Scratch coat render


Does your home need scratch coat rendering?

One of the best ways to improve the appearance of your home is by applying render to the external walls. You can add a fantastic finish to a modern property, or enhance the charm of your period house, simply by adding a coat of render to the outside walls.

Traditional lime renders can restore the elegance and appeal of older buildings, while modern, through-coloured renders with the added polymers (silicone and acrylic) make a bold and stylish statement.

But it isn't just about the look; rendering adds value to your property, protects it from weather erosion for many years, and increases the insulating qualities of the exterior walls.


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Is a scratch coat simple?

As simple as that sounds, there are many types of render available these days, and it is vital that you choose the right one for your needs. Then there's the actual task of applying it. Many keen DIY enthusiasts take on the job themselves, which is great. However, it's a job that has to be done right the first time if you want to avoid unnecessary delay, inconvenience and expense. It isn't just about the end result - although this is essential - it's also about following the correct steps and understanding the procedure so that you get perfect results every time.

This is why it's always better to use professionals like Render Hero, especially if you aren't confident or proficient enough to do the job yourself!

It isn't just the process that can be difficult to follow, either. There are many different names and terms involved that you might be unfamiliar with or find confusing, two of which are scratch coat and scratch coat render. So let's take a look at what they mean to help you get a better idea of the process.

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What is a Scratch Coat?

Most traditional renders are applied in two coats; a base coat and a topcoat. The first coat is known as the scratch coat, and is applied in a thin layer and left to cure. This can take from between 24 hours up to two days, depending on humidity, temperature, and airflow around the building.

You might wonder why you need two (or more) coats - as do many DIY enthusiasts - and the answer partly lies in the walls themselves. Most wall surfaces (or substrates) are porous, allowing water and moisture to pass through. This is both good and bad news. Walls need to breathe to some extent, allowing moisture from inside the bricks or masonry to evaporate.

Any plaster or render applied to these surfaces needs to allow for this, and this is where a scratch coat helps. By applying a very thin layer first (to a thickness of approximately 5mm) using a steel plastering trowel or sprayer, you provide a firm base over which the rendering finish can be added. This layer has most of the waterproof additive that protects against water ingress but still allows moisture out, as well as adding structural strength to the exterior walls. It also fills in any dips or imperfections to make the surface level, ready for the finishing render.

A cement mixer is often used, with a typical mix of one part concrete to three parts sand. Timber screed battens can be attached to the wall to ensure that the finish is level and that you get a straight edge.

While the render is still wet, it is marked with a scarifier (a special comb), hence the name. These scratches improve adhesion and make it easier for the second coat to bind onto the surface, forming a tight, secure bond with a level finish. Without this coat, the render is more likely to crack and 'blow', which means that there is air trapped behind. You will recognise these blown areas from bulges in the plaster or a hollow sound when you tap them. This is not good news as it means that your render is in danger of crumbling, leaving your walls exposed to the elements as well as looking awful.

Drying (or curing) times differ, depending on the climate and any additives in the mix. For example, if you add lime to the mixture then you'll need to allow for extra drying time.


Moist Curing

This involves spraying or misting the surface of the render with water to stop it from drying too quickly. This process improves the strength of the render and stops further layers from slipping off during application.

Direct Drying

This is an older method that's used more for 'patch-up' jobs or when speed is required. No extra moisture is added, and this can make any further coats difficult for plasterers to work with. Weather conditions need to be taken into account when using either method, as these can alter drying times considerably.


Is a Scratch Coat necessary?

Most tradesmen and experts agree that it is essential in most cases! Without it, any additional coats won't bond as well as they should. It provides extra structural strength and flexibility and helps the tradesmen to achieve a perfect, level finish.

There may be instances where it isn't absolutely necessary, but these will depend on the wall surface and whether it is an internal or external wall. Before beginning any project, it is always best to seek expert advice.

Do you need two coats of render?

Rendering should never be done in just one coat as cracks will form and will eventually blow off. The strength of the second coat should always be less than the first, but the thickness will vary according to the overall desired thickness, usually between 10-15mm. The second application should never be thicker than 20mm, and remember, if you need any additional coats, the drying time will be longer if the coat is thicker.


Is plastering sand good for rendering?

Rendering and plastering are similar processes, both used to cover walls. Rendering is used on external walls and plastering is used for internal walls. Rendering sand is shaper and coarser, and plastering sand is generally much finer. The strength of mix also differs between the two, but both products usually consist a mixture of cement, sand, water and lime gypsum. In general, plastering sand should be retained for internal use only.

Why is plastering or rendering a good idea?

Plastering and rendering both dramatically improves the appearance of a building and adds value to the house. Rendering is particularly recommended if you want to give the building an instant facelift, getting rid of the old and tired look, great for if you are looking to sell.


Do I need to PVA before rendering?

Some builders recommend applying PVA in between the first two coats to allow for greater suction on the second coat, but this is not actually advised by some professionals. PVA should not be used on external render and other alternative methods should be used to control suction. Suction control is important as the render could absorb too much water causing it to become too wet, but too little suction could cause problems in the bonding.

Scratch Coat Renders

Another type of scratch coat that some plasterers refer to concerns the final look of the render surface and is usually called a scratch coat render.

Modern, through-coloured renders are an excellent way to dramatically improve the appearance of your home, providing protection from sun, wind, rain, ice, and snow! They also work with your existing insulation to slow down heat loss and make your home more energy-efficient.

Scratch coat renders can be applied to a range of materials, whether brick, stone, or block work surfaces. They can then be scratched back with different tools to form a wide range of decorative finishes as the colour is present right through the mixture.

For more detailed information and advice on this subject, drop us a message and be sure to include your phone number for a fast response.


How long will my rendering last?

If the wall was properly cleaned before the render was applied then it has the best chance of avoiding any cracking. The more common type of sand and cement render is more prone to shrinking and cracking but this still lasts many years (8-15).

Is rendering my house a valuable investment?

Yes having your house rendered will certainly increase your houses value but more importantly if you are thinking of selling it has a much better look and gives older shabby houses a complete new build look of a face lift.

Does rendering prevent damp?

Yes especially silicone rendering & wet dash rendering which have waterproof properties that stop water from penetrating the render and brickwork.

What is the difference between plastering and rendering?

The difference quite literally is only the fact that plastering is done on internal walls and rendering is done on external walls. There are some slight differences in the materials used and a broader range of rendering types but none the less it is the same skill set.

Will rendering make my walls stronger?

Indeed it does as it acts as a binder to the older more crumbly brickwork and mortar. It also increases the thermal efficiency of your home reducing energy bills.
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