Water leakages are the general cause of moisture stress exceeding the moisture tolerances of constructions. Moisture stress causes biodeterioration (mould, rot and insect damage) in wooden constructions. The degree of damage is affected by the severity and duration of moisture exposure and temperature, together with the sensitivity of the wooden construction to environmental stress. The growth of mould fungi on wooden materials depends mainly on the air humidity, temperature and the surface quality of wood. Long term humidity conditions above a relative humidity (RH) of 80% are a risk for mould growth in pine and spruce sapwood when the temperature is between +5°C and +50°C. Between −5°C and +5°C the growth of mould fungi is slow, and possible only when the relative humidity is above 90%. High nutrient content of the surface can stimulate the mould growth. In fluctuating humidity conditions, the severity and cumulative time of conditions suitable for mould growth are critical factors for the development of mould fungi in constructions. Humidity conditions must be above RH 95% (around the fibre saturation point of wood, wood moisture content about 25–30%) for the spore germination and mycelium activation of brown rot fungi. Under unsuitable conditions, fungi are inactive, but they can become active quickly under suitable conditions. The degree of decay in wood depends on the total time the decay organism is active. The natural durability of wood varies with the environmental conditions, wood characteristics (wood species, sapwood or heartwood, etc.) and the decay organisms. The service time of wooden materials can be extended by modification of the wood.