Mobile chimney stack

[TheGables]

I have a 1900s cottage with a tall slender chimney stack (three bricks by two and a half) which rises about 15 feet from the eaves. From ground to the top of the pot is about 30 feet. I plan to re-point the brickwork and line the flue with a metal liner (for a wood burner). The lime mortar pointing is generally ok but the top 3 feet has seen considerable erosion and needs seeing to. While the scaffolding was up I thought it appropriate to re-point the whole stack rather than just attending to the top and the lining.

When standing on the scaffolding and attempting to remove the pot, it became apparent that, with very little effort, I could make the whole stack sway. There are no obvious cracks and the stack is straight and perpendicular. We have had recent gales and there did not seem to be a problem, although I have always been concerned that this stack would be vulnerable in high winds if I didn’t attend to the pointing soon.

Is a mobile stack normal? Will re-pointing with lime mortar reduce its mobility or should I be planning to rebuild it?

 

[Zebra]

I repointed my whole chimney stack this summer. I used a ready-mixed NHL 3.5 mortar, and although only one face was in really bad condition, and the whole thing wasn't moving, I was nonetheless quite unsure of its stability whilst I was doing it, because each mortar course had to be refilled with lime mortar all the way to the inner face.

I did each face in turn. On the better faces I did up to six courses at a time. On the bad face initally I only did one at a time, although sometimes I would do several individual courses some way apart. Clearly you're going to hvae to make your own judgement about how much you can do in one go without risking collapse. I was perhaps quite conservative in that regard. I scraped out the old mortar, all the way to the inside, leaving just a tiny bit on the very inside to give me something to push against. Hoovered out the joins (people in the pub garden were staring at this person up a ladder with a Henry on their back!), and repointed, as I said, with ready mixed mortar.

I used a ready mixed mortar to be sure of getting the mix right. It wasn't very easy to work with, and went off quite fast. I used NHL 3.5 - no harder because I have very soft bricks, and no softer because you should use a slightly harder mortar on a chimney than elsewhere because of wind-loading. The Old House Store have a very helpful chart on their website indiciting what type of mortar to use.

So I would say yes, perfectly feasible to repoint, and that will solve its mobility problems, because the mortar sets perfectly hard and fine. You'll have to suss it out as you go along though, some might not need doing, whereas certain bricks will no doubt fall apart in your hands and need replacing.

Mine took me ten days straight, forty courses, three faces, each face three bricks wide. But I was being very particular and it does look good!

 

[Zebra]

Kind of the wrong time of year right now though to be repointing in lime.......?

 

[Penners]

with very little effort, I could make the whole stack sway....
Is a mobile stack normal? Will re-pointing with lime mortar reduce its mobility or should I be planning to rebuild it?

I can only say that, if it were mine, I would definitely rebuild it. The cost of doing this will, I'm sure, be unwelcome, but compared with the cost of recovering from a chimney stack falling through the roof, it will be modest.

 

[Feltwell]

I agree with Penners, it sounds like it needs rebuilding to me. Zebra, you said that yours wasn't moving - that's the critical difference I think, and why yours was OK to be re-pointed.

You say "with very little effort, I could make the whole stack sway" - which makes me think the bond between bricks has gone completely at some point, and that re-pointing alone will not be enough. Not what you want to hear I know, but a chimney stack collapse can be an extremely dangerous thing - several tons of masonry coming down from on high could be fatal to you or a passer-by.

You won't necessarily have to take the whole stack down - just down to the point at which it is solid. If it is as unstable as you say, I would urge you to carefully take it down now, cleaning the bricks as you go, cap it off temporarily with a cut-down paving slab, then repair & rebuild it properly when the weather warms up.

I had a similar problem with a garden wall - at first it looked like it just needed repointing, but it was leaning quite badly at one end, and moved alarmingly when pushed - I was concerned about it coming down in high winds and causing someone an injury. I took it down to ground level, rebuilt it and am much happier now that it's vertical and has no movement.

 

[Penners]

it was leaning quite badly at one end, and moved alarmingly when pushed

Sounds a bit like me after the Camera Club dinner.

 

[Feltwell]

I suspect you had problems with both composure and your focus Penners, but I sincerely hope not your exposure. Especially in this weather.

 

[Penners]

:lol: :lol: :lol:

 

[Andrew Bodenham]

TheGables,

If your chimney stack is really as unstable as you say then it poses a very real danger and needs immediate (i.e. today) attention.

Get your scaffolding contractor to modify the scafold so that it is providing lateral support to the stack. a.s.a.p.
Then contact a structural engineer experienced in conservation work and ask for an inspection and recommendations. It sounds as though the stack will need to be taken down and rebuilt, perhaps with some reinforcement to cope with bending caused by lateral wind loads.

If you can move it by hand then it could easily blow down in strong wind - amazing that it hasn't already done so. The risk of damage to property or injury to anybody who gets in the way is too great not to take appropriate action.