Levelling hearth - what to use?

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wriggles

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Hi there,

I'm trying to fit an old cast iron fireplace in my Victorian house. So far, I have replaced the builder's arch with a concrete lintel (the floor level is apparently a few inches higher now than when the house was built, so I needed a few extra inches for the insert. I've also removed all rubble inside the chimney breast, including the original but now cracked quarry tiles that formed the base, so the back hearth is now down to bare (and slightly damp) earth.

See the attached pictures.

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The front (constructional) hearth is 2-3 cm below the level of the floorboards. The back hearth is approximately 8 cm below floor level.

I'm planning to fit a 30mm thick slate slab (in three pieces to prevent cracking) on top of the constructional hearth, extending slightly onto the floorboards on either side. Then I'll need to raise the level of the back hearth to sit flush with the slate. So probably 10cm in all at the back.

Here's where I'm stuck. There's a 2cm gap all around between the constructional hearth and the floorboards (which had been filled with a quite crumbly mortar that I've tried to remove), plus a large chunk missing on the right side. Given these gaps, what's going to be the best way to raise the constructional hearth by the required 2-3 cm to reach the floorboards?

For the back hearth, should I use concrete (4:2:1) or just sharp sand and cement (4:1) to raise the level to sit flush with the slate? This surface will need to be strong enough for the fireback and some of the cast iron insert to sit on, and I'd like it to be smooth enough to brush ash from. Will this be able to withstand the heat from the fire grate burning logs and perhaps coal just above it?

Thanks for any advice you can offer on this one.

Cheers,

Matt
 
Since the front section already has a crack in the middle why not just rip it out and put a one complete concrete construction hearth in complete with DPM. It can then be formed right up to your floorboards.
 
Or fill in the gaps around the hearth with mortar and set your decorative hearth inside the floorboards, which is how the tiled hearth would have been done originally.
 
I recently did the same.

Rip out the cracked hearth.

I dug down a brick course, cleared out all the rubble, tamp earth down, then add a layer of sand, then 1200 gauge DPM plastic, bring it up at the edges.
I used ballast (it's sharp sand and aggregate pre-mixed) just add cement and water. Fill DPM with concrete.
I stopped about an inch from the floorboard level, that way you can mix another concrete final layer or self levelling compound... or inset slate/marble hearth.

Just to say, months on, it's dry-as-a-bone!


PS, take note of where your damp proof course is... possibly bitchumen line in the mortar, try not to go above this, as it promotes a bridging point for damp to re-appear.
 
Mr Chibs, that's a really useful post, thank you. As it happens, 15 mins before I read this I gave the construction hearth a tug and managed to pull it up. So I'm down to bare earth there, too, and ready to fill it with a good layer of concrete. Good tip, too, to stop short of the top to leave room for a SLC.

Any idea how long I will need to leave the concrete before fitting the slate to it? I'm guessing the slab will be about 8-9cm deep.

Thanks again,

Matt
 
Oh - and is a SLC capable of withstanding the heat of a fireplace, especially in the back hearth, underneath the grate?
 
hmmm.... are you having fire grate or woodburner?

I would say yes, as it's just another form of cement type slurry.
I'd be inclined to make the hearth deeper for open fire. From memory mine is around 300mm, but this is a bit overkill I suspect.

I'm sure someone will tell you what the guidelines are.

Presume you are having a hearth on top of the SLC?

PS, if it's open fire, they are notoriously un-efficient, have you tried a fireback... metal back plate... I think the idea is it project more heat forward.
 
The adventure continues.

Having dug out the infill of soil and broken brick, I've discovered the joists around the hearth are rotten, probably because the soil level was too high and transferring moisture, as well as blocking any possible ventilation from the air bricks. 1 foot down and I still haven't found solid ground yet. I am wondering whether I need to build a fender wall to frame the hearth and to keep any soil or rubble infill away from the joists (I'm still not sure whether I want to replace the rotten joists, or to just forget I ever noticed them).
 
A lot depends on the extent of the rot. You may be able to build up some support a little further back if it is the ends that have gone. Support in the ideal world would be something like bricks with the top one a blue brick, but there is nothing to stop you using some concrete blocks some dpc and brick or slate to get new support. As for a wall to frame the hearth you could simply make up the level with bricks or blocks laid dry and put in a DPM before the concrete hearth it is unlikely to go anywhere!

If you need to replace a section of joist you can usually get additional support in on ground floors to support the joint between old and new
 
Here we go:

1. The most rotten joist is running along the left side of the hole as we look at it, to the right also rotten. These joists are probably only three floorboard widths long , judging by the nailing. There's also a joist running perpendicular to the other joists, along the front edge of the hearth pit. That's also quite soft at the bottom. The depth is currently around 1 foot, but I can see through a hole in the earth that there's probably at least another foot or two of earth before the bottom.

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2. This is the left side joist. There's a brick there that I guess the joist end might have been resting on. The end is completely rotten and already half disintegrated.

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3. This is the right side joist. 4" deep.

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4. View from inside the chimney breast looking out. I noticed what might be a sleeper wall (the fender wall?) running underneath this joist.

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Thoughts at this point:

- It wouldn't be a bad thing if I needed to replace the two (tail?) joists and move them further apart to create wider constructional hearth - that way, I wouldn't need to run my slate hearth over the floorboards as it could be contained entirely on top of the new concrete slab.
- It seems I'll need to find a way to provide a base for the concrete slab, but without that base (soil, sand?) touching the joists. I'm not sure what the best way to do this will be.
- Do I need to dig all the way to hard ground (which still seems a couple of feet away) before building any support walls?
- I have only two days left at home before going away with work for a long period, so if I'm to open another can of worms, I'd prefer it to be a smallest one possible!

Thanks a lot for your advice, this is a great community here.

All best,

Matt
 
Hi Wriggles, I am facing a similar problem in my Victorian terrace, with living and dining room fireplaces. Would you mind letting me know what course of action you took, and how it turned out? Thanks :-)
 
Jamhan,
the thread is 2017, two years old.

start a new thread, and say what's existing and what you propose to do?
posting pics to show whats existing always helps.
read the Related Threads on the bottom of this page, and on the search feature.
 
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