How to Mix the Perfect Concrete Render
Rendering a wall is the kind of thing usually best left to professionals, but in a world of endless YouTube videos tutorials and DIY blogs, it’s only natural that many people will feel the urge to take the occasional task on themselves.
When it comes to rendering a wall, there can be a lot of conflicting information out there, whether it is how you apply the render, what the best ingredients for your mix are, or what the ratio of that mix should be. As you may have guessed from the title of this post, it is the latter two that we are concerned with here.
Getting the perfect render mix involves not only using the right ingredients but mixing them in the correct ratios and applying them in good time. Getting any of these aspects wrong can result in a subpar render, and a costly mistake if any of your work needs to be redone.
There are certain things you should aim for with your render mix, and we’ll go into how you do that with your mix shortly. For now, let’s take a look at what those traits are and why.
The first thing is that your render mix should not be stronger than the materials that it is being applied to. This includes both the primary construction material—such as bricks—and the secondary material—such as mortar. The reason for this is that the structure will move over time, as things expand and contract at different rates in the changing temperatures. If your render mix for concrete blocks is stronger than those concrete blocks, the render will resist that movement and, eventually, crack.
Another thing that can contribute to render cracking is the amount of cement used in the mix. Some people mistakenly believe that more cement is better because it makes the render stronger. This isn’t necessarily a good thing, both for the reason mentioned above about being stronger than the structure below it, but also because cement contracts as it cures, and too much cement in your render can cause it to crack.
What Ingredients Should my Mix Have?
Not every job is the same, and some jobs will obviously call for slightly different mixes. That being said, for an excellent all-round mix, you should be aiming for the following;
- Six parts plastering sand
- One part cement
- One part hydrated lime
It is important that you use plastering sand in your mix, as building sand has a tendency to shrink which, as we’ve touched on above, generally leads to your render cracking.
It is also important to make sure you keep your cement and lime evenly balanced, as these two components work in tandem to provide different qualities to your render. It is possible to render using a plasticiser instead of lime, but a lime render mix has many beneficial qualities, such as giving the render a degree of elasticity. This elasticity will allow the render a certain amount of movement, which should prevent it from cracking as the underlying structure contracts and expands throughout its life. The lime in the mix can also help with cracks by calcifying over them, almost like a self-repairing feature. It is important to keep the lime cement render mix ratio one to one because they are essentially providing opposite qualities. The cement is providing the strength, while the lime is providing flexibility.
If you choose to go with a plasticiser instead, the render mix ratio without lime is basically the same, with plasticiser taking the place of the lime in the 6:1:1 ratio.
Render Mix Consistency
The consistency of your render mix is something that you will have to judge by eye. If it is too sloppy, you will have difficulty getting it to adhere to the surface. If it is too thick or dry, you will have trouble getting a nice even coat.
As a general rule, your render should be easily workable but cohesive enough that if you were to flip your hawk upside down, the render would not slough off. It is also crucial that you ensure the consistency is smooth, as lumps will be a big problem for you in the final stages if they find their way into the mix and are allowed to cure on the walls.
It is also worth noting that rendering in hot temperatures will generally cause problems over rendering in cooler temperatures, as the water in the mix will dry out far more quickly. This will lead to the mix becoming unworkable far more quickly than it otherwise would have done. If you find yourself taking on rendering work in the summer, consider getting up that little bit earlier and getting as much as you can do done before the sun really starts to cook.
There are many different applications for render, so you may find yourself wondering what you need to do differently for interior rendering or rendering where the surface needs to be waterproof.
If you are wondering what the internal render mix ratio would be, the good news is that it is exactly the same as the external mix. The purpose of the render is the same, whether it is on internal walls or external walls. This probably brings up the next obvious question; what about waterproof render mix? Exterior walls are subjected to the elements and need to be able to withstand rain, snow, and wind, surely they have a different mix.
Well, actually, they don’t. Cement-based render is not designed to be exposed to the elements in this way. Once your final render layer is complete, you should be looking to coat it in some kind of weather-proofing sealant, and you will likely want to give it a coat of paint as well. It is also standard practice to stop the render at least a couple of brick courses above ground level to prevent water from creeping up behind the render and causing all manner of problems.
Ready Mix Render
Convenience is the name of the game in our capitalist culture, so it should not be surprising that there are ready mix options available. For the most part, there is nothing wrong with ready mix render products—they will do the job just as well as a self-mixed render. In fact, if we are talking about DIY renderers, the ready mix option will often do a better job, since the ratios used in a ready mix render will be carefully managed—likely automated—and are guaranteed to be correct. Home mixes—especially by non-professionals—can be a bit out of balance, or not mixed properly.
That being said, the main argument against ready mix options is one of economics. You will nearly always be able to get the components parts for your render mix more cheaply than you can buy a ready mix alternative for the same quantity of render.
If you have a particularly small area to render—perhaps a touch up or repair—ready mix options start to look more appealing, as the cost benefits of mixing your own are less impressive when you only need a small amount. But if you are embarking on a large project, you will almost certainly be better off from a financial standpoint buying the ingredients and mixing your own.
Practice Makes Perfect
Since we assume most professional renderers will not need to read a post on the proper render mix ratios, we thought it worth including a little bit of advice for the DIY and beginner renderers out there.
As with most things in life, practice makes perfect. Now, you may be wondering how you realistically practice something like this without wasting a whole lot of render or potentially making a mess of someone’s wall. And the answer is the scratch coat.
There are multiple layers of render to apply, with the scratch serving as the interface between the underlying structure of the wall and the top render coat. The scratch coat is different because the goal here is to get the best adherence to the underlying bricks as possible. The scratch render is often thinner than the top renders, and the renderer will try to make sure the scratch render works its way into all the nooks and crannies of the bricks. The other aspect of this coat is that, because it will be covered up by the top render coat, the finish does not have to be perfect.
Now, don’t get us wrong, the rougher your scratch render finish ends up being, the harder a job your final render will be—there is no getting around that fact. But the point here is that you don’t need to have a perfect finish, which makes it an ideal vehicle for getting some practice in. Granted, you do need to make sure that your scratch coat is properly worked into the bricks, but you have a little more leeway when it comes to the finish.
Once you have had a bit of practice here and feel comfortable that you can pull off an acceptable finish, you can move on to your final render!