Concrete fire hearth held up by luck?

[compact]

1930's house
1st floor fire places have 2ft x 1ft concrete hearths ( that when I took the ceilings down are not held up by anything but the sides.

Is this normal?
What would you advise?
For one room the tiles are shot, so tempted to knock it out and replace with floorboards/cover with a slate hearth.
The other is nice.

So what to do?

Hope zoomed in image shows that it is just self bracing against the sides of the joists.

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• 28 Oct 2012
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[maltaron]

You will probably find that it is cantilevered from the chimney breast.

 

[compact]

Hi
So I have been doing more excavating of the fireplaces

I'm not sure if there is any metal in it and if you look at the quality of the cement/rubble (around the hole, which is hanging free), it is crumbling away.

1)What do you suggest, leave as it and cement up the hole, lay flags and put wood burner on top (hopefully won't fall)
2) screw some metal straps underneath just in case
3) Knock it out and put some wood/joist the cement and tile on top
4)?

All advice welcome
Cheers

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• 24 Nov 2012
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• 24 Nov 2012
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• 24 Nov 2012
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[Burnerman]

Officially, the hearth on an upper floor would be supported by a brick 'trimmer arch' - the trimmer being the main large timber joist at the front of the existing concrete slab. This trimmer would support the existing common joists.
The brick arch would have been held up by a former from below until the cement had set.
The trimmer arch would support a thick concrete back hearth (under the original fire) and this, due to the curve of the arch would support the thinner front hearth towards the front.
In effect, you have a concrete cantilever, as mentioned by maltaron.
The back hearth should have projected 2/3rds into the original brickwork, if constructed to the specifications of the day.
In your case due to crumbling and age, I'd be supporting the underside (somehow!) by steel.
The weight of the original fire would have been carried by the thick back hearth - you should consider this if your stove is intended to be further forward, i.e onto the thinner front hearth.
John

 

[compact]

Thanks for the replies.
The second set of photos were for the ground floor fire place.
I think I'm going to knock out all of that hearth, fill the space with joists and wood frame. Then lay a new granite hearth on top.
I doubt the concrete would have fallen through, but better option whilst doing lots of work.

For the 1st floor, the hearth is tiled, so will look at some steel straps bolted to the joists to form some support, just in case (I won't be putting much weight on it).

 

[Burnerman]

For the first floor hearth, the concrete would have been cast with shuttering supporting it from the joists below.....the shuttering was normally left in place, and the ceiling joists increased in depth to allow one ceiling level.
Who knows what has happened in the past here!
On the ground floor, the hearth was usually concrete laid on rubble, or any other non flammable infill.
John

 

[compact]

The fireplace for the back room is concrete on top of rubble, ontop of soil.
But at the front, it's concrete, on rubble, on thin air and blind faith

 

[Burnerman]

Nowt new there then!
When you're sub floor, do see that there are sufficient air bricks (you can't have enough with an earth floor) and see that there is a good DPC on top of the sleeper walls, below the wall plate. (Battling with such an issue as we speak - joist moisture content over 40%)
Be lucky
John