Do I need rubble in fill behind a new fireplace??

[Reginalddean]

I am about to fit a new cast iron fire place (solid fuel), its the sort that has a built in fire basket behind.

I have opened up the original builders gap (approx 36" square and 15" deep). The man in the shop suggested I fill the gap between the new fire basket and the builders gap with rubble.

Does anyone know why I need to do that? What would happen If I just put the new fire place in and left a void behind?

Cheers

 

[stuart45]

You need to fill in behind the new fireback or soot will fall down behind it.
You can build some brickwork behind to the shape of the fireback and leave a 3 inch gap. This can be filled with vermiculite and then form a smooth slope at the top with lime mortar. Alternatively you can just use rubble behind like most of us used to do. Vermiculite is better because it allows for heat expansion. It also has good insulation properties at high temperatures and so helps radiate heat into the room.

 

[Reginalddean]

Thanks for the response, can I ask,

What is vermiculite, can I buy this in B&Q

also,

Can you buy lime mortar in ready mixed bags?

My only other concern is how I would create a slope as the new fire has a build in dampner that opens towards the back of the chimney, Im imagining trying to slope it down is going to impossible as there is a tiny gap?

Thanks

 

[gday2uk]

I have just completed same job.
Vermiculite wasn't available from b&q where I am, had to get it from a proper merchant.
Just stack some bricks up in a horseshoe shape behind fire up to level of bottom of dampener, leave a 2-3" gap between bricks and fireback. Fix fireback in preferred way (I used strips of masterboard all stuck together and fixed with plumba flue sealant). Loose fill gap behind fireback with the Vermiculite level with the bricks and then mix Vermiculite 5 parts to cement 1 part add a bit of water and create a slope from about 4" above the bricks down to the back of the dampener so any soot will roll into the grate (in theory).
I also used the Vermiculite concrete mix to fill under my granite hearth to keep the heat out.

 

[Reginalddean]

Thanks for that,

Is Vermiculite a dry mix or some sort of aggregate?, so I simply pour down the gap and over time it will set hard?

Apologies for my ignorance!

 

[Lower]

You can buy vermiculite from the plumb centre. Fill opening with rubble or bricks leaving a gap between the back of the fire and the rubble and then fill the gap with the vermiculite. Don't make the mistake that i made of expecting to fill the gap with vermiculite alone. You'd need loads and its a pain in the arse then trying to get rubble into the vermiculite to increase its volume!

There is no easy way to get the cement in to form the slope. Its just a case of building up the slope with sand, rubble, vermiculite etc and then covering it with a layer of cement that you trowel in through the exhaust hole in the fireplace.

The fire damper should be removable for access.

Thanks for the response, can I ask,

What is vermiculite, can I buy this in B&Q

also,

Can you buy lime mortar in ready mixed bags?

My only other concern is how I would create a slope as the new fire has a build in dampner that opens towards the back of the chimney, Im imagining trying to slope it down is going to impossible as there is a tiny gap?

Thanks

 

[Lower]

Vermiculite is a type of aggregate. Its very light and comes looking either like Wotsits or cat litter depending on the type you buy.

Thanks for that,

Is Vermiculite a dry mix or some sort of aggregate?, so I simply pour down the gap and over time it will set hard?

Apologies for my ignorance!

 

[gday2uk]

Vermiculite is a dry mix.
You need to mix it with cement and water (like aggregate) to form a lightweight concrete.

In its dry form it can be uesd as loft insulation, fireback insulation, ragworm insulation

When mixed 5:1 with cement it forms a lightweight concrete perfect for forming the slope behind a fireback.

See here - www.dupre-vermiculite.co.uk/dupreprodmicafil.htm

 

[stuart45]

Reginalddean,
Remove the damper to form a 45 degree slope. Sometimes it was formed flat with the fireback to make a smoke ledge. This was said to help prevent down draughts, but i have never used it.
You can buy a bag of lime from most builders merchants. You can also use lime mortar if you build up the brickwork at the back. Even if you fill in behind with rubble its best to add some lime mortar in to stabalise it. Rubble can settle and form voids. Also when hot it may expand where its settled and put pressure on the fire back. As yours is cast iron I doubt if this would be a problem, but with an ordinary one it can crack it.
I have known insulation quilt to be used instead of vermiculite.

 

[Reginalddean]

Many thanks for all your help, I now feel confident to fit the thing myself.

One last question!

When I removed the horrible fireplace that was there it was a seventies brick thing with a gas fire. I removed all the bricks but for some reason they seem lighter than ordinary bricks, they almost seem internal bricks?

Can I reuse these bricks to build the horse shoe or are they not suitable?

Cheers

 

[stuart45]

The bricks should be OK for that job.

 

[Reginalddean]

Many thanks for all your help, fingers crossed!!!

 

[cottoncockerel1971]

Hope you don't mind me asking but would I need to back fill behind a Gallery Crown Cast Iron Insert - we are fitting a class 1 gas inset to sit inside so it is more for decorative feature and will not be used for solid fuel.

We have a full chimney the house was built in 1903 and have just taken out an emberglow outset in order to replace it with the crown.

We would like to fit the surrounds ourself to save money and get the gas safe engineer to fit the fire.

Would be grateful for any help or recommendations, Thank you for your time.
x

 

[jeds]

Depends how hot it will get? The purpose of backfilling is that without it cast iron will burn through very quickly. The backfilling dissipates heat and prevents that happening.

 

[stuart45]

I would backfill to prevent any soot falling in to the void, possibly igniting. It's not a big job. If using rubble, stabalise it with some lime mortar.