When Good Wood Goes Bad:
We've all been there: You go into the workshop on Saturday morning to dig into a project, only to find that beautiful lumber you bought a week ago warped from its straight and relatively-square shape. Unfortunately, wood often changes shape naturally due to a variety of factors, some of which you can control and some you can't. Before you get bent out of shape, take a deep breath and remember: There are simple guidelines to deal with warped wood.
What causes warping?
At its most basic level, a tree consists of cells containing water, a tree's lifeblood. When you cut a tree and saw it into lumber, it must be dried to remove most of that water to make it workable. When the amount of water left inside dried wood stabilizes, it has reached "equilibrium moisture content," or EMC. For hardwoods in the United States, that figure averages about 11 percent in the humid Southeast, to 6 percent in the desert Southwest, to 8 percent in the rest of the country.
Once at EMC, wood can be machined or worked by hand with faith it will remain stable. But there's no guarantee it will remain at that moisture level because wood continues to swell in response to upticks in humidity and shrink as the air around it dries and draws moisture out through the wood's pores.
Warping typically takes one of four forms: cup, bow, crook, or twist. (See the illustrations, above.) A number of factors cause warping. First, where a board gets cut from a log has the most influence on movement. Shrinking and warping can be predicted based on the grain pattern and its orientation to the pith (or center) of a tree. (See the illustrations, below.) Second, defects that occur naturally within the tree can cause warping. For more details on these defects, see "Less-than-ideal conditions cause stressed grain," bottom of story.
Third, perhaps the lumber dried too quickly (causing splitting) or with too much or too little humidity. By themselves these don't cause warping, but they can accentuate the wood's characteristics that do cause it. Fourth, the environment in which you place your wood can cause dramatic changes, even in short periods of time, as the boards adjust to the humidity level. This you can control.
To avoid or limit warping in your stock, the environment in your workshop and wood storage area should replicate the humidity and temperature levels of the setting in which your project will end up. A small window-unit air conditioner should suffice in most shops to reduce humidity. In the dry winter months, use a humidifier in conjunction with your heater. Avoid dramatic swings, maintaining these levels even when you're not in the shop.
You also can reduce the risk as follows:
* Select boards by hand before you buy. Avoid boards with large knots or areas where the spacing (on the end grain) between a few rows of grain rings is much wider than the majority of rings.
* Although more expensive, quartersawn lumber, below, tends to warp far less than plainsawn wood.
* Buy only enough wood needed for your current project. This way all the lumber will be at a consistent moisture level, and you won't have a big stack of leftovers in your way. Also, getting all your wood at one time increases the chances of it matching.
* When you purchase stock, store it horizontally with plenty of stickers (narrow strips of dried wood inserted between boards) to allow air to circulate. Be sure the stickers are directly on top of each other throughout your stack. Then, give the wood at least two days to acclimate to your shop's environment.
In the WOOD® magazine shop, Master Craftsman Chuck Hedlund prefers to break boards down right away rather than leaving them in their original state. With a cutting diagram at the ready, Chuck first cuts workpieces to rough length, leaving them a few inches longer than finished length. Then he joints one face flat, and planes the other to remove the rough face. This gives each board more open grain to absorb or release moisture. After that, he lets the lumber sit for at least two days to allow it to acclimate to the moisture level in the shop. Then he machines it to final dimensions.
For sheet goods stored vertically, Chuck suggests using a few scrap blocks between the wall and stock to allow air to circulate. Also, leave a fan running just to keep the air moving. Never store lumber on a concrete floor, which can transfer moisture to your stock. Instead, stack it on 4x4s with stickers between layers.
Saving warped boards
So let's say it's too late: You've got boards that have already warped. No need to panic. You won't be able to remove the warp and return the piece to its original condition, but you can, in most instances, still make good use of the lumber.
* CUP: If your board has a cup in it, you've got two options, below. First, create a thinner version of the piece by jointing the cupped face flat, and then surface-planing it to a consistent thickness. Or rip the board into three pieces—ripping with the cupped face up to avoid kickback on the tablesaw—and edge-glue them back together, inverting the grain of the middle piece. Joint and plane this new panel flat.
* CROOK: For a minor crook, below, joint the concave edge flat, and then rip the opposite edge parallel on the tablesaw. For long boards, snap a chalk line along one edge, cut along that line with a circular saw, joint that edge, and rip to width. For severe crook, crosscut into shorter pieces, and then joint and rip to width.
* BOW: A slightly bowed board can still be used in face frames, for example, intact if secured with screws, biscuits, or similar fasteners to negate the bow. For more substantial bows, crosscut into shorter pieces, below, and resurface if necessary.
* TWIST: Without a straight reference surface, boards with a twist prove difficult to use. These boards work best if machined into shorter, narrower pieces, below, for hidden use, such as cleats or test pieces.
Less-than-ideal conditions cause stressed grain
Trees have a natural attraction to the sun and its life-giving light. This makes trees grow, in most cases, straight up. When a tree gets forced out of that vertical plane, for example, by an intrusive neighboring tree or by growing on the side of a creek bank, as shown below, it will make every effort to correct itself and grow vertically again.
This "elbow" in the trunk produces stress that will result in warping should that part of the tree be sawn into lumber. Industry experts call this stressed grain "reaction wood," and it proves nearly impossible to detect in sawn lumber except to trained technicians who observe it on a regular basis.
Reaction wood is called tension wood in hardwood trees, and compression wood in softwood trees. Reaction wood tapers off as it moves away from the point of correction, so you'll be okay to save the log starting a couple of feet from the bend in the trunk.
You'll also find reaction wood where large limbs branch out from the trunk. This stressed wood lies in both the limb and the area of the trunk around that limb. Trying to save boards from a limb will only lead to frustration with warping later. As for the log, make your cuts a foot above and below the limb.
To spot reaction wood in a log, note areas with an asymmetrical or unusual change in ring thickness, as shown below. Such wood is best saved for the woodstove or chipped and used in pressboard.