The best way to avoid problems when building a wood project is to know how to buy lumber at a supply center. Always start with lumber that is straight, flat, and square. Woodworkers who own expensive shop tools have a definitive advantage in battling the common types of lumber defects. A wood planer, for example, can transform crooked and twisted board into a perfectly straight piece of lumber.

Without expensive shop tools, you need to select the best boards you can find. The inherent problem here is that most of the lumber at lumber supply centers are mixed with boards that are defective with cups, bows, crooks, and are often warped and twisted. This lumber is useable in ruff construction but is not so good for craft or furniture building projects. Unfortunately this leaves craft builders the challenge to figure out how to dig through the piles and pull out the best boards in the stack.

Always fully examine each board for signs of cup, bow, crook, and twist. Here are some tips on what to look for.

Cup:

Boards with cup have a "U" shape from edge to edge, and are nearly useless for building anything. Edges of a board that is not flat along that edge will not allow for tight joints. The best way to check for cup is to lay the board flat on the floor and check for a side-to-side rocking motion.

Bow:

A bow is easy to spot. It's the same kind of "sag" you might see in an old bookcase shelf that's been loaded with too many books. The best way to spot bow in lumber is to hold the board edge-side up and look down the full length of the board. If you don't see a straight line, the piece is warped from one end to the other. A certain amount of bow might be okay, especially if you plan to cut the board in small lengths. However, if you need the full length for projects like a workbench or table’ bowed lumber will not produce a quality finish.

Crook:

A board with no cup and no bow can still be crooked. The best way to spot a crook in the board is to hold the board surface-side up and peer down the full length of the board. A crook makes an otherwise straight board look like a road with a curve. Unless you plan to cut this board into a lot of smaller pieces, don’t buy boards with a crook in them.

Twist and warp:

Sometimes lumber defects are uneven and scattered along the length of the board. For example, you might find a board that is perfectly square at one end, but cupped on the other. Or you'll find a board that is straight through most of its length, but then curve a little towards one end. Although you might discard these boards, it might be worth your time to take a second look. For most many projects the pieces are relatively short lengths that you can cut from anywhere on a board. That means not every board you buy necessarily needs to be perfect. You may want to bring your project design with you, and look for boards that can yield just enough flat and straight pieces for the project.

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