Sand and cement render

SCRATCH-COAT-RENDER

Sand and cement render

Take a look at these four things:

  • Impressive Roman houses and temples from 2000 years ago.
  • Gleaming rows of white houses on stunning Greek islands.
  • Picturesque pink, white, blue, or yellow cottages in quaint English villages.
  • Sleek, contemporary homes with walls enveloped with crisp, clean colours.

What do they all have in common? They all use a coat of render to make them look beautiful! But it isn't just about how they look. From the ancient Romans right up to to the present, people have used sand and cement render to protect their buildings from the worst of the weather.

Constant changes in temperature cause walls to expand and contract. Cracks can appear, which allows moisture in. Freezing temperatures cause ice to form, forcing the cracks open. Porous materials, such as brick, stone, or concrete blocks, allow water to seep through exterior walls, causing problems with dampness inside the building. Mould, mildew, rising/penetrating damp, dry/wet rot all become a possibility.

Simply by applying a coat of render to your home, you add a protective shield that prevents the elements from taking their toll on your external walls and reduce the risk of damp problems, as well as improving insulation.

Rendering is also a simple way of increasing kerb appeal, adding value to your house, and covering ugly wall surfaces!

If you're planning on rendering your home, take a look at this guide first to get an idea of what to expect. You might be considering the possibility of tackling the project yourself. This is fine, for small-scale work, but even so, you would be wise to seek advice from professionals. For large projects, it's always best to leave it to the experts Render Hero if you want the very best results.

Let's take a look at what's involved in applying render.

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sand and cement render

The Rendering Process

Preparing the walls

Your old walls may have been rendered in the past. They might be chipped and cracked (bossed render), with bare patches showing. Walls that haven't been previously rendered might be dirty or have plants and algae growing on them, or have spots of grease, oil, and paint. This will have to be stripped completely and cleaned thoroughly. The wall should then be hosed or sprayed down with water to clean off any debris and dust.

The wall surface should be examined for any crumbling bricks or masonry. Loose mortar, brick, and any other material needs to be removed, and any gaps and cracks should be filled so that you have a level surface once the render is applied.

Superficial cracks can be repaired with levelling mortar, whereas more serious damage can be fixed by attaching metal mesh over the top of bricks. If you suspect that the cracks are caused by structural damage then you should get the house checked by a surveyor before proceeding.

Preparing the mix

The traditional mix for cement renders uses a ratio of 6 parts sand, 1 part cement, and 1 part hydrated lime. The addition of lime gives the mixture elasticity and prevents it from cracking and crumbling once it has dried (or cured). It is important to get the right mix; too wet and it will run, too dry and it will be difficult to work! You can achieve the best mix by adding water in small amounts until you get the right consistency.

Some sources will suggest that the type of sand or cement you use in the mix is not important. However, this will affect the strength and efficiency of the mixture, as well as having an effect on the final finished look.

Building sand is generally unsuitable for this task because it causes shrinkage and cracking. Instead, use sharp sand, or rendering/plastering sand. At this stage, you might want to add a waterproofing agent into the mix. This not only increases the protection provided by the render when it's dry but also extends the working time of the render mix, which is helpful when rendering large areas. Some people also add a mortar plasticiser instead of lime, but this comes down to personal preference. Both make the mix more workable, but lime is often favoured as it moves with the building, is far more breathable, and naturally fills any hairline cracks that might appear.

As for the cement, it is widely accepted that any general-purpose brand available in builders merchants can be used. It goes without saying, though, that the better the quality, the better the end result will be.

Applying the render

A very thin base coat, sometimes called a scratch coat, should be used first. This can be applied by hand, using a trowel, or sprayed on but should be pushed into any remaining gaps or cracks as tightly as possible. Ideally, it should have a thickness of between 3 and 5 mm.

To get as level a finish as possible, use a straight edge (sometimes called a Darby) to smooth over the surface. Before the mixture dries, use a rendering comb or similar tool to scrape the surface.

When the first layer has set - usually one or two days - the next coat should be applied. This should be a weaker mixture than the base coat.

Before applying the render, the dried surface should be sprayed lightly with water to make it slightly damp, which will help the second coat stick. The mixture should be trowelled on and smoothed down with a rendering float.

You can create a range of finishes using different tools and techniques:

  • Textured finish – the addition of coarse sand in the final mix can help to give more texture to the finish.
  • Sponge finish – using a sponge to mop the surface will give a different texture, but make sure the sponge isn't too wet or water will run down the wall and cause streaks.
  • Patterned finish – also called a 'bagged finish', this is where you use a balled hessian bag and roll it across the damp surface to give a unique pattern.
  • Trowel finish – you can get a really fine, smooth finish using a wooden float to skim the final coat.
  • Roughcast finish – this is achieved by flicking the mix onto the wall, sometimes with the addition of crush aggregates. Traditional Pebbledash is similar, but the aggregates are added to wet cement rendering afterwards, rather than being mixed in with the render before application.

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Do I Need A Scratch Coat?

Most expert plasterers agree that a scratch coat is essential!

In case you're not familiar with the term, a scratch coat is a thin coat of plaster, render, or mortar (no more than 5 mm thickness) applied to the substrate (wall surface) and then scraped with a comb to form grooves before the render dries. These grooves give the subsequent layers something to grip onto, forming a tight bond that adds strength and stability when dry.

Without it, there's a good chance that the render will slip down while curing or begin to blow once dry. This means that you'll have air gaps between the wall and the render. Eventually, the render will crack and fall off, exposing the wall beneath.

Is It Best To Use A Cement Mixer?

It is vital that the sand and cement render is mixed properly, and a cement mixer is by far the best tool for achieving this, as it distributes the ingredients evenly.

You may have seen people mixing sand and cement render in wheelbarrows, or you might even have done this yourself in the past. While it's handy for transporting the mixture, there's always a bit of unmixed sand or cement left at the bottom.

A spot board is a handy tool for keeping the wet mix handy while you work it, without having to make too many trips to and from the mixer.

Care must always be taken when mixing anything that involves the use of lime or cement as these can present a health hazard.

What Makes The Best Mix For Rendering?

As mentioned, above, the correct mix widely used by plasterers is a 6:1:1 ratio of sand, cement, and lime for external wall rendering. Some people prefer 4:1:1 mortar mixes, although this is not as widely used for exterior walls.

Building sand isn't really suitable, but a small amount can be added while mixing the base coat if preferred, but never to the topcoat. Plastering sand and sharp sand are by far the best choice as they add strength.

The addition of lime makes the mix more elastic, allowing for any movement in the building as it expands and contracts, which makes it less likely to crack. Lime also makes it self-curing and self-rendering, as it allows moisture and vapour to escape. It is important that walls are able to 'breathe' otherwise moisture will accumulate inside.

Finally, lime makes the mix creamy, giving a smooth, flat finish and a beautiful straight edge.

sand and cement render
sand and cement render

Sand & Cement Render vs. Acrylic Render

The render system you use will depend on several things, including the local climate, the age of the house, the building materials, and your budget!

Recent innovations have improved the quality and range of renders available. Many are waterproof but breathable, self-cleaning, through-coloured, algae/fungi resistant, and UV resistant.

Each type of render has its pros and cons, but every one of them is an excellent investment.

Acrylic render, for example, doesn't always require painting as you can get tinted mixes, whereas you will need to paint cement and sand render. However, it is generally not as cheap as cement and sand render and is more difficult to apply, although it is often premade and arrives on site ready to use.

It only takes a couple of days to dry fully, whereas traditional cement renders can take up to a month or more. Then again, acrylic render lacks the 'breathability' of traditional renders, although some modern versions have sought to resolve this - the technology is always being improved!

Final Thoughts

Sand and cement render has been used for centuries and is an excellent way of protecting a property from the elements while adding value and aesthetic appeal. Up until about a decade ago, you might have expected your render to last for only ten years or so before cracking and needing repair or replacement.

These days, however, with new additives that increase the waterproofing qualities, flexibility, and life of the product, the life of each render can be extended to as much as twenty, thirty, or even fifty years!

The choice of renders is also much wider than it used to be, with new types being added all the time.

Traditional sand and cement (and lime) render is still used today, often as a base for modern 'thin coat' types. These include mineral and silicone renders. Mineral renders use traditional sand and cement mortar as a base, with the addition of polymers and lime, which you will need to paint. Silicone thin coat render, on the other hand, is often through-coloured, meaning that there is no need to paint.

For guidance and advice on your project, speak our team at Render Hero. There are so many different products to choose from that it can be overwhelming, and it is important that you select the right one for your home.

Rendering a property is often regarded as a low-cost alternative to moving house or undertaking expensive renovations. Both of these latter options are fraught with stress and involve major upheaval and inconvenience.

Rendering, on the other hand, is a relatively quick and painless process, depending on the weather conditions and the size of the project, and the end results are stunning. What's more, this technique can be combined with ideas such as cladding to produce a beautiful contemporary look.

What better way to bring your home into the 21st century? It's a makeover that increases the value of your property, protects it from being damaged by the weather, and helps to reduce your carbon footprint by increasing its energy efficiency!

sand and cement render
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