Suspended Wooden Floor Rebuild Design

[Ayms]

I am currently renovating my front downstairs room and need to rebuild the floor. Its a 1930s end of terrace, solid wall construction. 

 

The joists were originally supported on three sleeper walls with an air gap between them and the main walls of the house. This design did a pretty good job at preventing too much damage to the woodwork from damp - a couple of the joists had woodworm damage but nothing too terrible. Unfortunately the sleeper walls were falling apart, so I’ve removed them and need to now build a new floor. 

 

The old joists were 100 x 50mm. I am planning to replace them with C24 200 x 50mm so that I don’t need a support in the middle- they need to span 3.4m (so I realise 170 x 50mm would also be fine, I just want the extra solidity). I am planning for the joists to be spaced at 400mm centres - the room is 4m wide so there will be 10 of them.

 

So I think my options are:

 

1) Rebuild the sleeper walls. I had assumed I would do this, but don’t really want to try it myself as I’ve never built a wall before and I don’t really want to start with one that has to be accurate to within a couple of mm to make sure it joins seemlessly to the hall floor.  Also there is some kind of aggregate floor base in the whole room, which the original walls were just build on top of. It is solid and would be difficult to remove for foundations, but its not at all flat. The new joists need to be supported 250-300mm off the ground if the joists will be resting on them directly, so only about 3 courses of brick. I have spoken to some builders and all of them have said that sleeper walls are old fashioned and they don’t think I should do this option, and don’t want to do it for me. 

 

2) Use a wall plate and joist hangers. This is the preferred solution of the builders I have spoken to, but I am worried about damp. The wall plate would need to be fixed to the external wall of the house, below DPC. The wall plate would also cover most of the ventilation (there are 2 ventilation grates in this room - see pic for wall plate position). 

 

I have more or less ruled out replacing with a solid floor as its quite a deep void (500mm) and it seems like a lot of material, plus I’d have to think about impacts on ventilation to the back of the house and the hallway, and damp. 

 

Really appreciate any advice- is the sleeper wall option really as bad as the builders seem to think? Is there a way to do their preferred wall plate/joist hanger option without causing damp and ventilation issues? Is there another good option I haven’t thought of yet?