I seem to get this question more and more. Wood rot begins with air pockets in wood that get moisture in them. Air pockets in wood very in size and abundance from various wood types. For example Pine is some of the softest wood available. Also “New Growth” or “Rapid Growth” wood can be very porous. As opposed to hardwoods, synthetic or treated lumber. So whenever the moisture gets in wood from hydrostatic pressure or simply is exposed to weather. The wood expands and then begins to rot from the inside out. This is basically what happens.