The Construction of the Thatched Roof

How a thatched roof reacts to the exposure to humidity depends mainly on the roof structure. Today we distinguish between ventilated and non-ventilated constructions. Especially for the thatched roof we think that the older terms „cold roof” (ventilated construction) and „hot roof” (non-ventilated construction) are very appropriate. That is why we will be using them in the following.

The Thatched Roof as a Cold Roof

The classic thatched roof is a cold roof: The attic is not developed. At the eaves the reed just lies on the wall edge or the tilting board and on all hipped sides there is an opening (Owl Hole). It is very draughty in the attic. As a consequence the thatched roof is very dry. The rain intrudes about 5-6 cm into the top layer and can dry quickly.
Even in combination with a fully converted attic the thatched roof can be constructed as a cold roof. A double partition is used and its shells are thermally separated by a ventilated air layer. The inner shell has a heat insulated build-up (normally between rafter insulation). The outer shell is the thatched roof. In between there is an air layer of 6-8 cm in turbulent flow. This ventilation only takes place if the air can enter the roof at the eaves and leave it at the ridge. Fresh outside air is, as a rule, already provided by the permeability of the reed eaves. However, if the reed is very fine, which makes it more steam-tight, additional vents at the eaves or in the walls are recommended. If the masonry is back ventilated, then the ventilation of the masonry should be connected to the ventilation of the thatched roof. In the ridge area the escape of air is ensured by cross ventilation in the cock loft (between the open frontispieces). The air below and above of dormers should be allowed to flow freely between the individual interspaces of the rafters in order to prevent standing air.
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.
Fundamental for the longevity of thatched hot roof is a working vapour barrier. In practice it has shown to be very difficult (e.g. due to floors and partition walls) to install a vapour barrier that is really completely water-tight. That is why special diligence should be taken.
Advantages of the Behaviour Under Fire
The most important advantage of the hot roof is its behaviour under fire which in comparison to the turbulently ventilated cold roof is much better. During an experiment the University of Applied Sciences Lübeck measured only 18°C below the burning roof of a thatched house with hot roof. 800°C more were reached in a ventilated area. That is why in the Netherlands thatched roofs that are directly attached to the substructure without a layer of air to ventilate them fall under the same regulations as hard-roofed houses and the same minimum distances apply. As the hot roof cannot dry on the inside, in addition to the vapour barrier, great care has to be given to the thatching process. It is essential that the reed applied is dry.