To AS/NZS 3500.3:2015 "Stormwater Drainage Acceptable Solutions".

This program is free and complies with the Australian Plumbing Code AS/NZS 3500.3:2015; but can be used for any Plumbing Code. Find out how to use this program for any Plumbing Code. It does however have the ability to far exceed the range of the Code Find out how to exceed the limits of the Charts in the Plumbing Code.
There is no limit to Downpipe size, or eaves gutter size. The code limit is 150mm for DP's and 26,800 sq mm for flat eaves gutters
The program will calculate the size and number of downpipes (downspouts, leaders) required, and the eaves gutter sizes for any roof.
You can choose a downpipe size and calculate the number required, and the corresponding Eaves Gutter size.
Or you can choose an Eaves Gutter size and calculate the corresponding down pipe size, and the required number.

To Calculate:
Enter roof area, roof slope, and rainfall intensity. Press the 'Calculate' button associated with that section.
Note: Pressing the 'Enter' button will also dismiss the Alert Box should it appear.

Watch the Video

Legend:  Boxes shown like this Indicate a calculated value. (no user input is required)

Impress your clients at meetings, get A simplified version for your Android phone.

Enter Details

Diagrammatic of eaves gutter and downpipe

Unit Conversions.

Warning! A lot of gutters have fronts higher than the back, and buildings are being designed without eaves. This can be a recipe for disaster.
There are three main things that can go wrong. Design, construction, and maintenance. Best not to be in the firing line for the design.
Read about overflow provisions for some extra design protection.
Also it doesn't hurt to get a copy of the calculations with all the necessary Plumbing Code references for your records.
View a typical project
Read about the calculation PDF file. Watch the online presentation.
Purchase unlimited Calculation files for your records. Plus activate the Extra Features on as many devices as you wish.

By the way this note disappears on activation. So that's one good thing.

You will require one of the following DP options :- (dimensions in mm)
(Assuming approximately equal catchment areas)


90   Dia:
100 Dia:
150 Dia:
225 Dia:
300 Dia:
arrow pointing to checkboxes  

 Select DP

For Non Equal Catchments.
The Plumbing Code method suggests creating roughly equal catchment areas for each downpipe, however if this is not possible, you can enter each down pipe catchment individually to determine the DP size, or the number of DP's required for that catchment.
Alternatively the following table will give the maximum downpipe capacity in terms of the horizontal catchment area for each DP.
It is then a simple matter to ensure that no catchment area exceeds the maximum allowable area for the Down pipe size selected.
Video on how to calculate catchment area from a PDF file.
The table also gives the corresponding gutter size for each downpipe when flowing at capacity. The calculation uses the above Intensity, roof slope, and gutter slope.
Check how Roof slope affects the catchment area.
Note: The DP capacity table requires activation for the results to appear.

(requires Activation) ACTIVATE NOW

Overflow over the back of eaves gutters Overflow over the front of eaves gutters Overflow over the front of eaves gutters with flashing


90   Dia:

100   Dia:

150   Dia:

225   Dia:

circular Downpipe diagram
Custom rain water head diagram
Rectangular Downpipe diagram

Custom rain water head diagram

Note: When using the printout for this section, please select a checkbox first.

* The maximum Plan area that this DP will drain, for the Roof slope and Intensity entered above.
** The maximum Plan area for each DP actually used.

What's this?

Valley Gutter

This calculation gives the total sheet width, which is folded in accordance with the diagram.
Roof slope as per the main program.

AS/NZS 3500 has the following limitations :-
  • Maximum catchment area 20 sqm. From table 3.6.2, with max intensity of 400mm/hr, this equates to a max flow of 2.2 L/s.
  • Roof slope not less than 12.5 degs (1:4.5)
National Construction Code (NCC) has the following requirements:-
  • For Roof slope greater than 12.5 deg the Minimum sheet width is 400mm. Accordingly this program will not give a sheet size less than this amount.
  • For Roof slope less than 12.5 degs design as a box gutter.
    (This is fascinating as the plumbing code does not allow box gutters to have a grade steeper than 1:40
    or more correctly, the Code does not have a means of designing a box gutter for these steeper grades.
    Although my program will do it. I haven't checked whether it will give satisfactory results for super-critical flows.
    So, rock and a hard place. However a box gutter designed at 1:40 will presumable work for all steeper grades.)
  • Roof sheeting to overhang a min of 150mm each side of the valley gutter.
Notes on location of downpipes and Valley gutters.

The Plumbing Code allows for the worst possible case. That is, with bends in the gutter and DP at the end. For all other cases, this method will achieve significant cost savings. It is based on formula derived by the CSIRO:-     CLICK HERE
Generate a pdf file for your records. Shows calculation steps and Code references. Allows insertion of Job numbers, Descriptions, etc. Designed for printing out on your own letterhead. Also includes the 'Extra features; CLICK to Activate

view a typical template (HTML)
Semi circular eaves gutters are more efficient than rectangular ones.
The code also requires a 10mm freeboard. Activating the "More Down Pipes" function will allow you to calculate the resultant diameter.

These programs can be activated on more than one computer for the same price.

I often get asked if I know someone in a certain location who can help with hydraulic problems/design work, and as I no longer do design work, I'm happy to pass on the name and email address of anyone who purchases the "Activate all programs" option.
So if you would like me to pass on your details, please add your area of expertise, ie Hydraulic designer, Architect, Building designer, Engineer, Plumber, etc. and your location, in the area of PayPal marked "Special Instructions/Add referral instructions to the seller". I can then give your email address to anyone looking for that profession in your area.

The More Downpipes function
This allows the calculation of Circular Diameters not listed, the calculation of Rectangular Downpipes, and Semi-circular eaves gutters.

This gives the opportunity to enter a downpipe size that is not on the list.
There are plenty of these. Different manufacturers have different sizes. There are metal and there are PVC Pipes.
There are Storm water pipes, and there are Drain waste vent pipes (DWV). In the bigger sizes there are a whole lot more materials to choose from.
You may even want to enter the exact internal dimensions of the chosen pipe, instead of accepting the 'Nominal' diameters of the pipes in the program. For example a 150 nominal diameter (150DN) PVC stormwater pipe is actually 154.5 mm internal diameter, and a 90DN is about 95 internal diameter. Slightly bigger. Carries more water. This may reduce the number of downpipes in some cases.

For rectangular downpipes it doesn't matter which is side 1 or side 2.

A semi-circular eaves gutter section is more efficient than a rectangular section.
Also the Code requires a 10mm freeboard. This can either be a vertical rise of 10mm, which makes folding harder, and may not look as good as continuing the circular arc.
This method gives the resulting diameter by continuing the circular arc to achieve the necessary 10mm freeboard.

Valley Gutters
Well its in the Code so I've included it here just for fun.
Admittedly it is an interesting method of calculation, with many limitations, but hey, that's the plumbing Code.

Valley Gutters and Downpipe location
The National construction Code requires valley gutters to be at a high point of the eaves gutter.
The plumbing Code suggests that a DP should be as close as possible to a valley gutter.

So is this a rock and a hard place or what?

Lets take a closer look. A valley gutter normally enters the eaves gutter at a bend. So when water comes tearing down the valley gutter it may split in two when hitting the eaves gutter with a proportion of the flow going in each direction around the bend.

If this is not a high point, there will be existing flow going through the bend.
The valley gutter will increase the flow leaving the bend, and impede (or stop) the flow coming into the bend.
Which is not good.

So giving the eaves gutter a high point at this location makes sense. Enabling the flow to go in both directions unimpeded.

The flow is concentrated at this point in the eaves gutter, has a high velocity and may produce waves. So best not to add too much more flow just in case. Hence the DP requirement to be as close as possible to the valley gutter entry point.
I believe that if the DP can be placed within a few meters of the valley gutter, and with no bends in between, then the "as close as possible" requirement is complied with.

The Calculation and results file for your records.
This program has more functions than just printing the Calculations and Code references.
It will tell you if there is a smaller DP available than the one you have selected.
It will advise if you have gone outside of the Code tables or graph by stating that the result has been "extrapolated".

Table 3.5.2 in the code only allows for standard down pipe sizes. There are no in-between sizes.
However if you choose the "More Down Pipes" function the in-between size is shown as "interpolated".
You are free to choose this, The table size, or use your own preferred size.
All these sizes are shown in the resultant PDF file.

On pressing the generate button, you are taken to the next page that allows you to add your Job numbers, Descriptions, Client Names, your own Name and address etc. Up to 14 text fields can be inserted. These fields are optional, and will disappear if nothing is entered.

After entering any relevant data, the next screen gives you a look at the final output before the PDF is generated.
If something is not to your liking you can go back to any screen as many times as you want and change something.
The PDF file is arranged with very wide margins that hopefully will allow printing out on your own letterhead.
There is also no colour or internet references.

There is also no limit on the number of different PDF files that can be created in the allotted time.

The PDF file shows the calculation steps, the results, summary, and Code references.
This file is ideal for your records, submission to the Local Authority, client, Builder, Architect, etc. (even a court of Law, but hopefully you won't find yourself in that situation).
But wait ...there's more...You can also have a warm inner glow, knowing that you have a backup copy of the calculations on your files.

Here is a typical template (PDF) Here is a typical template (HTML)
Here is an online Powerpoint presentation

There are no special downloads or passwords or login required. A cookie is placed on your computer which remembers your details. This saves the hassell of logging in each time you use the program.
However should the cookie be lost, or you wish to activate on more computers, you can place the cookie manually. Refer to the Login menu for how to do this.

The extra features will now work for the allotted time.

The time remaining is also depicted unobtrusively in the top left hand corner.

P.S. Any text you add is not stored or used in any way, and is lost on exiting the browser.

Activating this feature also activates all the Extra features functions.
However the valley gutter function is not included in the printout, as it requires a different catchment area.
The "More Down pipes", function is included in the results PDF file.

There is now the ability to activate the programs on all of your office computers for the same price.

The program is based on the Australian Plumbing Code, but can be used for anywhere in the world where water is still wet and flows downhill. The physics of water flow does not change. Well.. not enough to have any significant effect on our sizes. The changes relate to viscosity, and gravity changes. (So don't use this program in outer space.)

Therefore the only change required to operate the program for locations outside of Australia is the required design rainfall intensity for your local area. This is normally stated in your local Plumbing/building Code in terms of the frequency and duration of the design storm.

For instance, the International Plumbing Code, and the Uniform Plumbing Code, both require a storm frequency of once in 100 years, and a duration of 1 hour, for all catchments.
By the way, this happens to be far less than the Australian requirements, and is roughly equivalent to an 'once in 1 year' storm, of 5 min duration in Australia. Check it out here.

Enter data, press 'calculate' or the enter key when the cursor is blinking in any box.
You may also move between the boxes by using the 'tab' and 'shift+tab' key.
Note: Entering data will clear all previously calculated results. This is to prevent you from accidentally using the wrong results, and forces you to recalculate with the new data.

The program will calculate the total number downpipes required, in each DP size range, for the catchment area entered, and the recommended eaves gutter cross sectional area for the relevant downpipe size.
It is recommended that all downpipes on a project are the same size, so if you prefer 100 dia downpipes, the calculator will tell you how many 100 dia DP's you need.
Conversely if you can fit only say 2 downpipes, then the program will tell you what size you need.
To mix and match sizes, split the roof into different catchments for each size.

However, it is important when using this method, that all downpipes have a similar catchment area. Or the ability to share the flow. If this is not the case, it may be necessary to split the catchments, or calculate each downpipe catchment area individually.

Roof Catchment Area:
The roof catchment area (watch video on how to calculate from a PDF plan) is the plan area as shown on the drawings. Entering the roof slope in the next box makes the necessary adjustments for a sloping roof.

Roof Slope:
Enter the roof slope in degrees. Note: if your roof slope is shown as a percentage, or of the form 1 in ?, (Vertical:Horizontal) then use the 'conversion calculator' shown below.

Roof Slopes for combined Roofs
(Roof slope calculation diagram should be here.)
(Roof slope calculation diagram2 should be here.)

If the roof has multiple slopes, ie a steep slope draining to a flatter slope (or visa versa), the slope to use is the average slope.
Take the plan length from ridge to gutter, divide by the total fall, this will give the 'x' in terms of the roof slope being '1 in x'. Enter the 'x' in the conversion calculator to get degrees.
'Degrees' must be used for the 'roof slope' box.
For the 'Conversion calculator', enter in one box and the answer will appear in the relevant other.
More info on how the roof slope affects the catchment area.

Click the drop down box to enter a location. This sets the rainfall intensity for that town. If your location is not on the list, refer below on how to find, and add your own intensity,

Click the check box if the eaves gutter slope is steeper than 1:500 (eg 1:200).
Press 'calculate' (or 'enter') to obtain the required number of downpipes and eaves gutter cross sectional area. You may also move between the boxes by using the 'tab' and 'shift+tab' key.

Effective Area of Gutters:
The Gutter area shown in the program is the "effective" cross sectional Area.
Clause 3.5.4 in the Australian Plumbing Code dictates that the 'Effective' Gutter Cross Sectional Area shall be taken as the area beneath a line not less than 10 mm below the overflow, e.g., front bead, gutter back or bottom of overflow slots.
This should be taken into account when the Manufacturer quotes the "Effective" area.

If folding your own gutter, add 10mm freeboard to the suggested depth shown.

The eaves gutter cross sectional area is calculated on the flow in the gutter. That is, the total catchment flow divided by the actual number of downpipes used. Therefore if a 90 diameter downpipe is sufficient and a 150 or greater diameter is used, the gutter area will remain the same, as the flow in the gutter will not change by using a larger down pipe.

Eaves Gutters:
For sizing eaves gutters, the most efficient (and desirable) cross section, is width = twice the depth. The plumbing code assumes this in these calculations, so don't vary too much from this criteria.
The width and depth values shown above are for the "effective area" and are rounded up to the nearest 5mm. These values are only shown to give an idea of the size of the gutter. The actual size will depend on where the overflow is - front, back, or slots. (Refer "Effective Area" above.)
Pick a standard gutter that the Manufacturers quoted "effective" area is equal to, or greater than that shown in the program.

'Number of DP's Req'd', and, 'Number of DP's Used':
The number of downpipes required is the theoretical number. This may not be a whole number, eg 3.5, Not very practical using 1/2 a down pipe, so the number is rounded up to the next whole number.
If using your own DP dia, or a rectangular downpipe, the object is to get the number of DP's required, as close as possible to a whole number.

Down Pipe size not listed
A few of the uncommon pipe sizes are not listed, eg sizes smaller than 90mm, 125mm dia etc. However any size can be calculated by entering the diameter into the "Any circular DP dia" box.

Rectangular Downpipes:
Enter your chosen dimensions in the box given. It doesn't matter which is side1 or side2.
Play with the dimensions until you get the number of DP's you require (or the required gutter area).
It doesn't matter what dimensions you enter, the printout will tell you the size referred to in table 3.5.2 of the Plumbing code. You are then free to go back and change to this size, or use your preferred size. Either way your DP will either be equal to, or greater than, the code requirements.

Working back the other way:
Commercial gutters are made in standard sizes and shapes. An Architect may like the look of a particular gutter and specify that. So the designer must then find the most economical number of, and size of, downpipe to fit the specified gutter.

This is working back the other way from the free version of the program, which allows the designer to select the number and size of downpipes first, and then find the gutter size that suits.

However for those of us that prefer to work back the other way this program is provided.

The manufacturers catalogue will give the "effective gutter area" of each of their standard gutters.
Enter this figure in the gutter area box, and press calculate.
The results don't seem to make much sense at first glance, so let's see what is happening:-
We obviously need a whole number of downpipes to service the gutter. Standard Downpipes can increase in diameter by as much as 50%. So one number (n) of DP's may be not enough, and the next number (n+1) may be too many.

Looking at it another way, let's say the chosen gutter can handle 2 L/s; but there is 7L/s draining to the gutter.
If we split the gutter into 3 equal catchment areas (ie using 3 DP's), each will have 7/3 = 2.3L/s flowing into the DP and also flowing along that stretch of gutter. This flow is too much, as the gutter can only take 2L/s. Therefore we need a larger gutter, or more DP's.

If we split the gutter into 4 equal catchment areas (one more DP) we have 7/4 = 1.75L/s flowing along the gutter. However the gutter can handle 2 L/s therefore we can use a smaller gutter. This is why the "gutter area for this flow" result in the program is smaller than the chosen gutter.

Either way, in this example, we must use at least 4 downpipes . The most economical downpipe size to use is the "theoretical size" shown. This size can handle approximately the same flow that is flowing along the gutter.
Note: the flow along the gutter is not necessarily the gutter capacity. To utilise the full gutter capacity would require a non whole number of downpipes. For instance, the example above would require somewhere between 3 and 4 down pipes. (three is not enough, and 4 is too many)

However another solution is to use different size downpipes.
One size to suit the chosen gutter size, and one size to suit any left over areas. For instance, the maximum catchment area for the chosen gutter has been calculated. Entering this area in the main entry fields will give the required DP size for this catchment area. Divide the main roof into catchments of this size, and any left over area can be entered again to find a down pipe size for this smaller area.

The answers for downpipe size are in mm. The program will calculate the most suitable circular, and rectangular downpipe sizes.
The option to select the downpipe size to be used, is only necessary if you require a printout of the results. The size entered here has no effect on the capacity of the gutter. The eaves gutter capacity depends solely on the gutter area and the slope.
The printout program needs to know what size is required. For example, any size can be selected as long as it is greater than the "Code" size.

Flat Roofs and Surfaces
This function uses two formulas, the weir and the orifice formula. The formula that gives the biggest size is adopted.
The weir formula is taken from the Plumbing Code AS/NZS 3500.3-2015 clause
With the exception, the constant 1600 is changed to 1522. This agrees more closely with commercial rain water outlet grates when flowing under weir flow.

The orifice formula is taken from the Q'ld Urban Design Manual (QUDM) formula 7.5. with the following exceptions:-
The pressure loss coefficient is changed from 2.75 to 11.1. This has the effect of reducing the capacity by approximately 50%.
The formula in QUDM is designed for field inlets and not roof outlets. Roof rain water outlets (RWO's) are much smaller and seem to have a greater resistance to the flow.
This change gives results more consistent with commercial rain water outlets such as the Wade type 100dia.

Also commercial rain water outlets come complete with an outlet pipe size, either vertical or horizontal. This also affects the capacity when the grate enters the orifice phase, as there are now two orifices to contend with.
This formula used on this site does not take this interaction into account.
This has the effect of reducing the grate diameter and increasing the downpipe size in some instances. In other words, if you see the formula used is "orifice", then a commercial Rain water outlet (RWO) may have a smaller outlet and a larger grate that will pass the same flow.

Also the clear grate area used in the formula is assumed to be half the total area. This can vary substantially between manufacturers, and grate type.

So after all that, there are so many variables and assumptions, that these answers give only a general idea of sizes and where to start looking for a commercial RWO.
The answers give the theoretical sizes. The next highest available size should be adopted.

Both these formulas make assumptions as to the head loss coefficients and discharge coefficients. These depend on the grate configuration etc. If in doubt, or the numbers look weird, please adopt the Manufacturers recommendations as to the outlet capacity.

The formulas also assume that the flow can enter the outlet from all sides. If a square outlet is against a wall, only 3 sides will accept the flow under weir flow conditions. Therefore the user should calculate the perimeter of the square outlet given, and use that length over 3 sides instead of 4.

The Area to be drained is not the area of the entire roof, just the area draining to the particular outlet with no significant roof slope.

Ponding Depth A Flat roof normally has high and low points. The maximum depth of ponding will be the difference between these heights for the catchment in question.
However this may not be the appropriate value to use. Consideration should be given to the usability of the roof. For instance, if it is a habitable roof, or a car park, think about how much water pedestrians may have to walk through.

The blockage factor suggested in the Code is 0.5. That is, allow for 50% blockage if the area is subject to leaves, debris etc.

However the Code formula is confusing because a blockage factor of 0.8 (80%) means that the capacity of the inlet is 80% of the unblocked value. That is the capacity is reduced by 20%. Meaning the grate is blocked by 20%, not 80%.
In an effort to avoid this confusion I have used the term Percentage blocked instead. So if a grate is 80% blocked, it means that its capacity is only 20% of the unblocked value.

If nothing is entered for the Percentage blocked, the default 0% is used. That is, no blockage is allowed for.

The Graded Outlet Pipe Size is calculated for a graded pipe of slope 1 in 150, this will give a slightly larger pipe to allow for entry and exit losses. The constructed pipe grade should be as per the code. This size for a graded pipe should really only be used when there is no vertical pipe involved, as this pipe is calculated as flowing full, whereas a down pipe is not flowing full. If this size is smaller than the downpipe size, it is best to use the downpipe size.

The vertical downpipe size is calculated using the same formulas as mentioned above, only without obstructions due to grates and blockage, and with an extra head of 30mm.
Also as mentioned above, if you see the formula used is "orifice", then a commercial Rain water outlet (RWO) may have a smaller outlet and a larger grate that will pass the same flow.

Further reading. If you wish to know all there is to know about roof outlets, Wade have produced a very good document here WADE Roof Outlets
and Galvin HERE Galvin Rain water Outlets

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

Other Places not listed :
There is the ability to use any intensity you want.
How to find the Intensity for other places.

Ken Sutherland
Certified Practicing Engineer,
Bachelor of Technology
Member of the Institution of Engineers Australia.
Registered Professional Engineer Queensland.
contact me

Conversions (Rounded)

It is the users responsibility to ensure the program is suitable for the purpose intended and that all data is entered correctly as required by the program and the associated instructions. Copyright Ken Sutherland 2017
donations gratefully accepted