Bathroom floor re-build - levelling and sub-floor material?

[Sensors]

I've just ripped out the 1st floor bathroom in my 1920's terraced house in the UK which invovled replacing a floor joist that had rotten at the end. It sits in the brick wall at either end so I've been able to level that and get it to the height of the neighbouring joist by packing slate underneath the ends. The new bathroom will have a large shower, toilet, and sink with tiles floor and walls (see image below). I intend to install most of the bathroom myself, but hire a tiler.

My next step is to re-floor the room since the old floorboards need to go. For info, the joists are 6x2", ~400mm centres, and 2m span.
* What material should I use for the new flooring?
* Can I just put 22mm structural ply directly onto the exposed joists and depend on that to be a good enough surface to tile onto?
* Do I need to add additional structural support?


What I have also realised when I got the level onto the rest of the floor is that the joists have a bit of a slope.some are level along the length, but have slope aross the way, and some slope along their length down towards the door (see image below, arrow end is lowest point).
* Is this going to be a problem for installing the bathroom/tiling the floor?

 

[Deleted member 174758]

My next step is to re-floor the room since the old floorboards need to go. For info, the joists are 6x2", ~400mm centres, and 2m span

If your house in 1920s, then surely your joist centres are 16in or c.406mm, not 400mm? This is potentially a significant difference.

* What material should I use for the new flooring?

You sort of answered that in your next question. WBP structural hardwood ply from 19mm upwards will be fine

* Can I just put 22mm structural ply directly onto the exposed joists and depend on that to be a good enough surface to tile onto?

That partly depends on what the joists are like. Have you put a 6ft level down them and across them to see how level they are? Or better yet, can you beg, borrow or otherwise obtain a small self-levelling laser and a find a piece of straight 2 x 1in softwood lath about 4ft or 1220mm long to check them? Ideally, if you intend to tile you ideally want a flat, level surface to work off, or failing that at least a flat surface, so if your joists are all over the place you might want to consider taking out at least some of the discrepancies by sistering errant joists with something like 3 x 2in or 4 x 2in CLS whilst you have the chance.

* Do I need to add additional structural support?

No. Even 19mm ply would be sufficient unless you happen to want a cast iron his and hers bath in there at a future date

What I have also realised when I got the level onto the rest of the floor is that the joists have a bit of a slope some are level along the length, but have slope across the way, and some slope along their length down towards the door (see image below, arrow end is the lowest point).
* Is this going to be a problem for installing the bathroom/tiling the floor?

Here's the rub - if the floor currently slopes downwards towards the door and you were to correct the slope (by building up the joists or sub-floor at the door) you would end up with a step at the door opening. This means you'll need to trim the door, and that you'll have a trip risk to contend with. If you additionally lay cement fibre board on top of the plywood (always a good idea for a tiled bathroom floor), as well as the tiles and adhesive, that in itself will add a further 20 to 25mm to the floor height (assuming 12mm cement fibre, 10mm tile and 3mm adhesive). So just how bad is the floor? If it slopes but is flat, I'd seriously consider leaving it as is unless the slope is really bad. If there is a minor dip it may be worth flattening the dip by sistering the offending joists, but not necessarily levelling them

The point about joist spacing and plywood is that plywood sheets almost universally come in 2440 x 1220mm (or 8 x 4ft, aka 96 x 48in, in old money) sizes - which happen to fit onto 16in centre joists exactly. If the joists are on 400mm centres, you'll find yourself having to lop 40mm off the end of each sheet to ensure that the plywood sheets join end to end over a joist and are therefore fully supported.