Wet rot

Also called: wood rot, decay

Wet rot is decay in wood caused by fungi that need moisture to grow, breaking down the cellulose, the fibrous material that gives wood most of its strength, as they spread through it. It differs from mould and stain fungi, which colour or coat wood without seriously weakening it: decay fungi actually consume part of the wood's own structure.

How decay develops

The earliest phase of fungal attack is called incipient decay, and at this stage it shows up mainly as discoloration of the wood, with its strength not yet significantly reduced. Strength loss starts sooner than the look of the wood suggests, though: even newly begun decay already reduces its impact strength. As decay continues it does not weaken every property equally. It has the greatest effect on toughness, the ability to absorb a sudden blow without breaking, and on impact-bending strength, and barely affects shear strength, resistance to sliding forces, or hardness, resistance to surface indentation. Where decay stays confined to dark lines running through the wood rather than spreading generally, woodworkers sometimes call the result 'spalted', though this is an informal term rather than a technical one.

Where rot takes hold

Rot-causing organisms may already be present in timber before it is ever put into use. Setting new timber into fresh brick walls is a common cause of decay, since damp lime-based masonry actively promotes rot, and using road-sweepings instead of sand in the mortar mix raises this risk further. Timber kept constantly wet, such as piles set below the water table or logs stored in ponds, escapes attack by ordinary decay fungi.