concrete over corrugated iron floor

Hi Guys, hoping someone can help.

Currentley renovating my bathroom (50's frame house) and am relocating the bathtub. my problem is: after originally retiling over the space where the bath was the tiles cracked and on further inspection the flooring in that area is a sheet of corrugated tin nailed between two joists with concrete poured over the top with no joists running underneath. of course as soon as i walked over it a few times the tiles all cracked because there was a lot of movement in the floor.

In this area i ve removed the tin and am planning on adding in some new joists and laying compressed cement sheet over the area.

The real issue is this: the area that currently has the shower(it will be an empty walking space soon) is constructed in the same manner ie corrugated iron with concrete poured over, with the difference being that there are additional steel joists running between the girders to support the corrugated iron sheet. will this area need to be pulled up and wooden joists put in or will the steel "cross joists" be okay to stop movement and cause tiles to crack?


Regards,

Adam

PS, many appologies if (and i probably have) mixed up all the terms, i think i might get the term joist and girder muddled


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