Hardwood lumber cupping, twisting, warping is defined along with wive's tales of causes and solutions.
This is a layman’s discussion of wood warpage. Any bending of the wood is generally defined as warpage, but we will provide a further breakout.
Cupping: It is when the lumber has curved into a concave or convex bow from side-to-side. Usually caused by moisture getting into the wood from the top or bottom. Frequently found on old tabletops where the glue seams have also failed due to the moisture getting into the lumber. The moisture obtains access due to a minimal or deteriorated top finish and likely no finish on the bottom of the tabletop. Old desk or dresser tops can experience the same problem.
Since most lumber boards are only 4” to 8” wide it is basically impossible to reverse a problem board. It is basically impossible to reglue from side-to-side a series of cupped boards because the side edges are no longer perpendicular.
Conceptually the high spots can be surfaced planed or jointed away to obtain a new flat surface, whether the opposing side is flat or not. Once a flat surface is obtained the side edges can then be made perpendicular again. The board can then be reglued to a square edged adjoining board, but will likely be thinner at the ends.
Warping: It is when a board has bent from end-to-end whether being concave or convex. Usually caused for all the same reasons. But more importantly, usually because the board was no longer attached to its adjoining boards due to failed glue seams, and because the board was no longer tied down or attached to whatever created a rigid frame for it such as a table skirt system.
It is possible to bend most lumber back flat by simply attaching it again to its rigid back frame. The thicker the board the more stress that will be involved. Such warping can be pulled out in the regluing phase within reason. The “within reason” means that the surface elevations of the good board and the problem board might not end up truly level. The option is to then have the new slab wide belt sanded to obtain a total new flat surface.
Twisting: It is when a combination of the above cupping and warping occur causing the board to actually twist from end-to-end and side-to-side. The solution possibilities and solutions are somewhat the same.
Wives Tale – Add Water: Some literature says that you can take a cupped board and apply moisture to the curved side to cause it to re-bend flatter. The answer is that it can work to some limited degree to the extent that all finish has been removed and that the magical amount of moisture was added and that the reversed cupping stays that way when the added moisture eventually dries. Our experience is that it is not worth the effort.
Wives Tale – Add Weight: Some literature says that you can counter warp a board by bending it backwards with force, weight, or clamps and let it set that way for some magical period of time. Some even suggests adding the moisture mentioned above. The same experience holds true that you might be somewhat successful in un-warping a board from end-to-end but that once you take the clamps off and let the wood readjust that the final result will be less than acceptable.
Supporting Frame Failure: A more common experience is that on first assessment the top of something is warped or is sagging from end-to-end. An example might be a long desk top or library table that is supported by 4 legs on the corners and a minimal skirt frame along its length. Overtime the top’s center sags down. The real problem is not warpage of the top boards. The real problem is that the framework below the top has failed due to weight or inadequate original design.
Installing a rigid brace to the bottom side of the sagging top can usually reverse the problem. Such a brace might take the form of a 1 ¾” thick by 3” wide hard maple board installed on edge under the top.
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