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)

Check the number of DP's required, if the figure is say 0.1, then that DP is only 1/10 full.
You will be guilty of gross overdesign if you use this. A smaller DP should be used.

If you have the case of say, 3.25 number * 150 dia DP's required, then you may choose to split the catchments to have 3*150 & 1*100 dia DP's. However, it is normally better to keep all DP's the same size and accept 4 * 150 DP's.

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 DP's as close as possible to 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.)

Also, although not stated in the plumbing code, the Building Code requires DP spacing to be not greater than 12m. This would apply to straight runs of gutter to limit the expansion, it would also require an expansion joint every 12m in this case.

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. However for rectangular eaves gutters, the most efficient shape is when the width is approximately twice the depth. So try to select a gutter that is close to this.

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.

When designing pipework, we must always use the internal pipe diameter in all calculations, however some suppliers quote stormwater pipes as the Outside Diameter, so a 150 dia. pipe shown in the calculator is usually equivalent to a 160 dia storm water pipe in the catalogue.

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

Some Theory :
The time of concentration is the time it takes water to travel from the furtherest point in the catchment to the point under investigation. To generate peak flow from a catchment, a storm must last at least this long. Now the longer a storm lasts, the less is the average intensity. eg a storm may bucket down for 5 mins, but is not likely to keep up such an intensity for hours.

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, or over the entry edge (weir). 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.

Note: down pipes do not flow full (except in syphonic systems) so pipe flowing full formulas do not work.

Another way to increase the downpipe capacity, is to increase the throat diameter by having a conical entrance to the downpipe. Hence increasing the length of the entry weir.
However you should approach a consultant on this as it is not always applicable.

A down pipe designed to the code does not flow anywhere near full, it is the entry orifice/weir that limits the flow. If you want to investigate this further, check out the notes on the pipe size calculator Charged downpipes. and pipes flowing to tanks, or discharging under water.
A "charged" downpipe is one that flows full, or stays full because of a "U" shape.

Further Notes on the Code:
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 (Large industrial Sheds) it would be conservative (ie an overdesign).

For these projects, the CSIRO have produced formulas that take into account the location of down pipes and bends along the gutter. The relevant document is here Roof Drainage by K G Martin
I have used these formulas to create another calculator here.

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. (

Vino, Champagne, bottles of rum, cold hard cash also acceptable.