Sub-hearth renewal

[Robso]

Hi everyone.

I’m working on installing a 1.2 inch thick granite hearth in my Victorian‑era home in the UK (it has a suspended timber floor). The fireplace has a sub‑hearth already in place (see photo - sorry for bad quality, I added a plastic sheet to keep the wind out of the room in the cold weather).

I’ve removed the old tiles inside the firebox and cut out the floorboards where the granite hearth will sit.

I noticed that the back portion of the firebox sounded hollow, and after opening it up I found that it was basically about 2 inches of screed over mixed rubble and soil (see the larger hole in the photo). My plan is to excavate the area, compact the soil, then rebuild the base using crushed stone (hardcore/sub‑base), a layer of sand, then concrete, and finally a new screed layer to bring everything level before setting the granite. The section of the hearth that extends into the room is absolutely rock solid (took me and a buddy jumping up and down on with no problems), but the combination of the granite and stove I'm adding are 400lbs plus so I want to make sure the firebox is good and solid.

I know most folks here are in the US, and construction methods and terminology differ a bit between the UK and the States, but I’d really appreciate any advice or insight you might have.

Cheers.

Rob.

 

[begreen]

The main concern is that there be no combustible material underneath or supporting the hearth. As long as there is no framing material underneath or forms, then the plans should be ok. Normally, the hearth and hearth extension are cast as one unit so that there are no gaps of cracks. The hearth extension is typically tied into the hearth floor with rebar before pouring the cement.

Jumping up an down, while likely a funny sight, is not a proper test. There needs to be nothing combustible under this area and it should be mechanically tied to the hearth floor with no gaps possible.