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AD-TEX Avoiding Problems

How to Avoid Problems with Render

There are a number of problems that can arise in regard to finished renders. The addition of pigment may exaggerate the visual problem.

The first point that should be well established is, what is the clients end expectations? Some clients do expect a finish comparable in colour to a painted surface. This is highly unlikely. Finished renders are subject to many factors that can, under circumstances, cause enormous variations in final texture and colour.

Tradesmanship is possibly the place to start. Many tradespeople are site trained and unless training has been carried out by people experienced specifically in finished renders, the basic foundation is not instilled. Australia does not have an established history in this method of finishing walls, unlike some European countries where external (sometimes coloured) renders have been acceptable for centuries.

Another problem, which arises, relates to the client. Colour variations or slight efflorescence on a finished brick wall is most often accepted as being part and parcel of the nature of the product, but not with Renders.

The following are common complaints regarding finished renders and possible causes. Some of the complaints can be overcome easily by better planning and procedures, and some may be the result of one or more multiple causes. Problems are often difficult to diagnose after they become apparent due sometimes to the inability to collect accurate information on methods used and site conditions.

1. Colour Variation

The final colour or a render is dependent on many factors. As a general rule the entire wall should be applied at the one time. Extra plasterers should be used on very large walls, and breaks in work should only be carried out at internal or external corners where the refraction of light will vary with the angle. Variation of colour in large panels over a wall may indicate that the material has been applied and finished over small, more manageable areas. That is, a high wall rendered from a scaffold may show variation at each “lift” because starting at the top, the material was applied and finished; material, tools etc. passed down to the next level and that area is applied and finished.

A small area in the middle of a large panel, which shows colour variation, may be caused by:

The plasterer touching up an area sometime after the whole panel has been completed.

Variation in the absorption from the background. This may be commonly seen when a single finished coat is applied over brick or block work. At times a brick wall, which is to be rendered, may look like "Joseph′s coat" with the builder using bricks left over from many sites. Even bricks that appear the same may have totally different rates of absorption.

Generally, walls applied in cooler weather will dry slower and therefore be darker than walls applied during warm weather. Other factors that must be considered are the humidity and the wind. A warm dry wind will dry a render quickly and result in a lighter colour. Freestanding walls may tend to be lighter at the top. This is caused by wind blowing over the top of the wall affecting the upper section more than the bottom.

2. Colour Leaching

This is where very large areas of the wall are sometimes very much lighter in colour (sometimes almost white). This is most often caused by water. Rain shortly after application is a major cause.

This may be due to:
  • The wall being exposed to the conditions.
  • Collection of water on an overhang which eventually runs back then down the wall. A drip groove cut under the overhang may help to overcome this.
  • Ill fitting or non-existent downpipes.
The tradesman′s care/knowledge may also be a cause:
  • Water carelessly flicked from a brush.
  • While applying a sponge finish, if the plasterer cleans the sponge before completing the entire wall surface.

3. Grinning

This is where each brick/block can be seen in the finished rendered wall and is caused by:
  • Variations in absorption from the bricks/blocks and the mortar joints.
  • Subtle variations in thickness of the material because brick/block work can never be laid perfectly in line.
In some circumstances this may be a desired part of the texture, as is often the case with a bagged finish, but it usually caused problems as a one coat application is costed when a two coat finish is required. The only method, which minimises grinning is to first apply a base render coat. This provides a flatter base and therefore a more even thickness of finish coat can be applied which will tend to even up the absorption from the background.

4. Efflorescence

This is the appearance of white crystals on the surface of the wall. It commonly appears after excess dampness has affected the wall and is not necessarily from the render as it may be originating from the brick/block work. It is caused by a chemical reaction and will often brush off once the wall has thoroughly dried although a colour variation may remain.

5. Powdering

The common cause of a render being powdery is that proper curing has not been carried out. The material has literally dried out before the cement has had time to set. This is most common when renders (especially thin coat renders) are applied in very drying conditions.

Mid summer is an obvious example. During these conditions, curing of the wall can start almost as soon as it is finished, however, precautions should be taken. The wall can be periodically sprayed with water, the precaution being that a mist spray should be used. Spray the wall enough to darken it, in no way should water be allowed to run down the wall as leaching of colour could occur.

Excessive absorption from the background is also a factor. A simple dampening of the background may overcome this problem.

6. Cracking

There are two forms of cracking which can mar a rendered finish.

Structural − caused by the movement of the building. Correct placement of construction joints will not stop such cracking but will allow the cracks to be taken up in controlled positions. Many buildings have joints − above doorways, above and below windows and at strategic positions along large walls. Engineers would normally set their position.

Craze cracking − Two main reasons for this are:

Applying a top coat which is stronger than the base coat. This is not necessarily judged by the ratio of sand and cement. A strong sand/cement mix actually be weaker than the background because the "strong" mix is made up to a very wet consistency to achieve the desired texture.

Overworking the surface. This brings a concentrate of fine material (cement) to the surface of the render, which then basically follows the first reason.

7. Drumminess

This is where the render has lost adhesion from the background. Where multiple coats of render have been applied to achieve the desired finish, the loss of adhesion may be between any of them.

This may be caused by:
  • A stronger top coat pulling itself from the background.
  • Poor preparation of the background i.e. dusty, greasy, very smooth, very dense.
A drummy area should be carefully assessed and the question should be seriously asked, "Does it really need to be disturbed?". A lot of drummy area are needlessly chipped of and repaired. The major problem being is that the action of chipping out a drummy area will almost always cause it to spread, and so far larger patch to the wall will be required.

8. Variations in Texture

Admittedly, it is not always possible, but, one tradesman should carry out the actual completion of the texture finish. There is more possibility that one person can maintain a standard finish.

Never touch up part of a wall/panel after it has been completed.

9. Patching

General Rule: A finished render cannot be invisibly patched.

The join will almost always be visible and due to different drying conditions on the day of patching compared to the day the wall was first done, there will be a colour variation. The only method is to completely re-coat the wall.

A good deal of damage to the finished wall can be avoided.

Do not apply finished renders until as late as possible during construction.

Ensure all electrical and plumbing work is completed.
Hold relevant trades responsible for damage. Much damage is caused by pure carelessness, e.g. ladders against walls, walls being bumped, etc.
Telephone: 02 4872 2774  Cavendish Street, Mittagong  NSW 2575 Australia