deflections

I am looking for some advice on suitable bathroom subfloors on joists. I have had a look on the forum, and am struggling to find an answer. Currently the bathroom subfloor is some kind of 18mm chipboard screwed onto the joists (400mm centres). Previously carpet was laid on top of this so...

G

The proposed method is to use clear span secondary joists to support two layers of 12.5mm plasterboard with batts between the joists. I am trying to optimise joist spacing and depth. Joists to be support on hangers off new wall-plate under the existing sandtex-overpainted high-relief coving...