1920's chimney breast, suspended floor hearth construction

[Thepenetrator1]

Hi, I'm renovating my house and decided to replace the floor joists as they were bouncing. They were also rotten in my other room. I'm going to insulate in between them all and relay new floorboards. There was a lot of debris (sand, rubble etc) below the floor with I have cleared out. All the chimney breast base brickwork was loose so needs rebuilding. There was a solid ash box just sitting on sand/mud which I have removed and cleared out. I need to build up the height of the fire place internal /hearth to ffl. Is it just a case of shuttering off,putting a membrane underneath and up the sides and filling with concrete or will this cause damp to rise between the membrane and the brickwork coming out at the top. Would it be better to build it up out of blues leaving a gap all round and cementing in place a 2" concreteslab on the top? I intend to put a log burner in place once finished.Thanks

 

[vinn]

You first need measurements for the wood burner opening and the back and front hearths.
Check the fire opening against whatever dimensions the manufacturer says you will need.
Draw a rough section view, the top of the blues is your approx. FFL.

Then use a membrane over the hearth area soil and up the recess past the blues. You can trim it just below the FFL.
Use an offcut of the plastic sheeting you perhaps intend for the soil vapour barrier.
Build up the footprint of the front and back hearths with any handy, dry bricks: roughly lay them off/above the membrane. Finish off with a semi-dry mix of concrete or even a cut concrete flag.
Dont use soil as a filler.
Stop building up the sub-hearth just below the blues to leave room for a top hearth and whatever finish materials you've selected to go on top.