When plumbing a shower waste what is the required incline of the 40mm pipe to reach the soil pipe which is 2.5 m away?

I have been told 18mm per 1000mm.

My plumber says it needs to be 100mm per 1000mm and is using this as a reason not to plumb through joists.

Is there a standards document I can share with him please?

Clairol

Hi Clairol,

First I had to convert from metric to the standard we use in my area. The standard slope for drainage is 1/4"(6.35mm) to the foot,(304.8mm). In tight situations the plumber may drop the slope to 1/8",(3.175mm to the foot.
Your plumber wishes to slope the job at 100mm,(4") every 1000mm,.(40") which works out to 1" every 10". That's way too much fall.
Even 18mm,(3/4") to every 1000mm,(40") is too much. Not enough fall and the job won't drain. Too much fall and the liquid drains so fast the solids drop are are left behind to build up and clog the system.
Hope this helps, Tom