Rendering the Back Wall of a Fireplace

[divil]

I've just got rid of a back boiler and removed all the filler bricks, exposing the original construction hearth and Victorian brick lintel. The opening is quite big - 94cm wide by 130cm tall. The sides are in fairly good condition, made of brick, but the back is an ugly mixture of brick and stone.

I have raked out all the loose stuff on the back and will clean up what's left with a wire brush, but then I'm going to need to render it. The surface is VERY uneven, as much as 100mm between the front of the furthest-forward stone and the back of the deepest recess.

1. Is a 1 cement, 1 lime and 5 building sand an appropriate mixture for rendering in this situation?
2. What's the maximum thickness per coat, bearing in mind I have as much as 100mm to fill in one place?
3. Can I plaster on top of this, given the heat of the eventual wood-burning stove?

Thanks!

 

[marshman]

divil as you're got to dub out a 100mm I'm wondering have you got enough depth so you could build a brick wall at the back instead
using reclaimed bricks,
the problem with rendering fire places is the render needs to be soft 6.1.1 / 7.2.1 as it tends to crack with the heat,
it also won't hold well over soot stained bricks and if there is any damp the soot will leach through.
there is http://www.euroheat.co.uk/images/product_images/1710243172TB203 Heat proof plaster.pdf I've never used it
so I can't say if its any good or not, there is a white heat resistant fire board ( sorry can't think of its name ) that you could screw fix at the back then fill over the screw heads.

can some one name this board please, sold at most builders merchants, comes in 8x4 sheets.

as to your questions, if they are deep holes put bits of brick in the first coat of render, don't smooth the render off,
leave it with a rough open face between coats and dub it out in layers over the course of a day until it nearly out to level
then scratch the last dub coat and leave it to dry for a few days, the day you put the float coat of render on wet the scratch coat with water
so its damp this will help it bond.

 

[divil]

Thank you marshman.