Case 1

Cause of Defects

The mechanism of staining due to joint misalignment is as follows:

• Joints that run vertically or horizontally on the façade can be presented as irregularities on the façade and behave as crevices to trap and retain dirt particles within it.

• Runoff under the combined effects of gravity and wind action will concentrate at these places of irregularities (Figure 1). Wind causes a lateral migration of runoff, which concentrates the downward flow along the lines of vertical protrusions and depressions. Such areas are also points where dirt concentration is highest and would be carried away by runoff.
• When runoff concentrates to flow within façade joints, the construction of the joints will determine whether the runoff will be totally contained and directed to flow within the joints or overflow the joints to cause staining at defined areas.
• Joints that have the ability to effectively contain runoff so that it flows into and along it without overflowing the joints will cause little staining to occur. However, joints that channel runoff to flow into it easily but at the same time not able to retain runoff within it will encourage stains to form.
• Findings from the rain simulation experiment have shown that when water droplets first impact on a dry panel, the resultant runoff flow is undeterminable and random. As the speed and volume increases, the separate streams concentrate into larger streams near to any vertical obstruction such as a vertical window or panel joints and flow along its inner sides.
• When the vertical joint breaks at the next panel, runoff continues to flow downwards with a small volume being diverted sideways along the horizontal joint. This spilling over of runoff from the vertical joints onto the façade causes the rate of flow and volume to be reduced such that the runoff will no longer have the capacity and speed to carry the dirt particles along. Long stain marks will then form directly below the vertical joint and bounded at both sides by shorter and less distinct stains.
• When spraying lightens and stops, runoff flow becomes more distinct and corresponds to the existing stain marks. The limit of flow shortens as runoff dries up, allowing the dirt to build up onto the existing stain marks.

• The sequence of staining as illustrated in Figure 2 demonstrates that due to the mechanism of staining at misalign joints, stain marks will be most intense at the area immediately below where the vertical joint discontinues and decreases in intensity further away from the discontinuity.

Consequences

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 will be irremovable.

Staining on a facade has a lot to do with the pattern of rainwater runoff, which in turn is dependent on the facade configuration, colour, texture, as well as the “roughness” and “water absorption” properties of the materials used: