Hearth foundation crumbling - how to rebuild?

[FergusMacdonald]

The foundation under the hearth seems to be (in this order from bottom up):
1. Earth
2. Large sandstone blocks
3. Loose rubble
4. Concrete/rubble mixed
5. Cement
6. Granite hearth

We want to tile the hearth so I'm leaning towards removing it all and rebuilding. I had thought about supporting it under the cement with a joist but perhaps simpler to rebuild it.

Wondering what's the best way to rebuild it and end up with a level hearth?

 

[Deluks]

Fairly standard. When the house was built they just throw all the rubble in the ground and concrete on top. You could have tiled straight into the concrete without disturbing it, but here you are, so build it back up.
Put some well compacted hardcore, a few bags of type 1 MOT (available in small packs) then concrete the last few inches.
If you want it level, use a level.

Unlikely, but I take it you haven't gone through any damp membrane?

 

[JohnD]

How old is the house?

Lime mortar?

 

[FergusMacdonald]

Unfortunately I had to remove some rotten joists so had to disturb part of it and the centre of it just crumbles if you touch it.

No hadn't seen any damp proof membrane anywhere on the house. It's circa 1890.

I've got about 500-600mm to come up so would I need something to contain the MOT? Was thinking to build it up from the sandstone in blocks?

 

[JohnD]

Yes, you can build dwarf walls under the floor for your slab to sit on. If the slab sits on the walls ( no rubble infill) there will be less damp transmitted. You can put in DPC. Hearths and chimneybreasts are common sources of damp in old houses because they have no damp proof layer and it soaks up from the base. It was not a problem in the old days when the fire was lit every day, which dried it out.

Water can evaporate off exposed brickwork like a honeycombed dwarf wall and will not usually rise far in a well ventilated void.

If you want rubble infill, include a DPM all round it.

You can take the opportunity to clean out the void and its airbricks.

 

[FergusMacdonald]

Great thank you. Any idea if I should use cement or lime mortar or does it not matter?

 

[JohnD]

Cement.

It is below ground and liable to be damp. Cement strengthens better and is not weakened by damp.