Wet rot
Wet rot is the most common form of timber decay, caused by persistently damp timber. It stays localised to the wet area, softening and darkening the wood. Fix the source of moisture and the affected timber, and the problem is resolved.
Dry rot
Despite the name, dry rot also needs moisture to start, but it is more serious because it can spread through damp masonry and behind finishes to attack timber some distance from the original source. It requires prompt, thorough treatment and, above all, elimination of the moisture that is feeding it.
Woodworm
“Woodworm” refers to wood-boring beetles whose larvae tunnel through timber, leaving the familiar small round exit holes and fine dust. Some infestations are old and inactive; others are live and need treatment. Damp timber is far more vulnerable.
The common thread: moisture
In almost every case, the real fix is to identify and cure the source of dampness (a leak, defective detail or poor ventilation) and then deal with the damaged timber. Treating the symptoms without addressing the moisture rarely lasts.
Advice
A proper investigation establishes the type of decay, whether it is active, and the moisture source behind it, so the right, lasting repair can be made.