bathroom tiling - replace damaged floorboards or whole floor

[AlF2008]

Quick question

We are going to be doing our bathroom in the very near future and we need to replace at least 3 of the original tongue and groove boards under the bath due to a long term leak (rotted away). The plan is to tile the bathroom so I was going to get some marine ply for going over the existing boards and I just thought it might be better to remove all the original boards and then put down a new floor of thick ply and hence solve both problems?

Is this possible - i.e. would it be strong enough? Any advice on the thickness to use etc?

If not then whats the best way?

 

[Symptoms]

Alf - take all the old stuff up and replace with exterior grade ply (WBP) either 18mm or 22mm or 25mm thick - the thicker the better so as to stop flex/movement which WILL result in the tiles lifting/cracking. No need to use marine ply. Support all joins between ply sheets above joists and fit noggings (structural timbers between joists) below any other joins - this prevents flexing between ply sheets resulting in a straight line tile crack running the length of the join across the room. Use screws to fix the ply.

What thickness ply? Small room, small joist spans, joists close together use 18mm minimum. Big room, big joist spans, joists wide apart use 25mm; also consider level issues with adjoining floors and how you make the interface between the 2 ... step or slope. 22mm IMO is a good choice (also available in T&G but not really critical to use T&G)

 

[AlF2008]

thanks a lot for that. very helpful

The bathroom is approx 10' x 8' and from memory the joists are about 16" apart.

What size do the noggins have to be? I think the joists are 6" x 2" so do I need to use the same or would some 3" x 2" be sufficient?