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.