Case 3

Cause of Defects

1. Due to the downward flow of runoff, the area along and below the ledge is usually most prone to staining. Staining occurs at such areas due to:
   • Presence of horizontal or inclined surfaces on the ledge where dirt particles of all sizes can settle and accumulate easily.
   • The disruption of the otherwise even and regular flow of runoff as it flows over the ledge.
   • Concentration of runoff at some areas on the ledges such that washing effect is irregular.
2. The top surface of a ledge whether horizontal or inclined allows dirt particles to settle and accumulate easily. This concentration of dirt on the ledge is washed off when runoff flows over it.
3. The dirt particles will be carried along by the runoff as it flows over the ledge in a flow path determined by the profile of the ledge (Figure 1).
4. The width of the ledge will affect the intensity of the stain streaks that originates from the ledge since a wider ledge will allow more dirt to settle on its surface and hence cause more pronounced stain streaks. However, a slight protrusion from the general façade is adequate to retain dirt and cause staining.
5. Runoff is diverted to concentrate in streams as it flows over the ledge. The horizontal plane of the ledge also caused the speed of the flow to be slowed down considerably, reducing the washing effect of the runoff.

Consequences

Staining originates from the ledges as the runoff washes away the dirt particles accumulated on the ledge. Runoff, after it hits the ledge will drain away from the ledge either at its ends or directly over the front face of the ledge (Figure 2). This will result in long stain streaks originating from the end of the ledge and/or short stain marks on the vertical face of the ledge.

Staining if left unchecked results in loss in aesthetic value of the building. Long-term deposition of dirt stains on the facade may result in high cleaning costs and sometimes, the stains might become hard to remove.