Alternative to the Plumbing Code 'Worst Case' method
The Plumbing Code allows for the worst case. If you have no bends in the gutter, or the DP is not at the end,
this method will produce significant savings.
The formulas are based on this document: Roof Drainage by K G Martin Division of Building Research, CSIRO.

Outlet Location (from center) along gutter:
Bend Location from outlet:

The program requires a small fee to make it work.

Payment can be made with Paypal, or any credit card via PayPal as a guest.

There are no special downloads or passwords or login required.
Also the program can be activated on more than one computer in your office.
On activation you will receive an email advising on how to do this.
The time remaining is also depicted unobtrusively in the top left hand corner.

As this program is outside of the Plumbing code, you are encouraged to activate and obtain a printout of the calculations which may, or may not, help with certification.

Important Note
The program calculates the Theoretical Downpipe size.
However this is rarely an available size, so the program selects the next available larger size.
But, sometimes this larger DP size will not fit in the required "theoretical" gutter size, this means that the gutter needs to be increased as well.
Also as per AS/NZS 3500.3 the effective gutter area is taken from below a line 10 mm below the overflow level.
Therefore the program has added this 10mm to the Approx gutter height.
The Plumbing Code allows for the worst case. If you don't have a bend close to a downpipe, or your downpipe is not at the end of the gutter, this method will produce significant cost savings. Therefore it is very suitable for long straight gutter runs as in Industrial buildings.

All entry boxes have the same requirements as the main calculator. The intensity can also be found there for various locations, and advice on how to find intensity if the location is not shown.
There is also a conversion table for slopes and units.

The approximate width and height results are suggested sizes that give a rectangular gutter with the width = twice the depth. This is shown only to give some idea of the gutter size, that relates to the required cross sectional area.
It is not necessary to have this exact size, but it is necessary to have the required Gutter Area or larger.
It is also best to have the width equal to approximately twice the depth, as this is the most hydraulically efficient shape.

Downpipe Dia is the same size as required under the plumbing code.

Gutter Area (Rounded Sole)
From the CSIRO document a gutter with a rounded sole is taken as an ogee, half round, or quad style gutter.
A completely rectangular gutter has the area increased by 10%.

By the way, if you enter a downpipe at the end of a gutter, and a bend close to the Downpipe, (this is the worst case), you will get an answer very close to the Plumbing Code when using a curved sole gutter with slope greater than 1:500. Check it out.
e.g Using a flow of 3L/s slope>1:500
  • Code gutter Area = 9781 sq.mm
  • CSIRO gutter Area = 9850 sq.mm
Put a bend further than 3.6m from the DP, the resulting gutter area is 8416 sq.mm. a reduction of 14%.
Put the Downpipe in the centre of the gutter, the resulting gutter area is 6327 sq.mm, a reduction of 35%
This is because the gutter flow on each side of the DP is now effectively half the total catchment. So why design the gutter for the total catchment, when in reality it is accepting only half.

A typical Industrial Building Roof Catchments:-
Typical Industrial Building Roof Catchment For a rectangular building there are usually only two different scenarios. If the DP's are aranged as shown in the diagram, there are the DP's on the ends of the gutters, and the downpipes in the center of the gutters.

The down pipes on the ends are half the catchment area of the others.
This means that each gutter length has the same flow, and therefore can be made the same size.
However the down pipes in the centre of the gutters take twice the flow and could be larger.

So we only need two designs. One design for Down pipes 1 and 4, and one design for DP's 2 and 3.

Obviously there are cases when further designs may be necessary. But usually the grid lines are equally spaced, and the downpipes are on the grid lines. Meaning the majority of the catchment areas are equal.

A slope flatter than 1:500 will give a gutter size larger than the Code. So no need to go there.

The formulas are based on this document Roof Drainage by K G Martin Division of Building Research, CSIRO.

The program is outside the scope of the plumbing Code AS/NZS3500, however the new National Construction code (which overrides the Plumbing code) allows alternative Performance Solutions to be used. Refer to Clause AO.6, which states

A0.6 Assessment Methods The following Assessment Methods, or any combination of them, can be used to determine that a Performance Solution or a Deemed-to-Satisfy Solution complies with the Performance Requirements, as appropriate:
(a) Evidence to support that the use of a material or product, form of construction or design meets a Performance Requirement or Deemed-to-Satisfy Provision as described in A2.2.
(b) Verification Methods such as—
(i) the Verification Methods in the NCC; or
(ii) such other Verification Methods as the authority having jurisdiction accepts for determining compliance with the Performance Requirements.
(c) Expert Judgment.
(d) Comparison with the Deemed-to-Satisfy Provisions.

Also from Clause DV1:

Compliance with DP1.1 to DP1.4 (which is the performance requirements of roof drainage) is verified either—
(a) by calculation and certification by persons or organisations with recognised credentials in the design or testing of stormwater drainage systems; or
(b) by satisfying the required criteria when tested in accordance with a specified test method endorsed by a recognised certification body.

We are relying on the fact that the CSIRO can be classed either as expert judgement, or an Organisation with recognised credentials.

Ken Sutherland
CPEng BTech MIEAust.

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