Most types of timber will rot if a sufficient supply of moisture and air is present. Rotting is a form of fungal attack which can cause significant, permanent damage to timber, and by the time the spongy, damaged wood crumbles in your hand, the problem is usually well established.

Timber cannot be protected from airborne fungal spores, but the availability of moisture can be controlled. Timber that is rotting needs to be cut out and replaced or treated with a poison or preservative.

You may encounter great expense and difficulty when attempting to replace complex pieces of joinery, including timber windows. As a result, it is important and sensible to try to prevent the rot and repair where necessary. Wood consolidating products, such as those that contain acrylic and urethane adhesives, can be effective for this purpose as they are designed to penetrate the timber and stick to the fibres. This provides timber with structural integrity, limits the uptake of moisture and ultimately causes rot inactivity.

Products such as this can be used for rotting windows, weatherboards and cladding, outdoor furniture and fencing. They are not suitable for the restoration of structural timber.

Source:  http://au.lifestyle.yahoo.com/better-homes-gardens/diy/articles/a/-/8100264/how-to-fix-wood-rot/