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Normal minimum drain diameters are:
100mm for maximum 10 dwellings and
150mm for more than 10 residential units.
Older drainage systems (Victorian or thereabouts) are most likely to comprise salt-glazed clayware pipes of 100mm, 150mm or 215mm nominal bore diameters (i.e. 4 inch, 6 inch or 9 inch).
In many respects these pipes have...