Lime render mix for breathable granite and cob interior walls?

[AmyJ]

Hello,

We have a granite stone cottage with interior cob walls. We mistakenly... (previous post - but our only major mistake thankfully ) removed the lime render from two interior walls. Finally we are getting the walls re-rendered.

The renderer has said he will first do a scratch coat of lime render followed by a finish coat of lime render. The lime render he will use is 4 parts sand to 1 part hydrated lime. There will be zero concrete in the mix.

- Is that the correct lime render mix for breathable walls?
- Is hydrated lime okay to use instead of hydraulic lime?
- Is 4 parts sand recommended?
- Is that the recommended way for these interior walls - scratch then finish coat?

Thank you

 

[jamest]

I'm surprised no one has responded to your message yet.

Anyway, my view would be as follows: by hydrated lime, I assume you mean the usual hydrated lime you get from builder's yards mixed with water to produce a lime putty. If so, that's a perfectly reasonable type of lime to use, provided you've got a relatively new batch of hydrated lime and not one that's been sitting around for years.

There's a school of thought that hydraulic lime is not really appropriate for interiors, so using putty would get the thumbs up around here I suspect.

4 parts sand to 1 of lime seems a touch on the weak side, hopefully your plasterer knows what he's doing!

I would check with the plasterer what he's going to do because there is a common type of plaster that is made from 4 parts sand, 1 part hydrated lime powder and 1 part cement powder. That's not what you want. What I've done and what others on here have done too, is first to mix the hydrated lime powder with water to make lime putty and then add the aggregate. Never tried mixing the hydrated powder with the aggregate first and then adding water (done that with hydraulic lime though) and so can't comment on its appropriateness or otherwise, but can't see why it wouldn't be fine to do with hydrated lime.

I don't see a problem with a two coat system, though 3 coats seems to be more common. I confess that I've plastered some internal walls with just one coat of putty-based mortar, but the result is rather 'rustic', which was what I was after. Having said that, all my plastering is rather rustic because I'm not a very skilled plasterer.

Have you considered adding some insulation whilst you're at it?

 

[aockenden]

Hmmm, yeah 4 parts sand to 1 part dry hydrate or 1 part putty wouldn’t do for me. 3 to 1 is the most sand I’d ever go for. And these days I definitely prefer 2.5 or 2 to 1.

The only way that 4 to 1 ratio would be lime rich enough for me is if his 1 part lime quoted is actually quicklime (as opposed to dry hydrate or putty), which expands in volume radically when reacted with water. Fresh quicklime probably expands in volume by 30-50% when slaked / hot-mixed, so a 4 sand to 1 quicklime ratio is probably equivalent to a 2.5 sand to 1 putty ratio, which is right on the money in my book.

If he’s quoting his lime ratios in units of quicklime then he’s likely hot-mixing on site. You can dig into the details with him.

 

[AmyJ]

I'm surprised no one has responded to your message yet.

Anyway, my view would be as follows: by hydrated lime, I assume you mean the usual hydrated lime you get from builder's yards mixed with water to produce a lime putty. If so, that's a perfectly reasonable type of lime to use, provided you've got a relatively new batch of hydrated lime and not one that's been sitting around for years.

There's a school of thought that hydraulic lime is not really appropriate for interiors, so using putty would get the thumbs up around here I suspect.

4 parts sand to 1 of lime seems a touch on the weak side, hopefully your plasterer knows what he's doing!

I would check with the plasterer what he's going to do because there is a common type of plaster that is made from 4 parts sand, 1 part hydrated lime powder and 1 part cement powder. That's not what you want. What I've done and what others on here have done too, is first to mix the hydrated lime powder with water to make lime putty and then add the aggregate. Never tried mixing the hydrated powder with the aggregate first and then adding water (done that with hydraulic lime though) and so can't comment on its appropriateness or otherwise, but can't see why it wouldn't be fine to do with hydrated lime.

I don't see a problem with a two coat system, though 3 coats seems to be more common. I confess that I've plastered some internal walls with just one coat of putty-based mortar, but the result is rather 'rustic', which was what I was after. Having said that, all my plastering is rather rustic because I'm not a very skilled plasterer.

Have you considered adding some insulation whilst you're at it?

Brilliant reply. Thank you. We've decided not to add insulation. Just go with the breathable house and heating.

 

[AmyJ]

Hmmm, yeah 4 parts sand to 1 part dry hydrate or 1 part putty wouldn’t do for me. 3 to 1 is the most sand I’d ever go for. And these days I definitely prefer 2.5 or 2 to 1.

The only way that 4 to 1 ratio would be lime rich enough for me is if his 1 part lime quoted is actually quicklime (as opposed to dry hydrate or putty), which expands in volume radically when reacted with water. Fresh quicklime probably expands in volume by 30-50% when slaked / hot-mixed, so a 4 sand to 1 quicklime ratio is probably equivalent to a 2.5 sand to 1 putty ratio, which is right on the money in my book.

If he’s quoting his lime ratios in units of quicklime then he’s likely hot-mixing on site. You can dig into the details with him.

Another brilliant reply - thank you. I had one additional question. Is there an issue with hot-mixing on site?

 

[AmyJ]

I passed on all this information to the renderer. He replied that he was not using cement. And was going to add fibres - like fake horse hair that was used in the past - which will help strengthen the lime without using cement. Is that correct - that fibres will sufficiently strengthen a 4 sand to 1 hydrated lime ratio? Thank you

 

[jamest]

Good that he's not using cement, but given that he's not, the mix is pretty weak at 4:1. Aockenden has been keeping us all up to date with his plastering activities and the results always look really good, so you can take his ratios as being close to best practice. I usually get mine ready mixed, so I just go with the supplier's ratios and they are usually 3 to 1 or stronger. I haven't mixed much of my own, so haven't tested what difference it really makes.

I have added fibres to my plaster and it makes it less prone to cracking - it's a bit like adding a mesh (which I've also done). A weak mix makes the resulting cured plaster or mortar more crumbly or friable - if you rub a very weak cured mortar with your finger it will break down and turn to powder. If you do the same thing with a strong cured mortar, it won't. I assume that your plasterer has done this before and so knows what the characteristics of the cured plaster are. I guess if he's actually using quicklime powder rather than hydrated lime powder to make the plaster/mortar, then it's probably fine. But if he is, let's hope he adds the fibres once the mix has had time to cool down otherwise the fibres will melt - the reaction between water and calcium oxide is highly exothermic.

 

[housegolem]

I've used 2.5:1 on all my scratch coat plastering.

Following a friends advice who works on old buildings/churches as well as the two reference books he lent me. Neither books give a weaker mix than 3:1 (bar a mix with a suitable earth added)

 

[Janela]

Yes, I also think a 4:1 mix too weak unless it is hot mixed which I have no experience of.
I always use 3:1 for the scratch coat and 'float' coat (although I never try and flatten out the lumps and bumps of the stone walls but follow their contours as I want to see that I have lovely old walls)
For the top coat I would use 2.5:1 for walls and 2:1 for ceiling.
I have always used a good hydrated lime (putty difficult to get here) which I mix up with the agregate (0-4mm sand for base coat and 0-2mm sand for top coat) and water and then leave to fatten up for a few days.
Or I mix the hydrated lime with water and let it fatten up before mixing with the sand.
I always use real horse hair in the middle coat, this definitely strengthens the mix. I never get any cracks and I believe this will help hold the plaster in place over the years and why old plaster can be blown but not fall off. The problem with artificial fibres is that all those left in the muck that is left in the bucket will be washed out in to the enviroment - lots of little bits of plastic going into the soil.

Don't use hydraulic lime on cob walls it is too strong and rigid, the more flex in the mortar of the walls the more flex you need in the plaster covering them.

Plastering is fun to do with lime especially if you want a more rustic finish maybe have a go yourself (save a lot of money)

 

[aockenden]

Another brilliant reply - thank you. I had one additional question. Is there an issue with hot-mixing on site?

No, not really. It’s just the way certain types of lime practitioner do things.

 

[aockenden]

Good that he's not using cement, but given that he's not, the mix is pretty weak at 4:1. Aockenden has been keeping us all up to date with his plastering activities and the results always look really good, so you can take his ratios as being close to best practice. I usually get mine ready mixed, so I just go with the supplier's ratios and they are usually 3 to 1 or stronger. I haven't mixed much of my own, so haven't tested what difference it really makes.

I have added fibres to my plaster and it makes it less prone to cracking - it's a bit like adding a mesh (which I've also done). A weak mix makes the resulting cured plaster or mortar more crumbly or friable - if you rub a very weak cured mortar with your finger it will break down and turn to powder. If you do the same thing with a strong cured mortar, it won't. I assume that your plasterer has done this before and so knows what the characteristics of the cured plaster are. I guess if he's actually using quicklime powder rather than hydrated lime powder to make the plaster/mortar, then it's probably fine. But if he is, let's hope he adds the fibres once the mix has had time to cool down otherwise the fibres will melt - the reaction between water and calcium oxide is highly exothermic.

You flatter me mate. I’m just an enthusiastic DIYer whose willing to get his ass kicked by making mistakes and learn the hard way.

 

[jamest]

You flatter me mate. I’m just an enthusiastic DIYer whose willing to get his ass kicked by making mistakes and learn the hard way.

That's what I like - you make the mistakes so we don't have to!
It's why I read this forum, helps me avoid mistakes