Fireplace constructional hearth

[Robson77]

Hello.

I'm fitting a stove into an existing fireplace. 30mm granite T shaped hearth in the fireplace and room over constructional hearth.

The previous owners did not use the fireplace, and tiled the fireplace hearth area, while boarding over the room constructional hearth.

I've cut out the floorboards above the constructional hearth and all is good (it's solid concrete and rock solid under foot). On pulling up the tiles and mortar in the fireplace, the front 60% or so felt very solid, but the back 40% sounded really hollow, like a void is present.

To investigate I pulled up the back left corner (approx 25 x 15 x 20) and it was loose (some mortar, hardcore, some sand, some dirt - not a lot of concrete from the look of it). See picture (I can get a better one if needed).

I'm not happy that it will support the weight so I plan to pull up the whole fireplace element of the hearth and fill the whole space properly.

Could anyone suggest best approaches (process, materials, dpm requirement etc)? Cheers.

 

[Wayners]

No dpm under fireplace because of heat. A fire will melt it.
I've only ever seen them concreted or soild stone dropped in

 

[Robson77]

Thanks for your reply. DPM would be below 125 mm of concrete constructional hearth + 30 mm granite decorative hearth + mortar with a stove temp at decorative hearth of less than 100oC. I've read else where that DPM is required with hearth built into suspended timber floor (Victorian). I saw this decade old thread which gave some limited direction: https://www.diynot.com/diy/threads/constructional-hearth-construction-of.383668/

 

[Wayners]

Ok.
Interesting...
Follow that advice then.

 

[Deluks]

Is it actually damp down there?
Against a party wall or an external wall?
You'll likely have no problems without dpm, you could just screed on top of what's there.