Need to re-point an old sandstone boundary wall (ca 130 years old). My son, who is competent at these things, is going to tackle it and believes that unlike modern brickwork etc he will need to use lime mortar. This appear to introduce quite a lot more faff - is it definitely the way to go?
Also, assuming we do go that way, recommended mix proportions seem to vary. Anyone got a recommended recipe?
You can go with a more traditional mix yourself, but there are companies out there who can supply pre-mixed and dry-bagged, just like cement mortars and renders.
I am fairly sure there is a lime expert / contractor on the forum, but I cannot remember who...
https://baumit.co.uk/guide/internal-plasters/lime-cement-plasters
https://www.lime.org.uk/products/lime/natural-hydraulic-lime.html
Worth it to prevent spalling of the sandstone. I'd just be careful (more so than cement mortar) around frost this time of year, though you'll need to cover it anyway to stop it drying out while it cures. The pointing itself isn't much different, it's more the prep before and care after. Have a look on the Scottish Lime Centre and Ty-Mawr websites / YouTube as they have some good info.
He is right
Non hydraulic lime is the way to go
Plenty of stuff on YouTube if you look
Thanks all, some good pointers there (pun intended 😉)
Yes, my parents had modern cement pointing in a sandstone garden wall and also part of the house and it ****ed it up good and proper. The stone is soft enough that Portland cement can do a lot of damage pretty quickly.
Lime mortar isn't really complicated or difficult, there are just a few things to understand and then it's a straightforward DIY job. I have done quite a lot round my house and it's held up well for a few years so far.
PS Think I use NHL 3.5 for general-purpose round the house (indoor and out) and garden use.
https://www.k-rend.co.uk/products/k-lime/setting-213-klime
Used this before. Can get in Jewson TP etc
NHL is better than cement but not by much. Unless the wall is very exposed, you’d be better off using non hydraulic lime. https://www.limebase.co.uk/
I'm sure perspectives differ but everything I've read - and what I was recommended by the expert I took a short course with - was that NHL 3.5 was very suitable for a wide range of uses. Yes for sure some walls are exposed, not to the level of a lighthouse but it's certainly not specifically for indoor use. Some hard-to-access joints were completely void, you could see right through and the stones had shifted, so I didn't want anything that might require future maintenance any time soon.
I have been involved in a few restoration projects over the last 10 years or so, with virtually every one using a lime mortar and lime plaster, to be honest, I prefer it now to any cement-based product.
You tend to have to match the mix to the brick or stone that you are using, in the case of a soft sandstone I would maybe think about an NHL2 6+1 mix with a plastering/sharp sand aggregate, the reason being with the sandstone being quite porous and holding a fair amount of moisture its an advantage to have a mortar just a touch more breathable and a mix a little weaker than an NHL 3.5 mix would offer.
That sounds like good advice, and surely better-informed than me. TBH I might just have got the 3.5 as it was available at the local builders' merchant (which might also imply it's widely used in this area). But I was definitely told one or the other 🙂
Actually, that's another thing - asking at the local builders' merchant may also prove useful as they are likely to know what others do in that area.
Yeah we are specialist lime contractors in Scotland, we work exclusively with lime mortars, clay and stone. Boundary walls make up a good part of our work, I guess many thousands of lime mortar mixes for repair and rebuild over the years.
First of all where are you? If like us you are in Scotland you are going to use a different mix to one you might use in the south of England, because weather. That said a boundary wall is still a structure that has no roof or guttering, so regardless of where it is, it will nonetheless be subject to rain directly on it and weathering generally.
In terms of a suitable mix for a typical not especially exposed (Scottish) boundary wall, we would used a gauged hot lime mortar which would be a mix of quicklime (the hot bit) and a Naturally Hydraulic Lime (NHL) - the strength of the resulting mortar would depend on the ratio of quicklime to NHL but would typically be in the 2.5-3.5 range for general pointing and potentially approaching 5 for the copes and any mortar facing the sky. In other words a stronger mix on the top where weathering is anticipated.
Now for the amateur the prospect of using hot limes can be a daunting prospect, and while we have successfully taught enthusiastic groups and individuals how to mix them, you do not need to to carry out an effective and hopefully beautiful repair. So a suitable non hot lime mix would be two parts sharp/concrete sand to one part NHL 3.5. In other words a 2:1 mix. It won't be as nice to use as a hotmix but it will be perfectly fine. A couple of people have mentioned bagged ready to go sand and lime mixes which you can buy off the shelf - these are fine to use but with one caveat, they are outrageously expensive! So any more than a couple of bags for smallish repairs they are not economical. Be cautious of buying lime from general builders merchants - if they are not selling lots of it then beware lime that is shot and no good. Also they probably won't have a clue what they are talking about.
In terms of physically mixing, you need to think about how much mortar you can actually use in a day. We can get through many mixes so will bang them out using a bell mixer, but an enthusiastic DIYer will need much much less so hand mixing or more likely an electric whisk in a trug will be best. Mix the powder and sand dry then add water.
A some have already alluded too, the key to good lime work is the preparation and aftercare. To prep, remove all the cement and clean out all the joints back to sound mortar. Get all the dust out. Where joints are wide, you are going to need stone pinnings within the joint to avoid large gobs of mortar which will slump and cure poorly. So prep up some suitable pinnings before you start and search through the grob you have raked out of the joints to retrieve any original ones. Before you start pointing, wet the area you are working on and wait for the shine to go ie it's damp to touch not wet to touch. Once you've finished pointing, you absolutely MUST cover the work. If the mortar dries out too quickly it will not cure and will ultimately fail. We use a layer of hessian sheet over the work followed by a layer of plastic (monoflex) tarp. The hessian stops air movement over the mortar, the tarp keeps wind and rain off and together they will also protect from frost. Even on a cool day wind alone will dry a mortar too quickly so don't skimp on your coverings - if you do you are very possibly wasting all your efforts up to that point.
The next stage is to scrape back the mortar. Essentially you need to remove the entire top 1-2mm of the mortar using a scraper. This allows carbon dioxide to penetrate the mortar and continue the curing process, and also means the mortar will be pressed back and tidied visually. If you miss this stage the mortar underneath the surface will not cure and you will be left with a thin crust of scabby pointing over a failed mortar. The correct time to scrape will probably be from 24 hours after you finished pointing or it could be considerably longer in colder weather. As an indicator, when you scrape you should get minimum build up of mortar on you scraper - if it is getting cludged up it is too soon. Once you have scraped, put the covers back up again - this is very important as the curing process is continuing.
My top tips would be to get yourself suitable tools for pointing ie a small tool and scraper. Invest in some decent tarps and hessian. Unless you have your pointing ducks all in one row, start on a section of wall you will never have to look at. Don't smear lime across the stonework. Wait til the chances of frost have gone, and avoid hot sunny days.
Happy to answer any questions
Ok it has a wee bit cement in it, easy to use with sharp concrete sand, made in France and no faffing about
Thanks a lot for all the feedback and really useful tips! Just to answer some of the Qs raised by Vader, yes, in Scotland (Edinburgh)
Its a low wall - 6m long, less than 1m high high, but has got built up garden behind it - not quite level with top of wall. The exposed face is looking east, so out of the prevailing wind and its a sheltered location It has a line of coping stones along the top so it will never really get lashed with rain the way the side of a building might.
Got all the necessary tools, and thanks for recommendations about materials. Would you recommend Jewsons or a builders merchants for materials? I know this is difficult, but even a rough idea of quantities of materials likely to be needed?
Vader,
I've an exposed, SE facing gable end of an Angus sandstone cottage, @ early 19th Century. Scottish Lime Centre repointed it about 15 years back and it's holding up just grand. What's current best thinking around a white lime wash for re-coating the surface?
Is there a tactic here that will help it last beyond one typically harsh winter or is this simply always going to be an annual re-'painting' job?
What's the benefit of 'hot lime' over slaked lime?
Hi Vader here, I've had to sign in on Madame's log in.
Myopic if you're in Edinburgh you're in luck as you can get everything you need lime wise from mason's mortar down on salamander street in Leith. I would suggest you use Otterbein NHL 3.5 lime which they will have plenty of. They also sell bagged sand, it will probably be a bit more expensive that a general BM but will be your best chance of finding a well graded sharp sand with a good colour. Sounds like you have about 6sqm to point. I'd suggest buying a bag of Otterbein and 4 bags of sand to start and seeing how you go. You may well need more but you should try and avoid having to store lime if you aren't going to use it all straight away. Better to get fresh when you need it. When you measure out use a bucket or ice cream tub to get your ratios spot on, don't go by trowel scoops.
Re the wall it sounds like you may need to think about some weep holes, especially if it's wet behind.
Highlandman you have a couple of good choices for limewash. Starting from scratch you'd want to apply a minimum of 5 coats to bare wall. It sounds like you have some on already, but if it's well worn back then now's the time to get the coats on. You can either buy ready made or make your own. If you go ready made use Buxton Superfine limewash, it is very good. You can get it from mason's mortar branch in cowdenbeath. Make sure you water it down, it is far too thick out of the tub regardless of what anyone tells you. Ideally it will be double cream/full fat milk consistency. You want many thin coats not few thick. To improve the weather resistance you can add some st astier NHL 3.5 powder, mix it separately in a bucket with water and whisk so it's same consistency as the limewash and then add it about 1 to 4 IE not to much. Lastly if you are confident you can make your own by slaking Buxton quicklime. You will get a great hot limewash but ensure you take appropriate precautions IE goggles gloves etc, and slake in a metal bin or bucket and make sure you have plenty of water on hand. Things will get very hot very quick if you aren't very careful so use very small amounts to start. You can store what you make for ever really and water it up when you need it as it will look like cream cheese after a few months
Iscoffcake hot limes have several benefits, the heat created in the slaking process will create airspace in the mortar structure which helps reduce frost damage by moisture expansion. The hot mortar will expand into joints giving a good bond. The mortar becomes very hot and sticky, great adhesion and nice and plastic to work with. When you slake quicklime it doubles in volume which means you need less - in a guaged mix this means less powder and therefore cost.
Great, Vader, thank you! There weren't any weep holes before - ground area between the wall before you get to house that very big (less than 2 meters)and it is on a hill with the downslope running in direction of wall, so flow of any ground water would be parallel to the wall needing fixed, rather than towards it. I will check the other wall though now you have mentioned this!
@vader I have a house which needs repointing - just the ground floor is in stone and the cement based mortar is coming out in lumps. I’ve been quotes horrific amounts to have it repointed and was wondering how hard it would be for a confident DIYer to do. The pointing would be about 2cm wide at the most and is even as its square stone. I can see that the house was built with what we call “black lime” mortar underneath the rubbish cement mortar - would this cause a problem? It’s quite dusty when disturbed.
The one thing that concerns me is the covering after pointing - how do I cover the side of a house with hessian?
The other issue might be that I’ll have to do it slowly as I’ll be a novice - might take a while to complete.
If it’s a big cost saving I really want to do it myself, I should be able to get scaffolding in for a couple of sections which are too high.
b230ftw yes if you are competent the short answer is yes. To be fair homeowners we know who are handy have had a couple of days training by us or someone like Ty Mawr so worth thinking about that too. There's no need to go quickly, take your time, avoid hot weather and keep things under control.
I suspect your black lime has fly ash in it, I have never seen or used it but it was added as a pozzalan to the mortar and will increase the strength by virtue of a chemical set. I guess you live somewhere where it was easily available, maybe near mining so it may have been a free or very cheap by product. It won't cause a problem, just rake back to sound mortar before pointing. Ideally the joint should be a minimum of twice as deep as it is wide. If you want to repoint in a black mortar you could add an earth pigment from someone like Fiddes Supplies. I don't know where you get fly ash these days.
As far as covering, we would typically hammer a nail/peg type thing into a joint then set up a rope to hang from. Also down pipes, guttering that sort of thing. Don't forget a tarp over the Hessian too, aftercare is very important especially on smaller joints that can be prone to drying too quickly.
