Possibly Unsupported Chimney Stack

[Specificbumbless]

Hi everyone

Hoping someone can help. We are in the process of buying a 1900s Victorian property. Our level 2 survey has thrown up an issue in that the rear chimney stack is possibly unsupported. I have spoken to a structural engineer and they quoted £550 to have a look a cut a hole in the ceiling. We are feeling slight deflated as we don’t think we should be footing the bill as if the stack is unsupported, we can’t move in as it’s not safe. It’s a probate house and the lady was quite old and lived there for a long time, so I know it’s probably been like that for years, but still a hazard. Anyway, I have spent hours looking at various images online and none of them look quite like ours. Can anyone offer any advice as to why ours seems A. Open and not boxed in like all the other stacks, and B. It appears corbelled. Does they make any difference? I know we need professional advice, I’m just interested in what people think. Thank you!

 

[^woody^]

Brickwork self-supports. If the wall around it is sound, and the stack bonded in to the wall, the stack will be fine.

 

[mrrusty]

Is this tower and room unsupported too? https://www.stravaiging.com/history/castle/craigentinny-castle/

To me that looks like someone has taken the trouble to neatly finish it off so it's corbelled in to the wall. I expect it has been like that for a very long time, and will be fine for a very long time in the future.

Here's another example - is this structure unsupported?

 

[stuart45]

I'm not sure why the SE needs to cut a hole in the ceiling to see it, as it looks pretty obvious to be unsupported. Back in the 70's it was really common to take out the chimney breasts and lots of builders relied on the tying in to hold them up. Firms I worked for used to get me to cut into the party wall and corbel them in with solid brickwork.
However by the 80's it became part of Building regs to remove and support the chimney when taking out the masonry below, because a few of them failed and collapsed. Initially it was OK to corbel, or use gallows brackets, but steel beans are more common now.
Although there are probably thousands like that in the country, occasionally ones that have been OK for years can fail, usually when next door starts to knock theirs down.
As it's a probate sale, there's usually a better chance of the beneficiaries wanting a quick sale and agreeing to a lower price to cover the costs.
You have to remember that an unsupported chimney could hold up a sale when you come to sell. Lenders will often refuse a loan on it.
A rule for corbeling is you shouldn't corbel out more than the thickness of the supporting wall.

 

[mrrusty]

what @stuart45 says is the voice of more experience in this field than mine. When looking to buy an old house, you can't expect it not to have a few issues - it goes with the territory. Personally, unless it was holding up a mortgage, that wouldn't be bothering me, because it looks solid, even though it isn't up to current regs. Best case, you move in, forget about it, and never touch it. Worst case, it requires future attention for some reason - if your neighbours start similar works then you can look at it then.

If you like the house otherwise, and negotiation risks the seller looking elsewhere, then I wouldn't lose it just because of that.

Our last 30's house had a shared central stack corbelled just like that in to the party wall, and 3 of the 4 angled flues in to the base of it had been removed - cut back flush to the party wall. Prob done in the 80's. We were there 2005-2017 and resold without any issue.

 

[George_engineer]

Without having a good look at both the corbel and the wall it is not possible to say.

While corbels can be stable, whether they are or not depends on the overall geometry and stability system of the wall that supports it. To be self stable, corbels should only protrude 1/3 of the thickness of the wall - any more and it risks shifting the overall centre of gravity outside the wall footprint. However, even if it is greater than 1/3 this can remain stable when propped by the roof, but if that moves or restraint slips in future, the chimney can fall and it would be without warning. This is why we have moved away from corbels and gallow brackets.

Anyway... I would just allow for the cost of removing the chimney completely. Partially removed chimneys are just an unnecessary hole in the roof. You can try and negotiate a discount for that to be done or just accept it is sold as seen.

 

[stuart45]

I'd be surprised if an SE gave you a written report saying it was fine as it is, but you never know. You can go more than 1\3 for corbels on new work by using Bricktor in the joints, but it's not as practical on retro work.
Surveyors are often giving a nil valuation in these situations nowadays.
However as mrrusty says it's probably not worth losing the house over, as it can be fixed if it concerns you. The room below could be an ideal guest room for the mother-in-law.

 

[George_engineer]

I'd be surprised if an SE gave you a written report saying it was fine as it is, but you never know. You can go more than 1\3 for corbels on new work by using Bricktor in the joints, but it's not as practical on retro work.
Surveyors are often giving a nil valuation in these situations nowadays.
However as mrrusty says it's probably not worth losing the house over, as it can be fixed if it concerns you. The room below could be an ideal guest room for the mother-in-law.

Not disputing this (I can see how it could be made to work) but do you have a tech reference I could look at?

I would class that as reinforced masonry which does change the situation but the geometric limit on COG is hard to get around.

 

[stuart45]

Not disputing this (I can see how it could be made to work) but do you have a tech reference I could look at?

I would class that as reinforced masonry which does change the situation but the geometric limit on COG is hard to get around.

As I'm a bricklayer, not a designer or engineer, I normally built what's on the spec, so I'm generally relying on someone else to work out what is structurally sound. I can see the possible issues with corbelling out too much, although most of the old text books used to say the thickness of the wall, compared to modern building regs which mean the overhang of the corbel would be half the wall's thickness, to give 1\3 of the new overall width.
I only know from experience on a new build at the start of the gable end the Brick Tor was used to allow for the corbel to come out more. I would guess that this has been allowed for in the calculations by someone, although it's possibly wrong.

 

[^woody^]

While corbels can be stable, whether they are or not depends on the overall geometry and stability system of the wall that supports it

The corbel is not holding up that stack. The stack is (hopefully) bonded into the wall it is attached to, and that bond - effectively repetitive vertical corbels is the support and transfer of loads from the stack to the wall.

 

[George_engineer]

As I'm a bricklayer, not a designer or engineer, I normally built what's on the spec, so I'm generally relying on someone else to work out what is structurally sound. I can see the possible issues with corbelling out too much, although most of the old text books used to say the thickness of the wall, compared to modern building regs which mean the overhang of the corbel would be half the wall's thickness, to give 1\3 of the new overall width.
I only know from experience on a new build at the start of the gable end the Brick Tor was used to allow for the corbel to come out more. I would guess that this has been allowed for in the calculations by someone, although it's possibly wrong.

Thanks. I found the manufacturer's guidance. This is what their product allows:

"For tying and reinforcing corbelled masonry, Bricktie is widely used. It eliminates the need for
steel support systems and will simplify design and construction."

You can see the COG for the outerleaf will move out of the footprint of the outerleaf so is held back firmly by the Brickties.

(Interestingly enough I came across this situation last year where no additional reinforcement had been installed in the outerleaf so had to partially condemn and provide reinforcement to the stepped outerleaf. A typical solution would be to use a steel plate to support the outer cavity. where the step was needed. This does at least reassure me I wasn't being overly conservative).

The corbel is not holding up that stack. The stack is (hopefully) bonded into the wall it is attached to, and that bond - effectively repetitive vertical corbels is the support and transfer of loads from the stack to the wall.

Think more global stability. At some point the corbels goes too far out the COG leaves the middle third of the wall below. That then means the remaining bearing masonry starts increasing in stress. If it can withstand that (probably can), it'll remain stable. But if the COG leaves the wall below completely then it will fail without a prop from the roof. Can't rely on the tension capacity of mortar.

 

[Specificbumbless]

Thanks everyone for your responses. You’ve definitely given me food for thought.