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Hello @John-Annie
Thank you for sharing your question about your stormwater pipes. The 90mm and 100mm are the common sizes of downpipes in most houses. If you are adding another one to your existing set, I suggest using the same size already installed in your house. The 100mm is more often used in underground drainage. But, if the current pipes installed in your house are the 100mm setting, then I suggest going with that size instead.
However, before you attempt to modify your drain system, I recommend checking with your local council on the rules and regulations regarding adding and linking a new downpipe directly to your existing drain system. Some council regulations will sometimes specify that a registered plumber install the direct link to the drain system.
Would you be able to tell us a bit more about what your plans are? Are you replacing an existing set or are you repairing one? Will you be adding more downpipes and will you be connecting directly to the drain system in your home?
Any other information you can share about your project would be very much appreciated.
If you have any other questions we can help with, please let us know.
Eric
