Growth of Microorganisms on Thatched Roofs

According to current knowledge, algae, moss and fungi play an important role in the decay and the premature ageing of thatched roofs. In the past almost no control measures were taken against moss and algae on thatched roofs. Apparently, algae and moss did not have a negative effect on the lifespan of a roof (25 to 40 years for a „normal” house).
In recent years this seems to have changed. Thatched roofs are increasingly vulnerable against fungal decay, moss and algae. The increased formation of algae, moss and fungi has a negative effect on the lifespan of a roof, especially, if the algae start to form a sealing layer on top of the reed.

Reasons for the Increased Growth of Microorganisms on Thatched Roofs

Algae, moss and fungi are just the symptoms of the problem of the shortened lifespan of a thatched roof. They grow where they find ideal living conditions. Fungal decay has a preference for warm and moist areas – as anyone who has had the experience of catching athlete's foot in a swimming bath will know.
However, the reason for the growth are the optimal living conditions which make it possible for algae, moss and fungi to damage a thatched roof in the first place. If today an increased growth of algae, moss and fungi is detected, then the only reasons are the improved living conditions on thatched roofs. Today’s roofs do not get wetter from the outside or over a longer period of time than they did in the past. The reasons for the improved living conditions are the following:
  • Change in the shape of thatched roofs
  • Too short reed is used for thick roofs
  • Deficient ventilation of thatched houses
  • Flaws in the constructional design of the roof
  • Wrong combination of old and new materials and constructions
  • The quality of the thatching reed used
  • The location of the thatched house
  • Expert knowledge and expertise of the thatcher
  • Lack of care and maintenance of thatched roofs
Changes in the points named above are the best method of getting rid of algae and fungi.
Microorganisms are thus deprived of their livelihood.

How the Growth Works

As a result of the improved living conditions the algae succeed to populate a roof in no time. The algae cause the roof to stay wet considerably longer, which in turn boosts new algae growth. If you do not react in time a biofilm evolves, that changes the colour of the roof from a nice greyish-brown to a dirty green. This is not harmful as such, as algae and moss draw their nutrition from the air and the thatched roofs just serves as a basis. At the same time, however, especially algae contribute to the fact, that the roof stays moist for a longer time. Algae basically form a simple vapour-tight coat. This coat of algae minimises the evaporation that occurs, primarily, at the surface of the roof. Beneath the infested surface an optimal climate for the growth of mould develops. Especially white-rot and fungi of the micena family prosper here.
In this optimal microclimate these fungi are able to spread quickly over the whole roof. In contrast to algae and moss they feed from the decomposition products of cellulose which is one of the main components of reed. Without (preventive) maintenance a roof can thus decay within 10 to 15 years. It is also possible that a roof has to be renovated after 7 to 15 years (at the best) only to last some 5 to 10 years longer. That still is too short. A well thatched roof can last 25 to 40 years.

Controlling Microorganisms

If the maintenance of a thatched roof does not adapt to changing conditions, a part of the roof will not reach the „usual” age of thatched roofs. In such a case, longevity can only be attained by costly renovation. Experiences made in our neighbouring countries with the control of algae and moss show good results. The basic principle is to keep the roof as clean as possible, as then it will stay as dry as possible as well. The drier the roof, the longer it lasts. You can find more information on the control of algae, moss and fungi on thatched roofs here.