Bow (Lumber Defect)
Bow is a warp in lumber where the board curves along its face from end to end, like a shallow arch. It is measured by placing the board flat and checking the gap between the board's midpoint and a straight edge. Bow is caused by uneven drying, internal stress, or grain irregularities.
In-Depth Explanation
Bow is one of four primary warp defects in lumber, alongside crook, cup, and twist. A bowed board curves along its length when viewed from the edge — imagine a board that rocks like a canoe when laid flat. Bow typically results from uneven moisture loss during drying, where one face dries faster than the other, or from residual growth stresses released during sawing. Mild bow can sometimes be corrected by jointing and planing, though this reduces the board's final thickness. In framing lumber, slight bow is generally acceptable as long as the crown is oriented upward during installation, allowing the load to help straighten the member over time.
Usage Example
The framer checked each 2x12 for bow before installation, making sure to place any slight crown facing upward in the floor joist system.