Confused about mortar mix for fireplace

18 Nov 2014
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United Kingdom
Im making our builders opening smaller in order to install a gas fireplace, by installing a steel lintel (as concrete will crack) and bricking up the hole above. However Im confused about what mortar to use!

The DIY store sold me a bag of "Natural Mortar" which apparently contains lime, and said to mix it with cement -hey presto. However there are no instructions on ratio on the bag, and googling comes up with a confusing array of answers. Does anyone know??

I want to make a mortar to stick the bricks, and then a render to cover them on the outside and create a flat surface for my fireplace to be installed against. The whole lot will then be plastered over so none of it will be visible.

Any help greatly appreciated :)
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By the way the gas engineers will be installing the fire, and they told us to do this work first. I've done basic bricklaying, pointing, rendering and plastering before... but just not sure on what mortar mix to use as its a fireplace with heat.

Cheers x
A concrete lintel will not crack.
Has the fire opening (the builder's opening) been bricked around yet?
What kind of "fire" (make & model) are gas engineers installing?
By fire surround do you mean the decorative surround that goes on the chimney breast face and frames the fire opening?
A couple of photos would help.
Ive got a steel lintel so that's not an issue :)

Here's a pic of the builder's opening as it stands,

and the cast iron fire place that's being installed (a gas unit sits inside the basket to give a final finish like this:

The cast iron fireplace sits perfectly width-wise with the arch sitting back into the opening, however the opening is taller than the cast iron so there is a hole above it, between the top of the cast iron and the brick arch.

Ive had quotes from 4 professionals who have all given various options but basically the same thing -there needs to be a seal between the wall and the cast iron so the opening needs to be made smaller.... the easiest way being to take out the two half bricks that are 1 brick below the arch (which is in good condition so will be fine whilst we do this), insert a steel lintel, brick up the gap, and then render the entire front to give a flat surface for the cast iron to rest against, which is then sealed with fire rope/sealant etc which the installers will do.

So my only question is about the mortar for the bricks and render really.

Everything else already checked out - flue, hearth, decoration, gas line in, conversion of cast iron fireplace, suitability of gas fire etc... all of that is fine :)

I was just didn't want to use normal mortar only to find out it cracks under heat or something.

Does that make sense... any obvious errors in thinking?? There shouldn't be after having 4 companies round, but you never know!
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Any soot residue wire brush off.
Use a render of sand and lime or sand and cement on the face of the c/breast.
Why not use backer board on the inside of the opening - or render?

FWIW: your ref only gave a pic. I cant locate the manual for installation dimensions.
Use a render of sand and lime or sand and cement on the face of the c/breast.

Yes, this is what Im saying.... my question is what is the ratio of sand to lime to cement for this purpose?
Does anyone know?


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