What’s the Best Mix for Rendering?

Roger helps you get the best mix

The best mix for rendering depends on what you’re rendering on to and the golden rule is you never make the mix stronger than the material that you’re going on to.

For example, lightweight concrete block that’s 3.6kN so we don’t want a really strong mix for this because if the wall moves, which it will as you always get some expansion and contraction as the seasons change.

You don’t want the render to be the thing that stops the wall from moving otherwise the render will crack. If you’ve ever wondered why does render crack, often this is a reason.

Also if you put too much cement in render it shrinks back and it will crack. People don’t always believe that, lots of people do a 3:1 mix and say it’s nice and strong but that won’t stop it cracking because there’s no real tensile strength in it so there’s no point putting too much cement in.

Best Mix for Rendering

Why lime helps make the best mix for rendering

Instead, we make a 6:1:1 which is six parts of sand, one part of cement and one part of hydrated lime.

The sand must be plastering sand or rendering sand, not building sand – do not use building sand for rendering because it shrinks and cracks.

Sometimes you can add some building sand to the mix for the base coat but it shouldn’t be your primary sand. It’s important not to put more than one part lime to one part cement because hydrated lime won’t set on its own, you need the cement to help it set.

The lime does give the mix body and make it creamy and gives it a certain amount of elasticity so that if the building moves the render will go with it a bit and if you do get a crack the lime in the mix will move into the crack and re-calcify to some extent. In a way, it is self-curing and self rendering.

A lot of renderers don’t use it and prefer to use plasticiser which puts lots of bubbles into the mix which is a good thing but the lime is better as we demonstrate in this video.

Roger also discusses the best weather for rendering.

Your Comments

forbidden forest – September 2019

Too many young builders get caught in the ‘brand new transit van and nagging wife for a bigger house’ brigade and before they know it they’re working 7 days a week and ripping everyone off so they can take their spoilt kids on a dull tour of Disney Land because their wife thinks they’ll like them more. Well here’s a message to busy builders everywhere – forget Disney land and a dull pile of plastic presents: buy your kids some second-hand books to read as this’ll give them the best life imaginable, then work 4 days a week so you can spend time with your family as opposed to throwing sh*t on walls all day while listening to talk sport and talking about talk sport.

stewie3ify – October 2019

I’ve got a house that’s been rendered on to concrete blocks and I’ve had quite a few long cracks I’ve had to chop the cracks out and patch them and this time I used plastering sand cement and lime I’ve been a plasterer for 16 years and the first thing I was taught when floating and rendering was the mix it’s important to always put lime in the mix never use building sand I’ve always done a 5:1:1 mix I hardly render these days but it’s fact to much cement and the wrong sand will make your render crack in time.


See more on rendering on Skill Builder.

About Dylan Garton

Dylan Garton is a co-founder, video producer and editor for the Skill Builder social media platforms.

19 comments

  1. Hi can you help. I’ve recently rendered a house and had major problems with cracking.i was told to use a 3-1 base coat using washed sand and ordinary cement with waterproofer which I think was fine but the top coat I was told to use a 3.5-1 top coat using yellow Sherborne sand with white cement {which I did question but was told to carry on} and finished smooth using a plastic float and sponge as it was drying there were slight cracks appearing but I thought nothing of them I rubbed over with my float and they disappeared but a week or so after there was crazing everywhere and one or two hollow spots so it all had to be taken off Can you help

    • Hi
      It would have been good to see some pictures of the cracks. The eggshell crazing is shrinkage and it always happens. If you had used some lime in the mix it would have filled those cracks but most people just paint the render and the cracks are hidden. They are not doing any harm so long as the render is stuck firmly there is no need to worry about the crazing.

    • I’m about to do the same. There’s so much conflicting information out there but it seems the most important thing is not to make the mix too strong. Something like 5 parts sharp sand, 1 part cement, 1 part lime. Saying that Roger suggests 6:1:1 above so I think I’ll go with that.

      Does base coat need to be stronger than top coat?

  2. Hi do you add waterprofer in scratch coat or finish coat?

  3. What would be the best mix and type of sand for internal rendering over breeze blocks before skimming with multi finish.

  4. Hi..

    I have a wall in the garden that was built using lightweight aerated blocks. It looks pretty awful so I am thinking about rendering it and eventually painting it. I have read that due to the porous nature of the blocks they are difficult to render and the render is prone to cracking due to the blocks absorbing the water from the mix. Can you give me any advice on how to successfully render this wall? Thanks

  5. Hi Roger,
    I have just finished rendering my garden wall. I wish to paint it white, is there a primer or sealant I should use before I
    paint the wall? Can you please advise.

    Thanks
    Bilal

    • Use masonry paint and thin the first coat down with a little water if it says so on the tin. The paint will soak into the render and form a good surface for the next coat. There is no need to use a stabilising solution or primer on new render.

  6. Hi Roger I can’t get Hydrated Lime at present due to corona what mix would you recommend? I have plastering sand, cement, SBR and PVA I might even have some plasticiser.

  7. Hi Roger on an internal wall how long do you leave between coats and howling before you can plaster multi finish?
    thanks Gary

    • Hi Gary
      If you are using sand and cement you really need at least 7 days for the moisture to come out and the shrinkage to stop. There will be hairline cracks in the plaster if you don.t do this. If you can leave it longer then all to the good.

      Thistle Hardwall is an alternative which lets you get on it the next day.

      Regards
      Roger

  8. Hi Roger,

    I’m taking down a wall which was probably built with sand and lime. I’m leaving a brick and a half from the corner and want to add strength to the corner wall which now supports a concrete lintel. What is the best mix to help strengthen the wall which tbf seemed relatively easy to chisel some bricks out. I want to cut a straight line rather than going gung ho with a lump hammer so I don’t disturb the corner.

  9. shelley ricketts

    Hi Roger , my husband and I are rendering our leaky garage using your video for help !
    We have done the first coat 4:1 mix with a 3in1 waterproofer ( this came up on our merchants when I typed plasterciser ) . How long should we wait before applying the top coat for this method ?
    Also , we have hydrated lime for the top coat what will be the best mix based on my bottom coat ?
    Thanks 🙂

  10. I am having to render to cover the brickwork mess..
    Never done before, cant afford to pay builder prices.
    Many health issues too.
    3 sides of house approx 100m2
    Can you help me work out quantities and tips for beading?
    Ive got 2k for this project inc scaffolding.
    At least we can do 1 side at a time (maybe back first to practice)
    1920s house..ex miners..and brickwork is a mess and surface of some bricks flaking away.

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  12. How long should I let 6:1:1 render on a parapet wall dry before painting it?

  13. Hi, thankyou Roger for your skills and experience.. please keep up the good work.

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