Bathroom floor

it will be a trip hazard whatever you do I feel, unless you want to remove the boards and start from scratch again. Brian

Punch the nails below the surface then use a small block plane to flatten the raised portions of the boards, I did this when fitting oak flooring over floorboards (also replaced a couple of boards that someone had lifted/butchered).

replying to DerbyBorn, Iggy wrote: Yep, you can sand them quite flat or Plane them like Andy said. Presuming you'd be removing any cabinetry and the toilet for the new floor and therefore keeping the same height to the user, then you'd really just need a new (thicker or 2nd layer) door threshold. Yes, the new threshold would, possibly, need some getting used to, but there won't be any secondary floor lip that will catch most anyone.

Our builders simply stapled thin ply over the 9" floor boards here. 26 years ago so perhaps 0.25" prolly 5.5mm now. I didn't notice them doing any serious preparation work.

Replacing the boards with moisture resistant mdf would give you the same level. I wouldn't take a chance with ceramic because, however much you reinforce the floor, there may still be slight movement in the joists.

If you are going to tile you'll need a flat and stable surface. Replacing t he floorboards with 18mm WBP ply would be prudent but you'd still need to e nsure the foor is properly supported so there will be no movement which can promote cracked grout or worst case tiles.

Also, don't underestimate how cold ceramic tiles will be if you are walking in there with bare feet. I fitted electic UFH and with the insulation, the heating wires embedded into SLC, the tiles and adhesive there was a 20mm s tep up into the bathroom.

What sort of floor covering do you intend using? Makes a difference to how flat the original needs to be.

A 10mm step is OK if you put in a 3-4" threshold and chamfer it so there's no actual step to trip on. I've got several (in 3 cases it goes up nearer 20mm).