Biological growth on roofs
| Biological growth on roofs | |
| Organisms | Moss, lichen and algae |
|---|---|
| Distinguished here by | How they behave and how they respond to treatment |
| Taxonomy source | None; described as trade practice, not sourced botany |
| Favoured by | Shade, damp, and a porous or weathered surface |
Three kinds of growth are found on United Kingdom roofs and are treated differently: moss, lichen and algae. They are separate organisms, and the practical differences between them — how they attach, how much water they hold, how fast they return and how they respond to a biocide — are what determine how a roof is cleaned. This article describes them as the trade encounters them. It carries no citation for the biology itself: no source meeting this site's standard was found for the taxonomy, so what follows is established practice rather than sourced botany, and is written to that limitation.
Moss[edit]
Moss is the growth that matters most to a roof, because it holds water and has bulk. It forms cushions and mats that sit on the surface and in the laps between tiles, and it holds moisture against the covering long after the roof would otherwise have dried.
Its bulk is the practical problem. Moss sheds in weather, and what sheds goes into the gutters and downpipes; a roof heavy with moss produces a drainage problem before it produces a roof problem. Where growth is thick in the laps it can hold tiles apart. Moss favours the shaded and damp parts of a roof, so a north-facing slope or one overhung by trees will carry it while the opposite slope of the same roof is clear.
Lichen[edit]
Lichen appears as flat, crusty patches, often pale grey or yellow, adhering closely to the surface rather than sitting on it. It holds far less water than moss and does not have the bulk to block a gutter.
It is the most tenacious of the three. Because it bonds tightly to the surface it does not come away with the ease that moss does, and mechanical removal that would lift moss cleanly will often take some of the surface with the lichen. Its slow growth cuts the other way too: a lichen-covered roof took a long time to reach that state and will take a long time to return to it.
Algae[edit]
Algae present as staining rather than as growth with substance — green, or in some conditions black streaking, following the paths where water runs. There is nothing to scrape off.
Because the problem is a film rather than a mass, it is almost entirely cosmetic, and it is the growth most responsive to a biocidal treatment and the least suited to mechanical removal, which risks damaging a surface to address a stain.
Why the differences matter[edit]
The three respond differently enough that a single method is rarely right for all of them. Moss has bulk to remove and its removal produces visible results immediately. Lichen adheres and resists, and pressure sufficient to shift it is often pressure sufficient to damage the covering under it. Algae has nothing to remove mechanically at all.
This is why a roof carrying all three may be treated in stages, and why a claim that one method deals with everything on any roof should be read carefully.
Conditions that favour growth[edit]
Shade, damp and a surface that holds water are the common factors. Overhanging trees, north-facing slopes, low pitches that shed slowly, and older coverings whose surface has weathered to a rougher and more porous state all encourage growth, and the last of these is why an aging roof accumulates faster than it did when new.
It follows that cleaning does not address the cause. A roof cleaned but still shaded and damp will regrow, and how quickly depends on those conditions rather than on the quality of the clean.