Today we'll cover ASNZS 3500 Part 3 stormwater drainage.

I will outline to you the key amendments to the previous version and the new updates and their onsite implications for the design and installation.

Please use this session as a technical orientation.

You must consult the 2025 version in full and comply with other regulatory framework and manufacturer instructions for the design and installation.

Since the 2021, a few key things have changed in 2025 version.

First, clause 3.6. now increases the allowable catchment for valley gutters.

Next, clause 5.3.8 introduces new rules for how miscellaneous devices and appliance connect to the stormwater system.

Importantly, Appendix F is now mandatory.
It specifies eaves gutter overflow requirements for roof catchment up to 400 square metres.

Finally, there are editorial tidy ups to make the standard easier to read (while the technical intent stays the same).

Now let's dive into the detail.

Clause 2.2.1 now includes Note 3.
It requires plumbers to consider the quality and the quantity of stormwater discharge.

This is to ensure it does not negatively impact council infrastructure or on site systems, which mean that you must consult the local council that has jurisdiction over the legal point of discharge and confirm that the quality and the quantity of your site discharge suits their network.

If required, obtain consent from the relevant council.

Clause 2.2.2 is a new addition to the 2025 standard.
This clause requires that when we are selecting stormwater pipe work, fittings or products, we base our choice and two key factors, the anticipated discharge temperature and the quality of that the discharge.

In simple terms, this means you need to look what's being discharged into the stormwater system and how it might affect the materials used.

The best way to do that is to get the charge data from the manufacturer, things like temperature, composition or any chemicals present.

Once you have that information, you can then choose the right pipe material to suit those conditions.

It's also a good idea to record the basis of your material selection in your documentation.

That way, it is clear that you have considered the performance and compliance requirements of this new clause.

Now let's look at Table 3.3.4 “Annual exceedance probability”, which has been updated to now to include eaves gutter overflow measures.

In Australia, eaves gutter overflow must be designed for at least 1% annual exceedance probability (AEP) event.

If you are wondering what exactly AEP means, according to the Australian Water Information Dictionary, annual exceedance probability is the probability that a given rainfall total accumulated over a given duration would be exceeded in any one year.

That doesn't mean the event only happens 1 in 100 years. It means there is 1 in 100 chance of it is happening in any single year.

So the risk is always present, even if rare.

You can find rainfall intensity for the suburb either in Appendix D “Rainfall and density for Australia”, or directly from the Bureau of Meteorology website.

And when we move to Appendix F, in the next section, I will show you how exactly to use rainfall intensity values in a calculation example.

Clause 3.5.3 is a new clause that sets the basis for designing overflow measures in eaves gutter systems.

When designing for overflow, start by assuming that all down pipes are fully blocked.

Then determine the catchment area according to the method outlined in section 3.3.3.

Next calculate the inflow rate Q*   That's the inflow volume in litres per second.

Once you have established the inflow rate, move to Appendix F to select the appropriate overflow measure for your design.

We will go through the detailed calculation steps when we cover Appendix F in more depth.

Clause 3.5.3 requires you to include any upper roof that can discharge directly or overflow onto a lower roof.

This ensure that if all down pipes are blocked, including on the upper roof, the overflow still works and prevents water getting back into the building.

Of a higher 70 square metres of a roof area from an upper roof that can spill onto a lower roof of 80 square metres.
You will need to design the lower eaves gutter overflow for the combined catchment area, which is 150 square metres, not just the 80.

In other words, always make sure your overflow design accounts for any additional roof area that drains down from above.

Now there is a couple of scenarios where overflow measures are not actually required.

The reason is simple.

In these cases, any overflow water cannot make its way back into the building.

It just flows clear to the outside, so there is no risk of water ingress.

Here are two examples where overflow measures are not actually required.

The first scenario is when eaves gutter is fixed to a veranda, or eaves are wider than 450 millimetres and there is no lining underneath.

The second scenario is quite similar.

It is a rack, veranda or racked eve, again more than 450 millimetres wide, but in this case the lining slopes away from the building.

In both of these situations, any overflow water has a clear path to discharge to the outside and cannot make its way back toward the building, so no overflow measures are required.

Now let's move to clause 3.6 valley gutters.

The deemed-to-satisfy catchment limit has increased.

A valley gutter may now serve up to 40 square metres.

The limit was previously 20 square metres in 2021 edition.

A new figure 3.6.2 has been introduced to assist practitioner in selecting the appropriate valley gutter widths based on the project's catchment area and rainfall intensity.

Notably, the new figure now allows for catchment up to 40 square metres.

To clarify, the term EW refers to the effective widths of a valley gutter.

It is the portion of the gutter that carries the flow of water below the Freeboard level HF.

The effective width is measured across the valley gutter and the design flow depth below the freeboard, and it varies depending on the gutter profile and the side angle.

The table provided in the standard, Table 3.6.2, was developed based on a freeboard HF of 15 millimetre.

In 2025, Figure 3.6.2 adds an alternative method to calculate the effective widths of a valley gutter.

In addition, an explanatory note to clause 3.6.2, permits CFD computational fluid dynamics.

CFD is a numerical simulation of fluid flow and heat transfer that use mathematical models to analyse fluids in motion.

Accordingly, after you calculate the catchment area, choose the suitable method as follows.

If the catchment area is 20 square metres or less use Table 3.6.2, or alternatively you may use Figure 3.6.2.

If the catchment area is more than 20 and up to 40 square metres, you must use Figure 3.6.2 only. Not the table.

Now important note: the new Figure 3.6.2 is under review by Standards Australia and should not be used in its current form.

We will update you once a confirmed version is released.

Now let's move to into Section 5, Surface drainage.

In this section, a new clause 5.3.8.1 has been introduced.
It specifies the requirement to how to connect miscellaneous devices and appliances to the stormwater drainage system.

This clause introduced 2 new notes.

It is important to verify that the internal components can tolerate the discharge and confirm with the council that its quality, quantity and temperature are acceptable.

And if the council requires it, make sure to get their approval or consent before connecting to the stormwater network.

Now let's delve into the detail.

This clause says that if a device or appliance discharges into the stormwater system, that the discharge must occur through a tundish or a pit.

In simple terms, no direct connection straight into the stormwater line.

Now Part B goes a step further.

It requires that the discharge point be positioned so that steam or heated water doesn't cause nuisance and is easily visible, and does not damage the building or harm anyone nearby.

So in short, always discharge via tarnish or a pit and make sure the outlet is visible and safe.

Now still under clause 5.3.8.2 stormwater connection, this part focuses on what's required when you connect discharge from an appliance to a tundish or stormwater pit.

The standard now gives us four clear technical requirements to comply with.

First, it must be accessible.

Second, it must be securely fixed.

When the appliance discharges, there should be no vibration or movement that could damage the pipe work or cause water to splash out of the tundish.

Third – it must allow the discharge to be visible.
That means you should be able to see when water is being released.

So if something discharged unexpectedly, it can be noticed early before it becomes a problem.

Finally, a 25 millimetre air gap is required between the point of the discharge and the tundish or the pit.

That air gap protects against back flow and contamination.

So overall, this clause is really about safety and maintainability.

Now we come to the last part of Clause 5.3.8.

The new Clause 5.3.8.3 “Stormwater system design”.
It requires the stormwater system to be sized for the maximum discharge.

So when selecting the pipe size, you are designing for the peak flow, not the average.

There are three supporting notes, each linking back to earlier clauses.

Note 1 directs you to the manufacturer's specification for the discharge details such as flow rate, temperature and quality.

Note 2 refers back to clause 2.2.2,  reminding us the material must be suitable for discharge condition and comply with the standard.

Note 3 highlights the need to consider discharge quality and confirm with the local authority if approval is required before connecting to the stormwater network.

Now let's move to Appendix F overflow design for eaves gutters.

This appendix has undergone a significant change in 2025 edition.

In the 2021 versions, Appendix F was informative, meaning it was optional guidance.

But in 2025, Appendix F is now normative, which means it's mandatory for deemed-to-satisfy solutions.

Appendix F sets out the requirement for designing eaves gutter overflow systems for roof catchment up to 400 square metres.

The 400 square metres limit applies universally to all building types, both residential and commercial.
There is no limitation in the standard that restricts this to small buildings only.

Now a few key clarifications based on our regulatory position.

If the roof catchment area exceeds 400 square metres, a performance solution is required because this goes beyond the design parameters of the standard.

However, to comply with the standard, a plumber can divide the roof into separate eaves gutter systems, provided each system serves a distinct catchment area not exceeding 400 square metres, and the systems are generally independent (i.e. not just artificially divided on paper).

Now let's look at clause F.2 overflow volume, which explains how to calculate the design overflow volume Q* that must be managed by eaves overflow measures.

You can find this value by multiplying the catchment area A, by the rainfall intensity R and then dividing by 3600.

So the first step is to determine the catchment area A (i.e. the total roof area draining to eaves gutters).

Once you know your catchment area, Step 2 is to find design rainfall intensity R.

That's the 1% annual exceedance probability AEP for your project location in Australia.

After you have got both values A  and   R, you simply plug them into the formula shown on the screen.

Q*   =   A * R / 3600.

Now to understand why we divide by 3600.

Rainfall intensity is given in millimetres per hour.

The overflow volume is expressed in litres per second.

So dividing by 3600 converts hours into seconds.

That's how you get the correct flow rate which your overflow system must handle safely.

So to summarise simply:     A is your catchment area, the part collecting rain.

R is the rainfall intensity   (i.e. how hard it trains during the design storm).

Dividing by 3600 converts it to litres per second……giving you the overflow rate Q* that your eaves gutter overflow system must discharge to prevent flooding or backflow into the building.

Before we move into a detailed example, a quick but important reminder.

If you are using rainfall intensity data from Bureau of Meteorology website, be careful. Because the Bureau of Meteorology website provides two different units,

• millimetres per hour    (this is the rainfall intensity); and
• millimetres        (this is the total rainfall depth).

For the purpose of calculating in full intensity R in the overflow formula, you must use the millimetres per hour value.

As you can see on the screen, the Bureau of Meteorology table includes a drop down menu for the units.

Make sure it is “millimetre per hour”. Not “millimetre”.

If you accidentally choose millimetre, you will be using “rainfall depth” instead of “[rainfall] intensity”. And that will give you incorrect overflow results.

So always double check that your data is in “millimetres per hour” before entering into the formula.

Now let's see how you actually find it on the Bureau of Meteorology website.

Rainfall intensities or RFI's are shown in table like this.

To find the correct value, go to the 1% AEP column because that's what the standard requires.

Then look across to the 5 minutes duration row.

Where the two intersect – that's the rainfall intensity value.

In this example, the rainfall intensity is 218 millimetre per hour, and that is the correct figure to use in your overflow calculation.

Now let's do a quick calculation example, now using standard tables for the RFIs (instead of the Bureau of Meteorology website).

Let's say we are designing a roof drainage system for a house in Mildura and the roof catchment area is 200 square metres.

We already know the catchment area, so the only thing missing is the rainfall intensity.

Looking at Appendix D in the standard, we can see that for Mildura at 1% AEP, gives a rainfall intensity of 219 millimetres per hour.

That's the value we will use for R in our overflow calculation.

Now that we have all the values, let's plug them into the formula in Appendix F.2.

We have rainfall intensity R  =  219 millimetres per hour

catchment area AC  =  200 square metres

Using the formula Q* equal to area of catchment times rainfall intensity divided by 3600, we substitute the numbers and Q* will be equal to 12.17 litres per second.

So the design overflow volume for this catchment is 12.17 litres per second.

That means eaves gutter overflow measures – whichever method you choose – must be capable of safely discharging, let's say, 13 litres per second of stormwater, assuming all down pipes are blocked.

Under F.2, note 2 is important to highlight. The standard clearly states that the overflow design method does not account for the effects of debris buildup, or the presence of snow, hail or ice.

This means that even if your design works perfectly under normal rainfall, its performance can be significantly reduced if debris, snow or ice obstructs the flow path.

Now, while you cannot always predict when this might happen, you can assess the likelihood based on the site conditions.

For example, whether the property is near trees, vegetation, or planters that might shed leaves or debris into the gutters.

In such cases, designers should take a conservative approach.

Overflow measures that discharge toward the external edge of the gutters are generally a better choice, as they are less likely to cause overflow back toward the building.

And for a region that experiences snow, hail or frost, additional design consideration should be included to make sure the overflow path remains clear and functional under those conditions.

At the previous step, we have calculated the design overflow volume Q* which represents the amount of water that your overflow system needs to handle.

Now, under Clause F.3 “Overflow design”, we move to the next step.

This clause tells us that the overflow measure you select with a back gap, slotted gutter, or front bead must be selected so the capacity Q is equal to or greater than the calculated overflow volume Q*.

In simple terms, the selected overflow system must always be able to handle at least as much water as the calculated flow.

Now let's move into close F.4.1 “Continuous overflow measures”, and look at the general requirement for designing any overflow system.

First, overflow measure must be continuous.

This means the overflow runs along the length of the gutter rather than being located at a single point.

The key advantage of continuous overflow is that if one section of the gutter gets blocked, the remaining length can still provide overflow capacity, so it gives you a level of built in redundancy.

Clause F.4.1 also tells us that the overflow capacity is given per metre per length.

So to find the total capacity Q for selected overflow measure, you will need to multiply the per metre flow rate by the total length of eaves gutter serving the catchment.

If you selected a back gap gutter that can discharge 1 litre per second per metre, and your gutter is 15 metres long, you simply multiply 1 litre per second per metre by 15 metres, which means that the gutter can disperse 15 litres per second in total.

This is a simple but very important step, because it allows you to check whether your selected gutter can actually handle the overflow volume you calculated earlier.

Another general requirement to note here, regardless of which overflow measure you select, is the need for a freeboard margin above the maximum head of water in the gutter.

This margin is important because it provides a small safety buffer to allow for things like construction tolerance, building settlement that could affect the slope, turbulence in the water flow, or even wind effects.

The standard specify that you must have a minimum 3 millimetre margin for eaves gutter with a slope greater than 1 into 500 and a minimum 6 millimetre margin if the gutter slope is less than one into 500.

So for example if you have designed your back gap gutter for a 10 millimetre head of water and slope is less than one into 500, then you will need to add that six millimetre freeboard margin. Meaning the back of the gutter should sit 16 millimetre below the fissure.

This ensure the system has enough allowance for any minor issues while maintaining proper overflow performance.

We will talk more about the required head for each overflow measures in the following slides.

Now let's start with the first overflow measure for eaves gutter in the new revision of the standard.

“The gutter back gap overflow” – the gutter back gap is the small gap intentionally lift and continuously maintained between the gutter and the fissure.

It is important to ensure this gap is maintained along the gutter.

This may be achieved by using spacers or any other method recommended by the gutter manufacturer.

This gap acts as an overflow path when the primary outlet is blocked, allowing water to discharge safely instead of flowing back into the roof space or building structure.

Table F.1 helps to determine how much water a back gap can discharge if the primary outlet becomes blocked.

The discharge rate depends on the head of water above the back of the gutter.

The higher the head, the greater than the discharge capacity through the same gap.

For example, if the average bottom gap is 6 millimetre and the head of the water is 8 millimetre, the back gap can discharge approximately 1.8 litre per second per metre.

If the head of water increases to 12 millimetre, the discharge increases to around 2.6 litre per second per metre.

Now let's look to the second overflow measure of eaves gutters – “slotted gutters”.

Slotted gutters have a small horizontal slots along the front edge which act as a continuous overflow path when the main outlet is blocked.

The standard gives us this formula to calculate the flow rate through the slots.

It depends on two things, the total slot area (A),  and the head of the water above the slots (h).

In practice you don't need to calculate slot area manually.

The manufacturer usually provides you the slot area per metre.

Then you will need the head of the water above the slot.

That head should be selected to allow for discharge of the overflow before the water level rises high enough to flow back toward the building.

So in short, the design intent here is to ensure that when the primary outlet blocks – the slot safely release water outward, not inward.

Now let's look into the third overflow method – the “front bead overflow”.

The concept here is to position the front bead of the gutter at least 10 millimetres below the top of the fissure.

By doing this, any overflow will naturally discharge over the front edge rather than back toward the building.

The standard gives this method a fixed overflow capacity of 1.5 litres per second per metre of gutter length.

So if you multiply 1.5 by the gutter length, you will get the overflow capacity Q.

Using this method – at 10 metres, the eaves gutter can discharge 15 litres per second.

However, it's important to know that this method can only be used if – the required overflow capacity is 1.5 litre per second per metre or less.

If the calculated requirement is higher, or the 10 millimetre low front cannot be achieved, you must use another overflow method (such as slotted gutter or back gap overflow).

The last option is designed by computational methods such as Computational Fluid Dynamics CFD.

This approach uses advanced modelling to simulate how water flows and behaves under the real conditions.

It's intended for use only by qualified engineers or expert hydraulic designers with expertise in flow dynamics and numerical modelling.

CFD can support complex or non-standard gutter designs. But results must still meet all the standard performance criteria.

And that concludes our session today for the updates of ASNZS 3500 Part 3 Stormwater Drainage.

If you have any further questions or require further information, you can contact us via email at plumbingtechnicaladvice@bpc.vic.gov.au.