anyone seen a chimney supported like this?

[gday2uk]

Block or brick partially embedded in a wall, with one end projecting out from the face. The weight of added masonry above counterbalances the cantilever and keeps the block from falling out of the wall. Corbeling often occurs over several courses, with each block or brick overhanging the one below so as to resemble a set of inverted steps.

Thanks Google

 

[Shytalkz]

Yes and what it doesn't say is anything about the relationship of the height of the corbelling to the outstand. 3 or courses of corbelling sticking out a brick and a half, as in this case, is pointless. The stack would be standing there whether it had the corbelling or not; at the moment the stack brickwork is holding out, but there's no guarantee that it will continue to do so.

 

[^woody^]

We can't see the condition or bond etc, but in principle this is not a problem.

There would not be just one corbel course holding the whole lot up, but the flue and stack is bonded into the whole wall all the way up. Also there is [presumably] the same on the other side of the wall and the load is central. Also its only a 215mm overhang.

OK, the may be some softening of mortar to the flue, but the brickwork is quite wide and so will not all be affected, and will remain bonded

So in principle it is fine and its not uncommon to see something like this.

Whether it is acceptable in this particular situation would require further detail and/or investigation. But I would tick the "yes/maybe" box

 

[Shytalkz]

Like I said, 3 or 4 courses of corbelling with a 330 outstand is adding absolutely nothing to the stability of the stack, the more so when it's cobbled in afterwards as here - how well could that possibly have been constructed? Not very.

The stack stability would be no worse if there was no corbelling there whatsoever, that's how pointless it is in this instance. If it is only a 215 overhang, btw, that would mean the party wall would have to reduce in thickness to accommodate a 215 flue and, with the one from the other side, that would mean the two flues would be erm one...

If it was ever called upon to take the stack load entirely on its own as per the theory, it wouldn't - it's a simple maths check to prove that that would be the case. The reality is that, as you say, you're (still) relying on the bond of the stack/whiffs to the party wall to keep it there, which is not a good position to take imo: stacks can look ok from the loft space, but be a right cod's ear inside, per earlier post. Any collapse of unsupported flues will, more often than not, be as a result of failure of the whiffs and their bond to the party wall.

There is no substitute for supporting it underneath with a beam, or a gallows bracket, if you really must, although the latter relies on the integrity and capability of the party wall to cater for the horizontal forces applied by the bracket fixings.

 

[noseall]

the breasts below are gone

you did read this bit didn't you woods?

 

[^woody^]

It does look like a 215mm projection, so it does raise the question of where are the flues - the flue must be in the actual party wall?

Anyway, just looking at it, it seems that the majority load from the stack is on the party wall. Yes some loading will be transfered down the angled flue brickwork, but how much? Is the actual vertical stack so defendant on these, or will the vertical load to the wall and tied -in corbels enough to provide support - yes, I think it could

Never underestimate the power of a corbel!

And remember that there is a stack on the other side - yes it may be being removed, but thats another issue, and one which could cause stability problems.

But I'd still say, in principle it is OK.

There are factors which would make it not OK, but without more detail, we don't know

BTW, does a gallows bracket do anything different to a properly formed corbel?

 

[NS215]

post removed already answered

 

[Symptoms]

They say a picture's better than a 1000 words:

Untitled

• Symptoms
• 6 Nov 2008
• 1

 

[Shytalkz]

Woody, if the stack itself is capable of cantilevering either side of the party wall, then the inclined flues would not be taking much other than its own weight. However, would you go and stand under a stack where the inclined flues had been removed? Wouldn't inspire confidence would it, really? That's because you know intrinsically that the stack does indeed load the inclined flues.

Corbels in the right place and constructed as pert of the original work do work, that is true. Retrofitting such a large outstand over such a short vertical height doesn't. It's a sop.

It does, as you seem to suggest, impart a horizontal force to the supporting wall. However, it will tend to be more robust than a gallows bracket, which relies solely on the fixings into the wall to resist the vertical and horizontal forces.

Either way, if I was doing a survey of that property, there is no way that I would willingly put my PII at risk by saying that it is ok and will continue to be so - there are just too many imponderables.

 

[^woody^]

Oh yes, there are a few variables and without knowing all the facts then the "correct" (ie bum covering) response is that it may not be satisfactory.

But I don't think its something to be that alarmed about when taken at face value.

IMO, there are equal reasons as to why it would be OK as there are as to why it would not be. The principle seems fine, but in context and with other facts about the particular circumstances, then further support may be prudent - although not necessarily required.

Its an interesting thread

 

[Shytalkz]

I don't often err on the side of caution with structural issues, but hanging unlined stacks in old structures is one where I would and do. It's not a question of a*se covering, it just does not feel right as a long-term solution. That stack is still hanging there precisely because the bond to the party wall is currently capable of taking the load, not because of the corbelling.

A properly constructed corbel (ie about 35-50mm overhang per course) as part of an original structure can and does work quite happily; but I will never be convinced that a retrospective construction will achieve the same - a lot of structural situations work, but because the elements in reality work together, rather than in isolation, something that is more difficult to achieve retrospectively, especially with corbels.

Now, what about that purlin?!

 

[stuart45]

Corbelling used to be the method used when removing a breast. However in many cases they didn't bother to cut in to the wall properly and it was a poor job. Even when you did take out enough brickwork I doubt if was as good as being built in when first constructed. When I was younger some of the old bricklayers believed that you could take down a breast and the bonding in would hold it. One of them used to take it down and leave it without any corbel. Its very risky to rely on the bonding as not all chimney jambs are bonded in properly. It was much quicker to run in the party wall about 10 courses and then built the jambs and tie in the next course. All they did then was to smear a trowel full of mortar up the straight joint to cover it.
Some of the older chimneys only have a 4 inch skin between the neighbours fireplace, even though the party wall is 9 inch. The stack might then be 3 1/2 bricks wide.

 

[DCC]

Er, for those that dont know & possible other readers of this thread.
Oh and me...

What is "corbelling "

Basically lumps of hard stuff jutting out of a wall to support things above.

Think of those apparently hanging balcolnies and bays you see on old stately houses which are supported by rows of bricks below, with each row coming out a bit further from the main wall.

 

[^woody^]

with each row coming out a bit further from the main wall.

This is actually 'oversailing''

A corbel is one or several bricks projecting or a single masonry block

 

[Shytalkz]

Or on an old timber frame building where first floor oversails the ground floor walls, it's a jetty .