So what is cement rendering and solid plastering?
In Australian (depending on what state you live in) traditional cement rendering can also be called solid plastering. Solid plastering also insinuates more of an art focus and complex detailing.
Modern & traditional solid plastering is the application of a thin cement type product, consisting of sand, cement, lime, water (and sometimes proprietary ingredients). This render can be applied to such claddings as brick, concrete block, stone (and more recently) fibre cement sheeting. It is often finished with a texture and can be pre-coloured or painted after application. It is generally used on exterior walls, garden fences and interior walls.
A European Tradition
The rendering of brick, stone and concrete block homes has been used for centuries to improve aesthetics and waterproofing of exterior walls. Cement rendering can be seen in different forms all over Australia from suburban housing estates to highly detailed heritage building in our city centers.
Different countries have their own style and traditional colours. In France for example new houses continue to be rendered, and in some areas rendering is obligatory. The range of colours that can be used is also controlled by local councils.
Solid Plastering in Australia
Solid plastering first appeared in early Australia settlement and most stone and brick houses were actually rendered. Australia saw a resurgence of rendering during the 1950's in federation style homes and again recently over the last 20 years.
The simple rendering of walls can transform any style of house built of common brick or concrete block. Many new homes use a combination of render, horizontal cladding, brick or natural stone offering a modern take on a traditional art.
Solid plastering is commonly used in combination with PVC mouldings to create highly decorative wall finishes. Traditional cement render is made from clean sand, cement and lime, but depending on the 'look' required, renders may use fine or coarse sands, a textured or smooth finish, natural or coloured render, pigmented or painted finish.
Creating different effects is easy with finish specific trowels, sponges, carpet squares or brushes. Traditional solid plastering is an art and a professional renderer should be chosen based on their proficiency with their trade. Different tradesmen will have different skills and styles and are able to produce simple and complex texture coatings and decorative effects. Some of effects may need to be created from a thin finishing 'top coat' or from a finishing wash.
Engaging a traditional renderer or solid plasterer
Cement rendered surfaces are best done by experienced, licensed professionals. Like the engagement of any tradesmen, it's wise to do research - by either talking to manufacturers, previous clients and viewing their work (either online or in situ). When scrutinizing their work, take notice of the finished texture coating and any cracks or patches. Many Sydney, Brisbane and Melbourne based renderers are of European decent are very experienced in traditional rendering, producing interesting and excellent quality work.
If you are using a pigmented render, ask the plasterer to produce a sample, and make sure that it has time to dry. Often pigmented render will dry lighter from that of wet render. Also be aware that it is difficult to repair pre-coloured renders, as colour matching is impossible due to sun discolouration over the years.
Most external rendering jobs use a simple plaster mix as previously detailed but there is also a wide variety of premixed renders for different situations that are worth consideration. "Unitex" and "Rockcote" for example manufacture plaster that can stretch with changes in temperature and building movement (great for new subdivisions).
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 paneling. With the right preparation, they can also be used on smoother surfaces like cement sheeting ("Blue Board"), new high tech polymer exterior cladding such Uni-Base and expanded Polystyrene. Some of these premixed acrylic renders have a much smoother finish than traditional render, and some can even be sprayed on.
There are also a wide variety of acrylic bound pigmented 'designer' finishing coats that can be applied over acrylic render. The finish types vary greatly and include patterns and textures similar to 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
1. Preparing the surface
Ensure adhesion buy hosing off or (if internal) brushing off any free dirt, grease and loose particles. Old paint or old render may need to be scraped away and/or roughed up with a wire brush otherwise render will only adhere to existing surface for a very short time. Use an anti- fungal wash to remove any traces of mould, otherwise mould will continue to grow causing ugly patches on the wall.
2. Installing batten guides
For large areas that need to look as smooth as possible the installation of vertical battens is essential.
- Install timber battens (10mm deep x 50mm wide) to the wall at 1 to 1.5 metres centres (creating sections). This will ensure that the end result is a flat and even depth render.
- Fix the battens to the wall with a very small amount of silicon filler and ensure the battens are secure before rendering starts (see manufactures instructions for drying times).
3. Apply plaster and texture
Cement render consists of:
- 6 parts clean sharp fine sand - any general purpose cement can be used
- 1 part cement
- 1 part lime - the lime makes the render more workable and reduces cracking when the render dries.
- Water to bind
Before applying the plaster, read the manufactures instructions carefully as different products have different drying times. Apply render one section at a time with a trowel or similar and finish surface as smooth as possible. A large light piece of timber can also be used adjacent to battens to smooth out thick areas of render. Remove battens one at a time as you work your way across the wall so that the edge of the previously rendered section becomes the guide depth for the next section. As a general rule materials should be mixed mix well and only enough should be made that will be used in a half-hour period.
Once all sections have been rendered and cured it's finally time to paint. It is advisable that the surface first be painted with a bonding agent to increase adhesion of paint to the wall.
- Good weather - Rendering should ideally be done in good weather when conditions are not extreme (i.e. normal humidity and not too hot or windy). As much as possible, avoid working in direct sunlight and ensure the surface is damp but not wet and spray it water if necessary.
- Multiple applications - 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.
- To avoid cracking - Keep the top coat slightly damp for at least three days to allow render to cure and gain its full strength. Note: you don't want the render too damp either otherwise it will dry too fast and will be prone to cracking. If necessary the drying process can be slowed down by covering the render with plastic sheeting.
- Understand material movement - It is very important to understand materials and the way they work and you should discuss your cladding with the render supplier before you commit to a product. If you have a typical brick home for example applying a standard cement render is not wise based on the way these two products work against each other. Clay bricks expand and contact with heat and moisture, cement render is in a constant state of contraction. Given time the render will crack and fall away from the brick work. Rendering clay based brickwork is better done with an acrylic render that expands and contract as well.
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, fibre cement or concrete block houses. It can also be applied to brick boundary fences, creating a uniformed 'look' to the property.