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I'm installing a new foul water drain fed by one shower/toilet/sink for occasional use. The guide lines I've read for this application either say 1:40 or 'minimum 1:40' slope on the drain. I know that solids can get left behind if too steep but nowhere can I find a figure for the maximum slope. The word 'minimum' that is so often used suggests that it can be steeper. Going steeper would be a benefit as it is in steeply sloping ground. Can anyone advise a suitable maximum slope?