S4S (Surfaced Four Sides)
S4S stands for Surfaced Four Sides, meaning the lumber has been planed smooth on both faces and both edges to a specific finished dimension. S4S lumber is ready to use without additional milling. It is the standard finish for dimensional lumber at retail lumberyards and home centers.
In-Depth Explanation
S4S lumber has been processed through a planer on all four faces, producing a board with smooth, uniform surfaces and consistent dimensions. This is the most common surfacing specification for retail lumber — when you buy a 1x6 or 2x4 at a home center, it is S4S. The surfacing process removes material from the rough-sawn dimensions, which is why nominal and actual sizes differ (a 2x4 measures 1-1/2 by 3-1/2 inches because material was removed from all four sides). For hardwood lumber, S4S is less common at the wholesale level because buyers typically prefer to purchase rough or S2S material and mill it to their own specifications. However, hardwood S4S is increasingly available at retail outlets and online suppliers catering to hobbyist woodworkers who may not have a jointer and planer. S4S hardwood typically costs more per board foot than rough or S2S because of the additional processing.
Usage Example
The weekend woodworker ordered S4S poplar boards from the lumberyard since she didn't own a jointer or planer to mill rough-sawn lumber.