House is 1930s  clay brick construction with suspended timber first floor.  Floor joists appear to be 3" deep, which are supported mid, and or third-span with a perpendicular 4" timber.  Orientations switches from left to right side of the house.

We would like to remove the chimney stack to ground level, freeing up some space (stack has been removed in the loft already), and hopefully remedying the damp issue I think is rising from it.   This would leave a fair amount of floor unsupported.  Am I looking at having to replace the entire floor structure?  Joists/ crosmembers and boards?  

 

CAD sketches attached, first floor removed apart from internal masonry (there's one 1 internal wall that's not mad of cardboard upstairs!)  the sketch is derived from various floorboard lifting and poking with a tape measure.

Ground floor ceilings are 2.5m, 2.4 to the underside of the crossmember, which is an exposed (yet badly painted) feature in both downstairs rooms.

Extension to the rear is shown for context, but isn't relevant to this issue.

You could do this with a steel support structure(s) that takes the loads the stack currently takes. That in itself will need supported but will let you proceed without taking your house apart.

 

Looking at the joists on the left hand side, I would imagine they could be caught with a steel running approximately where the stack sits, and if suitable, sit on the walls of the room that the stack is on - left hand side. Another steel or part of the same system will then need to pickup that bearer that keys into the stack on the right hand side. 

 

Go and find a friendly local structural engineer and set him to work.

 

I believe you will need to take up a fair bit of boarding but am sure with adding  joists lapping and bolted to the existing they could be extended. Yes a structural engineers input would be good (but I would do it myself as I have a history of building and have an understanding of loads etc) .

3" joists sound really weedy.  Is the floor bouncy or deflecting?

31 minutes ago, Mr Punter said:

3" joists sound really weedy.  Is the floor bouncy or deflecting?

there's a bit of bounce, but it's not terrible.  You can walk across the a room without furniture toppling over, but if you were to jump around, you may have to reposition a few ornaments afterwards.

I think the timber used back in the 30's was a bit tougher than the fast grown stuff of today- certainly of the stuff I've seen the grain is much denser.

2 hours ago, Crowbar hero said:

certainly of the stuff I've seen the grain is much denser.

Most definitely 👍.