"Christian McArdle" wrote in message et...
Given that my new timbers (I hesitate to call them joists, cos they
aren't!) are not a structural part of the house as such, I was going
to use much less substantial wood for them than the real joists;

You should calculate the dead load of your proposed timbers and floorboards
and ensure that this loading does not make the existing joists too small.
What is the span and dimensions of the existing joists?

Hmm, that aspect hadn't occurred to me!

The overall span is about 12 feet; the raised area where the timbers
would sit is about half that. The existing joists are varied in size,
but are all at least 5.5 x 2.5 inches.

As regards loading; there is a stud partition down the middle of the
span, ie at right angles, and the non-raised area will need to bear a
bath and shower.

What d'you reckon?!

Thanks
David