Green lumber is lumber which has not been dried or treated, making it as close to the original tree as it is possible to get. Typically, lumber is dried and treated before it reaches the market, so that it is ready to use. It is possible to obtain green lumber, however, and some individuals actively seek it out for specific projects and needs. When sold, this type of lumber is usually clearly identified, so that people understand that the lumber has not been dried.
The most important thing to know about green lumber is that it has a very high moisture content. Wood is filled with natural capillaries which are used by the tree to transport water and nutrients, and when it is freshly cut, it is full of water. As it dries, wood shrinks significantly, and it can also warp. If green wood is used in construction, it may cause problems as loses its moisture, warping the underlying structure and causing structural instability.
This type of lumber has also not been treated with any substances which are designed to promote water and insect resistance. Green lumber is more subject to rot, and it can be especially appealing to insect life and fungus. It also tends to be difficult to paint or stain and may ooze sap and other liquids, causing discoloration.
One of the most common reasons to buy green wood is a desire to shape the boards for specific projects. It is very flexible, so it can be bent into place for projects like ship building and custom homes, and then dried in a kiln or in a drying shed. As the wood dries, it will tighten up, but it will also retain the bent shape, as long as it is dried in clamps. To create bended timbers with treated lumber, people would need to steam or soak the lumber, bend it, and then subject it to drying again, a time-consuming process.
Some people also like to work with green lumber because they prefer to process their own wood as much as possible. In these cases, the lumber will be dried in a kiln or allowed to age, and then treated, if desired. For construction companies concerned about leakage of the toxic chemicals used to treat wood, curing their own green wood can ensure that their structures are free of such chemicals, which can be a selling point for some consumers.
Construction companies sometimes use the term "green" to indicate materials that are sustainable and environmentally friendly. In these cases, green lumber might refer to lumber that is harvested from areas that are managed with the long-term health and survival of the forest in mind. If a company is selling green lumber, it's important for a buyer to make sure he or she understands if the term refers to lumber that comes from sustainable forests or that which has not been seasoned or treated.