Hi there, welcome to my blog.
I am a Certified Practicing Engineer with an embarrassing amount of ( years ) experience.
For more details check out the "About Me" link at the top of this page or for some great free calculators check out the web site.

When designing box gutters to the Australian Plumbing Code AS/NZS 3500 we design them for a once in 100 year storm event.

Now, box gutters have always had a bad reputation for causing damage. But with the advent of AS/NZS 3500 giving a concise method of designing box gutters, there should be no damage for all storms up to at least a 1 in 100 year event. (tested in the lab BTW)

Now the strange logic part. What if a hurricane or a cyclone, etc is taken as a 1 in 100 year storm event.

Our box gutter may be the least of our worries.

However just to protect one’s backside, in these days of litigation etc. we should make sure our box gutters are designed for a once in 100 year storm. (Besides which it is mandatory under the code)

Just in case we get a wind free cyclone.

Alternatively, I hear you ask, what about the recent floods in Dungog? Surely that was a 1 in 100 year event?

They got a lot of rain there. But not much wind.

So, when your house is floating down the river, are you going to sue your Hydraulic Consultant if your box gutter springs a leak?

But for those of us who like to delve into things, and work out how things were derived. I will attempt to offer an explanation.

The crucial thing to understand is:-

Rainfall measurements are taken in inches or millimeters falling on a horizontal surface. The angle of the rain is not important. All that matters is the quantity of rain over a given area.

So when thinking about this, we need to calculate the area on a horizontal plane, where the rainfall would have fallen, if the roof wasn’t there. The roof intersects this amount of rainfall.

However to do this, somewhere along the line, someone has to dream up at what angle the rain is falling.

Fortunately for us, the powers that be have come up with an angle of 2:1 as shown in the diagram.

Just like anything to do with rainfall, there is no standard rainfall event. All we can do is base the design on averages, and figures pulled out of the air. For instance we design eaves gutters on a rainfall event that may, or may not, occur once in every 20 years. And a rainfall angle of 2:1 is as good as any, and in fact, as you will see later, this makes the calculations much easier.

Looking at the diagram, a roof from A to D also intersects the same amount of rain as the main roof.

In fact any roof between rainfall lines B and C, will intersect the same amount of rain, and therefore have the same catchment area.

But what is really interesting, it doesn’t matter what the roof does to get from point A to point D. It can go up and down. or round and round. As long as the starting point is A, and the ending point is D, it will have the same catchment area.

Now, to determine what the real catchment area is, we must determine the area of the slope effect that must be added.

For a straight roof the Architect has normally shown this slope on the drawings. But if there are vertical drops, or different slopes we take the average as shown in the diagram. because this will intersect the same amount of rain.

Now the hard part, we have to do some mathematics.

We know the rain falls at an angle of 2:1, therefore in the diagram above, the length of the “slope effect added”, is half the “vertical rise” ( 2:1 remember). This is also true for the roof areas, that is, the area of the slope effect is half the area of the vertical rise, as both these lengths are multiplied by the same roof width to find the area.

So all we have to do now is find the area of the vertical rise. If you can remember your trigonometry, the vertical rise area = (roof plan area) * tan( roof slope).

Catchment area (CA) = roof plan area + 1/2 (vertical rise area)

= roof plan area + 1/2 *( roof plan area * tan (roof slope)

Ah, on second thoughts, its probably just as easy to look up the “slope factor” in the Plumbing Code, or simply measure the area from the Architects Elevations.

When designing roof gutters, or surface drainage for building sites, The rain shadow can make a big difference. Especially for a tall building.

The Plumbing Codes assume rain is coming down at an angle of 2:1.

That is 2 units vertical to 1 unit horizontal. (63.4 degrees).

So from the diagram on the left, you can see the effect of the shadow.

The area of the shadow is half the area of the vertical face of the building. (2:1remember).

But what if rain comes from the other direction?

From the diagram on the right, you can see that half the vertical face of the building has been added to the catchment area.

The catchment area is always measured on the horizontal plane. Because that’s how they measure rainfall when they talk about ‘mm’ or ‘inches’ of rain. So to make all our hydraulic formulas work, we must also use this method if we want to use rainfall figures calculated by the local Weather Bureau.

Now consider the interesting case where we have a building on both sides. One side has the shadow, and the other has the added catchment. If the buildings were of equal height, the effects would cancel each other out. No matter which direction the rain came from. Even if the rain came down the middle.

Got your head around that one yet?

However if the rain came from all directions at once, we are in diabolical trouble, and our roof gutter design, or site storm water design would be the least of our worries.