On 2013-08-29, Java Jive wrote:

On Thu, 29 Aug 2013 12:15:18 +0100, Adam Funk
wrote:

No particular comment to make about the feeds.

The two waste pipes currently meet at a T above the kitchen floor; the
merged pipe goes under the floor then runs under the kitchen and
through the wall, where it opens over a drain. I haven't been under
the kitchen floor in a long time, & I can't remember whether there's a
trap in there.

There are at least two primary purposes to a trap ...
1) To keep out drain smells
2) To trap objects that may cause a blockage
... but a third is to prevent waste from one source adversely
affecting the others.

I would say that you need a trap for each individual item whether it
be sink/basin, or one or other machine.

I think I'll go for that, although #2 isn't relevant he the
existing pipe runs out the wall and opens over a grate outside that
goes to a 4" pipe.