Green Dimensions
Green dimensions are the size of lumber before drying, when the wood still contains its natural moisture from the living tree. Green lumber is larger than kiln-dried lumber because it has not yet shrunk from moisture loss. Softwood lumber standards specify slightly larger dressed dimensions for green lumber to account for the expected shrinkage.
In-Depth Explanation
When lumber is freshly sawn from a log, it contains a high percentage of water — often 30 to 80 percent moisture content depending on the species and whether it's heartwood or sapwood. At this stage, the wood is considered green and its dimensions are at their largest. As the wood dries, it shrinks — primarily across the grain (width and thickness) rather than along the grain (length). The American Softwood Lumber Standard (PS 20) accounts for this by specifying larger dressed dimensions for lumber sold in the green condition. For example, a green 2x4 is dressed to 1-9/16 by 3-9/16 inches, compared to 1-1/2 by 3-1/2 inches for dry (KD) lumber. The 1/16-inch difference on each dimension accommodates the expected shrinkage as the board dries in service. For hardwood lumber, green dimensions follow similar principles but are less rigidly standardized. Builders using green lumber must account for the fact that the members will shrink, potentially opening gaps in joints and connections as the wood seasons in place.
Usage Example
The timber framer ordered green Douglas fir beams knowing they would shrink slightly as they dried in the structure, planning his joinery to accommodate the movement.