Tech Note: Removal of Biological Growth

Types of organisms

There are several types of organisms that are considered potentially damaging to masonry substrates include fungi, algae, lichens, and mosses and other higher plants.

Molds, Mildews, Yeasts
Fungi, which include molds, mildews and yeasts, are typically green, brown, or black in color. From the spot or patch that initially appears, the fungus develops into a “fuzzy” layer. Most of the damage directly attributable to fungi has involved staining of interior plaster. Fungi are known to be the necessary precursors of some organisms and are thus indirectly responsible for other types of damage. Although several types of fungi have been isolated from exterior stonework, their role in its deterioration has not been firmly established. However, because fungi produce organic acids that can solubilize mineral constituents (e.g., oxalic and citric), their potential for directly damaging substrates is well recognized.

Algae
Many different species of algae have been isolated from limestone and sandstone substrates. Algae are green, red, or brown in color and often appear “slimy". The unsightly appearance of algae is worsened when dust and soot are entrapped in the algal mass. Algae develop on masonry materials during wet conditions and, because they retain moisture, normal drying of underlying substrates is retarded. This often results in the acceleration of water-related deterioration. Damage attributable to algae includes pitting, discoloration and staining, and the solubilization of calcium carbonate.

Lichen
A lichen is actually algae and a fungus growing together in a close association. Lichen are usually orange or gray in color. The hard, leathering appearance of some lichen is disfiguring to masonry surfaces. Lichen damage masonry substrates in several ways. Mechanical damage is caused by increases in size of the lichen mass. As with algae, the retention of water and production of acidic metabolic products often accelerates deterioration of masonry. In addition, most lichens are resistant to severe climatic conditions and survive during droughts and high and low temperature extremes. During drought conditions lichens shrink in size, creating shear stresses that can be damaging.

Moss
Mosses are small plants that typically form dense green clumps or mats, often in damp or shady locations and can attach themselves on a variety of surfaces including many types of masonry. Moss grows on exterior concrete surfaces that are not exposed to sunlight and remain in a persistently moist state. While moss is not harmful to concrete, its presence on concrete patios and walkways can make them more slippery, causing a safety hazard. Moss growth can be inhibited by decreasing availability of water through proper drainage or increasing direct sunlight exposure.

Other Higher Plants
Other higher plants such as ivies are often seen growing up the side of buildings. Ivies climb by producing aerial roots or little, suction cup-like pads along their roots and stems that “grab” onto surfaces. Ivy can easily damage old bricks, wood, stucco and even vinyl siding. The roots easily find seams and small cracks in brick, mortar, concrete, and stucco, growing into them and causing damage. With stucco, when ivy is pulled off, the stucco may be pulled off as well. Ivy is occasionally responsible for moisture issues with older homes because covered exterior walls can hold moisture in.