Facts and Information about Dry Rot and Wet Rot

DRY ROT

Dry rot is the term given to brown rot decay caused by certain fungi that deteriorate timber in buildings and other wooden construction without an apparent source of moisture. The term is a misnomer because all wood decaying fungi need a minimum amount of moisture (circa. 25-28% moisture content) before decay begins.  The decayed wood takes on a dark or browner crumbly appearance, with cubical like cracking or checking, that becomes brittle and can eventually crush the wood into powder. Chemically, wood attacked by dry rot fungi is decayed by the same process as other brown rot fungi (Wet Rots).

An outbreak of dry rot within a building can be an extremely serious infestation that is hard to eradicate, often requiring drastic remedies to correct.  Part of the reason for this is because dry rot has the ability to grow behind and within non-organic materials such as plaster, mortar joints, hardcore under solid floors etc. to find new sources of food.

The term dry rot, refers to the decay of timbers from only certain species of fungi that are thought to provide their own source of moisture and nutrients to cause decay in otherwise relatively dry timber.   In modern texts, the term ‘dry rot’ is used in reference to damage inflicted by Serpula lacrymans.  This species of fungi cause decay, by removing cellulose and hemicellulose from the timber, leaving a brittle matrix of modified lignin.

An explanation of the term “dry rot” circles around boatyards periodically.  In the age of wooden ships, boats were sometimes hauled for the winter and placed in sheds or dry dock for repair.  The boats already had some amount of rot occurring in the wood members, but the wood cellular structure was full of water making it still function structurally.  As the wood dried out, the cell walls would crumble.  In other words, the wood was already rotten and as the boat dried, the wood collapsed and crumbled, causing the workers in the yard to determine it was “dry rot”, when in fact, the wood had been rotten all along.

Identifying the source of water and allowing the affected timbers to dry will kill dry rot, as it is a fungus and requires water as all fungi do. This will not, however, kill any spores left behind, which will remain viable and cause the rot to return upon wetting.  This approach, while necessary, will not in itself repair structurally damaged timbers, or building fabrics.

WET ROT

Although there is only one type of Dry Rot, which is in the “brown rot” category, there are numerous forms of Wet Rot, which fall into two categories “brown rots and white rots”.  Wet rot is a debilitating fungal infection of timber, which can affect the strength of structural timber, resulting in severe damage to a building.  Wet rot occurs when there are high levels of moisture, damp or condensation in a building, which result in moisture contents in excess of 25-28% in the affected timbers.

There are a number of different types of wet rot fungi, although the most common types are:

Cellar fungus (Coniophora Puteana) – The most common type of Wet Rot

This type of wet rot fungi is usually found in damp basements, under floors and in skirting boards. It causes timber to darken and produce cracking both along and across the grain of the wood. It prefers very damp conditions in areas like basements, leaking roofs and wood floors where there is insufficient ventilation, or measures such as damp proofing have not yet been taken to prevent a wet rot outbreak.  This is classed as a brown rot.

Mine fungus (Poria Vaillantii)

This wet rot fungi causes wood to shrink and split into cuboidal sectors. Its strands are white and sometimes fern-like.  Where this has affected buildings, often cellars and basements, due to the higher levels of dampness and humidity, the necessary treatment may be similar to that required for Dry Rot.  This is classed as a brown rot.

Phellinus Contiguus

This type of wet rot, white in colour, bleaches wood, which becomes fibrous and stringy. This is a common type of wood rot decay in external joinery timbers such as door and window frames.  This is classed as a white rot.

Asterostrama

This type of wet rot, where the mycelium or hyphal strands are generally a whitish, or cream colour, is often found in joinery timbers, such as damp affected skirting boards, along with the undersides of flat roof timbers where water ingress has occurred.

What to do if you suspect a wet rot infestation in your home?

Check for damp or water ingress in places where you think this is most likely to occur, such as roofing, gutters, windows, floors, and cellars or basements. Check the walls for signs of possible rising damp – there may be a discoloured or white ‘tide mark’ at the level the damp has risen to.  Check for possible plumbing leaks which may be concealed.