Why is lichen bad for my roof?

Greening on roofs - a natural process!

  • Improved environmental conditions favor the growth of algae, moss and lichens
  • But: Inadequate ventilation of the roofing is also a common cause
  • Tiled Roof Working Group publishes the folder "Green Formation on Tiled Roofs"

(07/11/2004) Germany's homeowners are worried and fear for their roofs: Green patina shimmers on many pitched roofs. "No reason to worry", so the Arbeitsgemeinschaft Ziegeldach e.V., Bonn. "Despite the green formation, the protective function of the roof is guaranteed over the long term. Green formation is neither harmful to the health of the house residents, nor does it disturb the longevity of the tiled roof." Almost every surface is affected by green formation from algae, lichen and moss. The reason for this lies on the one hand in changed, i.e. improved, environmental conditions. "In the past 'acid rain' and pesticides were responsible for clean roofs, today it seems as if nature is reclaiming lost terrain. Thanks to better climatic conditions, microorganisms can spread faster than before. Greening takes place on the building materials." , explains the Tiled Roof Working Group.

Although only a visual impairment, the problem still drives countless homeowners, according to the association of leading roof tile producers. "Consultations and the provision of information on this topic are a long-term favorite with us." Reason enough to put out a leaflet "Green Formation on Tiled Roofs" and thus to contribute to the proper information of the professional public and end users as well as to clarify causes, prevention and remedies.

As you can read in the eight-page fanfold folder, greening is possible on almost all materials. The roughness of the covering materials promotes the formation of green. But the microorganisms do not stop at smooth or coated surfaces in the long run. Like on glass roofs, greening can also develop on glazed roof tiles in the long term. Decisive for this are, among other things, the location of the building, proximity to water and high humidity, shading, nearby trees, drifting from fertilized fields or from mowing the lawn. All of this forms an ideal livelihood for the "green pioneers".

But the building construction also has a significant influence on the algae growth: the steeper the roof and the higher the building, the less green there is. "In addition to other factors, the main cause of 90% of the objectionable roofs with greening is a lack of or insufficient ventilation of the covering," warns the working group. Poor air exchange behind the roof tiles ensures that the drying process is decidedly slower. The roof stays moist longer - the necessary breeding ground for microorganisms can develop.

Therefore: proper roof ventilation is an essential prerequisite for avoiding greenery. In the case of extended roofs, the subsequent installation of ventilation elements on the eaves and ridge usually has a positive effect. Cutting back green growth and reducing shading in the area surrounding the building can also counteract the formation of greenery. Mechanical collection of moss and lichen is also useful. However, the Tiled Roof Working Group advises caution in the event of an intended high-pressure treatment with hot water or a surface sealing with chemical varnish that is only effective to a limited extent. The use of poisons should also not take place for reasons of health and environmental protection. A checklist in the folder for your own and expert assessment of greenery also reduces existing uncertainties. Conclusion of the author: "Even ecologically minded homeowners who recognize a patina with natural charm in a green formation can be sure that their tiled roof will offer generations of residents protection and beauty with professional care and maintenance."

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