Building sub floor wall

[sjee6]

Hi, does anyone know anything about the different methods of building a sub floor wall?

The first picture is from my own house (1920s terrace). I've knocked it down and intend to build another one preferably like the one in the second picture but I can't figure out what the top layer is. I guess you need that layer to stabilize the wall - I don't know anything about building but I have to fix up this room myself because everyone keeps quoting me unbelievably rip off prices -

- that's the link to the video in case it's not clear.

Also, I destroyed the hearth for the fireplace as it was sitting on a bed of sand which was getting saturated and bridging the DPC, so I scrapped everything and my plan is to cover up the fireplace using plasterboard, only I don't know if I should leave it open at the bottom when I install the joists and floorboards as the chimney stack has been removed and I've read that unused chimneys need to be vented to prevent hydroscopic salts forming. So I'd appreciate any ideas about what I should do about that. Also, what would be the best way to install the joists? Up to the wall with joist hangers or should I build a wall at the edge of the breast and sit them on that?

Thanks.

 

[freddiemercurystwin]

It's a timber wall plate eg:

 

[sjee6]

would the dpc be a layer of slate?

 

[freddiemercurystwin]

normally a plastic DPM nowadays.