The Manufacturers produce catalogues of all their available roof sheeting profiles.
Buried in there somewhere is a water carrying capacity table.
This gives the maximum length that a roof sheet can have for a certain slope and rainfall intensity.
From this data we can work out the maximum flow in Litres/sec using a standard hydraulic formula.
Some profiles have wide, deep pans and can carry a lot of water.
And others, for instance all the corrugated types, can not carry much water at all.
And of course the steeper the roof slope, the more water can be carried.
This program calculates the maximum capacity of each individual roof sheet given the Manufacturers data.
Then calculates the actual flow on that sheet from the users's entered data.
Can you see where this is going?
Subtract one from the other and we have the excess water carrying capacity (if any).
Surprisingly, it is possible to select a profile that won't work for the existing design flow.
If you see a negative available flow capacity, then you may have fallen into this trap, and it's time to select a bigger profile.
So we now know how many litres/sec we can put from our spreader on to each roof sheet.
From this, the program calculates the maximum catchment area of the upper roof that delivers this flow. Here is an example calculation
Doing it on a per sheet basis, ensures that there is no water overflowing the sheeting overlaps.
Supposedly this is a good thing.
So when building the spreader, try to keep the discharge holes within about the centre 400mm of each roof sheet.
If one sheet is not enough, try discharging into two sheets with one spreader as shown in the picture.
This will allow you double the upper roof area given in this program. Two sheets will give twice the capacity.
If still not enough, try a larger lower roof prolile, or add more spreaders.
This will be a better solution as it help spread the load in the lower gutter.
Sheet Length of the lower roof
This is taken as the longest sheet length from the ridge line, or top of the sheet to the gutter. Measured on the slope.
The overall sheet capacity does not change with a change of sheet length, as this is related to the cross sect area,
the rainfall, and the roof slope.
Changing the length only changes the catchment area of the sheet, and hence the existing flow in the sheet.
Not the total flow capacity of the sheet.
Therefore the longest sheet gives the worst/max case of existing flow.
This is the length the user should enter in the above form.
If you drop a spreader 1/2 way down the existing sheet, from a side roof,
this will not change the existing flow, or the maximum capacity in the lower sheet. (Unless there is no rainfall on the upper 1/2 of this sheet).
Sarking
Clause 3.4.5(b) from AS/NZS 3500.3-2021 states:-"For a corrugated metal roof in Australia,
a minimum width of 1800mm on either side of the point of discharge shall be sealed
for the full length of side laps."
The sarking is to be carried down to the eaves gutter.
Overflow arrangements
The Australian Standard AS/NZS 3500.3 requires eaves gutters (and box and valley gutters) to be designed for an
overflow of 1%AEP. That is a 1 in 100 year storm. No mention of sheeting
However HB 39 clause 7.6.(d)(i)requires roof sheeting to be designed for 1% AEP.
Therefore you must check the upper and lower gutter for this overflow from the total
catchment in each gutter.
However if you want to lay straight in bed at night, it might be a good idea to run a 1% AEP flow through this program
and see what happens.
You can get the 1% AEP value from any of the box gutter programs or directly from the BOM site.
The Next Step
Use the downpipe program to design the DP and gutter sizes,
Don't forget to add both contributing roof areas to design the lower gutter, and the overflow provisions.
Users Responsibility
As it is up to the User to enter the correct data in the right places, you may wish to satisfy yourself
that your answers are in the ball park.
Here is how to do it.
It is the users responsibility to ensure the program is suitable for the purpose intended and that all data is entered correctly
as required by the program and the associated instructions.
Copyright Ken Sutherland 2025
Sheet Length of the lower roof
The Australian Standard AS/NZS 3500.3 requires eaves gutters (and box and valley gutters) to be designed for an
overflow of 1%AEP. That is a 1 in 100 year storm. No mention of sheeting