Levelling floor over metal+wood



I am trying to fix a shower room floor which has been left in a poor state by previous tradesmen. The floor is constructed of structural steel beams plus wooden joists on 400mm centres. This has been overlaid with 18mm WBP ply. The problem is that the metal beams finish higher than the wooden joists, which means that the overlaid ply is uneven, especially at the joints between boards.

My preliminary plan is to cut the ply back so that it only covers the wooden joists, this will leave a surface which is reasonably level and with no obvious bounce.

I would then need to fill the gap where the metal beam is, finishing at the same level as the top of the ply. The gap would be 230mm wide, just over 2 metres long and between 8mm and 18mm deep. Any ideas?

I don't think a levelling compound would work because of the 10mm-ish gaps around the beam and the requirement to finish at the same level as the ply.

My first thought is to use a 6mm sheet product like ply or No More Ply, but I'm not sure how to bond it to the metal underneath or how to deal with the varying depth I would have to fill.

I'm planning to finish the floor with a vinyl product like Karndean or Polyflor.

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