Is this strange Logic?


Cyclone Yasi
Cyclone Yasi

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.


Cyclone Tacy
Cyclone Tracy

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?


On second thought, yea probably, these days.


Dungog flood
Dungog Flood
rainfall event
Dungog Rainfall event


  1. Steve Opper says:

    Hi Ken,
    just a short note of thanks for your amazing website. I first found you about 2yrs ago when thinking about a box gutter roof design on an extension I am building on my home in Wollongong. Given that this area was the site of the 1986 Wongawilly Storm rainfall event, I have since decided that this is NOT a great location for a box gutter. I stuck to a conventional pitched roof with external gutters. Your website has been a great help in understanding the NCC tables and charts. I just read your explanation about the 2:1 slope factor in surface area calcs. Knowing that rainfall depth in IFD tables is based on plan area but NCC considers ridge to gutter distance, this was something which had me puzzled. BTW, I used to head up the flood planning area for the NSW SES until retiring a couple of years ago, so rainfall and floods still fascinate me.
    Kind Regards, Steve Opper

Leave a Reply

Your email address will not be published.