I'm about to do some repointing and general repair to some brickwork. At this stage, all to the internal face of a 9" external wall, Victorian, fairly soft/crappy bricks with the original lime mortar. It doesn't really matter what it looks like as it won't be visible once the project is finished.

So, have been trying to read up on lime mortar and get my head around the options. Unfortunately it seems the more I read the more conflicting advice I see.

That cement mortar is a big no-no I get, and understand the reasons.

As far as I can see, the options for lime mortar are:

1) Lime putty. Non-hydraulic. Mix 1:3 with sand, slow setting.

2) Natural Hydraulic Lime (NHL). Again mix approx. 1:3 with sand. Fast setting. NHL3.5 seems most appropriate to my needs.

3) Cement:Lime:Sand mix using hydrated lime(ie non-hydraulic, bagged lime as available widely from builders merchants). Mix 1:2:9 (or 1:1:6 where more strength desirable). Fairly fast setting on account of inclusion of cement.

I think I'm going to rule out (1) because I'd like something fairly fast setting, especially at the moment with cold weather, and because I've seen comment that it's a bit more difficult to work with and I'm not a bricklayer.

Option (3) is attractive because the hydrated lime is cheap and easy to come by. The potential concern is the inclusion of the dreaded cement, but this kind of mix seems fairly widely recommended for repointing historic brickwork. I am however aware that thinking changes over time and this advice might be outdated.

The reason I'm a bit sceptical about option (2) is that most of the advice saying that I should use this comes from people who are trying to sell NHL. Flexibility and breathability are mentioned as the benefits but is there really a big advantage over say a 1:2:9 mix where the cement content is pretty low?


Any thoughts appreciated!

I'm by no means an expert but sources I'd trust include SPAB and Building Conservation. Here are some links (there are probably others on the sites, as well as references):

http://www.buildingconservation.com/articles/pointing/pointing.htm
http://www.buildingconservation.com/articles/plaster/lime-plaster.htm
http://www.spab.org.uk/advice/technical-qas/technical-qa-11-repointing/
http://www.spab.org.uk/advice/technical-qas/technical-qa-21-inappropriate-cement-pointing/
http://www.spab.org.uk/advice/technical-qas/technical-qa-14-lime/

There's also http://www.shetland.gov.uk/planningcontrol/advicenotes/documents/AdviceNote13-LimeMortarsandRenders.pdf

As far as very low temperatures go, the rule of thumb seems to be - just don't, wait for warmer weather.

The articles say the mortar should be softer than the bricks, so if you've got soft bricks, you need soft mortar, which tends to indicate (1). The articles recommend against cement and against the use of bagged hydrated lime (though I understand it can be OK if it is really fresh). Instead of considering adding cement to speed up setting, consider adding a pozzolan.

how about slaking your own lime!

hydraulic lime in tubs is better than bags.

Avoiding cement = avoiding future problems, fast set is not a relevant consideration more so if you cant see it all when finished

buy your bagged lime now, put it in a plastic tub and cover with water.

When the weather improves use your new lime putty.

Thanks for the various comments/links so far.

So the consensus seems to be that putty is better than NHL, and that any kind of mix involving cement is completely out.

Fair enough, but I find it quite frustrating that so much of the advice is stated without anything terribly objective to back it up.

Why exactly is putty preferable to NHL for example? How critical actually is the difference...? several of those articles just make a kind of vague statement that if that's what was originally used, then that's what should be used now, but that in itself doesn't seem to be a particularly logical reason to make the decision.

And as for the inclusion of cement in mixes...so cement decreases flexibilty and breathability - that's fine - but in a mix with a small amount of cement how much impact does it really have? Does it make it 95 percent as permeable or 50 percent as permeable....has any actual objective research been done?

This article is kind of interesting...seems to imply that guaging with cement may be ok in some instances and also makes the point about relative costs of different options.

http://www.ihbc.org.uk/context_archive/63/limemortars/hydraulicity.html

How much of this insistence on, say, the superiority of (expensive) lime putty is drawn from aesthetic preference rather than objective assessment, I wonder.

I'm not in any way having a go at anyone who has posted on this thread by the way....just expressing my frustration as someone who just wants a straightforward, objective answer to a seemingly straightforward question.

Lime pointing is crucially involved in the processes involving moisture in the wall, it sucks up any moisture, water, wet or damp as it is highly hygroscopic and transports that moisture away to the outside.

It keeps the bricks dry too by the same process.

Lime is much better to work with when it is fully hydrated, it will work from bags, cheaper out of bags but sometimes it can be quite old too.

Posted By: tonyLime pointing is crucially involved in the processes involving moisture in the wall, it sucks up any moisture, water, wet or damp as it is highly hygroscopic and transports that moisture away to the outside.

It keeps the bricks dry too by the same process.

Sure, I understand this, but how much more hygroscopic is it than say a 1:2:9 cement/lime/sand mix? I can't find anything that actually states in figures how much of a difference it makes.

Posted By: Peter_Sthis article sets out most of the points you raised, but if you need more science, then your getting into the building physics and chemistry fields..

http://www.buildingconservation.com/articles/cement/cement.htm

Thanks, that clarifies a few things. Amongst others that a 1:1:6 mix is better than a 1:2:9 mix if you are using cement.

Interesting that it implies that hydraulic lime is somewhat impermeable....although it doesn't state *how* impermeable compared to a cement or non-hydraulic mix.

I wonder what people think of this piece - expresses some similar doubts to those that I have -

http://markbrinkley.blogspot.com/2005/12/why-build-with-lime-why-indeed.html

It sounds just dreadful, doesn’t it. “Major hazard”, “clogging”, “greatly reduced porosity,” “seriously weakened”, “poorly formed pore structure”, “susceptible to frost damage and deterioration.” What a catalogue of woe! What we have here is an attempt to drive a wedge between the purists, who are delighted with the results from the Smeaton Project, and the mixers. But there is something just a little bit too alarming about these findings for my liking. Whilst they may have found segregation taking place on certain mixes, how can they be so sure that all these horrors will result? After all, the test of a good mortar is how it behaves after 50 years in place. The choice of language used by Graham in this piece is, to my mind, a dead giveaway that we are not seeing pure science at work here, with its understated conclusions and room for doubt, but an attempt to rubbish a practice that has been in widespread use since the 1830s.

This is interesting....

http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/nrcc20148/nrcc20148.pdf

Posted By: Nick Parsons''Once you have tried it you will wonder what all the fuss is about, and will not want to use anything else again.'' So yes, why not?

Well, the extra cost, mainly, would appear to be the main downside.

I've always found these document to be an interesting read, mainly because of the illustrations -

The use of lime & cement in traditional buildings

http://www.historic-scotland.gov.uk/informguide-limeandcement.pdf

http://cadw.wales.gov.uk/docs/cadw/publications/Tech_Con_2_Lime_Mortar_EN.pdf

6:1:1 is the old "restoration" mix, used by me (and MANY others) for years without apparent problems.

But note the "apparent". I become good friends with a local assistent-CO during his tenure on our patch. He'd done his master's dissertation on lime and took a lot of time to explain in detail why an unadulterated mix was better all round. He didn't lecture in the way that lime purists do when talking to those plebs who lack the capacity to understand natural processes, that capacity which makes them so superior to heavy-handed trades people. (HEAVY IRONY WARNING!)

Of course you can't see what's going on unless you get it completely wrong and you go back the next week to find half the wall on the floor or the next year to find half the wall spalled all to hell. The processes are more subtle than that for all the reasons explained in previous posts - and other threads ad nauseum.

Having said that, the Italians don't seem to be quite so particular. They'll happily bond masonry (brick or stone) in historic buildings with cement and then POINT with lime to achieve the "authentic look". Even in earthquake zones! And they discovered/invented lime mortar!

But having tried it the "proper" way I'm inclined to stick with it now because it's much nicer to use. Significantly, the two "traditional British brickie" guys who've worked with me on a regular basis have, after moaning about "all that twaddle" and how they "couldn't guarantee it" have come around to seeing the sense of it and have actually both said how nice it is to work, to the point where they now recommend lime rendering to all their customers. I won't say anything about how they've also caught on to the fact that they can charge more for it by cashing-in on the premium that attaches to the green-cache. Love it.

Define 5% in shovel equivalents.

Likewise Limegreen of Much Wenlock. Always helpful.