Lime Discussion

[OddJob]

You're taking me too literally Ff.
If I ever advocate anyone building a stack like the one you took down then you have my permission the put me in a home for the feeble minded :shock:
You did exactly the right thing with your chimney in terms of its appearance.
It's what I'd call a sympathetic repair.
I'm never going to agree with the mortar spec though so in answer to your last question yes you should be alarmed.
I'd suggest you present your LBC and CO with a spec for a proper traditional lime mortar based on your own research,I'd be interested to see what they have to say.

 

[Flyfisher]

I'm glad you like the new stack - I was beginning to panic that I'd made a conservation disaster!

I guess I'll just have to wait and see how long the mortar lasts, although it's still looking good after the recent hard winter. Fingers crossed, but even if the worse does happen, it won't be detrimental to the main structure and can easily be rebuilt (ugh!).

As for 'proper' repair mortars for the existing structure, I really don't think a pure lime-based mix is going to be a problem. Indeed, I'm expecting it will be insisted upon, which is why the 9:3:1 mix was expressly only approved for 'new work'.

 

[Gervase]

Before we get too carried away, it is important to bear in mind that the stack is essentially a new-build rather than a repair. As such, and given that white OPC is actually different to bog-standard OPC (Wikipedia isn't always right), perhaps we're all being a little precious here.

Me particularly; I hadn't noticed the tag in flyfisher's post stating where he was from. Given that the CO for that area is extremely well-qualified and has, in the past, been a valued contributor to this very forum (and has been of great assistance to me personally when arguing a particularly tricky point with intransigent, lime-averse Building Control Officers), I'm happy to accept there was a darned good reason for the specification for the stack.
As such, when I wrote:

The CO's recommendation of a 9:3:1 mix indicates that s/he's working from the old English Heritage guidelines from the Eighties; guidelines which the Smeaton Project have pretty well kicked into touch. EH now concedes that no cement should be added to lime mortars. Perhaps the CO needs to engage in a little CPD (and if he or she is a member of the IHBC, as one would hope any conservation officer would be, then CPD to keep up with best practice is mandatory).

I was writing under a misapprehension and did him a grave disservice. OK, I'd be happy to put on an anorak and argue with him the pros and cons of a fat-lime/pozzolan mix, but I know that his recommendation would not be made without a good knowledge of the relevant facts and research.

I'm also certain that the mortar recommendation for the repair of the remainder of the fabric will be a lime mix; should one have the temerity to present him with a spec for a proper lime mortar I think he'd probably make some remark about grandmothers and eggs.

 

[Gareth Hughes]

I think at this point I should break cover and say that I (and quite a few of the longer-term inmates here) know who the CO is, and are also aware that the conditions in which he now has to work have been rendered pretty intolerable because of a (in this case Tory-inspired, but I won't take sides - just vote Lib Dem and see if it's any better, it can't be any worse than Labservative) sell-off of the entire planning department to a well known commercial management company, which means that the chances of going on a course to see latest practice and thinking are, for the CO in question, severely limited.

 

[Flyfisher]

I didn't feel it appropriate to name the CO in question (and still don't) but I've posted before about how helpful he has been during the planning and implementation of my project. That's also the reason I posted the details of how the mortar in question was specified and why it was specifically restricted to completely 'new' work.

As for CPD, knowing the person in question, I'd be confident he maintains a detailed and knowledgeable interest in all such conservation matters, whether or not his employers pay for such courses. I think it unlikely he'd be associated with so many award-winning projects, including at least one that I know of in his own right, without knowing his stuff.

I didn't know about the workplace situation that Gareth mentioned but I can only add that it has never seemed to affect the service and advice that I've been given; further testament to his commitment to conservation?

 

[MdB]

There is a also a good chance that your CO knows something that we don't and there is a good justification for the 9:3:1 mix. Would you be able to ask?

 

[OddJob]

Before we get too carried away, it is important to bear in mind that the stack is essentially a new-build rather than a repair.

I completely disagree,a new build is new from the ground up,building on top of the original stack is a repair/rebuild.
Not knowing the CO in question I would still like to know why a proper lime mortar was not specified.

 

[Gervase]

It may not be too clear from the photos but the "before" chimney was built with hard bricks and a hard cement mortar.

Which may be one reason why the CO suggested a slightly less aggressive mortar to match the new, softer brick. None of us actually knows the building in question, but I do know the CO's bona fides.

 

[OddJob]

It may not be too clear from the photos but the "before" chimney was built with hard bricks and a hard cement mortar.

Which may be one reason why the CO suggested a slightly less aggressive mortar to match the new, softer brick. None of us actually knows the building in question, but I do know the CO's bona fides.

Yes but my guess is only the stack above roof level was built in the hard cement mortar and was quite obviously not original,beneath that is most likely the original chimney in a softer mortar,so IMHO a 9:3:1 mix is still inappropriate.
Perhaps Ff could elucidate as to what the new stack was rebuilt upon?

 

[Flyfisher]

The new stack was built upon the original chimney stack, which is part of the original gable wall, and opens to a large inglenook. We're not entirely sure if the gable wall was built at the same time as the original timber frame, but it's certainly hundreds of years old.

However, isn't this 'new build' vs 'repair work' distinction a bit academic? Apart from the first course of new bricks that sit on the original fabric, the rebuilt section of chimney is most definitely completely new. If I was to build an extension it would surely be regarded as a new build and not a 'repair', even though it would be similarly bonded to the original fabric. We can call the 'rebuild' what we like but it doesn't change the facts of the matter.

The bottom line is that the rebuilt stack is far more sympathetic than the old one and no harm has been done to any of the original fabric of the building.

 

[OddJob]

A bit academic it may be,new the stack may be,but the fact is it's a rebuild on top of the original and my question is still why the 9:3:1 mortar?
Of course the rebuilt stack is a million times more sympathetic than the old one but I'm questioning the COs choice of mortar which can't be anything like the original.

If you built an extension you would be putting in footings and building from the ground up,that's new work.
If for example you were building on top of a collapsed section of wall that's a repair/rebuild.

In recent years I've had to rebuild large sections of two walled gardens,one circular.
In both cases the collapsed sections had been rebuilt at some stage(one having been blown down in the '87 storm,the other affected by roots)both had been rebuilt in a cement mortar and due to subsequent movement in the wall the cement sections collapsed again because they couldn't move with the rest of the wall,one of the repairs cost £25,000,expensive mistake to use cement.
Nice work for me.
I'm not for one minute suggesting the same will happen to your stack,I'm simply trying to make a point about what can happen if you use the wrong mortar.
Of course I don't know all the facts but I really can't see why a proper lime mortar wasn't used.

A few years ago a retired architect asked me to do some work on his Victorian house and greenhouse,as soon as I started work and he saw the mortar I was using he was on the phone to a colleague,both said they'd never heard of lime putty or hydraulic lime and said I'd have to put some PVA in it to "make it stick",trying to explain lime mortar to him was useless,he'd never heard of it so it didn't exist,so I lent him the Holmes/Wingate book and went away for the weekend,when I came back he still wasn't having it and insisted on a weak cement/hydrate mix,the same as or similar to yours,I refused so he said he'd do it himself which he did,that winter it all blew out.