figure 3.5.5 and Table 3.5.2 for the lower gutter; Clause 3.5.3 for its overflow
  • The roofing makers' drainage tables, for the optional lower roofing check (an engineering check, not an AS/NZS 3500.3 method)

The Standard's preference

Water from a higher roof goes to a rainhead or a sized sump. A spreader onto the lower roof is permitted when the lower roof is protected: tiles sarked 1 800 mm either side of the discharge down to the eaves gutter, or corrugated metal with its side laps sealed for 1 800 mm either side.

The lower gutter carries both

The lower gutter and downpipe are sized for the total flow of both catchments (Clause 3.4.5), and the lower gutter's overflow should allow for the upper gutter's overflow landing on it (Note 3).

Victoria

NCC 2025 Volume Three VIC E3D2 calls up SA HB 39 beside AS/NZS 3500.3 (the Standard prevails where the two conflict). HB 39 Cl 5.7.7 allows a spreader from an upper catchment of up to 15 m², or more where the lower roofing maker's data carries the extra flow, and bars discharge onto ridge or mortar-jointed tiles, flashings, timber fascias or sheet side laps, into the building, or onto sheets that drain to a valley. The calculator checks the 15 m² when the site is in Victoria.

Walls and the lower roofing

A wall above the upper roof, or between the roofs, adds its wind-driven rain to the roof below it (Clause 3.4.4). The optional lower roofing check compares the flow each lower sheet carries, its own roof plus its share of the spreader, with the run the roofing maker's table allows at the 1% AEP intensity and the roof's pitch. Enter the maker's figure, or pick the roofing profile and the calculator reads the maker's table at the next printed rainfall intensity up and slope down (for a table of minimum slopes, the next printed run up), never between printed values. The lower sheets' run is increased by the spreader's water spread over the sheets it lands on, Leff = L + Acu / W, as the makers count the roof upstream of a pan in a sheet's run. It is an engineering check using the maker's table, not an AS/NZS 3500.3 method, and the result quotes what the maker says about roof runs.

A worked example

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

Flow in the lower gutter

2.02 L/s

The 7 300 mm² lower gutter drains up to 36.7 m² per downpipe here, so it carries both roofs (36.2 m², 2.02 L/s at 201 mm/h) into a 100 mm internal round or 100 × 75 mm internal downpipe; its overflow measures must shed 2.63 L/s.

  1. Design rainfall intensity, 5% AEP, 5 minutes = 201 mm/hAS/NZS 3500.3:2025 Table D.1, Sydney City
  2. Upper roof: slope multiplierF from roof slope = 22.5° = 1.205AS/NZS 3500.3:2025 Table 3.4.3.2
  3. Upper roof: catchment areaAc = Ah × F = 12.0 × 1.205 = 14.5 m²AS/NZS 3500.3:2025 Eq 3.4.3.2(2)
  4. Lower roof to the same downpipe: slope multiplierF from roof slope = 22.5° = 1.205AS/NZS 3500.3:2025 Table 3.4.3.2
  5. Lower roof to the same downpipe: catchment areaAc = Ah × F = 18.0 × 1.205 = 21.7 m²AS/NZS 3500.3:2025 Eq 3.4.3.2(2)
  6. Total catchment on the lower gutterAc = upper + lower = 14.46 + 21.69 = 36.2 m²AS/NZS 3500.3:2025 Clause 3.4.5
  7. Flow in the lower gutterQ = Ac × I / 3600 = 36.15 × 201 / 3600 = 2.02 L/s
  8. Share arriving through the spreaderQu = Acu × I / 3600 = 14.46 × 201 / 3600 = 0.807 L/s
  9. Lower 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 (rounded up, the safe side)
  10. Largest catchment per downpipe for the lower gutterAcdp 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)
  11. Lower downpipeTable 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)
  12. Design rainfall intensity, 1% AEP, 5 minutes = 262 mm/hAS/NZS 3500.3:2025 Table D.1, Sydney City
  13. Overflow volume for the lower gutter, both roofsQ* = Ac × R / 3600 = 36.15 × 262 / 3600 = 2.63 L/sAS/NZS 3500.3:2025 Clause 3.5.3; Eq F.2

Questions

Can I discharge a downpipe onto a lower roof?

Yes, through a spreader, if the lower roof is protected (sarked 1 800 mm either side for tiles, or side laps sealed 1 800 mm either side for corrugated metal) and the lower gutter and downpipe are sized for both roofs (AS/NZS 3500.3:2025 Clause 3.4.5). In Victoria, HB 39 Cl 5.7.7 also applies: an upper catchment of no more than 15 m² unless the lower roofing maker's data allows more, and no discharge onto ridge or mortar-jointed tiles, flashings, timber fascias or sheet side laps, into the building, or onto sheets or tiles that drain to a valley.

How big can the upper roof be?

AS/NZS 3500.3 sets no limit of its own: the lower gutter must carry the combined flow, and the lower roofing must carry the spreader's water down to it, which the roofing maker's data shows (the calculator's optional lower roofing check, from the figure you enter or the maker's table). In Victoria, HB 39 Cl 5.7.7 also applies: an upper catchment of no more than 15 m² unless the lower roofing maker's data allows more.

Which roofing makers' tables does the lower roofing check read?

Lysaght, Stramit, Stratco, Metroll, Fielders and Steeline, for each profile they publish a drainage table for: the longest run by rainfall and slope, Stramit's minimum slope for a run, or Steeline's capacity per metre width of sheet. Each result cites the maker's table and page and says what rainfall the maker's figures are for; the check reads them at the 1% AEP intensity, the basis the makers that state one use. For other roofing, enter the maker's longest run yourself.

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