fireplace render mix

[letinman]

Hi
I have a fireplace where i will be installing a wood stove, its currently sooty brick good on one side and rear but needs squaring on one side at the front that is adjacent to the front ie breast wall
i intend to use a cement sand and lime mix inside but i am not sure what to do with the breast wall as the stove will be very close to the side and the edge is going to get hot thus gypsum is out of the question , not sure what to do and how to blend it in to the rest of the skim etc
can anyone help
also should i use sharp or soft sand for the render there seems to be mixed views on this

thanks

 

[tim00]

I'm not sure of your directions, but typically, inside the opening and on the cheeks, wire brush off all soot and loose deposits, and render ( 3-1 S&L mix) as you propose over a blinding of one dry wash of PVA and second coat of tacky PVA. Skim where necessary with board finish. We bang in whatever sand is available, no come-backs so far.

Unless you have an unusual fire opening then most modern stoves should comfortably fit.

If the above doesn't answer your post , perhaps you could post a pic?

Maybe you already know about installing a solid fuel app. swept flues etc. if not, then research on the Building forum.

 

[letinman]

thanks for the reply Tim

I wondered about pva was just going to wet wall, but will coat in pva first ,
what is the best method of rendering the wall with the lime mix do you throw it on? or trowel it on, i have read that you cant steel trowel off as it brings the fat to the top, this worries me as i am unsure how to get a good flat finnish if you cant trowel off like a modern gypsum

the opening is small so i will only have 100mm to the side and therefore i am worried about the cheeks and breast wall, do you use fire proof plaster for those bits or normal gypsum

thanks for the help

 

[tim00]

1. You apply like any powder plaster, from a hawk with a steel trowel.

2. Apply a very tight scratch coat to start, and while the scratch is wet lay on over that to pull it out.

3. Leave to set up a little, and then rub up with a wood or plastic float, or even a block of 4x2 wood. Rub up in circular motions. Dont be fussy, most spreads bang it on with a steel trowel and straighten it up with a derby, leave it overnight and finish it next day.

I'm bewildered about your directions for what bit of brickwork you intend to render, please post a pic or diagram, or stick to the terminology that i introduced above.

 

[cotswoldbuilders]

Use Victas, Heat resistant plaster.

 

[letinman]

thanks for the advice
cant post a picture due to lost lead !
basicaly i intend to render inside opening
was also wondering about the breast wall , will use victas plaster now as advised

 

[letinman]

edited double post

 

[DoubleDPlasterers]

Wire brush off all traces of soot.
pva optional depending on type of brick.
Scratch coat- 5 plastering sand, 1 lime, 1 cement.
top coat- 6 plastering sand, 1 lime, 1 cement.

never skim with multi it'll just crack.
use lime as it can expand to heat temps.
i've always just floated then light spounge over top coat.
you can even use ''snowcrete'' cement to get a nice traditional looking fireplace opening.
done a few now and i've never had any comebacks.

 

[ampthill]

I have done loads of them the last 3 or 4 years but just recently some have become covered in loads of hairline cracks .. I have always done them in one day rationing pva to allow the scratch coat to go off enough to top them in the afternoon. I use a very similar to DoubleD above, the only thing i have been doing different is the mix may be a bit stronger and i have stopped using febmix because someone said you dont need it with lime!
I cant work out why they were cracking .. I even scratched the last one the night before but it still covered in hairline cracks!
My suspicion is its either the second mix is too strong or its the lack of Feb.
Any advice please its doin ma noggin !

 

[DoubleDPlasterers]

I have done loads of them the last 3 or 4 years but just recently some have become covered in loads of hairline cracks .. I have always done them in one day rationing pva to allow the scratch coat to go off enough to top them in the afternoon. I use a very similar to DoubleD above, the only thing i have been doing different is the mix may be a bit stronger and i have stopped using febmix because someone said you dont need it with lime!
I cant work out why they were cracking .. I even scratched the last one the night before but it still covered in hairline cracks!
My suspicion is its either the second mix is too strong or its the lack of Feb.
Any advice please its doin ma noggin !

your right lime will cream the mix up... i was always taught that the top coat needs to be weaker so that the lime can expand more to the heat? sometimes you have to keep floating to feather in cracks but keep on top and it should cure. you dont need to pva prep before, maybe thats why its cracking? just get scratch on in morn then top coat in the afternoon.

Hope this helps .....

 

[peaps2]

Clean up the area and wet the wall well. Apply a 4.1 sand, lime (NHL 5) scratch coat and the float coat needs to be 5.1 sand and lime.

Lime cracks when it dries quick but you don't need to worry about air line crack if you have done it in lime. Give the wall a good soak with clean water and give it a good rub with a wood float or plastic equivalent. Lime "self repairs" when you apply water.

Heavy mixes will also cause cracking.

PVA will fail at the heat produced from the fire and so will gypsum plaster so lime is the best to use in this situation.

 

[Jamesy78]

Not sure I agree with this ratio at all.

If you read the manufacturers advice for limes, specifically NHL 3.5 lime, such as limelite, they recommend a ratio of 2.5:1 for base coat and then 3:1 for top coat

I’ve tried mixes at 5:1 and they are very grainy with not a lot of workability

So I would suggest 3:1 will give a much nicer mix like that you would find of a modern sand cement render with feb mix in them.