Wet rot is caused by wood that rapidly changes from wet to dry frequently. This usually occurs from a leak in plumbing lines, or it can also occur through rain water or improperly sealed wood, such as gaps in outdoor siding and other similar problems. Wet rot is a soft portion of the wood that easily crumbles away or indents. You can treat wet rot by finding the cause of the rot and fixing that problem, cutting away the rotted wood, adding a wood preservative to the wood and then filling the hole with an epoxy wood filler.
To prevent wet rot it is advisable to ensure all timber including window and door frames are well painted/protected from rain water.