Biofilm Usually Found on Properties

The genus Trentepohlia would not, at first glance, be taken as a green alga. Free-living species are mostly yellow to bright orange or red-brown in colour, due to the orange pigment, haematochrome (β-carotene), which usually hides the green of the chlorophyll. The genus is terrestrial and is often found in Europe on rocks, walls and tree bark. Where they are found on buildings, they can cause severe mechanical degradation and deterioration.

Phototrophic microorganisms (algae, cyanobacteria) can grow on stone surfaces or may penetrate the pore system of the stone itself. Phototrophic microorganisms have a direct effect on the deterioration of stone due to their pigments which cause an aesthetically detrimental effect.

Filamentous cyanobacteria (Nostoc sp, Leptolyngbia sp., Stigonema ocellatum) and green algae (Desmococcus olivaceus and Haemaotococcus pluvialis) are capable of forming dense mucous-like layers on surfaces which produce characteristic colourations on the substrata. 

Nostoc. sp can produce blackish-green pigmentation which leaches into the substrate on which it is growing.

Chemoorganotrophic fungi are especially concentrated in stone crusts. They are able to penetrate into the rock material by hyphal growth and by biocorrosive activity, due to the excretion of organic acids or by the oxidation of mineral forming cations, preferably iron and manganese. 

This can often leave a rust-like stain on paving slabs even after washing. The deterioration activities of dematiaceous fungi also include the discolouration of stone surfaces, due to the excretion of dark pigmented melanin.

Botanically, mosses are non-vascular plants in the land plant division Bryophyta. They require sufficient moisture, access to nutrients and harvest sunlight to create food by photosynthesis. There are approximately 14,500 species of mosses. The most common species in urban Ireland are the greycushioned grimmia, Grimmia pulvinata; thickpoint grimmia, Schistidium crassipilum; and the wall screwmoss Tortula muralis.

Although mosses do not cause direct damage to roof tiles, they increase water capillary creep causing in the fullness of time corrosion of the fixings and ageing of the overall structure. They can cause some lifting when growing in the vertical bond. As they absorb moisture, they also add weight to roofs and when they become heavy enough or dislodged by bird activity etc, they can fall in gutters and down pipes and cause blockages.