External Rendering

Rendering Application Method

It is essential the correct type of render be used to protect and enhance any particular building. Using the wrong type of render on the wrong type of wall will result in the inability of the render to adhere to the substrate, build up of damp within the substrate etc.

Examples to illustrate the importance of using the correct render when applied to a substrate are the examples of rendering traditional buildings whose walls are constructed of solid stone and rubble or timber framed buildings rendered with cement render/ cement pebbledashed finish.

Fig.1: render applied to a wall of an old building:

After a short time the exterior brick or stone breaks up into smaller pieces and begins to fall apart which in turn causes the rendering or pebble dashing to deteriorate and eventually delaminate from the substrate. The internal walls begin showing signs of damp problems with the growth of mildew in corners and damp patches on the plastering. This is caused because the cement render has a low rate of vapour exchange.

All the moisture that the building absorbs from the ground or produced by cooking and washing would usually soak into the lime render (which acts like a sponge) then evaporate into the environment. Because the cement render cannot exchange vapour as quickly as lime render, high levels of damp build up within the wall structure. High levels of damp within the wall causes the external render to crack and delaminate especially during the winter months when the walls are continually freezing and thawing, eventually the damp has no choice but to vent internally. Once internal damp is evident to the human eye it is too late to cure and often the plasterwork internally needs replacing as well as the external render.

Before applying the render that you have chosen, you must ensure you have the correct tools at hand. One important tool is the float type (a tool used to smooth the surface of cement, plaster etc.)

Other tools required are: drill and mixer (use a drill powerful enough not to blow the engine), scaffold / ladder, shovel / hawk / small tool, brickies’ trowel / rectangular trowel, corner trowel (edging tool for corners), floats – polyester, plastic, wood, etc, sponge, flick Brush or spray bottle (high quality), buckets and brushes for cleaning, paint brush / roller / extension poles / paint tray.

Float types along with the type of render chosen have a large impact on the overall texture of the finished render so the desired overall texture of the finished render should be decided beforehand e.g smooth finish, scraped appearance etc.

The next step is to ensure clean water is mixed in with the render as any chemicals or contaminants may cause the render to undergo a premature chemical reaction and degrade prior to being applied to the substrate surface

Step 3: The bricks and mortar have to be clean before the render is applied because any dirt or mould will disrupt any attempts by the render to adhere onto the surface of the substrate, this surface being brick, stone etc. Any dirt or mould found on the substrate surface can be removed by the use of a high pressure air spray.

After any remains of the dirt or mould is removed from the substrate, black plastic bags are put around areas of the property surface where the owner does not wish the render to be present such as windows, doors etc.

Calculating the total area of the surface to be rendered (in square metre units) is important because from this value, you can calculate how much area one bag of premixed render covers on average and by dividing the total area to be rendered by the average area covered by one bag of premixed render, you can find the number of bags required for the project and the total cost in accomplishing the rendereing project.

Note: most of the render applied to the surface of the substrate will not adhere and will instead fall to the ground. Hence when calculating the number of premixed render bags required for a project, extra bags need to be bought over the estimated value of bags needed because of the loss of render during the application stage.

TIP: When using render, mix to instructions. One thing to consider when rendering is the climate of the area, as this will effect how long you will have to complete a wall. If the area you live in has a high in temperature and humid climate, you should start early in the morning and always try to stick to the shaded walls as when you start on a wall you should not stop until it is finished, this is because it is very difficult to blend together the dry render with the new moist render and is very visible even when painted over. Understandably this can not always be kept to, and I myself have been forced to work on walls for over 2 days in complete view of the sun. If this happens, keep a spray can nearby to keep render moist and if you have to stop half way, stop on a straight line up and down (for tying in the two parts cleanly).

Step 4: Wet the wall 30minutes before first coat is applied to give the wall some moisture; this will stop the moisture in the render from being sucked up to quickly by the brick and mortar. Again start early and for your very first coat, start on the wall that is least seen so you can perfect your technique. Start from the top of the wall and work your way down, using a rectangular trowel. Apply render in long upward strokes in thin and smooth layers. Between bricks will appear some raised bumps; do not worry about smoothing these over as this is normal on the first coat. When applying second coat you will go over them and any that pop up again will be blended in with the float.

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