I get a lot of queries from people who have suffered some sort of property damage due to water getting into their house. There is usually the possibility of a court case, and they are trying to determine if the roof drainage has been designed in accordance with the Plumbing Code, building Code, and any other applicable code.
If you are a designer, you do not want to be in this situation.

Fail Safe Design
Fail safe design is the key.
It is no good just designing the roof system to the plumbing code, walking away, and hoping for the best. There are a lot of things that can go wrong.
Horribly wrong.
I always say that a roof gutter system will always work perfectly.... until it rains.
Now, what if the next door neighbour's kids kick a ball on to your roof? And the ball just happens to be the exact same diameter of the downpipe.
And it goes down and gets stuck. This is not going to be good, or pretty.
What about hail? leaves? etc.
This is before we get to all the problems that can be associated with the construction.
The bottom line is: If it all goes horribly wrong, and all downpipes get completely blocked, where does the water go?
And how does the water go?

Thinking about where the water goes in this situation is imperative.
How does the water get out of the gutter? is it over the edge? or through a dedicated overflow? Can that overflow get blocked as well?
When it does get out of the gutter, where does it go then?
Not much good if it hits the ground and runs into the lounge room.
Sort of counter productive.
Also not much good if it hits the ground, causes enormous erosion, undermines the footings, and the whole house tips to one side.
That could be embarrassing for someone. The property owner would probably not be impressed, and you would probably end up on sixty minutes.
A similar case to this actually happened by the way, the building cracked, everyone got sued, the lawyers claimed it was due to wet ground causing some sort of subsidence, (and it was probably true). But still not pretty for everyone on the design team.

So what can we do?

Eaves gutters
If all the downpipes are blocked, the water can either go over the front edge, the back edge, through spitter slots, or out the end, if there is no end.
If a DP was at the end of the gutter run, the gutter can extend beyond a little and have no "stop end". So if the DP becomes blocked the water just keeps going. The only problem with this is, if the gutter is under designed, or the storm is greater than the 20 year return period, it is still going to go over the edges.
So this is normally only good for very large gutters.
When allowing water to flow out the end (or through spitters) we provide scour protection wherever the water is likely to fall. Concrete slab, rocks, etc. and then make sure the water will run away from the house with a suitable overland flow path. I have provided a means of calculating sizes for this. Overland flow path

Various methods of providing overflows for eaves gutters have been depicted in the Plumbing Code AS/NZS3500.3, and the National Construction code (NCC). I have provided a calculator for some of these methods HERE.

High Fronted Eaves gutters
Ever noticed that almost all the standard eaves gutters on the market have the front higher than the back? So that when things go pear shaped, the water goes over the back of the gutter.
Why do they make these gutters? Well the answer is, its what the market wants. People do not like the look of jagged roof edges, with bits of sarking, and insulation showing and poking out. A high front on the gutter hides all this, and also looks a lot neater.
OK so we are stuck with that one.
So lets look at how we can let the water go over the back of the gutter and not cause any damage.
By the way, in the past, roofs always had eaves. These eaves were lined with fibro, or nowadays FC sheet. This lining was always lower that the inside ceiling, and also lower than the outside wall. So if water got into this space, no one noticed, as major damage usually would not occur. (except for mildew, and maybe rotting after 10 years) Nowadays there are houses with no eaves, and water getting into this space could get into the ceiling or the walls.
The code has a solution for this, and that is to set the gutter off the fascia, so that there is a slot for the water to go, between the gutter and the fascia.
Also for this to work as intended, the back of the gutter should be below the top of the fascia.
I have provided a calculator for this HERE.

Slotted gutters
The Code classes the effective cross sectional area as being from the bottom of the gutter to 10mm below the slots.
So check if this is the 'effective' area quoted in the data.
Also, if slotted gutters are to be the only overflow arrangement, the Manufacturter should provide documentation that the overflow through the slots per metre run of gutter, equals the inflow to the gutter per metre.

Design for a larger storm, say 1 in 100
We could do this, but the Downpipes can still get blocked by balls, hail, snow, and dead birds that have randomly dropped out of the sky, and the whole thing can still overflow in a heavy dew. So what's the point of spending all that extra money. So either way, we come back to IT MUST BE FAIL SAFE
If there is no way to make it fail safe, then we should design for the 1:100 year event.
The theory is, if we are in a 1 in 100 year storm, it would probably be a cyclone, and the gutters would be the least of our worries.

Box gutters
Box gutters under the code come with complete overflow designs. The most fail safe are the ones with no ends.
Any box gutter relying on a pipe or a slot for overflow can get blocked. So in this situation, consider a larger than required opening, especially if the roof is likely to catch leaves or balls.
In high risk situations, consider an overflow at both ends of the gutter (or more than one).

Spitters and overflows
Consider where these discharge the water, and whether they can get blocked as well. If discharging to ground, ensure that the water runs away from the building, and has an overland flow path to somewhere else.
I have provided a means of calculating sizes for this. Overland flow path

Also if not discharging onto rock, erosion may be likely, consider piping or rock protection, or discharging onto a concrete slab.
Also not good to have a spitter or overflow discharging over an entry door; but the Architect will normally pick this one up.

Extra Design Protection
It is always good practice to have a copy of the design on your files. If something goes wrong, everyone on the design and construction teams will be in the firing line. For a few dollars you can get yourself a copy of the design (with all the plumbing Code references) from this site. This design can then be verified by the relevant experts, and hopefully the blame will be deflected to the next poor sod.
Other Hydraulic Sizing Calculators
Copyright 2017 Ken Sutherland