Help for my fireplace

[garob]

I want to install a heavy cast iron log burner in an old fireplace.
I have had to excavate where this burner will stand as it was very poor base. I have dug down to a about 10 inches to a soil base and the adjoining walls are all in stone & brick.
I will be using a lime mortar(mix 1 lime & 4 sand) for the stone walls but I want to know what can I do with the base? Can I use concrete to provide strenght ? Whatever I do I will be using a large piece of slate on the slab afterwards for the burner to stand on.
Many thanks for any advice or help you can provide.
Gareth

 

[Lime]

I don't think a cement based mortar will offer any further strength whatever just make it's future removal very difficult.

 

[FamilyWiggs]

I think you'll be fine with the same lime screed and then a hefty slate slab for the hearth. Anything over an inch and a half thick and you'll be OK.

10 inches is quite an excavation though... I'd fill the first 6 inches or so with compacted rubble/hardcore.

 

[chuckey]

"I will vbe using 1:4 lime & sand for the walls". Don't know if you mean casting a block or mortar between stones/bricks. 1:4 is too lean, should be 2 : 5 with sharp sand, when using lime. Providing the top slab is thick enough to spread the weight of the stove, whats undeneath it hardly matters. Say weight of slab = 3 cwt,weight of stove = 3 cwt, size of hearth = 18" deep X 36" wide, number of lbs = 672, number of square inches = 18 X 36 = 648, therefore ground pressure = 672/648 ~ 1 lb/sq. ins, much less then the pressure you exert when you stand on the ground!
Frank

 

[Penners]

I agree in every respect. Rammed hardcore under a bed of lime mortar under a good thick slate slab should stand up to anything short of a modest Hadron Collider.

But do heed Frank's warning, and don't stand on it, whatever you do! :wink:

 

[MdB]

If it was me, I would be tempted to use leca as a hardcore base and then mix more leca in with the lime to turn it into limecrete. It would then give some measure of insulation to the floor which would help keep the heat from the burner from heating the ground. Leca is around £8 for 50 litres so isn't that expensive especially for a small area.

You may also want to read up on the BC regs for hearths.

For the lime experts: If the ground was a bit damp, would it be best to use a weak NHL lime rather than putty in this application?

 

[garob]

Thank you all so much for your kind replies.
I will take your advice and I am will use a lime mortar mix of 2 lime & 5 sharp sand for the surrounding stone/brick walls of the fireplace.
I would however like more information about using Leca with the lime/sand for the base of the fireplace.Is this a good thing to use and what proportion should I use with my 2 lime & 5 sand?
Thanks again for any assistance
Gareth

 

[Lime]

http://en.wikipedia.org/wiki/LECA

 

[MdB]

I've not used it myself yet but I do have a couple of bags of Leca waiting to go into our two fireplaces. My plan was to mix the lime mortar as normal and then add as much Leca as I can. The important bit, I believe, is to make sure that the lime fills the gaps between the Leca.

http://www.limecrete.org.uk/
http://www.mikewye.co.uk/naturalproducts.htm#leca