hearth collapsing

[Nawen]

I think that's what the bit in front of my fire is called?

We moved in a few months ago (1920s house) and one of the first things (of many) we noticed is that the hearth of the fire is sinking in the middle, the tiles are cracked.


So I lifted the lose tiles today to find cracked, sunken concrete underneath, removing the loose bits I found... cracked sinking concrete!

Photos attached, looks like it began to sink and someone poured in more 'crete to fix it and that sunk and broke too.

I can feel a strong draught through the largest crack in the concrete.

I'm assuming under this (joisted floor) there should be some sort of base that the concrete was originally poured into, and I'm assuming this has failed, maybe someone has better a understanding of the construction and could advise? Essentially the fireplace will be covered and vented this year so I wont be replacing the hearth as such, but a suitable foundation for carpet will be needed here.

 

[Nawen]

Some better pictures now I have hovered the dust and removed the small debris.

 

[oldbutnotdead]

Is this downstairs or upstairs? Upstairs there's usually some timber to support the hearth, downstairs its usual for ground to support it.
Whichever, break out the failed concrete, check the void for damp or woodworm or ground heave or subsidence (something caused the concrete to fail) and post pics of what you find
If you're carpeting, plywood will be fine as a replacement (thickness depends on floorboard thickness, 25mm wld be ideal, 18mm might be a bit floppy without some framing across the gaps)

 

[bobasd]

read the Related Threads below and search for other related threads or, fwiw, follow my posts on hearths - how they are constructed, and how they can go wrong?
check around the chimney breast for any signs of damp.

why not stand back and pic the whole chimney breast?

remove the piece of ply.
you will have to lift all the green hearth tiles and expose the sub-strata then pic what you've revealed.
be very careful lifting the tiles - if you have a multi-tool then cut the grout lines before attempting to gently prise each tile up. those tiles are possibly original.

 

[Nawen]

Thanks for the replies both

It's a downstairs fireplace.

I tried to get the tiles up without damaging them but many of them had hairline cracks which caused them to fail under the slightest movement, I saved a few, I'll see if any salvage places nearby want them.

After removing the first layer of what looks like self levelling concrete, I uncovered more concrete with larger agrigate in it, still a fairly modern looking concrete for a 1920s house.

After going down about 5-6 inches and panicking thinking the chimney and house were going to collapse if I went any deeper, I found a very sandy mortar-style mix under everything that had a little (almost unnoticable) damp, weirdly the damp hadnt penetrated the concrete at all and the dust on top of this mix was bone dry. The fire hasnt been used in months so it's not being held back by that, the damp wasn't evident when I looked under the floor joists (which are on their way out... yay.)

Ends of joists were bone dry too, but beginning to crumble, I assume this means they have at some point been wet?

In the end I in-filled the hole with concrete back up to just below the top of the joists, I'll use a self levelling compound to finish it before using ply to the thickness of the floor boards, I'm not too worried about making a permanent job of it now I know I'll need to rip it back up in the months to come in order to replace those joists.

Thanks to both of you for your help and advice

 

[John D v2.0]

Ours was exactly the same although not sinking as badly, and the nearby joists were damp from the wet piole of rubble and water dripping down the chimney.
My solution was just dig the rubble out completely down to the oversite, put a new treated joist across, and put a new plywood floor over the whole thing.