If anybody has ever suggested to you that your house might benefit from a scratch coat render, then you might still be a little confused by what they mean. In a nutshell it’s a way to brighten up your home exterior.
But, today we’re going to fully explain what a scratch coat render is. We’ll also tackle why it’s important, and see why it plays such a big part in protecting our exterior walls.
What is a scratch coat render?
All external walls need to be an external rendered wall. The external render is what protects exterior walls from weather erosion and harsh weather conditions. Hence, it keeps your property in top shape.
All renders come in two coats: a base coat (or first coat) and a top coat. The two coats work to protect the wall surface and brick beneath.
A scratch coat is another name for the base coat.
Why are two coats necessary?
You need both coats because brick is made up of porous materials. That means these materials can absorb water and moisture. So having just the brick alone isn’t enough to protect your property from moisture. It’ll just act like a damp sponge, soaking in all the water.
When your wall starts letting water in at the surface level, it won’t be long until your whole wall, internal walls and exterior, are suffering.
What does the scratch coat do?
A thin layer is applied first, usually of around 5mm thickness. This first coat is the scratch coat. It’s applied using a steel plastering trowel or sprayer. The point is that the render forms a solid base for the top coat.
The scratch coat is the layer with the most waterproof additive to protect against moisture and damp, but it cleverly allows water out. So the internal walls don’t become damp because there’s nowhere for the moisture inside to escape.
The external walls are reinforced by the external render, providing more structural strength. The surface will also become level when the scratch coat is applied, smoothing out any bumps and dips in the walls as the render is used to fill in these uneven surfaces.
How is it done?
A cement render is created in a cement mixer. Mix one part concrete with three parts rendering sand (not plastering sand, which is for internal use). This sand and cement render is then applied to the walls using timber screed battens. This ensures a level finish and straight edge once applied.
By getting the correct mix of sharp sand and cement, you can create a render that’s effective and aesthetically pleasing.
Whilst the render is drying, it is marked with a scarifier in a circular motion, giving this bottom coat its common name – scratch coat. The scratches are an important part of the process, because they make it easier for the second coat to stick to the first coat by acting as a mechanical key. This helps create a secure bond between the two layers, which makes the render more effective.
Without applying the first coat, there is more chance of the render not making the wall flat. This usually means there is air trapped behind the render and the wall. This can then cause the render to crack or blow out. It will eventually lead to the render crumbling, leaving the walls beneath exposed to the weather.
With the scratch coat, you guarantee a straight edge, flat finish, and a smooth rendering that’s ready to paint with whichever colour you prefer, and any other process you want to do next to make the building look great.
It will need time to dry, which will vary based on thickness, climate, and any other additives such as added lime. Sometimes even the type of builders sand you’ve bought from the builders merchants is enough to make drying times longer or shorter.
Other factors affecting drying time
This is important to consider because most builders will want to get a second coat on top of the topcoat as quickly as possible. And for good reason – to protect the wall beneath and complete the render with the finish coat.
Here’s a general rule of thumb. Gather detailed information about the time it might take to complete the job before starting and mixing any render together. That’s because the whole process can be delayed by weather changes.
If you want the perfect render, then it’s best to do this when it’s dry and fine out, or else the building may simply have the render washed off in the rain, making the job more difficult and the render unsuccessful.
But there are other things that make a difference to the render success:
- Moist curing – this method of drying the render involves keeping it moister for longer to stop it from drying too quickly. This actually improves the strength of the render and is the best option if you’re looking for a long-term fix. It’ll keep the render soft for longer, but once it’s dry, it’ll be stronger than ever.
- Direct drying – this involves drying the scratch coat (and later the finish coat) with direct heat to quicken up the drying process. This will stop gravity from trying to pull off the soft scratch coat, but it’ll also leave the render vulnerable to cracks and compromise in the future.
A quick note on the top coat
Generally, the second coat will be made up of the same sharp sand and cement mix as before, occasionally with lime added in, etc. This rendering doesn’t have as much waterproof additive in it, but it will certainly still have some.
The mortar is then applied on top of the scratch coat. This topcoat is applied in an even thickness to the surface of the bottom coat, although it is thicker to help prevent damp. The process is usually the same. And it’s the key difference between a building that’s protected against the elements, and one that isn’t.
Usually the finish coat is smooth, but occasionally building owners and designers will ask for the scratch coat finish on the top coat of the render too, because it often makes the building more interesting to look at.
Once both coats are applied, the rendering is complete and the wall beneath is protected.
Hopefully, today, you’ve been able to see that scratch coats are essential before applying a second coat when rendering. This creates an even surface for the render to stick to and protect the wall.
It’s a scratch coat’s job to act as the protective waterproof layer between the wall and the second coat.
And without them, our homes wouldn’t be properly protected from the elements…