Replacing floor boards under internal block wall

[SimonH2]

There seem to be a few related threads, but I just wanted to confirm that my plan is "sensible".
It's an ex-council house, we believe built in the late 40s. A previous owner has (badly) replaced the original T&G floor boards in the bathroom with sheet chipboard - or weetabix board as I prefer to call it. The dividing wall between bathroom and bedroom run parallel to, and between, the (concrete) joists. Since the weetabix is thinner than the original boards, this leaves a step in the bedroom floor - but that's by the way. While I was pulling up the tiles and putting carpet down in the bathroom, I did notice that it looked like there was a gap under the wall about the thickness of the difference in thickness between the old and new boards - if so then I don't know what's actually holding it up !
I am planning to put back T&G boards - TBH I expect to be replacing most of the floors since much has been badly replaced and I "quite dislike" the unnecessary impediment to ongoing maintenance caused by the large sheets. As I expect to be effectively fully rewiring and re-plumbing the house bit by bit over the years, I want the convenience that lifting a single (or two) normal floor boards brings.

Anyway, am I right in thinking that I will be OK replacing the boards in sections, packing a weak sand/cement mix underneath each section when I've got the boards in so that the wall is properly supported ? I know I'll need to take up more of the boarding in the bedroom to give me room to slide the new boards in, but that's OK.
Also, any tips on where to look for new boards that are closer to the thickness of the originals - modern T&G flooring always seems to be a bit thinner ?

 

[oldbutnotdead]

The wall is probably being supported by the outside masonry wall and the corner post- if it isn't structural then all it is carrying is its own weight.
Plan sounds good- I'd probably shim the wall rather than use very thin mix but there ya go
If you're looking for 25mm finished then be prepared for a nasty price shock. 20mm finished find a timber merchant (not DIY sheds).
If it is 25mm you need and you can only get 20mm then get your friendly timber merchant to rip some CLS or similar down to 5mm thick x 50mm wide and spike that to the joists before putting your boards down

 

[John D v2.0]

we have exactly the same and the floorboards have been cut out similarly, I panicked at first, but then I just shoved some plywood under from the joist on one side to the other just in case it drops for any reason. The archway effect keeps it all in place pretty well though, the outside wall is one side and the remains of the chimney on the other, so I don't expect it to drop much by itself.

 

[SimonH2]

In this case, one end of the wall is joined to the party wall - BUT there is a big opening from floor to ceiling at that end where the shower has been built (taking a corner out of the bedroom). At the other end, the block wall stops at the bedroom door, and above the door is filled with studwork. TBH I expected to find evidence of cracking above the door, but haven't found any yet - unlike our last house.
We've plenty of timber and builders merchants locally - I've even trade accounts with a couple of them.
AFAIK none of them have the facilities to do bespoke cutting - I've asked in the past (wanted some ply cutting down to 8'x2' sheets when I did the loft at the last house) - but I could do that myself. TBH I think I'd just accept whatever new height I get and accept that I'll have small steps in places until I get round to/have access for replacing some of the older boards (of which there isn't much left).

it is carrying more than it's own weight - it's intermediate support for the ceiling joists and hence loft floor. A PO has also boarded the whole loft without any strengthening/additional support, and above the main bedroom (not the one next to the bathroom) the ceiling has a visible sag of several inches as there's a 19' span there. Another of those larger jobs on the ever lengthening list

 

[John D v2.0]

Yep ours is carrying more weight too, tiles, rafters, the lot., the new roof has a post carrying a beam on the high flat part of the roof. That post rests on the other part of the wall that's between the old chimney and the landing ie only buttressed at one side and free standing at the other.
I was surprised the se was happy to rest the roof on what I considered to be an unsupported wall, but after triple checking he was completely fine with it! The only thing was the old chimney once removed had to be made good and a wind post to stop the wall crumpling sideways.

 

[SimonH2]

OK, this may seem a daft add-on question ...
Certainly for the bathroom I fancy engineered boards for their resistance to cupping. But I don't need a fancy (aka expensive) hardwood top layer - just ordinary redwood will do.
Anyone know a source of such boards ? Engineered, non decorative top layer, 7/8" (22mm) thick, T&G.
Apart from cost and availability, any reason to NOT use engineered boards ?