Rebuilding subsided constructional hearth - 1900s terrace

[Ethoen]

Hi all,

I'm looking for advice on how to rebuild a constructional hearth.

Renovating a 1900s terraced house. The constructional hearth in the living room had collapsed and spilled under the joists in the suspended timber floor. The previous owners has covered over this collapse with a marble superimposed hearth, however seen as we are replacing the floor joists, I'd like to use this opportunity to replace the hearth, lay tiles at floor level (and eventually get a working period fire surround installed).

I've started to clear out the subsided hearth, but information online about how to rebuild it is limited. Any advice/guidance would be appreciated.

Thanks in advance

 

[blup]

Search constructional hearth in this link for the building regs guidance. You're new joists might be too close but difficult to tell from the pic.

The Merged Approved Documents (publishing.service.gov.uk)

Blup

 

[Ethoen]

Search constructional hearth in this link for the building regs guidance. You're new joists might be too close but difficult to

Blup

Thanks, pages 37 & 38 of doc J have been my guide so far. The spacing left between the joists is 600mm from the jamb and 1050mm wide. I'm assuming when the fireplace insert is installed, that will narrow the opening of the fireplace, so that the edge of the hearth will be more than 150mm from the edge of the appliance (assuming narrowing the fireplace in that way is allowed?)

The bit I'm a little unclear about is the composition of the hearth from ground up - hardcore to the top of the sleeper walls, then screed to floor level?
(If it helps, the sleeper walls are two courses of bricks, then 20mm bearers, then 120mm joists, then 20.5mm floorboards.)

 

[blup]

I'm no expert, the way I did mine was dpm, concrete blocks around the edge, filled with concrete, with allowances for the finished floor level.

Blup

 

[JohnD]

Old hearths tend to be damp because the rubble underneath is in contact with the earth. A DPM under your new fill and agajnst the damp brickwork will help, or you could leave it mostly empty with enough dwarf wall to support a slab.

Your pics suggest there is no ventilated void beneath the floor so it is likely to be damp. Has it been filled with builders rubble?

 

[Ethoen]

Has it been filled with builders rubble?

Yes, under the original joists was completely filled with builders rubble (plus blocked up air bricks) - I've cleared to 200mm+ under the joists now. The hearth is a mix of smashed bricks, earth and other rubble on top of a layer of slate.