Dowels in stone chimney stack?

[88v8]

The top two courses of this Georgian* chimney stack



are made of four ashlar Cotswold stone slabs, each approx. 18"h x 15"w and 3" thick. This is a typical Cotswold construction.

On top is an elaborately moulded two-piece stone cap, approx 8x10" section



Here you see the joints at 12 & 6 o'clock.

Currently there is a scaffolding, in progress here, up against the stack which I had put there to replace the Victorian pot with something taller.



Here you see on the left the projected 42" replacement, with the twin of the present 24" incumbent on the right.



Annoyingly, this idea has rather foundered, as the footprint of the 'new' pot is an inch too wide for the capstones and it would be difficult and possibly damaging to carve the necessary niches in-situ, so I will probably end up just putting up the twin of the previous pot, but set a little higher in the stack.



Which is not worth the £1,000 spent on the scaffold, but heyho.

From the scaffold, I can see that one of the stone slabs looks rather decayed on the outside and I am toying with the idea of replacing it.

It appears that the four slabs are just butted up against each other, a straight vertical joint. I had the idea that I could insert a stone saw, cut through the mortar and simply lean the decayed slab out. The weight would be manageable - an online calculator has 68lb (or 897 tablespooons !) and it seems a shame to waste the opportunity while the scaffold is there.

But... it occurs to me that there are likely to be iron dowels in the joints. I can't believe the slabs are simply butted, relying on the weight of the capstones to hold the whole caboodle together.
There is no sign of the cracking I normally associate with old dowels, but if there are dowels there, on the one hand this makes the whole structure much stronger, on the other hand I'm sunk as regards replacing the slab as the capstones would have to come off, which ain't going to happen.

Any experience?
Dowels?

Ivor

* I suspect the whole stack was a Georgian era transplant from another, older and grander, estate building as it is quite out of keeping with what would then have been an umble workers' cottage.

 

[dandrew]

Why do you think that there are iron dowels in there Ivor ?
Any even if there were, I can't see what benefit they would have if placed in the vertical joints between the masonry.

Where I have seen what you describe they are dovetail-shaped or U-shaped and let into the top surface of the masonry and leaded in placed (at 3 and 9 o'clock at each joint). This would stop individual stones in the top course of the masonry from leaning outwards from each other. Come to think of it I've only ever seen this used at gable ends where there is a capping parapet that oversails the roof covering

David

 

[malcolm]

My pots sit on top of bits of slate bridging the big hole below and are held in by the flaunching. Would that be an option with your big pot? Or have I misunderstood? (I am getting old these days and recently bought an automatic car with a lazy V6 and I can't remember what happened 5 minutes ago any longer). It seems odd for a pot to have to fit inside a big hole at the top of the chimney and more odd that you want to mess with the lovely chimney.

 

[a twig]

My stacks are very similar but in Bath stone. No dowels or joints, just ashlar and lime mortar butted together. There is then a large "slab" of stone capping the whole thing with a large circular hole in. Pot is flaunched on top of slab - a la malcom's post.

One of them needed some work, as a few of the ashlar were too far gone. The mason that did some repairs took down all the courses that were affected, and rebuilt, keeping as many as possible of the original blocks that were still in good nick, and shaping new as required. His opinion was that replacing a single block without affecting any of the others was unlikely - and it was worth re-doing it all properly almost from roof height back up to ensure the construction was all solid and sound. There was no additional supporting / doweling etc what so ever.

The recessing of the pot inside the stack seems unusual to me - but in your quest for extra height I would suggest malcom's solution seems easiest and very much mimics what I have?

 

[88v8]

When it was built, there was no pot.
For perhaps 180 years, the rain came in. The flue is straight, looking up from the fireplace, one can see the sky. When we bought the cottage, the 1" thick fire bars in the newish grate were nearly rusted through with the rain that came in.

So we had that 24" Victorian pot put on top, to restrict the opening.
The builder we used then, the one and only time we used him, cleverly sat the pot on some stainless angle brackets so it was recessed about 3" into the cap.
Unless a pot is very short, like Malcolm's, it's usual to recess it, generally the top course of bricks sits around the base of the pot.

Forward a couple of years, we hardly ever use the inglenook, so we have a stove installed. The pot has to come off. The angle brackets fall down the stack and are lost. In a rush to finish the job, our new and generally much better builder sits the pot on top of the closure plate so now it's sunk nearly 8". You can see the tide mark in the photo. It looks silly and has annoyed me ever since.

Forward a couple more years, our insurers are nagging us because our chimneys don't give what they regard as the minimum height above the ridge - six foot. So now I want to kill two birds with one stone - replace the pot that's sat too low, and install the taller and rather magnificent pot to give the 6' height. I also need to mesh over the bees' nest that's sprouted this year in the stack.
Hence the scaffold.

The plan was to sit the tall pot about 4" down inside the cap. After all, one can't perch a 42" pot on top of the stack and just rely on the flaunching to hold it steady, it has to be recessed.
But, the big pot has roughly a 15" footprint, and the capstone tapers internally so while it's 16" at the top, it's more like 13" lower down. That odd couple of inches is hard to take out without a lot of chiselling, and I'm nervous of damaging the stone. If I'd used a suitable builder I dare say it could all be taken down and redone, but it seemed like a perfectly doable DIY job, and to find a builder now would take way too long.

The possibly decayed ashlar is an add-on, mission creep doncha know.
I would probably agree with Twig, it would be hard to get that one slab out and back without disturbance. Remarkable though that it could be built with no dowels, just 3" slabs resting on their edge and glued together with mortar. One of the vertical joints has cracked. It's been repointed before and cracked again. I'll see if I can repoint it with lime - what's in there is rather cementy.

Why are things never as simple as they look.

Ivor

 

[Flyfisher]

Which is not worth the £1,000 spent on the scaffold, but heyho.

Scaffold can indeed be expensive. I spent £500 on a rather less elaborate scaffold when repairing one of our stacks. Subsequently, I hired a cherry picker for work on another stack for £200. A cherry picker cannot always be used, especially with a low stack in the middle of a roof, but for a gable end stack it's usually possible to get close enough to the work.

 

[88v8]

That would be nice.
But the cottage is up a long flight of steps.

Would also be handy to pick the apples at the top of the adjacent tree.

Maybe in a few years I'll be able to hang from a drone.

Ivor

 

[a twig]

But, the big pot has roughly a 15" footprint, and the capstone tapers internally so while it's 16" at the top, it's more like 13" lower down. That odd couple of inches is hard to take out without a lot of chiselling, and I'm nervous of damaging the stone. If I'd used a suitable builder I dare say it could all be taken down and redone, but it seemed like a perfectly doable DIY job, and to find a builder now would take way too long.

So can you not just put the new pot as far down as it will go and then secure it with a generous amount of mortar and flaunching? You could even use two halves of slate or block as caps to hold it. I've thrown together a quick exploded diagram to try and explain better:

 

[88v8]

Thrown together? Your CAD skills are on another level.
The top of the capstone slopes - imagine a flat pyramid - so sadly there isn't a flat surface on which to sit any additional stone, slate or flaunch.

I don't think the pot would sit down far enough inside the capstone to be secure. It needs to be down 3-4".
I made a template of the somewhat skewhiff footprint, but the flue liner protrudes above the capstone so I have to make a big hole in the template in order to sit the template down properly, then I can see exactly how far out I'd be.

Although yesterday I was reading the 160 page book that came with my new Tormek sharpener, and it mentioned stone chisels, so that has slightly revived thoughts of putting up the big pot. I'd been thinking to go about it with the angle grinder and a scutch. The flue liner is also in the way when it comes to carving a flat bed in any niches. Maybe I should buy a decent set of stone chisels. Or just sharpen the ancient collection of abused iron lumps that I already own.

In the end, it's not just the problem of sitting the pot up there, it's how I'd feel when we next have a storm, thinking of 150lb of tall pot catching the wind. That's a lot of leverage, especially if there are no dowels in the stack. Have to bear in mind that this stack was not designed to carry any pot at all.

It all looked so easy from the ground.

Ivor

 

[88v8]

To sit the tall pot inside the capstone I'd have had to carve a niche into each of the four sides. Equipped with my newly sharp chisels and my angle grinder, I prepared to start.
Then I realised that abaft the niches, there would only be an inch width for flaunching.
So, that was the end of that. The pot I spent ages searching for, drove 250 miles to pick up, will end up as a garden ornament. A failure of planning and fact gathering before the event. The not-triumph of hope.

I hung the liner from a scaffold pole, cut the closure plate with the angle grinder and removed it.



Fortunately, the bees' nest was immediately beneath so all the comb could be carefully removed. There was less of it than I'd expected, and the honey had already been robbed, but even so the wax is quite flammable and anyway I'd have been worrying about it, so all in all it was sort of worthwhile......
See the fragments of comb at the top



hanging from the supports


This lot


This is the side of the stack to which I was referring. See how thin the slabs are. The disintegration is not that apparent in this pic, and I took the perhaps selfish decision that given my likely tenure it's not urgent and I'll leave it for someone else.



The pot, which I could even have set an inch higher, at least is not submerged like its predecessor. From the ground one can see the moulded foot.
The cowl was custom-made by Brewer, with an oversize bottom plate.



Here are the vent holes through which the bees got in. Inch diameter. I covered them in stainless wire mesh. Which of course I should have done in the first place.



It's much nicer in the sitting room now. No bees coming down the chimney. No wasps. Nature is wonderful. On the whole.

Ivor