After a period of time the exterior surface of buildings like roofs and facades will become dirty, uneven discoloration will appear, often in patterns revealing how rain washes off the surfaces. Depending on the kind of dirt and the risk of deterioration of the surface by the dirt, the building owner after some time might choose to clean the surface. Cleaning, however, can be expensive and introduce other problems to the surface.
There are two causes of why building exterior becomes dirty:
Dust and dirt accumulation
Floating dust is usually under one micron particle size and generated by combustion of fossil fuel burning engines. Particles smaller than 0.01 µm are not stabile in the atmosphere; they will either react with oxygen or coagulate to larger particles. Emission of gases and particles by industry and cars generate particles up to 1 µm. coarser dust, with particles larger than 1 µm, is generally generated by abrasion. The particle size distribution varies with the environment, but generally the bulk of the particles are < 1 µm, both measured on mass, volume and on numbers.
While larger particles sediment quite soon because of their weight, smaller particles < 1 µm are floated in the air for days or weeks. During that time, they can be transported over 1000 km before they deposit. The particles are washed out in clouds or rain, a high content of water-soluble salts, especially in “urban” particles, lead to wet deposition. Small particles do not sediment in the same way; instead the particles are drawn by forces of adsorption of oil content in hydrocarbon particles and electrostatics.
A fast indication of whether discoloration is caused by biological growth or environmental particles can be obtained by observing the smudging pattern. Environmental particles are transported by water and to some extent also washed away by rain, leaving exposed surfaces cleaner than other parts. Biological growth, on the other hand, is typically seen on exposed surfaces that for some reasons are moist. Through a microscope the difference is often clear, because of the spores and mycelium in molds. However, sometimes it can be useful to apply the unknown dirt layer to agar plates, especially when deciding whether the matter is alive. This method is also useful in identifying the genius of the biological matter. Biological growth on surfaces can be moss, lichen, algae or mold. These are hardy species; nutritious matter can be found everywhere in nature, and if only the right moisture and heat conditions occur often enough, and the environment is not toxic, biological growth will appear. Biological growth is often seen near leaking down pipes or gutters, where the moisture content of the material is very high, but also on surfaces without unusual water supply, biological growth is seen.
Consequences of Dirty Buildings
Dirt on surfaces is generally not seen as a major problem because it does not involve any safety consequences unless it covers signs of deterioration. But the dirt is visible, and it is therefore natural to ask what effect the dirt has on the building.
Aside from some romantic ideas on how old houses should look, a too visible dirt layer on a building is an aesthetically failure, regardless of its origin. This kind of aesthetically problem is to some building owners unacceptable, as it can be seen as neglect and ignorance. Dirt on surfaces can probably not cause this alone, but is an important factor in the course.
Although there is probably no other damage than aesthetically, the reason for the growth must be determined and the facades will have to be cleaned.
Deteriorated areas are often covered with biological growth, but this does not mean that the biological growth has caused the deterioration; it is more likely that the cause is water damage. It is likely that heavy biological growth may enhance deterioration; biological growth keeps the surface moist, increasing the risk of frost damage.
Depending on the surface and the composition of environmental particles, a dirt layer can prevent the surface from deteriorating or react with the surface in an irreversible way, e.g. limestone and concrete can react with sulphate, forming a gypsum layer, which will appear as a crust, sometimes just visible to the naked eye as a discoloration.