Rough sawn lumber is lumber which is not finished before it is shipped for sale. It is left literally rough and needs to be dried, planed, and otherwise dressed by the recipient. This lumber tends to be less costly and some people prefer working with it because it gives them greater control. There are also some important cautions and caveats to be aware of when working with rough sawn lumber or purchasing lumber for a project. Handling lumber badly or choosing the wrong lumber can be a costly mistake.
When lumber is processed at a sawmill, the rough sawn stage is one of the earliest steps, when boards have been cut from a tree but not yet planed and dried. When it is shipped in this state, the lumber is still green, which means that it has not dried and cured. Consequently, it will shrink as it settles and it must be allowed to dry before it can be used. Drying is often done by leaving the lumber outside, but it can also be dried in a lumber kiln or in a workshop.
Rough sawn lumber is slightly larger than finished lumber. This is because of the shrinkage associated with planing and drying; it is cut rough and large to leave room to plane the lumber to smooth it and to allow it to shrink as it dries. Many different types of lumber can be sold as rough sawn wood, including both hard and soft woods, and it is available in a range of sizes.
One reason to work with rough sawn lumber is to save money. For people who have the time and space to plane and dry the lumber, it may be more cost effective to use this type of lumber than it would be to purchase finished lumber for projects. Some people also appreciate the ability to have greater control over how lumber is handled during finishing. Woodworkers, for example, are often very concerned with the handling of their lumber because small mistakes can be magnified in custom projects; a board which is poorly handled can ruin months of hard work.
Lumber yards are very careful about labeling their products so that people understand what kind of wood they are buying. Rough sawn lumber will be kept separate from finished or partially finished lumber such as lumber which has been planed on one side. It will also be separated by tree species so that people can purchase lumber from the right kind of tree for a project.