Hi guys, I was calculating the height for the waste that serves 4 vaniety units in 4 bedrooms. I see that the minimum fall is 20mm pre m or 1 in 40 (according to marley)

I was just wondering what in practice do you guys see as the minimum fall one could get away with. Total run is 13m so I want to keep it as low as possible. Am I pushing it with a 1/50?

No, but you're pushing it with 40mm.

Have a look in approved document H.

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ok just looked at it, I read it as 18mm per m

These vaniety units would never be used at once, people only use them to brush their teeth etc.

Problem is that you brush you teeth and turn the tap off shortly after so there is little water flow to push the crap along the pipe to the outside.