Simplified Box Gutter, Rain water Head & Downpipe Size Calculator
To AS/NZS 3500.3.2:2003 "Stormwater Drainage Acceptable Solutions".

FOR SIMPLE BOX GUTTERS, NOT ALLOWING FOR RAIN SHADOW EFFECTS,
AND DRAINING TO A RAINWATER HEAD AT ONE END

LOAD AND RUN BOX GUTTER CALCULATOR

MINIMUM WIDTH = 300 mm for Commercial construction, and 200 mm for Domestic consruction.
REQUIRED GUTTER SLOPE 1:40 TO 1:200
MAXIMUM ALLOWABLE LENGTH
Material Base Metal Thickness (mm) Max Length (metres) Minimum expansion space (mm) To use Rectangular DP (mm*mm) Enter Equivalent circular DP dia(mm), To find RWH sizes
Aluminum 0.90 12 50 100 * 75 97.5
Copper 0.60 9 50 100 * 100 112.5
Copper 0.80 15 50 150 * 100 137.5
Copper 1.0 26 50 125 * 125 140
Steel Colorbond Zincalume 0.55 20 50 150 * 150 168
Steel 0.75 25 50
Stainless Steel 0.55 20 50
PVC - 10 30
Zinc 0.80 10 50

Instructions & Notes: This program incorportes a simplified version of calculating the roof cachment area, and makes no allowance for rain shadow effects. Hence it is perfect for a box gutter with roof on one side only and no vertical faces.
If it is used with a roof entering from both sides, or with a small vertical face or parapet, then the result will have a slightly greator factor of safety than would otherwise be the case. This is because the possible rain shadow effects have not been subtracted from the total catchment area. For more information on rain shadow effects refer to box gutter with vertical faces

Also, This calculation is only suitable when the box gutter drains to a rainwater head, (with overflow) at the end of the box gutter, as shown.
For other conditions, eg a sump somewhere along the length of the box gutter, a separate formula applies, usually resulting in a larger gutter.
Click on the OTHER HYDRAULIC CALCULATORS link below.

Note: The roof catchment area, is the total of all roof areas draining to all sides of the box gutter, Including half the area of any vertical faces that may run off into the box gutter.
Choose a location, or alternatively enter a known intensity for a 1 in 100 year rainfall, with a five minute time of concentration.
For New Zealand, you require a 1:50 year rainfall intensity, with a 10 minute time of concentration.

Choose a trial width of box gutter (minimun width is 300mm for commercial or 200mm for domestic). If this gives a depth of Box Gutter that is too deep for the roof consruction, try a larger width, etc etc.
Then press the calculate button.
The most efficient hydraulic shape is width = twice depth.

This calculation also gives a suggested trial size for the downpipe, which is needed for the rainwater head design.

Rainwater head design: Enter the closest available DP size to the trial DP size calculated by the first button. Don't go too much smaller.
Use trial and error of all variables to obtain a result that suits the construction.

IMPORTANT NOTE
The code only allows for flows up to 16 litres/sec total runoff in the Down pipe. It recommends that if this figure is exceeded, more downpipes should be added, or catchments reduced.
The graphs in the Code were confirmed by experimentation only up to a flow of 16 litres/sec, and for this reason do not go beyond this flow.
However my calculators are based on standard hydraulic formulas, weir, orifice, trajectories, spatially varied flow etc, with the constants adjusted to give an answer consistant with the code. For this reason I believe that my formulas will stay true regardless of the flow.
However, just bear in mind that if you use flows above 16 litres/sec you are beyond the scope of the Code.

I would also be interested in any modifications, or suggestions that you would like incorporated.
If you need more info, or you would like other areas of Australia, or New Zealand, added to the list, please

Regards
Ken Sutherland
B.Tech MIEAust CPEng RPEQ

DISCLAIMER: It is the users responsibility to ensure input data is calculated and entered correctly, and that the program is suitable for the roof configuration required.
OTHER HYDRAULIC CALCULATORS

Copyright 2006 Ken Sutherland