Dave wrote:

Can I run the toilet soil pipe between the joists to the SVP? what's the
minimum fall/slope? Is there any type of pipe that would make this easier?

You can, between 18 and 90mm / meter according to the building regs
document.

I'm going to sit the shower tray on the floor (bedded on marine ply and
mortar?) and run the waste between the joists too. Is there any reason
not to join-up with the bath waste before the SVP? Is there a shower

so long as there is not a large vertical displacement between them all
then no.

waste that can be cleaned from above? Is there any point using one of
these "vortex" shower wastes? What's the minimum slope?

You can get some there the grate and inner section of the waste can be
removed from above. This gives you some access to clean or retrieve
stuff from the trap.

I want to run the basin waste in a new stud wall but it's going to be a
real pain to drill the (big) holes in all the studs, the alternative is
to saw (big) notches once it's built. Is there a narrower pipe that
people use in these circumstances?

34mm is the minimum for a basin IIRC. Aside from the weakening of the
studs (which may not matter if the wall is only a partition), a hole saw
will do the job easy enough.

--
Cheers,

John.

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