waiting for calculations

Step 1 - Enter roof and spreader details

Enter Details

Note: Catchment areas relate to the plan area. Not the sloping area.

Diagrammatic of a rainwater spreader to a lower roof

Step2 - Choose a roof profile

Profile image Profile name Manufacturer
Stratco CGI Corrugated 16.png CGI Corrugated 16 Stratco Stratco1
Stratco Maximus 22.png Maximus 22 Stratco Stratco2
Stratco Maximus 33.png Maximus 33 Stratco Stratco3
Stratco Superdek.png Superdek Stratco Stratco4
Stratco Prodek.png Prodek Stratco Stratco5
Stratco Topdek.png Topdek Stratco Stratco6
Stratco Spacedek.png Spacedek Stratco Stratco7
Stratco Smartspan.png Smartspan Stratco Stratco8
Profile image Profile name Manufacturer
Stramit Corrugated.png Corrugated Stramit Stramit1
Stramit Monoclad Monoclad Stramit Stramit2
Stramit Longspan Longspan Stramit Stramit3
Stramit Capacity Plus Capacity Plus Stramit Stramit4
Stramit Speed Deck Ultra Speed Deck Ultra Stramit Stramit5
Stramit Speed Deck 500 Speed Deck 500 Stramit Stramit6
Stramit MegaClad Megaclad Stramit Stramit7
Profile image Profile name Manufacturer
Lysaght CustomOrb CustomOrb Lysaght Lysaght1
Lysaght Custom Orb Accent21 roof sheeting.png Custom Orb Accent21 Lysaght Lysaght2
Lysaght Custom Orb Accent35 roof sheeting.png Custom Orb Accent35 Lysaght Lysaght3
Lysaght Integrity820 roof sheeting Integrity820 Lysaght Lysaght4
Lysaght Kliplok406 roof sheeting Kliplok406 Lysaght Lysaght5
Lysaght Kliplok700 HiStrength roof sheeting Kliplok700 HiStrength Lysaght Lysaght6
Lysaght Kliplok Classic700 roof sheeting Kliplok Classic700 Lysaght Lysaght7
Lysaght Longline305 roof sheeting Longline305 Lysaght Lysaght8
Lysaght spandek roof sheeting spandek Lysaght Lysaght9
Lysaght Trimdek roof sheeting Trimdek Lysaght Lysaght10

Step 3 - Show the results


Lower Roof

How does this program work?

The Manufacturers produce catalogues of all their available roof sheeting profiles.
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 entered roof parameters.
Then calculates the actual flow on that sheet.
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.

How to spread to 2 sheets 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.

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.

Sheet Length of the lower roof
This is taken as the longest sheet length from the ridge line 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 existing flow in the sheet.

Therefore the longest sheet gives the worst/max case of existing flow.
Meaning the user can then put the spreader anywhere on the lower roof and be safe.

For instance, if you drop a spreader 1/2 way down the existing sheet, from a side roof, this will not change the existing flow in the lower sheet. (Unless there is no rainfall on the upper 1/2 of this sheet).

Sarking Under AS/NZS 3500.3 Sarking is required for a distance of 1800mm either side of the point of discharge of the spreader. The sarking is to be carried down to the eaves gutter.

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.
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 2024