Horticulture Magazine

How To Render Exterior Walls

man using trowel to render an exterior wall

Do you get the urge to get your tools out when the warm weather rolls around?

Heading out into your garden and finally getting round to those jobs you can’t face doing in the chilly autumn and winter months is a great British tradition, and rendering an exterior wall is a job that may very well be on your to-do list.

close up of a wall being rendered in grey
This is a real photo, not a render

In this article we’re going to run you through the why and how of rendering exterior walls. If you’ve never done this job before there should be enough information here to get you started, or if you just need a recap we’ve got you covered as well.

What is render?

Render – also called stucco – is a construction material used to cover surfaces. When you mix render it’s a very thick liquid, then it sets into a hard and dense solid.

The surface coating that render provides is strong and waterproof, making it a popular choice for exterior walls.

Why do you render a wall?

As we’ve just touched on, render is very strong and durable.

The tough coating it provides can protect underlying surfaces from damage, and also from the ingress of water. For a breezeblock wall facing the elements, for example, a layer of render can prevent damage, provide insulation, and protect against moisture.

Render is also versatile in aesthetic terms. You can apply it plain for a flat and smooth surface, or you can use various techniques to create interesting ripple effects.

It’s also possible to mix different colours of render, and to paint the render surface to access an even broader palette.

worker applying mottled effect exterior concrete
This decorative mottled effect is easy to achieve

How to render an exterior wall

Now that we’ve introduced you to the concept, let’s take a look at the actual job of rendering an exterior wall.

First, we’ll tell you what you need, and then we’ll run you through the steps.

What you need

  • Render mix for the scratch coat in your desired colour: you can buy this premade, or you can mix lime, sharp sand (rendering sand or plastering sand for a cleaner mix), and water
  • Render mix for the top coat: again, you can buy this premade or you can mix 5 parts sand, 1 part cement, 1 part lime
  • A container for the mix to go in. A big bucket is probably favourite
  • A shovel for mixing
  • Dust sheets or something similar that you don’t mind getting messy
  • A steel trowel
  • A hawk (not the bird – see the picture below)
  • A straight edge
  • A scratching comb
  • A sponge
  • A spray bottle
a man with a hawk and trowel
The big square on the right is called a hawk: that’s where the mixture sits

How to render a wall: Step by step

1. Prepare the area

First, chip away any render or paint that’s already on the surface, and then scrape the wall with a stiff-bristled broom to remove any dirt, mould, moss, or other bits that might be lingering.

Next, spray the surface with water to slow down the rate at which the cement dries out when you apply it. This makes it easier to work with it.

Then, lay sheets or wooden planks on the floor near the wall to catch any bits of mixture that fall. This is less important outside than when you’re working with indoor floors, but we still recommend it.

2. Get your render mix ready

Create your mix in a bucket or similar container by following the instructions on the packet. This should involve pouring water into the container, adding powder, and then mixing thoroughly to achieve a pasty consistency.

3. Apply the scratch coat to your wall

First you’ll want to scoop a nice dollop from the container with the end of the trowel. Then, whack it on the hawk so you have something similar to the happy fella above.

Get a little bit of mix on the trowel, then apply to the bottom of the surface and smooth gradually upwards.

At first you’re applying something called a ‘scratch coat’, and it should be about half a centimetre thick. This coat seals the surface beneath, and provides a textured surface for the top coat to hold onto.

While you’re applying the scratch coat, hold your hawk underneath the section you’re applying to catch any mix that falls. Although you’ve got dust sheets down, catching it on the hawk makes it easier to reuse.

Cover the whole wall in this step.

4. Check the surface is flat

Note: smooth and flat are different. Your surface can look smooth, but actually have distortions. When the render sets and the sunlight lands on these distortions it can create unsightly shadows, so it’s important to take the time to flatten the surface.

At this stage you’ll want to run your straight edge up the wall to get rid of high spots (sections raised above the surface). The length of the straight edge means that it’s unlikely to sink into the low spots, and instead will scrape the wet-mix from the high spots.

If you have a straight with a serrated edge, this will tussle up the surface and reveal low spots. Then, apply more mix to any low spots: you don’t need to change the mix for these corrective layers.

5. Scratch the surface of the scratch coat

Before applying your topcoat, go over the surface of the scratch coat with your scratching comb. The lines will provide an anchor for the topcoat to hold onto.

Although you can use straight lines (horizontal or vertical), wavy lines are recommended as they reduce the risk of vertical cracks by allowing tension to dissipate out across different angles as the topcoat dries.

lines in a scratch coat of render
Aim for lines like those at the top

6. Let it dry

Go and have a cup of tea, and leave the scratch coat alone until it dries: usually between 30 minutes and 2 hours.

7. Add the top coat

This coat is where you get the smooth finish. Switch to your topcoat mix, and then apply in the same way as the scratch coat to a depth of about 10mm.

The topcoat should go on smoother if you’ve spent time making the scratch coat flat.

Once you’ve finished, smooth it again just to be sure.

8. Give it a once over

The final job is to give the top coat a going over with a damp sponge to even the surface. This takes a while but the smooth surface is well worth the effort.

9. Spray often

Spray your wall with room temperature water daily for five days. This gives the mix the best chance of curing rather than drying prematurely and cracking. Also try to keep temperature as consistent as possible, because sudden changes can disrupt the drying process. We appreciate this may be tricky when working outdoors.

Strong and stable

There you have it: after following these steps you should have a nice, strong outer coat on your exterior wall. Follow steps 8 and 9 and the surface should be smooth, attractive, and a source of serious protection against the elements.

DIY isn’t for everyone, but we hope this guide has shown you that some jobs, at least, are a little easier than they might initially seem.

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