Lime to sand mix ratio?

[Keithj]

Our builder has "dispensed with the services" of the chap who was doing the lime rendering on our house. We were happy, but the builder has his own reasons that we wot not of. We've insisted that the person who takes over must be an expert in limework, as the previous one was.

The new chap has arrived, and tells us that the previous man was using the wrong mix on the outside of the house. He thinks it was haired lime putty and sand on a 1:1 mix, applied with a metal float. He wants to finish the job with haired 1 lime to 3 sand for the scratch coat, and unhaired 1 : 3 for the finish coat, with a wooden float to apply it.

I know nothing, and Google results on this confuse me more than a little.

Does the ratio make a difference, and who is right?

 

[Penners]

The following should give you some relevant guidance:

http://www.periodpropertyshop.co.uk/acatalog/Plastering.html

 

[Nigel Watts]

1:1 sounds too rich for an exterior finish. 3:1 seems closer to the mark.

 

[Keithj]

Thank you, Penners! All I wanted to know and a lot more besides.

A printed copy is now in my instruction book for care and feeding of a Tudor house.

 

[Keithj]

... and thank you, Nigel. Yes, 3:1 is clearly the way to go.

 

[robgil]

On a finish coat I would have thought 2 lime/5 sand would have been better? Unhaired. A scratch coat 3 sand/1 lime haired.
Also the environment and weather can play a part in deciding on a mix ratio,not to mention thickness of the layer to be applied and how big a job it is and how many people are working on it.
Then also each plasterer will have preferences as to the mix they like to work with.
Just my very unexpert opinion mind, I could be wrong.

 

[MdB]

I don't know the "correct" answer but there has been a debate about ratios in the past here. If I remember correctly analysis of old mortar shows ratios in the range 2:1 to 1.5:1 though it is unlikely the quality of the lime was quite as good as it is now so more sand in the mix is probably fine. If you are using hydrate or hydraulic lime (bagged lime) rather than putty then I would definitely be closer to the 2:1 (by volume) range rather than 3:1 due to the air that is in dry lime.

The ratio also depends on the type of sand used with the finer stuff requiring more lime to fill in the gaps. A well graded sand of different sizes will require less.

Mike Wye (a manufacturer of lime mortar) sells 3:1, 3.5:1 and 4:1 so that gives you another data point as well.

 

[Keithj]

We now have the answer. It's 3 chalk, one lime, goat hair for scratch and finish coat. That's what was being used before - not sand and lime. That probably explains why it didn't look very sandy - cos there wasn't any in there!

 

[robgil]

Isn't the chalk thing just an East Anglia thing and as far as I am aware they sometimes also put some sand in with the chalk.

 

[biffvernon]

It's 3 chalk, one lime, goat hair for scratch and finish coat. That's what was being used before - not sand and lime. That probably explains why it didn't look very sandy - cos there wasn't any in there!

I think that will give a weaker material, particularly vulnerable to frost action when damp.

 

[robgil]

Anglia Lime swear by the stuff, they reckon it's a traditional mix. There is a house here rendered in the stuff and ten years on its still doing a grand job.
I always thought it was a 50/50 mix chalk and putty, that's what I use anyway, you shouldn't need to add water in a 50/5t0 mix.
But I do agree, sand in there would provide a better fix.

 

[Keithj]

I think the original mix idea came from Anglia Lime.

The chap now doing the rendering worked with R J Hogg, and it seems their "standard" for top coat is 3 sand, 1 lime, no hair. He reckons that goes on more quickly, mustn't be trowelled with a metal trowel, and will be fine once it's carbonated. He also made all the comments about wetting the surface, covering it to prevent it drying too fast, etc.

I think it's a case where there is more than one way of doing it that will work fine - but also lots of ways that won't.