Ensuring a solid sub floor

[uluru]

We're just in the process of ripping out our bathroom and starting from scratch.

Currently there is vinyl on top of ply on top of the original floorboards.

The ply will be coming up so that we can access the plumbing to reroute some of it.

We then need to make a decision about whether to tile or replace with vinyl.

I know from other DIY works that the joists under the bathroom floor are not as deep as elsewhere in the house and I presume this has an impact on whether tiles are a sensible idea or not.

What do I need to be looking for to ensure that we're able to create a solid enough subfloor for tiling? I know that marine ply goes on top, but is the spacing/size of joists underneath important?

Thanks in advance

 

[AdamCH]

Presumably the joists are thinner because the span is shorter? Test is really whether they flex.

 

[uluru]

Yes, span is only 2.1m ish.