I've an old (~100 years) stone-build house (sandstone? - not completely sure) which may or may not need repointing. And I'm not sure whether it should be lime, cement, or a lime-and-cement mortar that needs to be used.
Period property websites, and lime sellers insist that it has to be lime, or the building won't breath, become damp, and the stonework will crack.
The pointing man says the stone is hard enough. Cement mortar is fine - that's what he always uses - and it doesn't matter when you're doing 'sunken' pointing rather than the old-style 'raised' pointing. (Can't remember the proper terms he used..)
I'm not sure who to believe. Has anyone got any experience?
You need to find a builder who is knowlegable dealing with buildings of your type/age. If it really is sandstone and originally built with lime mortar, I would be very concerned about his suggestion to use cement based mortar. Most likely he has no knowlege of old buildings or experence of lime mortars.
Lime _renders_ are important (especially for "harling" in Scotland), but lime _mortars_ make zilch difference to any "breathability". They're too little surface, too thick in section.
If it is a lime mortar, stick with lime. This is for mechanical reasons though, not permeability.
We've got an old stone house (~ 200 years or more) with lime mortar between the stones and clay / cob / rubble infill between the inner and outer walls. When I looked into the repointing issue I too was told not to use cement based mortar. Looking at part of our house bears this out as the previous owner had pointed a small section using cement and the stone work around the mortar has blown. As I understand it moisture becomes trapped in the wall because cement mortar cannot breath. In cold weather this moisture freezes and blows the stone.
The purists say you should use slaked lime mortar, not the CaOH stuff sold in bags because it has partly gone off absorbing CO2 from the atmosphere. However, slaking your own lime from CaO is a tad over the top for me. Someone else recommended using hydraulic lime (grade 3.5) and I've been using that for pointing. It seems to work well. The main problem is the colour. It is very pale so I needed to add cement dye to match the colour of the ancient aged mortar.
I agree with the previous posters about it being the type of stone that matters. Soft sandstone or limestone will def require lime mortar, harder stones may not.
AIUI the problem with cement mortars is that they may block moisture wicking away from particularly wet areas of stonework. It's not their action in releasing moisture directly back to the atmosphere that's so important as not preventing moisture migration from wetter to drier areas and allowing the larger surface to do its work.
I've made up my own lime putty from builders lime, and given a sufficiently long soaking period has been successful. However I'm more likely to use a commercial lime putty in future.
Hydraulic lime usage seems more common in France - certainly it seemed commonly available in builders merchants in Southern Brittany - though the local building stone was granite.
I've also successfully used cement dyes in lime mortars.
Dear AJ The other posts are quite right. I recommend a lime putty as opposed to the bagged lime for reasons given. Advice generally is to be found in "Mortars, Plasters and Renders in Conservation" by Prof John Ashurst obtainable from the SPAB but I am told that some of his mixes he no longer recommends You cannot go far wrong with 3 to 1 sand lime Ignore your builder as clearly he has not a clue and go an find someone who has worked with lime ie knows the origin of the expression "knock up" appertaining to mortar. chris
The message from snipped-for-privacy@atics.co.uk contains these words:
A mixture of lime putty and hydraulic lime will give both a good initial set and maximum strength. That's the latest recommendations from the historic building preservation people. Source? One of my sons is an apprentice traditional stone mason with a firm specialising in conservation work.
For some time "experts" were recommending mixing a small amount of cement with the lime putty as it makes the mortar set quicker. However I've since read somewhere that this leads to "microcrystaline cracks" forming which weaken the mortar.
It seems like this topic is an ongoing issue with recommendations changing all the time. I do like the hydraulic lime to work with though as this does "set" fairly quickly. I still haven't been able to source any lime putty, but it is possible to buy CaO here and make your own but it seems like lots of extra work for unknown benefits.
Most of them have no real understanding of teh physics chemistry and mechanics of buildings. Like most builders in gact.
Not direct with stone, but the issues with l,ime versus portland are the portland is string, brittle and relatively impervious, whereas lime is wek, never truly sets hard, and is fully porous.
Where teh myths come from are that if there is damp inisde, the lime lets it out. Where the structure is rickety and foundations are poor, the lime will adjust. Whereas portland wont, and you end up cracking your fine crumbly tudor bricks etc.
Otherwise limes a crap material IMHO.
Id the foundations are good, and the stone is tough, and there is no evidence of movement, it doesn't matter except aesthetically: I managed to make the brickwork LOOK old by using a 1:1:4 (very roughly) mix of white cement, hydrated lime and sand.
If the stone is clunch, or maybe soft limestone or sandstone, then you may benefit from using a lime mix. Especially if the foundations are not that good. Otherwise really don't worry. If the stone itself is imepervious, the mortar isn't going to let it breathe anyway, and if its strong enough, the cement wont break it if movement happens.
There needs ti be a balane between 'museum restoration' and plain refurbishment of old, and not really that distinguished properties.
If todays 'period' crew had been around in the 16th and 17th century all those wonderful timber farmed houses that were infilled with - gasp - 'modern brick', and which have as a result survived when many others perished, wouldn't have been around for them to drool over today.
The question that is never addressed is to what extent the addition of pozzolans to make a hydraulic lime modify its basic properties. If it sets in a reasonable time, I don't see how it retains its self-healing properties
The message from Stuart Noble contains these words:
Which is why the mix must have lime putty. An initial set of some sort is desirable within a reasonable timeframe to keep the pointing in place, but the ability to soften when moistened and heal again on drying is vital.
Yes, but experience isnt what you need here. Either will work, so experience wont tell you much. Best thing to do is look at the writings of the researchers and orgs that use their work (eg SPAB),, and to a lesser extent sites that discuss and explain it (eg
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The short answer is use lime because:
Cement mortar frequently pulls the edges of the stone away when it finally gives up, in 20 years or more time. Theres a fair bit of such cement damage about.
Lime wicks and evaporates water from the wall, whereas with cement the evaporation will mostly occur from the stone faces, causing salt damage in time. Mortar otoh is expendable.
Minor building movement, which is common in old builds, will crack the soft lime, which is expendable. With cement OTOH the stone breaks, which is not good. 2ndly cracked lime can self heal on exposure to CO2 in the air, whereas cement can't.
The downsides of lime are:
very slow setting. dont add cement.
vulnerable to frost when setting: avoid exterior lime work this time of year
Overly sandy mixes are prone to rising damp, but there is no reason to use a bad mix of course. 1:2.5 - 3 is fine.
Bagged lime is fine if its not old and wrongly stored. Cement and lime can both go off, but lime looks exactly the same when it has, whereas with cement its obvious.
And yes, theres plenty of BS about. 'Cement has no problems' and 'you need putty' are the main variants. Just read the experts, eg the leaflets at SPAB, or the research papers if you want unnecessary depth.
Oh, ribbon pointing: it causes water splash onto the stone and reduces run-off. Consequently it makes the stone more vulnerable to freeze-thaw damage. Its also totally out of character. Its not recommended for any type of period property.
The message from The Natural Philosopher contains these words:
Portland cement is in many ways easier to use and is of generally consistent quality.
Not actually.
But on one of our very large buildings there is only one are where there is efflorescence on the interior plaster. And that's the one area where on the outisde the cement pointing of recent years has not been replaced by lime.
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