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DOWNPIPE AND EAVES GUTTER CALCULATOR

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

**Info:**

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. It does however have the ability to far exceed the range of the 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.

**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)

### Enter Details

Roof Catchment (Plan) Area (sq.m) (info)
Roof Slope (degrees) (info)
Rainfall:Either choose a Location
I prefer to enter a known intensity Adelaide Alice Springs Auckland Brisbane Cairns Canberra Christchurch Darwin Dunedin Gold Coast Gympie Hobart Invercargill Launceston Mackay Melbourne Napier Palmerston North Perth Sydney Sunshine Coast Toowoomba Townsville Wellington
or enter known intensity (mm/hr)

Tick if gutter slope is steeper than 1:500 (ie 1:200)

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.

Buy/Activate Now

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

Flow (L/s)
**You will require **__one__ of the following DP options :- (dimensions in mm)

(Assuming approximately equal catchment areas)

Number Req'd
Number Used
Gutter Width
**90 ** Dia:
**100 ** Dia:
**150 ** Dia:
**225 ** Dia:
**300 ** Dia:

Select DP

first.

**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.

**
EXTRA FEATURES
**

**DOWNPIPE CAPACITY **
In terms of Plan area of roof.

90 Dia
100 Dia
150 Dia
225 Dia
300 Dia
Max Catchment Area (sq.m)
Gutter Area (sq.mm)

**MORE DOWNPIPES ** calculate gutter sizes using any downpipe size

**Any circular DP dia**
Max C'ment Area*
Number Req'd
Number Used
Gutter Area
Gutter Width
Gutter Depth
Semi Circular Gutter Dia.
**Rectangular DP**
side1

side2
Max C'ment Area*
Number Req'd
Number Used
Gutter Area
Gutter Width
Gutter Depth
Semi Circular Gutter Dia

**WORKING BACK THE OTHER WAY **
By selecting desired gutter area first.

Gutter Area sq.mm.
Number of DP's req'd
Flow per DP (L/s)
Gutter Area for this flow
Theoretical DP size(mm)
DP size from Code(mm)
Actual DP size to be used. (this size will appear in the printout.
Please select a standard size that is greater than the Code size.)
**
**
More Info.

**FLAT ROOFS & SURFACES ** Find Outlet size for each outlet catchment area

Roof outlets can have vastly different capacities depending on the configuration. These are generic results, only intended to give you an idea of sizing. The manufacturers catalogue should generally be consulted. More Info.
(sizes in mm)

Formula used
Outlet Grate Diameter
Square Outlet Grate side
Graded Outlet Pipe size
Vertical DP size
**
**

**VALLEY GUTTERS **

sheet Width (mm)

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

**Referrals. **

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 Stormwater 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.

**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.

Buy/Activate Now

More Information and Instructions

Down Pipes to tanks, & pipe sizes etc.

Overflow precautions

More Storm Water Calculators

Plumbing Water Pipe Size Calculators

If all else fails there is still the ability to donate

**Info:**

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.

**Instructions: **

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.

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.

**Intensity: **

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)

The answers for**downpipe size ** are in mm for a circular downpipe.
If you require a rectangular DP, use trial and error on the "More Downpipes" function until you get the gutter size you require.

The option to select the downpipe size to be used, is only necessary if you require a printout of the results.

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 "theoretical" size. However to be strictly correct you should select a size that is equal to, or 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 5.4.10.1.

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 consistant 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

**Overflowing**

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.

**Conversions (Rounded)**

Degrees

Percent

1 in ?
mm

inch

sq.metres

sq. feet

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.

donations gratefully accepted

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. It does however have the ability to far exceed the range of the 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.

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

Unit Conversions.

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.

(Assuming approximately equal catchment areas)

Select DP

first.

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

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 that this DP will drain, for the Roof slope and Intensity entered above.

Roof outlets can have vastly different capacities depending on the configuration. These are generic results, only intended to give you an idea of sizing. The manufacturers catalogue should generally be consulted. More Info.

This calculation will give the total sheet width,

which is folded in accordance with the diagram.

Catchment area, Roof slope and intensity as per

the main program.

AS/NZS 3500 has the following limitations :-

which is folded in accordance with the diagram.

Catchment area, Roof slope and intensity as per

the main program.

AS/NZS 3500 has the following limitations :-

- Maximum catchment area 20 sqm. However

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)

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

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.

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.

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.

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 Stormwater 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

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.

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.

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.

More Information and Instructions

Down Pipes to tanks, & pipe sizes etc.

Overflow precautions

More Storm Water Calculators

Plumbing Water Pipe Size Calculators

If all else fails there is still the ability to donate

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.

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.

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.

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.

The

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

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.

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.

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.

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.

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.

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)

The answers for

The option to select the downpipe size to be used, is only necessary if you require a printout of the results.

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 "theoretical" size. However to be strictly correct you should select a size that is equal to, or greater than the Code size.

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 5.4.10.1.

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 consistant 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

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

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

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

The

The

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.

and Galvin HERE Galvin Rain water Outlets

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

There is the ability to use any intensity you want.

How to find the Intensity for other places.

More Information and Instructions

Down Pipes to tanks, & pipe sizes etc.

Storm Water Network Design

Overflow precautions

More Calculators

Plumbing Water Pipe Size Calculators

Down Pipes to tanks, & pipe sizes etc.

Storm Water Network Design

Overflow precautions

More Calculators

Plumbing Water Pipe Size Calculators

Enjoy,

Ken Sutherland

Ken Sutherland

Certified Practicing Engineer,

Bachelor of Technology

Member of the Institution of Engineers Australia.

Registered Professional Engineer Queensland.

contact me
Bachelor of Technology

Member of the Institution of Engineers Australia.

Registered Professional Engineer Queensland.

donations gratefully accepted