LOAD AND RUN BOX GUTTER CALCULATOR
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Instructions & Notes:
As you can see there are four possible catchment areas that can contribute to the flow.
Enter the variables in the number of catchments that correspond to your roof, (you may have 1 to 4 catchments, leave unused catchments with zeros). It doesn't matter which side you call the left hand side (LHS) or the RHS, as long as the entered slope, area, and vertical face are grouped within their respective areas, and all areas are grouped as shown in the diagram.
The program will compute which wind direction will give the worst effect, when allowing for rain shadow.
If you're feeling lazy, or you have a large area for the upper, and a small area for the lower, or a flatish roof with no vertical faces, you may add the upper and lower areas together resulting in just a LHS upper and RHS upper.
The result of this will be a slight over design, as the slope effects, and vertical face effects, from both sides will be added, and nothing will be subtracted for the shadow effects. Equivalent to the rain coming from all directions at once.
If your roof slope is given in the form 1:?, use the blue boxes to convert to degrees before entering in the program.
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.
If your box gutter drains to one end with a rainwater head, click on the OTHER HYDRAULIC CALCULATORS link below.

Overflow
The overflow is designed to carry the full box gutter flow in the event of a blocked down pipe. eg hail, snow, debris, etc.
As can be seen from the diagram, the overflow size can govern the size of everything else. The overflow must be just above the operating water level to prevent overflowing during the designed rainfall. This is the reason for clearances Ioc and B.
The total depth of the box gutter is also be governed by the height of the overflow.
The overflow is designed as a weir, therefore the wider it becomes, the less height is required, and visa versa.

Trial suggestions
The trial suggestion button will calculate a suggested width of overflow that corresponds to an overflow depth of 65mm, up to a max overflow width of 350 mm. The wider the overflow from this point, the longer the sump will become.
The sump has a minimum required length of 400 mm, which equates approximately to an overflow width of 350mm.
Trial DP size is a typical size that can handle the flow.

Making adjustments
The overflow is a major factor in determining the length of the sump, and the depth of the box gutter. (Note: minimum sump length is 400mm). Sump width is the width of the box gutter.
Increasing the length of overflow will decrease the depth of the overflow, and therefore generally decrease the depth of the box gutter.
Increasing the width of the box gutter decreases the depth of flow, lowers clearance B, and hence decreases the depth of the box gutter.
The down pipe size only controls the depth of the sump. A greater depth of water on the down pipe entrance results in more water being forced into the down pipe opening.
In other words, increasing the down pipe size results in a shallower sump, and decreasing the DP size results in a deeper sump.
The box gutter slope has no effect on the design as long as it is equal to, or steeper than 1:200. (eg 1:150)
In most cases, some of the variables are fixed by construction constraints or aesthetics. Therefore, the first step is to find out these constraints and adjust the other variables to suit.

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.

Also, and amendment to graph I6 was issued after the publication of the 1998 standard. This amendment disapeared on the publication of the 2003 code, instead reverting to the original graph. (both graphs are within the parameters of standard formulars, and either could be the one intended) However, the calculator above relates to the amended graph and will give a height of the overflow spitter slightly higher than the graph in the current code. Which is at least erring on the safe side.

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 send me an email. Click here to send email

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