Case 4

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Cause of Defects

Rain water flowing down the surface of the rendered façade carries with it dirt particles (Table 1) that are being retained by the façade material. The dirt particles are re-deposited at its limit of flow.
Upon drying out, dirt particles that are not washed off will manifest as stains.
The stain marks are dependant on the type of rendered mix used, the architectural feature and design, the influence of wind and rain and lastly the amount and type of staining agents used.
For the type of rendered mix used, it is usually dependent on the mix of rendered uses which will affect the absorptivity of water by the rendering, the texture of the rendering.
A textured surface will have a higher tendency to retain dirt than one which is smoother this may result in greater amount of staining due to the larger amount of that is being carried by the run-off and subsequently deposited down the façade.
Architectural feature and design will also affect the amount of staining. For example staining is usually predominantly found below the window ledge or and protruding feature.
Staining in the form of black mark or discolouration of the rendering surface may result and can be very aesthetically unpleasing. If left unattended over long period of time, the dirt will be very difficult to clean off.

Pollutants	Examples	Sizes	Sources	Possible effects
Aerosols	Sulphur dioxide (SO2)	Gaseous	Burning of fuels	Formation of acid rain. Reaction with water and other materials to form sulphates and chlorides to cause destruction to façade materials.
Soot or black smoke	Smoke from tobacco, coal, fuel oil, metallurgical industry	<1 pm	Imperfect combustion	Transported by air and deposited on facades to cause stains
Coarse particulate matter	Ash, dust, rock debris, mineral dust.	>1 pm	Unburnt fuel and dust from roads and industries	Transported by air for short distances and deposited more easily on horizontal or sloping surfaces to form stains.

Table 1: Sources of pollutants