There are two kinds of rot which affect wood and timbers; dry and wet rot. Dry rot is actually the result of the infestation of a living fungus, whereas wet rot is simply the natural decay of timber in the presence of high levels of moisture. Of the two, dry rot is far more serious and is usually treated by the removal of all infected timbers as well as the application of chemical fungicide. Wet rot is treated depending on the severity and placement of the rot; sometimes simple reapplications of water-repellent finishes may stop the rot, while on other occasions the entire timber may need to be replaced. In all cases of wet rot, the source of the moisture which causes the rot must be addressed to prevent further issues.
Identifying Dry Rot
In the earliest stages, dry rot will look like off-white, cottony sheets on brickwork and timber. It may have a yellowish tinge where exposed to direct light.
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Some mushroom-like bodies may grow over surfaces and finishes that are concealing dry-rotting timber, such as plaster or paint. Often, they can be the first visible signs of dry rot.
Inspect timbers for long, deep cracks running across the grain. These cracks may also evidence the off-white fibers of the fungus.
Where possible, apply direct pressure to wood which you suspect to be suffering from dry rot. Timber decaying from dry rot will buckle under pressure, often crumbling under even slight touches.
Identifying Wet Rot
Check for wet rot in areas typically vulnerable to moisture, such as those which may be exposed to the most moisture or that are nearest to the ground or other areas where water collects.
Paint and finishes may look sound while concealing rotting wood underneath. Professional builders and practiced do-it-yourselfers may use a thin-bladed knife to press into the timber, applying only very slight pressure. The knife should not penetrate very deeply at all. Deep penetration is a sign of rot.
Feel the wood in question. Timber affected by wet rot will feel spongy, even despite finishes and paint, and will look darker than timber unaffected by rot.