Joints on hearth slabs cracking.

[Soggy Bottom]

I know this is an in the garden forum but this is the hearth in my living room. I put it here as it's still essentially paving slabs & the joints on them so figured it may be the best spot to ask this?

Basically our living room suffers terrible damp. Those that installed the fire installed these black limestone paving slabs and from what i remember they put bonding in the mix which popped the front right flag up.
They came out, re-did it with a sand and cement mix, no bonding this time. They also laid down a DPM. I can't remember whether it was wide DPC or whether it was visqueen. I've a funny feeling it was a fairly wide DPC. Obviously this was to stop the damp getting through since ours is quite bad.



Now we've needed the fire out because of a separate ongoing issue but you can see there the joints have gone.


Is there a way to stop that happening again or due to it being damp underneath, even with DPC strips there, and then the heat from the fire, it's just going to happen again & again & there's nothing you can do about it?

 

[Ian H]

It looks a bit shallow to me.

Could you raise it so there is a good thick layer of concrete under it?

Or use 1 big slab?

 

[conny]

Is the floor below the slabs concrete or wooden suspended?

 

[Soggy Bottom]

1 big slab is something i've been thinking of. It would surely be have to cut to size though but it's something i've thought of. Not sure of material though. We went for black Indian limestone as we liked the look of it.

I've just been & measured the bed and it seems to be 20mm in most places, 15mm in some. No idea if that's not thick enough or what.

Regards the floor below the slabs - the living room is generally suspended timber floor however the hearth is on a stack of bricks which go in to the foundations. It's not actual brick under the flags. There must be some kind of material between the bricks & the flags but not too sure what it is.

House was built early 1930s if that helps any with what it may be.

 

[Munroast]

A full size slab would probably crack with the heat of the fire (esp if you use coal). That is why hearths are usually laid in two or three pieces, (our marble hearth is a 4 piece)

As for the pointing coming loose - have they used a proper 'fire cement putty' ? if they skimped and just used regular cement then that fails pretty quickly.

My guess is the damp has little to do with it .

 

[Mr Chibs]

Some of the hearth could be on the suspended wooden floor, if this is rock steady all well and good.
If there's a little spring/movement and mortar has been used, then you'll get cracking in the pointing.

There could be a damp issue, as well, but probably not what's caused the problem you have.

Presume you have a woodburner? Does it throw a lot of heat down to the hearth, so do some don't.
I've one piece slate hearth 30mm and it's fine, but heavy, so less likely to suffer from flex/movement, especially when the weight of the burner is on it.

 

[Soggy Bottom]

I've one piece slate hearth 30mm and it's fine

That's exactly what's going down. The paving slabs have been taken out.




I knocked out the old cracked cement and it had this layer of sand like stuff underneath in the middle.

Regards fire cement putty, i don't think so. I'm pretty sure they sat it on a bed of sand & cement. I didn't think you'd sit paving on putty? I thought that was for things like around your windows? It's all over my head anyway.


Yes the wood burner does kick out a fair bit of heat on to the paving slabs. After it's been going for a while & you touch them it'll burn your hand that's for sure.

I don't think the hearth was on the suspended floor. Perhaps a little of the bed may have been but i didn't think it was. Difficult to tell, but there is flexion in the flooring, or at least there was in the old flooring and i don't imagine this would be any different.

 

[Soggy Bottom]

I just went looking for a brickwork forum but i guess this is probably the most suitable part? So may as well ask this here...



I don't know what that chalky like coating is on the bricks. I'm guessing possibly what was left over from the dabs when we had boards attached to the walls. I can't get it off at any rate.

We'll be leaving the wall exposed. I know you're not going to get a perfect looking brick as it's had stuff applied on top of it over time and it's an original 1930s wall but i was wondering if it'd be possible to get the chalky looking stuff off the sides? If so then how?