Roof, Eaves Gutter & Downpipe Sizing Calculator
To AS 3500.3.2:2003 "Stormwater Drainage Acceptable Solutions".
(This program will calculate the size of eaves gutters, and the number of downpipes required for roof drainage)

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Rectangular Down Pipes From AS3500 :-
a 90 dia, down pipe is equivalent to a 5175 rectangular down pipe. (cross sectional area)
a 100 dia, down pipe is equivalent to a 6409 rectangular down pipe.
a 150 dia, down pipe is equivalent to a 14,365 rectangular down pipe.
This means that to carry the same flow, a rectangular Downpipe cross sectional area must be approx 1.23 times the area of a cicular DP.
This is because it is a less efficient cross section.
So, if you wish to use a rectangular xsect, the reverse applies.
Find the area of the rectangular section you wish to use,(length*breadth).
Divide by 1.23 to find the equivalent circular cross sectional area.
Then find the equivalent circular diameter, Dia = 1.128*squareroot(Area).
Enter this diameter in the trial above to find out how many are required.

Instructions: Enter a roof catchment area for the section of roof you desire. Enter the roof slope.
Choose a location, click the check box if the eaves gutter slope is steeper than 1:500 (eg 1:200).
Then press calculate to obtain the required number of downpipes and eaves gutter cross sectional area.

Roof design: If using the number of downpipes calculated above, try to have approximately equal catchment areas draining
to each down pipe, with high points approx midway between downpipes and at valley gutters. (Don't forget,
if there are no stop ends in the gutter, water may flow a little between catchments. i.e. if one downpipe is overloaded,
excess water may continue to the next downpipe.)

If it is not possible to have equal catchments, and a catchment area is much larger than the others,
then run the program again for just that larger catchment, it may require an extra downpipe.
Any size, or shape eaves gutter may be used, as long as the cross sectional area is equal to, or greater than, the size calculated.

The number of downpipes required is the theoretical number required. This is not always a whole number.
So the number used in the eaves gutter area calculation, is the theoretical number rounded up,
if the fraction of a downpipe is greator than 0.1, or rounded down otherwise.

Also, the Code only refers to the "Nominal Diameter". The actual Internal diameter may be more or less, depending on the material chosen.

Towns not listed : For towns not listed, you may add your own entensity; but you must select the location choice to:-
"I prefer to enter a known intensity"
Also the intensity required should be for a 1 in 20 year storm with a 5 minute time of concentration.

Some Theory :
The flow of water in a down pipe is restricted by the size of the entry (ie the entry diameter, throat, or orifice.)
Water starts to enter a downpipe as though it was flowing over a weir into the mouth of the downpipe. The weir formula is used to calculate the downpipe size.
As the flow builds up, the water level over this weir increases until the entire mouth of the downpipe is submerged, just like your bath tub when you pull the plug. The downpipe entry now acts like an orifice, and the orifice formula is used to calculate the downpipe size.
The greator the depth of water over the down pipe, the more water can be forced through this entry orifice.
This is why we have a rain water head over some downpipes, to increase the depth of water over the entry and hence force more water into the downpipe.

Another way to increase the downpipe capacity is to increase the throat diameter. However you should approach a consultant on this as it is not always applicable.

The above figures are based on:-
Storm frequency of 1:20 years (code requirement).

Notes: AS3500 does not take into account the location of the downpipe along the gutter, nor does it adjust the formula for bends in the gutter.
This can make a big difference. The code only allows for the worst possible case. This makes it ideal for residential buildings with many turns and bends in the roof. However for projects with long straight roofs it would be very conservative (ie an overdesign).
For these projects, we can design a system based on research by the CSIRO, that will produce considerable cost savings, by allowing a more detailed study into the location of down pipes and bends.

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 send me an email. Click here to send email

Ken Sutherland

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Copyright 2006 Ken Sutherland