+61 2 9418 7750 d.hall@buildreport.com.au


We have inspected several buildings re cently where previous poor building practices are now starting to show up. Those practices include the installation of metal external angles to cement render. The main areas were the rust is becoming evident is in external locations, such as retaining walls, planter boxes and walls external of the main building. It is a matter of time before the rust becomes evident in other locations, such as to the building itself.

External angles are a recent phenomena in external rendering of a building. They are utilised as they provide a straight line for the cement render and also allow the renderer to render more quickly, such as the four sides of a brick pier in one day. This can speed up the work flow and increase productivity of the cement renderer. It is now becoming evident that some renderers have used cheaper metal angles which are not designed for external usage.

The types of external angles available, both currently and previously include the following :

  • Cadmium plated metal angles. These are designed for internal plasterboard lining usage and not external use. It does appear that these angles have been used on a regular basis on many buildings and have resulted in the rust that is now becoming evident.
  • Hot dip galvanised metal angles. These angles are a more recent phenomena and we are unable to comment on the durability of these angles in an external situation.
  • PVC or plastic angles. These angles are now far more common place than previously evident and are suitable for external applications. These angles may be subject to minor buckling during the construction period, however this would be visually evident within the building.
  • Stainless steel angles. These angles are far more expensive and generally specified in coastal areas, or a salt laden environment.


Repairs where rusted angles are evident involve engaging a cement renderer tradesperson to physically remove the render and the angle in the affected areas and either replace with a suitable external angle or completion of the render using a more traditional method.

The undertaking of this work, however, is relatively detailed and time consuming and would also require external repainting to be completed to provide an acceptable overall finish.

Once rust is evident to the metal angles it is not possible to rectify the rust by sanding down, application of a rust neutraliser coating, application of a metal primer or similar. The rust mainly commences at the external edge as it is exposed by the weather and becomes obvious as the paint deteriorates with age. From various site inspections and works examined, the rust is also evident to the perforated areas of the angle, which is located behind the render. It is a matter of time before the rust in this area becomes obvious.

A few examples as seen from site investigations include :

2010 06-image002-rust 2010 06-image003-rust

2010 06-image004-rust 2010 06-image005-rust

2010 06-image006-rust 2010 06-image007-rust