Case 3

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Cause of Defects

roof1
Figure 1a
roof2
Figure 1b
roof3
Figure 1c
  • Stain streaks at the wall near to the roof top can be attributed to the accumulation of dirt on the horizontal ledge which are then washed off the edge. The deposition of dirt (Table 1) on the wall is especially severe during light rains where little runoff is generated and most of the rainwater is absorbed by the concrete wall (Figure 1a to c).
  • Rain water flowing down the surface of the rendered façade carries with it dirt particles that are being retained by the façade material. The dirt particles are redeposit at its limit of flow. Upon drying out, dirt particles that are not washed off will manifest as stains.
  • The stain marks observed in the photo originates from the parapet wall on the roof top. This may be due to the washing off of dirt particles that have accumulated on the horizontal face of the parapet wall.
  • The localised stain streaks may be due to an uneven surface or depression on the horizontal surface which creates a channel through which rainwater flows.
  • Stain streaks formed on concrete surfaces cannot be easily removed like those on metal or glass surfaces. They would be washed cleaned during heavy rainfalls and would most likely manifest as dark stain streaks.
PollutantsExamplesSizesSourcesPossible effects
AerosolsSulphur dioxide (SO2)GaseousBurning of fuelsFormation of acid rain. Reaction with water and other materials to form sulphates and chlorides to cause destruction to façade materials.
Soot or black smokeSmoke from tobacco, coal, fuel oil, metallurgical industry<1 pmImperfect combustionTransported by air and deposited on facades to cause stains
Coarse particulate matterAsh, dust, rock debris, mineral dust.>1 pmUnburnt fuel and dust from roads and industriesTransported by air for short distances and deposited more easily on horizontal or sloping surfaces to form stains.

Table 1: Sources of pollutants