2.5: Lumber Defects and Warp

2.5 Lumber Defects and Warp

Every board should be inspected before you buy it and again before you machine it.

Defects may affect appearance, yield, strength, machining, or safety.

Some defects can be worked around. Others may make a board unsuitable for a particular project.

Learning Outcomes

After completing this section, you should be able to:

• Identify the four common types of warp.
• Recognize several common lumber defects.
• Explain how defects affect material selection.
• Inspect a board before machining it.

Four Types of Warp

Bow — A lengthwise curve along the face of a board.

Cup — A curve across the width of the face.

Crook — A lengthwise curve along the edge of the board. Crook is sometimes also called crown.

Twist — A condition in which the four corners of the board do not lie in the same plane.

Four boards illustrating bow, cup, crook, and twist, showing the direction of distortion for each type of warp.

Figure: Types of Warp

Other Common Defects

Knot — A portion of a branch or limb that appears in the lumber.

Knots can be visually interesting, but they can also interrupt the grain and affect machining or strength.

Check — A crack running along the length of the board that does not extend completely through to the opposite face.

Split — A crack that extends through the board to the opposite face.

Shake — A separation between growth rings.

Wane — Missing wood or remaining bark along an edge or corner of the board.

Other defects may include:

• stains;
• decay; and
• insect damage.

Illustrations of common lumber defects including bow, cup, crook, knot, split, twist, check, shake, and wane.

Figure: Lumber Defects

Inspect the Whole Board

Do not look only at one face.

Before selecting or machining lumber:

1. Sight down the face for bow.
2. Sight down the edge for crook.
3. Look across the width for cup.
4. Check whether the corners lie in the same plane to identify twist.
5. Inspect both faces and edges for knots, checks, splits, shake, wane, or other defects.
6. Consider where your project parts can be cut from the board.

As your experience grows, you will learn to look at the entire board as a source of project parts rather than simply as one long rectangular piece of wood.

Defects Affect Yield

A 10-foot board does not necessarily provide 10 feet of usable project material.

You may need to cut around:

• checks near the ends;
• knots;
• areas of poor grain;
• sapwood or heartwood color differences;
• wane; or
• other defects.

This is one reason woodworkers buy more rough lumber than the exact finished volume shown on a cut list.

Defects and Safety

Some defects can also make machining more dangerous.

Loose knots, severe splits, or badly warped material may not be safe to run through a particular machine.

Defects may also prevent the stock from sitting securely against a machine table or fence.

When you are unsure whether a board is safe to machine, ask your instructor.

Check Yourself

1. What is the difference between bow and crook?
2. What type of warp occurs across the width of the board?
3. What is the difference between a check and a split?
4. What is wane?
5. Why should defects be considered when estimating how much lumber to purchase?
6. Why can some defects become a safety concern?