Instructions & Notes:
This program incorporates a simplified version of calculating the roof catchment area, and makes no allowance for rain shadow effects from vertical faces.
That is, if you decide to include vertical faces on all sides (refer below on how to do this), Runoff will be calculated from all the faces at once. That is, rain will come from all directions at once.
This is not a problem, but may give you a box gutter slightly larger than required.
The other box gutter programs on this site calculate which is the worst rain direction by allowing shadowing effects from all the vertical faces.
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.
Box gutter size
Choose a trial width of box gutter (minimum width is 300mm for commercial or 200mm for domestic). If this gives a depth of Box Gutter that is too deep for the roof construction, try a larger width, etc etc.
The most efficient hydraulic shape is width = twice depth. This is what the code formulas are based on. On top of this there is an allowance for freeboard, wave motion etc. So do not deviate too much from the results shown.
Critical Down pipe diameter & Rainwater head design :
The Plumbing Code has a requirement that the rainwater head depth should not be less than 1.25*Downpipe diameter.
This is dictated by a cryptic note, called note 1 on fig I2 (fig I2 is the diagram of the rainwater head shown above)
The note states: "This figure (I2) applies for total depth of RWH > = 1.25*downpipe diameter."
In the worked examples item J2.2(step 11) there is a requirement to adjust the RWH depth to comply with this note.
So the "Critical Diameter" shown in the calculator, is the diameter where the rainwater head depth = 1.25*DP dia.
This means that reducing this critical diameter will increase the rainwater head depth due to the orifice formula requiring more head to push the water through.
However increasing this critical diameter will also increase the depth of the rain water head due to the requirement that the rain water head depth must not be less than 1.25*downpipe dia.
So to make a long story short the "Critical Diameter" is the down pipe diameter that will give the shallowest rain water head for that particular flow in the box gutter.
I believe that what is implied, is that if the depth of the rain water head is reduced below 1.25 * DP dia. by using a larger down pipe, then the orifice formula no longer applies, and nor does the weir formula. So best not to go there.
However in real life a bigger pipe carries more water, so it needs less depth in the rain water head to carry the same flow. The water probably goes straight through the hole and doesn't need a formula. Anyway that's the code.
For very small flows, this diameter can be well below 50mm, and it is certainly not advisable to have a downpipe below this size, as it is too easily blocked. In fact the relevant graph in the Code (fig I3) does not go below a flow of 3.6 L/s and a DP size of 90mm.
This program puts equations to those curves allowing any flow, or diameter, to be calculated.
Down pipes below 90mm dia should only be used on small awnings and the like, where overflowing will not cause any damage.
By the way if you have a flow greater than 16L/s the program reverts to formulas developed by the CSIRO and this requirement does not apply.
If you wish to play around with rain water heads without this restriction try the custom rainwater head calculator.
Anyway, moving on, use trial and error of all variables to obtain a result that suits the construction.
It is important that the dowwnpipe is positioned closer than one diameter to a side wall of the collector to prevent swirl.
Check out the notes on overflowing and fail safe design here, Making overflow provisions
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, etc, with the constants adjusted to give an answer consistent with the code. For this reason I believe that my formulas will stay true regardless of the flow.
However if you choose to generate a PDF of the results, and if the flow is greater than 16 L/s the formulas revert to formulas developed by the CSIRO Division of Building Research. The printable version has full reference to the particular CSIRO document.
Also, the new National Construction Code (NCC) has provided a solution for us all.
Clause DV1, Under Roof Drainage Systems/Verification Methods/ states:-
DISCLAIMER: It is the users responsibility to ensure input data is calculated
and entered correctly, and that the program is suitable for the roof
OTHER HYDRAULIC CALCULATORS