Using formulas developed by the CSIRO.
AS/NZS 3500.3 allows for all roof designs, and as such must allow for the worst case.
This method allows for the location of bends and downpipe, along the line of the gutter.
For instance, if a bend was positioned close to the DP, and the DP situated at the end of the gutter, this method will give a result equivalent to the Plumbing Code. This Design complies with "Roof Drainage by K G Martin" Division of Building Research, CSIRO.
Catchment area A = 100 sq.m
Intensity I = 222 mm/hr
Coefficient of Runoff I = 1
Roof Slope S = 5 deg.
Roof Slope Factor Sf = 1 + 0.5*tan(Roof Slope)
= 1.04
Design Catchment Area = Sf * A
= 104.37
Design Flow Q = C*I*A/3600 L/s
= 1 * 222 * 104.37/3600
= 6.44 L/s
Find Gutter Design Coefficient "x"
Is gutter slopping of level? = sloping
Bend distance from DP = >3.6 m
DP location along gutter = end
Using the above data from Table 2 x = 0.4
Find Gutter Area (ga)
From equation 2, Q=ga^1.25(1+x)
converting to SI units and solving for ga (rounded sole)
taken as Ogee, 1/2 round or Quad style gutter.




From CSIRO document P7, increase ga by 10% for rectangular gutter. Therefore rectangular gutter area



Find DP dia
Method: Use AS/NZS 3500.3 to find the DP size.
then from Figure 3.5.2(b) or (c) Using the above flow, find the gutter area.
Then from Figure 3.5.2 find the DP dia relating to that flow.
Down pipe Dia Dia = 150 mm