Care and Maintenance of a Thatched Roof

Care, maintenance and repair are decisive factors which determine the durability of a thatched roof. Preventive care and maintenance is of great importance for a roof. Proper care can help to defer costly renovation. If damages from birds or storms occur immediate action very rarely costs much. A crack in the masonry of the ridge is easily fixed.
However, if you fail to react water can penetrate at the affected places. In the course of time the amount of money needed to mend the damages will multiply and unpredictable secondary damages might occur.
Algae, moss and fungi are considered to be the main cause for damages on thatched roofs or for their premature ageing. That is why in the last years the control of algae and, to a minor degree, as well of moss has become a part of the maintenance works.

General Care and Maintenance of a Thatched Roof

In the course of the overall care of a thatched roof it is advised to clean the roof and to re-tighten it once a year. The better a thatched roof is cared for and the better it is maintained, the more durable it will be. A roof dirty with fir needles, leaves, moss and algae needs more time to dry than a clean roof. The wet, damp condition of the roof does in turn promote the growth of fungi which feed from cellulose and thus decompose the thatching reed. Timely and professional maintenance will multiply the lifespan of a roof. That is why in the long run the maintenance works are less expensive than they might seem to be at first. The opposite is also true. A lack of care will always have a negative effect on the longevity of the roof.

The following measures can help your thatched roof:

  • Cleaning the thatched roof
  • Redressing the thatched roof
  • Controlling algae and moss on the thatched roof
  • Trimming surrounding trees

Cleaning a Thatched Roof:
A thatched roof has to be cleaned by hand. With help of different types of rakes the roof is freed from leaves, fir needles and moss. The easiest way to get rid of moss is to wait for a dry spell. The reason is that under dry weather conditions the moss loosens its grip on the reed. It is a common misbelief that the removal of moss damages the thatched roof. If the thatched roof is cleaned professionally only the butt ends of stalks can fall off that have been previously damaged by weather exposure. Also the assumption that harking spreads the fungal spores is objectively wrong. Reed, being a natural building material, contains per se a certain amount of fungal spores. If fungi can actually develop depends on the spores finding ideal living conditions.

Re-dressing the Roof Covering:
With the help of a legget the reed stalks are re-dressed.

Protecting the Thatched Roof from Excessive Tree Growth:
It is also necessary to restrict the growth of trees and bushes around the house. Trees and bushes shade the thatched roof and thus interfere with the drying effect of the wind and the sun.
Trees overlapping a thatched roof collect rain and dew and drip off onto the reed. Permanent dampness and moisture is the consequence which encourages the growth of fungi. Needles and leaves from trees should be removed once a year.

Controlling Algae and Moss:
You can learn more about the control of algae and moss on thatched roofs in the section: Controlling algae and moss on thatched roofs.

Controlling Algae and Moss on Thatched Roofs

The drier a roof is, the longer it will last. Mainly the wind and the sun are responsible for the „natural drying” of the roof. So, if a roof is not exposed to these two elements it cannot dry efficiently and the probability of the emergence of an algae film increases.
This layer can hermetically seal the surface of the roof and causes the roof to stay wet for longer. A reduced lifespan of the roof is the result. That is the reason why it is essential to try to control the growth of algae. Under dry conditions an algaecide can be applied
The controlling agent is applied on the roof by means of a spray gun (avoiding high pressure). It kills algae and moss and as the thatched roof will be drier on average the fungi will lose their means of existence.
Measures exclusively aimed at the fungi would at best fight the symptoms but not the cause for their growth. This procedure is not expensive and is worth the while as you can save money with regards to necessary renovations and as it will make your roof last longer (an additional 10-15 years). A welcome side effect is that the roof will look better.
If the algae are not removed in time you will easily spend ten times more on renovation works. After performing maintenance works an additional protective treatment has to take place. Should you fail to do so the remaining moss and algae will need less than a year to recover and again spread over the entire roof.

The Dutch Federation of Thatchers (Vakfederatie Rietdekkers, Henk Horlings, knows more than ten different (and apparently effective) controlling agents that are regularly employed in the Netherlands. These products show a very variable pricing and the manufacturers give different descriptions of the way they work and of their effectiveness.
Even if these products have already been tested, there is no information available on how long they remain effective. Of course, each manufacturer claims that his product works over a long period. In practice, however, it has been shown that these products do not properly adhere to the surface of the roof but are washed away by rain. As a result, these products are actually unable to prevent the future growth of algae – they can only kill off already existing organisms.
Currently the most suitable controlling agents against algae are so called quats (quaternary ammonium compounds). This product has a pre-emptive effect as a wood preservative and curative.
Different ways of applying controlling agents: immersion, impregnation, brushing, spraying and injecting. Quats are relatively new controlling agents. They are non-volatile and therefore no harmful side effects caused by evaporation are to be expected.
The product is surface-acting and is also used in many detergents and cosmetics (to disinfect for instance canteen kitchens, hospitals, stables) Quats have a soapy smell.

Best is to spray the roof after some dry days and with temperatures lying above 15°C. That is when algae and moss are very active and in a period of growth. Thus, they can absorb enough of the product sprayed. It is important that the roof is sufficiently moistened. The plants need at least 30 minutes to absorb the moisture. During this time the roof should be kept moist, especially in summer when the roof can get very hot. About 1 litre/m² has proven to be enough.

Quaternary ammonium compounds are less aggressive than agents containing Diuron and on a thatched roof they have to be applied in a different way. The active constituent penetrates the cell wall of the fungus, the algae and the moss and causes a hole in the membrane of the cell which leads to the death of the organism. As quats destroy the cell walls of algae, moss and fungi care has to be taken to ensure, that these organisms absorb a sufficient quantity of the active constituent.
If the agent does not produce the effect desired the organisms have not absorbed enough of the product. This can have two reasons: Either the organism was already saturated with moisture or too little or a too small quantity of the agent has been applied.

The quats are harmless for the environment and the thatched roof, provided that they are dosed properly. The agent has almost no negative effects on higher vegetation (normal garden plants and grasses). Thus it does not pose a threat to expensive gardens. Ponds, situated directly beneath the roof, should be covered, as water fleas and fishes are very susceptible to quats.

If quats come into contact with anionic detergents (e.g. soap) they totally lose their effectiveness. Take care that your sprayer is clean!

Health Risks:
The product is lightly acidic. In case of contact a low concentration can lead to skin and mucosa irritation. Only exposure to the product in form of foam or mist can be dangerous, especially in closed rooms. If the agent is not applied according to instructions and e.g. protective clothing, gloves and mask are not worn cough, a dry throat and even nausea can occur.
Quats are partly degradable. For most quats neither a complete degradation nor an accumulation in the environment could be shown. Microorganisms in surface water, mud or soil can be negatively affected by quats (booklice, for instance, die).

There are quats which are obtainable on the market under many different names. One of the best known is Dimanin. Which product should be chosen for a thatched roof and how should it be applied?

Formalin, which is very commonly used in agriculture, is not suited for the cleaning of thatched roofs. The algae die and the roof gets very clean but the reed is damaged. High concentrations of Formalin definitely have disastrous effects on the reed.

Hiss Reet in cooperation with Henk Horlins, Vakfederatie Rietdekkers NL,