Cavity behind constructional hearth unsure how to bridge/fill

[Sprocker]

Hi all
I recently removed my old fireplace and fire back and got to my builders opening ready for my stove fitter to come In and install a multi fuel stove for me, but once i removed everything and got to the opening I have got the constructional hearth but then there is the cavity behind it that goes straight down under the sub floor, it would seem the flue runs through the middlle of the cavity walls and the stove fitter hasn't seen this before and is unsure how to get around it (he is hetas registered) I was thinking of bridging the gap with a piece of slate then screeding but I'm a bit concerned about bridging the dpc and getting damp! Especially as the dpc runs right through the chimney breast and is currently higher than the constructional hearth Also the slate would only be resting on the lip of the dpc on the outer leaf wall so not ideal

Any advice greatly appreciated
Thanks

 

[blup]

Not clear about the flue and the cavity, but I dug mine out (the constructional hearth) and rebuilt it with the dpm lapping the dpc in the wall.

Blup

 

[Tigercubrider]

I don't know either but maybe the intention was to use the chimney as a device to draw damp from under the floor and upwards?
Presumably there was a fireback in there at one point?

I'd be tempted to lay a larger piece of roof slate over it so that the majority of the weight is on the house side and chuck concrete onto it. The concrete will form a self supporting slab once dry with the excess being a sort of cantilevered bit

 

[Sprocker]

Hi all sorry for the delayed reply, I was thinking of using roof slate then screeding over but I'm a little bit concerned about bridging the dpc? What's you guys opinions?
Thanks

 

[conny]

DPM over the base and lapped up to meet/overlap the original DPM. You could then screed over with no fear of bridging.
Are you going to skim the walls or have the bare brick on show as a feature?

 

[Sprocker]

Thanks Donny
I'm a bit unsure what you mean by lapping up the the dpm, also I was considering leaving exposed brick but that does depend on how the brickwork cleans up
Thanks