Box Gutter, Rainwater Head & Downpipe Calculator allowing for rainshadow effects.
To AS 3500.3.2:2003 "Stormwater Drainage Acceptable Solutions".

ALLOWING FOR VERTICAL FACES AND RAIN ENTERING FROM BOTH SIDES OF BOX GUTTER

LOAD AND RUN BOX GUTTER CALCULATOR Method of calculation catchment area

MINIMUM WIDTH = 300 mm for Commercial construction, and 200 mm for Domestic consruction.
REQUIRED 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) To find RWH sizes, enter Equivalent circular DP dia (mm)
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:
The program makes allowance for rain shadow effects as shown in the diagram. It doesn't matter which side you call the LHS or the RHS as long as you are consistant with slope, area, and vertical face. The program will compute which wind direction will give the worst effect.
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.

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 and 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. The most efficient hydraulic shape is width = twice depth.
Then press the calculate button.

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.

Overflowing
Check out the notes on overflowing and fail safe design here, Making overflow provisions

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, open channel 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, or you require a precise design 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 2005 Ken Sutherland