First floor structure issues.

Right the answer to your question is that the joists should normally be at 400 mm centres on a normal built house and they would probably be 9x2 joists but if the span is over a certain length and if you asked building control at your local council they would specify the maximum length for the size of your joists and once the span is to long for 2 inch joists you would have to use a 3 inch wide joist the joists should over lap the wall by 4 inches from both rooms and also they should be along side the joists from the other room and the wall below should be beam filled to the top of the joists for fire protection and also to stop the joists from warping and then the flooring above can be screwed down upto the stud wall above what would be fixed solid to the joists I hope that you can understand my answer thanks

Needs sorting due to not being fire safe and the stud wall upstairs doesnt sound the safest because majority of people would use 100ml or bigger screws and screw them straight into the floor joists and brick work should follow upto the timbers to make sure there is no gap for fire to spread

beam with block or brick infill needed between the joists for fire protection and for wall up stair to be fixed solid will have to be screwed down solid to make a good solid stud walls 150mm heavey duty screws straight into the floor joist