Mortar mix for chimney

I have scaffolding Stuart, coming next week.

I also have a few spare slates I can use. So I strip back flaunching to expose brickwork around flue entrances and then sit broken slate around the edges to sit the pots on. I then do the new flaunching. So the slate remains?

What is the reason for the slate Stuart? And would you recommend a flue liner for my chimney? I plan to have an open fire

Sorry for all the questions but I really appreciate the knowledge. .
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I now have the scaffolding up and plan to do this job at the weekend. I went up today for the first time and could see the real state of the pointing and flaunching. It's terrible and I don't think I need to bother too much about grinding the mortar back an inch as in most places it's weathered back there far already. The shear amount of weathering shocked me and my initial plan was to use the lime NHS 3.5 and sand mix for the stack pointing only only introduce cement for the flaunching. But given how much the stack has weathered I was wondering weather I should use a cement:lime:sand mix fir the stack too. My only worry is that the mortar mix would be too strong for the brick which would encourage the brick to deteriorate.

What do you think?
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I'm worried if I use a cement mortar on the stack then the mortar won't be able to breathe and moisture won't be able to evaporate so instead the bricks will remain damp and will deteriate quicker.

What about using a stronger lime:sand mix so that there's no cement and the mortar can breathe but with a greater concentration of lime being used it might be a bit stronger and last a bit longer? Say a x2 sand to x1 lime
i think the principles behind breathable walls/mortars is that air can pass through a structure from within. obviously because lime is porous it will drain water but you dont need your chimney to breathe from the top outwards.
Yes I understand what you mean.

Therefore do you think adding cement is advisable for the stack.

X3 sand
X1 lime
X1 white cement.
The main reason stacks deteriorate a lot is because they have been left redundant, if you are planning to use your chimney then I would think that is more beneficial than what mix you use. The last chimney I pointed I used hansons hydrated lime which is a mix of sand, cement and lime. You do not need to always use a grinder, you can use plugging chisel and a bolster to get the old mortar out if its soft enough.
Thanks Catlad, I'm going to move ahead with this first thing in the morning. I'll go with the sand:lime:cement mix and yes we plan to use two if the flues and I'll be putting down flexible liners at the same time. I'll just use a hammer and bolster chisel as you say to clear back remaining mortar (there isn't a lot left).

If I do chisel back one whole face of a chimney first and then repoint before moving round to the next face will that be ok? I worry if I chisel a whole face out I'll make it structurally unsafe
If I was doing that work I would remove the chimney pots stuff a large enough ball of rock wool down each flue to block it, so as to catch the old bits of mortar and re bed the top 3or4 coarse of brick work. And remember to take the rock wool out as soon as possible and tell anyone else in the house not to use the fires until you say so.
Ok. I'll remove all pots and old flaunching, stuff rockwool down flues, re bed top few courses of brick if it's loose and then just repoint the rest. After that I'll remove rock wool, put liners down flues and them rebed pots with new flaunching.

Sound like a good plan of attack?
The idea with lime mortar in the joints is not so much that it breathes from within the stack, but any moisture in the outside of the brick will go into the joints and evaporate. With a hard cement mortar the moisture will stay in the edge of the brick and break up the brick with freeze/thaw. Lime mortar is sacrificed in time and can be repointed.
It depends on the bricks whether or not cement will damage them. Soft ones are more at risk. It can take a long time for the damage to show, so a lot of builders think the pointing they did with cement was OK.
Thanks Stuart, this was my initial concern. I guess if I use a cement lime sand mix it could be too hard and the brick could suffer or if I use a lime sand mix then the mortar might not last as long.


The bricks are known as a Cambridge white in this area and they have about 9 holes in each brick. A big like a type of air brick I guess.

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