Most structural damages occur due to a lack of in or outgoing air in a ventilated construction. In this case what is supposed to be a cold roof becomes a hot roof with the consequences we will describe below.
The Thatched Roof as a Hot Roof
A thatched roof is called a hot roof if it does not have a ventilation or if it only features a layer of standing, non-ventilating air between the substructure and the reed. The layer of reed, which at the time of thatching is normally 35 cm thick, is used as heat insulation. The problem is that this layer of reed does not ensure a stable level of vapour permeability and of heat conductivity.
In dry conditions we can assume a diffusion resistance coefficient μ of 2-5 W and a heat conductivity of λ = 0.04-0.073 W/mK. During or after a rain shower however, the outer 5-6 cm of the thatched roof are wet. As a consequence the thatched roof becomes almost perfectly tight. That is why it is necessary to implement a vapour barrier on the inside of the roof construction, if the roof does not have a ventilation. If this vapour barrier is missing in a thatched hot roof or if it is damaged, then the humidity ascending due to vapour pressure condenses at the outer 5 cm of the rainy roof. In the course of the months November to March the humidity penetrates deeper and deeper into the roof. During the winter this roof – at least on the north side – will not get dry again.
If this problem is not identified in time and measures are taken, then the north side of the roof can already be severely damaged after little more than10 years.
Research in the Netherlands showed that approx. 90% of all cases of drastic premature ageing are caused by flaws in the substructure of the thatched roofs.