Racking

Also called: rack, out of square, distortion

Racking is the tendency of a frame or joint to distort out of square under a sideways, skewing load, the whole shape leaning like a parallelogram instead of holding its right angles.

How joints resist it

Joint strength differs greatly depending on the type of load involved: compression, tension, shear, a sliding force between two touching surfaces, or the twisting, skewing action that causes racking. A snugly fitted joint, even with no glue, can resist compression, shear and skewing loads simply through the wood's strength across the grain. In a mortise-and-tenon joint, where a tongue, the tenon, fits into a matching socket, the mortise, glue-line shear strength on the tenon's cheeks, its two broad flat faces, limits how far the joint can rotate under a skewing load. A mortise-and-tenon joint that has been wedged, pinned, or glued resists both being pulled apart and being racked out of square, and a well-fitted example can stay sound after years of skewing stress, shrinkage of the tenon, and even a split opening up in the wood around it.

Weaker and stronger joints against racking

Not every joint resists racking equally well. A full housing joint, where one piece sits in a groove cut across the full width of another, offers little resistance to a frame twisting out of square, and resists being pulled apart even less well than it resists racking; for that reason it should be chosen only after weighing the whole structure's needs against its weakness under a skewing load. Adding a rabbet, a stepped recess, to a through housing joint makes it more rigid and less likely to twist. Where a joint is likely to twist under load, reshaping the tenon and its shoulders, the flat faces that sit against the surrounding wood, is the chief way to stop it skewing out of true.

Racking in frames and structures

Frames are especially vulnerable while unfixed. An open, three-sided frame such as a door frame or a jamb lining, the frame set around a doorway to line the opening, is braced diagonally on the bench so it will not rack out of square while being handled or moved before fixing. In timber-framed walls, diagonal bracing becomes necessary wherever the wall sheathing, the boarding fixed over the frame, cannot on its own supply enough resistance to racking; where a whole frame is loaded so it tends to skew, the braces working in compression likely carry most of that load.

Other causes and workshop practice

Racking is not always caused by an outside load. Uneven shrinkage across the grain between solid case sides and a solid corner piece can pull a case badly out of square, or break the joint, as the wood dries. During glue-up, clamp blocks matched to a rail's thickness focus the clamping force right at the joint and stop it skewing out of true, and a heavy bench itself resists shifting out of square while its owner works on it. Frames left out of square on site, along with other poor work, are sometimes let go uncorrected because a later stage of the job is expected to put them right.