About Rendering and Bagging
by Pietro Scalise
How is render put on?
The substrate, or brick wall, needs to be free from dust, paint, oil or any extraneous material that will impede the render from adhering. On standard brick walls, rake the mortar joints to remove any brittle mortar between the joint. Wash the whole wall face whilst brooming off any residual dust of sand. There are on the market a wide variety of bonding agents such as Bond Crete that can be diluted with water and applied to brick walls prior to rendering but these need to be followed according to manufacturers’ directions.
Walls need to be dampened and allowed to dry prior to rendering. On internal walls all that is required is one coat of render but on external two may be required. Single coat applications should never exceed 15 millimetres in thickness and a minimum of three days needs to be allowed between coats. If applying an undercoat, this needs to be between 10 and 15 millimetres and its surfaces scratched when the surface is firm to provide a key for the next coating. Final coats are 10 millimetres maximum.
The mix ratios (cement:lime:sand) should be:
- Internal work 1:1/4:3 for a single coat or for an undercoat and 1:1/4:5 for the finish coat
- external work 1:16 for weak and porous surfaces such as brick walls
Prepare the wall surface as discussed then secure steel beading to outer and inner corners on walls. This will allow for a clean finish to all corners.
A mechanical mixer is highly recommended for large scale rendering work.
Turn on the mixer and prime it with one litre of water.
Add the sand, then the pigments if required then the cement.
Blend together until a uniform colour is achieved then add the rest of the water slowly until a stiff mix is obtained. At this point continue mixing it for two minutes then discharge into a wheelbarrow.
Renders need to be applied within half an hour of mixing.
The number of coats will depend on the evenness and alignment of the substrate. If you require more than one coat to correct the surface, do not exceed 15mm per coat, allowing three days between coats.
The mixture is applied to walls with throwing motions with a renderers’ steel trowel and a holding hawk, a holding wooden palette that has a handle on the underside. When the mixture is firm, usually 1-2 hours depending on weather conditions, then a steel straight edge is run on the surface of the wall to straighten and smooth the surface. Any holes or imperfections can be compensated with more render. A wood float can be used to smooth the surface further and a sponge to deliver a flat line finish.
Control joints need to be applied to firm render. Render will shrink as it dries and where the there are joints in the background material, these need to be reflected in the render. Cut a groove in the render at these positions with a v-joint with the aid of a straight edge.
Render coatings also need to be cured and not dry out too quickly. Keep the walls damp for a minimum of three days sheltering them from the sun with plastic sheeting but not allowing them to touch the surface. Protection from the weather is not necessary for internal render.
How much does it cost?
Cement rendering your home can cost up to $28 per square metre on your internal walls for a one coat of 10mm all inclusive of GST and profit margin for your licensed and reputable tradesman. Up to $30.00 per square metre for a one coat of 13mm thick render and $42 per square metre on the external façade if it has to have two coats. Add $13 per lineal metre for arris beading. This is where a metal bead is secured to corners of wall so that the render is finished straight and clean against it, giving a perfectly straight finish. Add $12 per lineal metre if the renderer has to run any v-joints or internal angles in the render surface and about $4.50 per square metre for the waterproofing additive in the cement render that will further protect the wall from water penetration.
How do you bag a wall?
A rough “bagged wall” requires one coating of render mix applied thinly and usually in circular patterns by hand with rolled up Hessian sack material; by steel trowel, sponge or wood float. A bagged finish usually entails a rough, rustic finish but this can be left up to personal taste. It can be thick or thin but usually void of the care taken with a smooth and thicker render finish. From a quick smearing of render mix by a bricklayer, to a thicker pattern by a plasterer, a bagged finish conjures up a rustic look. It also needs to be noted that there are a vast array of proprietary products on the market that include colour pigments and come in pre-mixed formats that a homeowner or professional can purchase.
Is it cheaper than rendering?
If a bagged finish is required to, say, a standard 13mm thick cement render layer, then much the same rates apply as per render.
If bagging is required on the external face of brickwork then either a bricklayer or a renderer can accomplish this for about $5.00 per square metre and up to around $17.00 per square metre using a pre-mixed and coloured bagging mix readily available from numerous companies off the shelf. It is relatively inexpensive because it essentially requires the smearing of a thin and uncalculated layer of mortar mix to the face and mortar joints of a wall to achieve a rustic finish.
How do you care for bagged walls?
Curing, or allowing setting for a bagged wall is similar to that of render. A sealer is normally used when it has set in order to protect the coating and the wall from rain penetration.