Box Gutter and Sump with Vertical Overflow & Downpipe Calculator
To AS 3500.3.2:2003 "Stormwater Drainage Acceptable Solutions".



Roof Diagramatic

MINIMUM WIDTH = 300 mm for Commercial construction, and 200 mm for Domestic consruction.
Material Base Metal Thickness (mm) Max Length (metres) Minimum expansion space (mm) To use Rectangular DP (mm*mm) To find Sump sizes, enter Equivalent circular DP dia (mm)
Aluminum 0.90 12 50 100 * 75 97.5
Copper 0.60 9 50 100 * 100 112.5
Copper 0.80 15 50 150 * 100 137.5
Copper 1.0 26 50 125 * 125 140
Steel Colorbond Zincalume 0.55 20 50 150 * 150 168
Steel 0.75 25 50
Stainless Steel 0.55 20 50
PVC - 10 30
Zinc 0.80 10 50

Instructions & Notes:
As you can see there are four possible catchment areas that can contribute to the flow.
Enter the variables in the number of catchments that correspond to your roof, (you may have 1 to 4 catchments, leave unused catchments with zeros). It doesn't matter which side you call the left hand side (LHS) or the RHS, as long as the entered slope, area, and vertical face are grouped within their respective areas, and all areas are grouped as shown in the diagram.
The program will compute which wind direction will give the worst effect, when allowing for rain shadow.
If you're feeling lazy, or you have a large area for the upper, and a small area for the lower, or a flatish roof with no vertical faces, you may add the upper and lower areas together resulting in just a LHS upper and RHS upper.
The result of this will be a slight over design, as the slope effects, and vertical face effects, from both sides will be added, and nothing will be subtracted for the shadow effects. Equivalent to the rain coming from all directions at once.
If your roof slope is given in the form 1:?, use the blue boxes to convert to degrees before entering in the program.
Choose a location, or alternatively enter a known intensity for a 1 in 100 year rainfall, with a five minute time of concentration.
For New Zealand, you require a 1:50 year rainfall intensity, with a 10 minute time of concentration.
If your box gutter drains to one end with a rainwater head, click on the OTHER HYDRAULIC CALCULATORS link below.

The overflow is designed to carry the full box gutter flow in the event of a blocked down pipe. eg hail, snow, debris, etc.
As can be seen from the diagram, the overflow is really a vertical pipe, the same diameter as the down pipe, raised up through the bottom of the main sump. The entry to this downpipe is then flared out into a rectangular shape the width of the box gutter, and a minimum of 200 mm wide. Imagine a rectangular funnel on the top of the overflow down pipe.
The total depth of the box gutter is also governed by the height of the overflow.
The overflow weirs are raised above the bottom of the box gutter to prevent overflowing during normal operation.

Trial suggestions
Trial DP size is a typical size that can handle the flow.

Making adjustments
Increasing the width of the box gutter decreases the depth of flow, lowers clearance Ioc, and hence decreases the depth of the box gutter.
The down pipe size only controls the depth of the sump. A greater depth of water on the down pipe entrance results in more water being forced into the down pipe opening.
In other words, increasing the down pipe size results in a shallower sump, and decreasing the DP size results in a deeper sump.
The box gutter slope has no effect on the design as long as it is equal to, or steeper than 1:200.
In most cases, some of the variables are fixed by construction constraints or aesthetics. Therefore, the first step is to find out these constraints and adjust the other variables to suit.

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

The code only allows for flows up to 16 litres/sec total runoff in the Down pipe. It recommends that if this figure is exceeded, more downpipes should be added, or catchments reduced.
The graphs in the Code were confirmed by experimentation only up to a flow of 16 litres/sec, and for this reason do not go beyond this flow.
However my calculators are based on standard hydraulic formulas, weir, orifice, trajectories, spatially varied flow etc with the constants adjusted to give an answer consistant with the code. For this reason I believe that my formulas will stay true regardless of the flow.
However, just bear in mind that if you use flows above 16 litres/sec you are beyond the scope of the Code.

I would also be interested in any modifications, or suggestions that you would like incorporated.
If you need more info, or you would like other areas of Australia, or New Zealand, added to the list, or you require a precise design please

Ken Sutherland

DISCLAIMER: It is the users responsibility to ensure input data is calculated and entered correctly, and that the program is suitable for the roof configuration required.


Copyright 2005 Ken Sutherland