The moisture content in trees decreases once felled. While growing, the moisture content can be 96% (Green), whereas when dry it can go to 12%.
Loss of moisture from the cell walls will reduce the girth of the tree substantially in some species, slightly in others.
To prevent major shrinkage and possibly change of shape in use the wood is usually dried down before it is converted into a building material.
If wood is to be used outside and therefore exposed directly to rain, sunlight and wind it should either have sufficient natural durability or be treated in some way to enhance its durability and to achieve an adequate service life.