Render A Stone Wall: What, Why, How?

Rendering stone walls is a great way to breathe new life into an otherwise tired or outdated wall. In short, a render gives masonry and stone a thin layer of concrete mixture to give a single, cohesive look. Among the many benefits of rendering are its adaptability, ease of application, and the number of customisation options you have when doing so. We recently ran an article on doing an internal render, but what about one outside? Is it still viable, and if so, how do you do it? There’s only one way to find out – keep reading about how to render a stone wall.

To render a stone wall, you need a clean surface, proper render mixture, and a steady hand. After that, all that’s needed is time.

While the above-bolded statement is true, it also leaves out a few important details, so don’t go anywhere quite yet. We want that render to look beautiful, and that comes with time and preparation.

Render a Stone Wall – Basics

So first things first – what is a stone wall render? It’s (basically) concrete with a few added ingredients. Your mixture will comprise of water, concrete, and sand of varying grit. Before you can get started on the render, though, we need to clean the wall. Just like applying a layer of plaster or paint, it’s vital that you thoroughly clean your stone wall. Otherwise, you risk the render not sticking, or worse, gaps where mould and gunk can grow between the render and wall – that’s no good. But we’ll get to that in time.

Into The Mix

Usually, you apply render to create a strong, solid, and waterproof layer between your stone (or brick/masonry) and the outside elements. Ensuring you have the proper mixture for the job is vital to success. Coarse sand provides a grittier texture and added support, while fine sand is good for ensuring the finish is nice and smooth.

Depending on what you’re rendering, your mixture will need to change ever so slightly. For example, if you’re rendering a chimney stack, you’ll need to add a bit more coarse sand to ensure it’s stronger than the average render.

If you’re unsure of how exactly to mix your render, you’ll want to consult with a professional. You don’t have to pay them to do it, but asking for basic mixing guides will prevent mistakes further down the line. You can find a list of the minimal required ingredients and tools in the article linked above, along with some very basic guidelines for how to do so. Nonetheless, I highly recommend getting your render mixed by a professional or at least having the mixture percentages approved by one.

Factors to Consider

Beyond dirt and grime (an important factor to keep in mind), the most important part of mixing and applying render is to know how cold things are going to get. Renders are best left to be done in the morning of warm days. This is because concrete render has water in it – and what does water do when it gets cold?

That’s right! It freezes and expands. And if you’re not careful, it will expand inside your render. To prevent this, you’ll need to either:

  1. Save the render for a warmer day. This is the best choice if you’re considering rendering in the middle of winter. A warm day will allow the render to set properly and reduce any expansion that may occur on freezing (or near freezing) nights.
  2. Cover your work. This is best if you can’t just wait (or you applied the render before reading this). You can use hessian sacking or an old blanket, really anything that will insulate your render from the elements. Add a bit of plastic sheeting or a tarp if you’re expecting rain or snow.

Applying Render to Stone Wall

Now we’re on to the fun part – the actual rendering! Let’s get to it because there’s still a bit of ground to cover before we wrap for the day.

Render #1: Prep

This is the most important part of the whole render. Before you start, be sure to thoroughly clean your work surface before beginning. You need to get off anything that’s tuck to the stone: moss or plant matter, dirt, debris, paint, and mould. If it’s moulded, you can use fungicide or diluted bleach to kill the mould.

After washing it down, be sure to wait until the surface is damp, but not dripping. That may mean waiting up to a day or just wiping it down very well. Some moisture is needed to bond the render to the stone, but too much can result in it not fully setting. This means that rain will straight up wash the render away if it hits before it’s fully dried. That makes picking your time even more important than you may have thought.

In short, clean your surface and make sure you have enough time to apply and dry the render.

Render #1: The Layer

Okay, your surface is cleaned and your mixture mixed – now what? We render, of course! While it’s not technically necessary, I highly recommend installing temporary 15 mm wood battens as a framework. You can also use wire mesh, but that’s gonna be attached to the surface, whereas you can remove the battens if desired.

Place your battens roughly 900 mm to a metre apart and ensure the screws are flush with the face of the timber. You’ll be using these to level out the render later down the line, so this is important. Crooked batten equals crooked render later on.

Apply the render to the damp stone with a plastering trowel. Press firmly and make sure it’s stuck before moving on to another section. This first layer is called the “scratch coat” and should be relatively thin – roughly 5 mm. If it’s too thick, the stone won’t suck in that first layer, which is vital to creating a proper seal. This is the same as applying a mist coat when painting – it’s more of a primer than an actual layer.

Scratch your work surface (hence the name scratch coat) with a comb, brush, really anything works. Hell, I’ve seen people use nails hammered into scrap wood -as long as it’s roughed up, you’re good.

Be sure not to scratch all the way down to the base layer. You really only need to scratch up the scratch coat of render.

Render #2

Give your scratch coat time to set. This should really only be about an hour at most, assuming you didn’t layer it too thickly.

The second coat should be roughly 1 cm thick. Build it up to the batten level (slightly over) and level it once finished with a straight edge and level. You can use scrap wood or a trowel to do this.

Work bottom to top to prevent dripping.

You will likely leave a few small holes as you level the render. Just slap a bit of extra render in the hole and level it like the rest of the layer. Just be careful not to use too much, or it’ll set with a weird texture.

Leave the second layer of render for an hour or so, and remove the battens. This is when you fill in the gaps the battens left, using a trowel and level to ensure it’s all even. Leave it to dry for at least one more hour.

Floating the Render

The test to see if you’re ready for this process is simple. Push a rendering float onto the surface. If the render resists and doesn’t bulge around the float, you’re set to start floating.

Take your render float and push it firmly onto the surface. In a slow, circular motion, cover the whole surface. This smooths everything out, creating a cohesive final look for your finished render. It’ll take a bit of practice and failure, but you’ll eventually get the ideal movement and pressure down. If you’re struggling, check out this video for guidance.

If you opted to not use battens, this is likely where you’re kicking yourself. Their biggest benefit is that you’re able to get an approximately level surface before this stage. If not, you’ll be left with a bit of extra levelling and detail work.

Failure to get a truly level and even layer of render can result in cracking when it dries. This will occur most often in areas where thick and thin bits of render meet.

When finished, sponge down your final coat of render. Use a car sponge that’s lightly dampened to give it a final pass. You’ll find that this results in a silky-smooth finish with no holes. The benefits of this include waterproofing, appearance, and it’s easier to paint over – which you’ll likely want to do.

Other Tips

It doesn’t matter how long you have between coats of render. You can allow the skim coat to fully dry over a week if desired, just be sure to do one thing. What is that you may ask? Well – you already know. Dampen your work surface. This creates a seal and prevents the wall from absorbing the water in your mix. If this happens, you’ll have it crumble off in pieces. Treating your surface with a slight bit of moisture will also result in a slower drying but stronger render when completed.

Another important thing to note is that homes that used lime concrete (older homes, usually) will require that you use lime mortar. Lime does the same thing that concrete does (holding things together), but it has a more important function. Lime acts as a breathable surface in relation to evaporation. This gives the render a bit more flexibility in terms of moisture (and actual flexibility). Lime is generally mixed 3:1 with water, though each professional has their own secret formula.

Alternatively, you could purchase or home-mix a concrete/lime mixture for rendering. Some people sell these mixtures, while other professionals do it themself – it’s up to what you can find around you. I don’t recommend trying to DIY mix the two unless you’re very confident in the mixture ratio, as messing it up can cause the render to fail or underperform.

Final Thoughts

Putting a concrete render on a stone wall is a great way to breathe a bit of life into old, tired walls. Whether you want a modern, smooth concrete look, or just want to paint over it, it’s a wonderful option. There are, however, a lot of things to take into account when laying render. The cleanliness and moisture of the wall will affect how well the render sets. Cold temperatures and rain can ruin or wash away undried render, and more.

To lay render, it’s best to use battens to get a level render if you’re inexperienced. This will also help the floating stage go a bit more smoothly (pun absolutely intended). Be sure to do a scratch coat that’s roughly half the thickness of the second layer, which should be roughly 10mm. And if you’re unsure of your ability to properly do this, hire a professional! A good render takes a skilled and steady hand, so pay someone with the experience to get the best possible final product.

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About the Author Harry Thompson

Involved in home renovations throughout his life, Harry is an expert in everything to do with home and garden DIY. In his spare time, he enjoys cooking and tending to his garden.