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When replacing floor joists, is it important to ensure all timbers are the same grade? Obviously if the load requirements call for c24 then using c16 would be foolish, but is there any harm in putting c24 in the mix if the other joists are c16? I'm just wondering whether the increased stiffness would somehow focus load onto the supporting walls in a detrimental way...

Fwiw, this is in service of a bathroom refit with a pretty bad woodworm problem. Most of the joists can simply be treated, but two or three are too far gone. The joists are 47x175x4200mm w/spans between 260mm and 380mm.

Hi founditlikethat,
Given that you have woodworm in the joists it is unlikely that they are metric sized joists. They are probably the old 7”x2” imperial size. The nearest metric size joists are 170x47mm and you would need to use C24 given the span you describe but this would only meet regulation for a lightly loaded floor. If the replacement joists are going under the bath I would suggest you use 63x170 or even 75x170 especially if you are going to fit a cast iron bath. In any case you will have to fit packing pieces to make up the difference in depth of the joists.
Regards S