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Especially in densely populated areas, facades are subject to deposits of sand, dust and soot. Acid rain, a combination of water and airborne pollutants such as sulfur dioxide, is particularly aggressive because it penetrates deep into the masonry pores, leading to discoloration and decomposition. Facades with a rough, coarse surface texture are more prone to large-area dirt pick-up or graying than facades with a smooth, fine surface.
Algae are unicellular organisms which are disseminated by the wind and get deposited on facades. To grow, they need light, moisture, carbon dioxide from the air and temperatures of between 20 and 25 °C. Particularly on severely weathered facades that do not dry out well, the algae proliferate and colonize large areas that turn green, brown or red. In many cases, algae only impair the outward appearance of a facade, while the underlying building fabric is not affected.
Salts promote water ingress
Salts contained in the earth or the construction materials dissolve in water and will migrate into the masonry if dampproofing is inadequate. As the water evaporates, the salts become more concentrated, especially in the plaster. There, they crystallize out, effloresce or even force the plaster and masonry to flake off. Damage of this kind occurs most frequently in the ground course and lower areas of the facade.
In addition, all facades are prone to streaking in places that the rain cannot wash clean, for example under cornices, eaves, windowsills, balconies and all kinds of facade projections. The height of the building and the off-vertical slant of the facade also influence dirt pick-up behavior.
Cracks make facades susceptible to weathering. Hair cracks and shrinkage cracks due to water loss are fine, web-like cracks that occur in the facade’s exterior coating and sometimes also in the plaster. They are caused by applying the paint too thickly, having too much binder in the mortar, by surface concentration of binder (caused by smoothing the plaster), by fine sand in the uppermost layer of plaster or by the plaster’s drying too quickly. Water can penetrate through the cracks and lead to moisture and frost damage in the substrate.
Spalling is a sign of poor adhesion. Surface coating systems with high internal stresses sometimes cause adhesion problems. This particularly applies if the coatings are applied to insufficiently firm substrates, which, in the absence of a primer, for example, are highly absorbent. The internal stresses in the paint lead to the formation of cracks, allowing water to ingress and accumulate beneath the coating. This causes the paint to flake off. The high pigment content of intensely-colored paints tends to increase internal stresses still further.
Facades exposed to hygrothermal (change in properties due to moisture absorption and temperature change) stresses for any length of time may be subject to blistering. This is caused by seasonal cycles of warming and cooling and of rain ingress and drying out. A film-forming exterior paint that blisters will adhere less well and will detach from the substrate at weakpoints.