The term dry rot is familiar to most people and is usually associated with dilapidated wooden fences and decks exposed to the elements for too long. More serious problems can result when structural members are attacked by undetected rot. The deterioration of wooden structures in moist or periodically wet environments can often be traced to rot problems.
The name "dry rot" is a misnomer as wood decaying fungi will only flourish in moist environments. In most building construction moisture contents remain well below the levels necessary for the development of dry rot. Typically, rot problems occur where wood is in contact with the ground, or where condensation or leakage problems exist. Wooden roof decks are a common example. Boats constructed of wood are notoriously vulnerable to dry rot and in most cases eventually succumb to its effects. Rot can be inhibited by the application of preservative solutions. However, in harsh environments these provide protection for only a limited period.
Dry rot development can indicate either poor design or poor maintenance and inspection. Both are usually due to ignorance of the potential for rot development.
For the development of dry rot, a special set of conditions must exist. These are:
A source of fungal spores. This requirement is easily fulfilled, since spores are present almost everywhere wood is used. They are produced in the millions by a single infected area. Even new wood can be infected.
An available food source. All woods, including plywood, act as a food source and so are vulnerable to fungal attack. In general, the lighter coloured sap woods are most vulnerable, while darker coloured hardwoods and root woods are more resistant.
Moisture. Wood moisture content of at least 25 to 30 percent is necessary for the development of rot.
Moderate temperatures. Temperatures between freezing and about 100 degrees fahrenheit are necessary for the growth of rot. At temperatures between 65 and 90 degrees fahrenheit, dry rot will flourish. During the colder months, it becomes dormant, but is not killed.
Oxygen. Oxygen is necessary for rot development, although very low concentrations will suffice. Wood will not rot if buried below the water table.
Rot grows best in low light to dark conditions and in moderately acidic environments. It will not grow at a ph above 7.5, therefore, sea water inhibits the development of dry rot. Early detection of dry rot is quite difficult. Incipient decay can spread through the wood causing reduced strength before rot becomes visible.
The early signs of decay are abnormal brown colouring in "brown rot", or a mottled pale appearance to wood attacked by "white rot". These effects can be quite subtle and require knowledge of normal appearance of the wood to be distinguished. In more advanced stages of dry rot, the wood begins to shrink and crack and eventually collapses. Even these effects are not always visible, since the surface of the wood may be hidden under another material.
Rot can also occur on the interior of an apparently unaffected wooden member. More advanced cases of rot may be detected by:
Although dry rot is well understood and the measures needed to avoid it, well known, it still causes substantial property damage every year.
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Walters Forensic Engineering | 277 Wellington
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