Wood decay in house framing can be classed as either wet rot or dry rot. Wet rot is caused by fungi that can only spread as far as the moisture has penetrated and their mycelia (root-like growths) cannot penetrate brickwork. There are several fungi that can cause this type of rot. In floors the most common one is the cellar fungus, Conio-phora puteana (formerly C. cerebelld). This makes affected wood darken and leads to cracks forming which mainly run along the grain of the wood. The fungus often forms narrow, dark brown mycelia on the surface of the wood.
Dry rot is another serious type of decay which is caused by the dry rot fungus Serpula (formerly known as Merulius) lacrymans. Like all other fungi, it needs damp conditions to become established, but once it gets a hold it forms root-like mycelia, sometimes as thick as a pencil, which enable it to spread into surrounding less damp timber and to penetrate through walls and masonry. The infection can spread from one room to another, and the mycelia can be found in walls between the plaster and the brickwork. They can also travel through mortar joints seeking out any pieces of timber in the walls. Wood attacked by dry rot tends to break up into dry brick-shaped pieces and usually has grey, fell-like growths of fungus on its surface.
However, if the air around the fungus is very moist, it can form white, fluffy cotton wool-like growths which often have tinges of lilac or yellow on them. When the fungus has been growing for a year or two, it produces fruiting bodies on the surface of the wood or on adjacent brickwork. These are pancake-shaped with a wrinkled surface that soon becomes a bright, rusty red color as the spores (seeds) develop. These spores are minute, oval cells about 0.01mm long. They are produced in such abundance that the surrounding surfaces soon become covered with what looks like a fine, rust-colored dust.
When this dust is found on the surface of a floor, it is an indication that the dry rot fungus is present somewhere nearby. Sometimes the appearance of the fruiting body is the first indication that dry rot has attacked your wood. Mold is the least worrying fungal growth that is likely to attack your house. Heavy growths of mildew and dark-colored molds may develop because of condensation on cold walls. This indicates that there is lack of ventilation and insufficient heating.