On 02/06/2019 22:15, GB wrote:
Is that sketch accurate? Not much overlap where the joists are joined.
or is there a support underneath?

.........I believe it maybe a support wall underneath, it is not to scale.

A very large tray with the drain in the middle is not a great idea. They
can snap in the middle, because that is the thinnest point and that's
also the most stressed point.

Mira do a 1400 x 800 with the waste at one end. Too long for you?

............I will check that out.

I thought you were going to raise everything up a bit, so does it matter
where the joist is?

...No, I wanted low profile and the plumber has had a look and can
accommodate the waste a bit lower down in the stack. I think he is going
to add an additional piece in to the waste stack but a bit lower than
the current one.