Control of the environment within a thatched roof is the interaction between material qualities and thatching technique, the skill of the thatcher, roof design and material quality combine to keep a thatched roof dry on the inside. Under favourable conditions water can evaporate but air movement, temperature and relative humidity will influence its movement. Air movement is temperature dependent, roofs tend to dry out when the wind is blowing; extreme weather conditions will drive water into thatch, the speed at which thatch absorbs water and then dries out is an important factor in controlling the rate that thatch may decay. The easiest and most important, test is moisture measurements; any reading above 17% moisture taken at more that 100mm depth is an indication that water may be penetrating the thatch, which has the potential to support fungal growth below the surface. Changes in weather patterns in the first decade of the twenty first century, to wet summers and warmer wet winters means that thatch is not getting periods of hot sun necessary to dry the thatch and to desiccate and kill microorganisms.
An early indication of the potential for straw thatch degradation is fungal breakdown of the nodes. Nodes are the strengthening points in a growing stem and are the point from which leaf sheaths develop. In thatching straw nodes keep the stem straight and form a restriction point which prevents water from travelling along the inside of the stem and so into the roof. Nodes are higher in sugar content that other portions of a mature stem and for this reason are the first point on a stem to be colonised by micro-organisms and the first point to show signs of degradation making node condition a good indicator of the potential for early degradation in thatch. Fungal attack at the nodes will eventually weaken the joint and cause the stem to fracture; in a roof where many stems are attacked in this way, the fixings will eventually become loose and the roof will fail.