Permissible Limits for Bow, Spring, Twist & Cup
The nature of timber as a hygroscopic product (absorbs moisture from the air) means that distortions in boards is an inevitability. Managing and minimizing these distortions requires an understanding of what distortions are and what the Australian Standards requirements are for what is acceptable before a board is considered unusable. There are four main distortions that can occur in a board:
Spring – a deviation in the longitudinal direction of the piece causing the edge to curve away from its intended flat plane.
Bow – a deviation in the longitudinal direction of the piece causing the face to curve away from its intended flat plane.
Twist – a spiral distortion along the length of a piece of timber.
Cup – a deformation across the width of a piece of timber causing curvature perpendicular to the face of the board.
Measurements for these distortions are performed using the methods outlined in the images below.
Under the requirements set out in the National Construction Code and Standards Australia there are maximum limits for these distortions, these are outlined in the tables below.