What’s the best mix for rendering?

Roger helps you get the perfect ratio for your rendering mix.

So what’s the best mix for rendering?

It all depends on what you’re rendering on to.

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.

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. But 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 plastisier 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.

About Georgina Bisby

Georgina Bisby
Georgina Bisby is a journalist and editor with extensive experience covering construction and industrial news and technology, including a particular focus on health and safety, energy and environmental issues.

6 comments

  1. Avatar

    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

    • Roger Bisby

      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.

  2. Avatar

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

  3. Avatar

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

  4. Avatar

    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

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