Another criterion for the durability of a thatched roof is the quality of the reed used. When choosing the thatching reed, the house builder has to rely mainly on the expertise of the thatcher who has to find a compromise between aesthetics, workability and durability.
His decision depends on the available goods on the market (and thus it depends on the reed trader/trading company) and it is influenced and limited by the financial possibilities of the house builder.
But the best thatching reed is of no use if the conditions for a high durability are negatively affected by flaws in the building physic.
Requirements of Thatching Reed
The properties required of thatching reed are stated in the German Roofing Trade´s data sheet on reed:
Generally speaking, thatching reed of good quality should be free from grass, old and bent canes as well as from other impurities. Moreover, the thatching reed should be strong and flexible. The canes should be dead at the moment of harvesting and they should be cut between the second and the third section. Reed canes can be divided into short, medium and long reed. The diameter of reed stalks should lie between 6 and 12 mm. A bundle of reed of good quality is cone shaped.
Reed bundles have to be processed, packaged, stored and handled under dry conditions. The natural moisture of a reed bundle can be compared with the natural moisture of construction timber and should not exceed 20% wood moisture.
Depending on the weather conditions, the outer reed bundles can temporarily show a higher humidity, which normally should dry out quite quickly. The inside of the reed bundles, however, should be dry at all times
Quality Criteria
The quality of reed is influenced by many factors. Reed is a completely natural product, this means, that its quality (as for example in wine growing) can vary according to harvest, weather and the quality of the soil and the water etc. There is no objective method of measuring the quality of reed and thus to determine its potential life span.
When judging his raw material the thatcher has to draw on his own expertise. In order to choose the most adequate reed for the roof the coordination of all people involved is required.
However, in the majority of cases, the durability of the reed only shows much later, when the reed has already been thatched.
Below, we list the most important factors that influence the quality of reed:
The price of Thatching Reed
If the house builder sets a tight limit to expenditure, then the thatcher will have to take this into consideration when choosing the reed. As on other markets, for the reed market the rule holds true that first-class products are more expensive than products of average quality.
When to Thatch
The reed harvest takes place from December to March. That is why the reed on offer is not always of the same quality. In years of low crop returns there is considerably less choice in the months of October, November and December than in the rest of the year. As there is only a limited amount of reed available, its quality tends to fall at the end of the year.
That is why some thatchers buy the reed in time and then put it in storage in order to have enough first-quality reed by the end of the year. Naturally, the storage has an influence on the price per square meter. If a house is thatched in winter, the reed, due to weather conditions, will be moister than in midsummer. Professional storage, however, can avoid this effect.
The Provenience of Thatching Reed
Depending on where it has been grown the reed shows different chemical and physical properties. Of all the varieties of reed that have been on the market for the last years none is known to lead to a premature ageing of the thatched roof. Premature ageing of a thatched roof cannot be attributed to the country of origin, but always has to do with additional problems as for example constructional flaws and errors (e.g. a hot roof without a vapour-barrier) or the careless handling of the material. Under the right conditions the natural process of wear and tear should span between 25 and 40 years, depending on the reed species.
Chemical Composition of Thatching Reed
The robust reed grows on sandy, nutrient-poor soil. Furthermore, the plant profits from continental climate with hot summers and an interruption of the growing season by strong frost (cp. Rodewald Rodescu: Das Schilfrohr). It is assumed, that a high concentration of silicate and a low concentration of nitrogen have a positive on the durability.
Scientific research on the correlation of the composition of reed and its durability are under way in both Germany and the Netherlands.
Structure and the Composition of Thatching Reed
The structure and the composition of the individual reed stalks has an effect on its perviousness of vapour diffusion. The thicker the stalks are, the less vapour-retarding is the roof. The finer the stalks are, the tighter the roof is, also in wet conditions.
That is why, in principle, a thatched roof with a high percentage of coarse reed is more durable than a thatched roof with a high percentage of fine reed. Here a conflict of goals arises between the contemporary taste for an aesthetically appealing fine and soft surface and the desired durability.
Under favourable constructional conditions, however, fine reed has a high life expectancy, too.
