Rendering Your Home
by Pietro Scalise
What is rendering?
Cement render on internal and external wall surfaces improve on the waterproofing, fire-rating and even appearance of a home and can include pigments, admixtures and various textured finishes.
Materials include general purpose Portland cement; hydrated lime that will make the mix creamier and more workable and provide a degree of elasticity in its hardened state to prevent crazing or cracking of the render; clean, sharp sand free of clay or impurities (coarser sands are suitable for bagging); clean water; admixtures, if required, such as pigments that should never exceed 10% of the weight of the cement in the mix and additives such as Bycol that will slow the drying process of the mix (added in minute amounts and strictly following manufacturer’s directions. Bonding agents such as Bond Crete can be used diluted with water as per directions of manufacturer to increase the adherence of the render mix to the brick walls.
Cement rendering is carried out by a plasterer and refers to the process of covering internal or external walls with a layer, or gradual layers, of cement based render mixes. This is one of the quickest and easiest ways of updating a home, and depending on the period characteristics, the initial outlay of cement rendering is exceeded by the increase in market value of a property.
Are more people choosing to render their homes?
Throughout the residential construction sector, more people are generally choosing a rendered finish to their homes.
Rendering was very popular initially on the facades of terrace houses. Colonial homes and Californian bungalows tend to have face brick façades, where the external brick walls are left untouched whilst between the 1960’s and 1980’s there was a resurgence of rendered external brick walls.
Generally now, since it is important to recognise the period of a house in order to maintain its characteristics and maintain its value in the eyes of savvy homebuyers, homeowners who are carrying out renovations tend to do so in harmony to those styles. Therefore people owning existing housing stock tend to choose render only if it is appropriate whereas if considering a new home to be built, people will generally choose a rendered external façade for its clean looks. It is also important to note that project home builders, who build a great deal of homes, commonly tend to favour a standard rendered external finish and this is what the general public is confronted with when visiting display home villages. Many of them baptise their various home designs with Mediterranean or Tuscan monikers and simply add external wall render that has colour through it or that is then painted. These then emulate Mediterranean villas that are all rendered and painted white which helps to keep a home cool during the stifling heat of the Mediterranean sun.
A lot of people also choose a rendered look for their new home simply because there is a considerable saving when buying bricks. Face bricks, which are laid and left as they are, tend to cost a lot more than bricks that are subsequently covered by a layer or layers of cement render.
Why choose to render a home rather than bagging?
A rendered finish is essentially a more elegant, cleaner and more sophisticated finish to the external faces of a home. A rendered wall can be painted to achieve a smooth surface finish whereas a bagged wall is more rustic in its appearance. Bagged walls can be left as they are or painted but, in both cases, the contours of the bricks and mortar joints will still be clearly visible from near or afar.
If you’re trying to recreate a rustic look or the distant cousin of the Tuscan farmhouse, then a bagged finish is suitable. Colour can even be added to the bagging mix to give the home more Tuscan credibility foregoing on the need to paint.
Anyone can bag a wall armed with Hessian sack material, steel trowel, a wood float or a sponge, it doesn’t require any formal training or experience but rendering is definitely to be left to the toils of professionals or the experienced home renovators.
In general, more people opt for a rendered finish. From experience, with an architect designed home where a client has had a lot of input at the design stage, a rendered finish will be specified.
Is render long lasting?
There is absolutely no reason why a rendered home should not have the rendered finish intact for 15-20 years if properly applied and painted over to further protect it from the elements. A plasterer carrying out rendering work should give a guarantee that his work will last that far.
What are the potential problems? Does it crack or flake?
It is important the render mix be suitable for the background surface, its substrate, that it is applied in the correct number of coatings and that appropriate time is given for it to cure, or harden properly.
Rendered surfaces need to be cut and given control joints to minimise surface cracks. Minute surface cracks, crazing, are usually due to render drying out too quickly therefore it is important to allow for proper curing conditions, dampening the walls and protecting them from the harsh sun.
Poor substrate preparation can lead to render spalling off the wall quite quickly, a matter of days in some cases. These areas are easily located; they will sound hollow when tapped and rise off the surface, eventually falling off. They can be patched but ideally the problem should not occur.
It is a difficult exercise to keep correct proportions of colour additives in the render mix. If these are not exact, then a patchy finish will be the end result on the external walls. To further complicate matters, even if an even finish is obtained in the render mix throughout the external façade, the colour oxides can fade quickly and even show up in darker patches in some areas where, say, the wall beneath the render is affected by rising damp or is suffering from a waterproofing problem within the home. Ideally, the finished render should be painted to protect it from the elements.
Render will simply fall away if it has been applied to a wall or part of a wall affected by either rising damp or falling damp. The application of waterproof render coatings to a damp wall will simply push the dampness to another part of the wall, not fix the dampness. Essentially a wall has to be treated of any such issues before rendering is considered.
What’s the difference between bagging and rendering? What is bagging?
A rendered wall will stand out from a bagged one in so far as its impeccable surface finish is concerned. A house that has been rendered and painted will have clean, smooth likes; precise corners; detailed finishes to window sills and the likes. A straight edge placed against it should not reveal any bumps or imperfections.
Bagging highlights any imperfections rather than hiding them like render does.
A bagged wall is rustic in its appearance and the pigments used in the mixture will run parallels to those found in Tuscany. Applied to a wall without much precision, it will allow for irregularities, bumps and a degree of calculated chaos.
In general, for the purposes of bagging, coarser graded sands are used, whilst for rendering, finer ones are mandatory.
There are companies that sell coloured bagging render in a vast array of colours that simply need to be added water on-site but essentially these are applied thinly to a thickness of about 5mm. Standard rendering, on the other hand, are applied in layers if required, each layer between 10-15mm depending on substrate conditions or degree of smoothness.
Can you bag or render over old walls? If so, what preparation do you need to do?
It is quite easy to bag or render over old walls. That is one of the primary reasons that a renovator may want to render - a wall may be old. But old walls may have value in so far as their period of construction is concerned. It is always more advisable to keep the façade of a home in harmony to its period of construction than simply hide surface blemishes. If bricks are loose, crumbling away or damaged, these can be matched and replaced. If the mortar between the bricks is crumbling, then there are companies that will re-point the brick by scraping away 25mm into the mortar joints and replacing it with new mortar that has an additive that will prevent any further cracking or crumbling of mortar joints.
What is important is that the substrate, the bare brick wall, needs to be clean of any loose material, including oil, paint, dust; that the mortar joints are free of any crumbling mortar; that the wall is free of any loose or cracked bricks; that there is no soil subsidence that gives rise to cracking of walls and may continue to do so after the render is applied; that the walls are free of any dampness problems and that the walls be free of any efflorescence, the white, salty discharge usually associated with dampness in walls.
For a new home, where I would be liasing closely with the architect at the design stage, I would specify a smooth rendered façade then a painted finish. In an existing home, if the brickwork of a façade is in poor state I would do the utmost to preserve it, thus keeping it in harmony with possible period or historic significance and in harmony with neighbouring homes. The local council may also have a say in what can or cannot be done as far as altering the external appearance of a home.
If brick walls cannot be fixed or repaired to bring them to their former glory, then I would consider rendering then painting. If rendering were not possible, due to financial or other constraints or in the case where a home did not merit from render, I would then consider bagging.