Cement render

What is cement rendering?

Traditional cement rendering is the application of a thin premixed surface of sand, cement and lime to brick, cement, stone or mud brick. It is often textured, coloured or painted after application. It is generally used on exterior walls but can be used to feature an interior wall.

The European Tradition

The cement rendering of brick, concrete and mud houses has been used for centuries to improve the appearance (and sometimes weather resistance) of exterior walls. It can be seen in different forms all over southern Europe. Different countries have their own style and traditional colours.

In France, new houses continue to be rendered, and is in fact obligatory in many areas, in particular in the South. The range of colours that can be used is also often controlled by the local council. This control over the appearance of domestic building is why so much of modern architecture in regional France is so agreeable. The French have in fact produced a modern aesthetic of rendering that is sadly missing in Australia.

Rendering first appeared in Australian on early settler's stone and mud brick houses, and then later in the 50's on federation style homes. However rendering domestic houses here has unfortunately always been the exception. For a long time, and for some inexplicable reason, brick in its basic form has generally been considered a finished material. It remains so, unpainted, and often ugly (though more interesting 'designer' bricks are now just starting to become available.)

The simple rendering of walls can transform any style of house built of common brick or concrete. While many new apartment blocks are now rendered and are aesthetically superior to traditional exposed brick, too few new homes in Australia receive this treatment. Most are post modern 'Seidler' style homes or innovate multi-clad homes that combine rendered brick, fibre-cement, timber and even galvanized iron. Thankfully, a few of the triple fronted, two story 'MacManions' that dominate new housing developments are now being built with this enhancing treatment.

Traditional cement render is made from clean sand, cement and lime. Depending on the 'look' required, rendering can be fine or coarse, textured or smooth, natural or coloured, pigmented or painted.

Different finishes can be created by using different tools such as trowels , sponges, or brushes. The art in traditional rendering is, (apart from getting the mix right), the appearance of the top coat. Different tradesmen will have different finishing styles and be able to produce different textures and decorative effects. Some of these special finishing effects may need to be created from a thin finishing 'top coat' or from a finishing wash.

Engaging a traditional renderer.

Rendering is best done by experienced, licensed professionals. Like the engagement of any tradesmen, it is circumspect to research their work- by either talking to manufacturers or to their previous clients, ideally those who have had work done some time ago (when any possible cracking has become evident.) When scrutinizing their work, take notice of the finished texture and evidence of more decorative touches. Many renderers are of European decent, are very experienced in traditional rendering and produce interesting, excellent quality work.

If you are using a pigmented render, ask the renderer for a sample to be produced. Make sure that it has time to dry. Often pigmented render will dry a different colour from that when it's wet.

Acrylic rendering

There are also a wide variety of premixed renders for different situations. (See www.unitex.com.au and www.rockcote.com.au)

Some have a polymer additive to the traditional cement, lime and sand mix for enhanced water resistance, flexibility and adhesion.

Acrylic premixed renders have even higher enhanced water resistance and strength. They can be used on a wider variety of surfaces, including concrete, cement blocks, and AAC concrete panelling. With the right preparation, they can used on smoother surfaces like cement sheeting (HardiTex/ Fibro), new high tech polymer exterior cladding such Uni-Base, and expanded Polystyrene. A few of these require activation with cement just prior to application.

Some of these premixed acrylic renders have a smoother complexion than traditional renders. Others can even be spayed on.

There are also a wide variety of acrylic bound pigmented 'designer' finishing coats that can be applied over acrylic render. Depending upon the product, they can be rolled, trowelled or sponged on. A limited number can also be spayed on. Various finishes, patterns and textures are possible such as sand, sandstone, marble, stone, stone chip, lime wash or clay like finishes. There are stipple, glistening finishes, and those with enhanced water resistance and anti fungal properties. Acrylic renders take only 2 days to dry and cure- much faster than the 28 days for traditional render.

The variety of effects available from acrylic renders requires a renderer who is familiar with the product and style required. It would be wise for clients to seek recommendations for renderers from the product manufacturer.

DIY Traditional Rendering

To ensure adhesion, the surface to be rendered must be hosed off to ensure it is free of any dirt and loose particles. Old paint or old render may need to be scraped away- render will not stick to paint and all traces of it must be removed. Use an anti- fungal wash to remove any traces of mould. Roughen the surface to improve adhesion. For large areas, vertical battens, about 10mm wide should be fixed to the wall every 1 to 1.5 metres, to keep the render flat and even. Finally, paint the surface and batons with Bondcrete to increase adhesion.

Cement render consists of 6 parts clean sharp fine sand, 1 part cement and 1 part lime. (This is a medium strength mix used for porous or weaker backgrounds.) The lime makes the render more workable and reduces cracking when the render dries. Any general purpose cement can be used. Bondcrete can be added to the mix for increasing adhesion.

Coarser sand is used as the base layer and slightly finer sand for the top layer. Materials should be mixed well and only enough render that can be used in half-an-hour should be mixed.

Application

Rendering should ideally be done in good weather when conditions are not extreme (ie normal humidity) and there is no wind. As much as possible, avoid working in direct sunlight. Ensure the surface is damp but not wet and spray it water if necessary.

Usually at least two coats are needed- the coarser sand for the first coast and the finer sand for the top coat. Apply the first coat to a thickness of 10mm. The cement render should be kept slightly damp in between coats. When the surface has become firm (which may take up to three days), roughen the surface for the next coat. Usually two layers are sufficient. A third coat may be required for thicker renders, or surfaces exposed to extreme wet weather conditions.

Keep the top coat slightly damp for three days to allow it to cure and gain its full strength, especially prior to any further work being done. Fast drying render is prone to cracking. If necessary the drying process can be slowed down by covering the render with plastic sheeting. Rendering can take up to as much as 28 days to cure and dry depending on the conditions.

While rendering is a little more cost effective carried out during building, it can also be easily done as a home improvement to existing brick or concrete houses. It can also be applied to brick boundary fences, creating a uniformed 'look' to the property. Rendering will generally add thousands of dollars to a house's resale value and make it more appealing for sale.