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Lumber Defects 101

Unfortunately, there are very few perfect pieces of lumber out there. If you are going to build a deck you might as well become familiar with the different types of defects so you know what to watch out for.

Defects in lumber are faults that detract from the appearance and utility of the wood. Some are present when the wood is produced and others develop or become more sever as the wood ages. As a general rule you can reduce the amount of defects by purchasing higher grade materials at the outset.

Properly drying the wood before it is installed will also help. Using protective stains on your deck can prevent some damage to the wood by slowing down the drying process in order to limit checking. Stains can also conceal visual defects by covering them with a consistent color throughout the deck.

Warping is any variation from a true plane surface on a piece of lumber. This occurs due to the differences between radial, tangential, and longitude shrinkage in a piece of wood or growth stresses that encourage the board to reform its original shape.

Checks and Splits

A check is a term used to describe a crack that runs through a board, usually running lengthwise. It is usually caused when lumber dries out too quickly. Adding a UV protective stain can help protect decking and rails. Checking often affects pressure treated framing materials as they are usually sold very green and are put under a lot of stress as they dry out in the sun. Thicker members like 6x6 posts tend to dry out unevenly and are especially susceptible to extreme checking.

Wood reacts differently in the various climates across the US, and different species and grades also have differing appearances. Straight from the lumberyard, lumber components likely already have some cracking and checking present. Lumber grading procedures evaluate these conditions piece by piece and rate the lumber accordingly. A grade one lumber is of better quality, and thus great structural performance, than a grade two, for example. When it comes to posts, grade two is most often accepted and the market norm.

Exterior environments have fluctuating moisture content in the air and materials. Whether in a dry or wet climate, there will still be annual variation. With the typically large cross-sections of posts, commonly 4x4 or 6x6, the lumber does not dry uniformly. As the wood dries, it shrinks, and the shrinking often causes existing cracks to open or twisting to occur. This is normal and structurally acceptable in most cases.

Pecking is a strange phenomenon found in certain kinds of wood, like cypress. This appears on lumber as a cavity or pitted area in a board caused by a parasitic fungus when the tree is still alive. The decay stops after the tree is cut. The result is an unusual textured surface.

Rot and Decay

Rot and decay is the general description for the disintegration of wood fiber. Decay is caused by microscopic organisms like mold and fungus that feed off the natural substances found in wood. Fungi require four conditions in order to survive: an oxygen supply, 40-100 F temperatures, a supply of moisture (most fungi need a moisture content of 30% or more to thrive), and a food source. Some woods like cedar and redwood have extracts in their heartwood that make them naturally resistant to decay. Pressure treated wood uses man made chemicals that are toxic to fungus and mold. Many stains have mildew inhibitors that should help prevent decay. The most effective method of avoiding decay is to keep your deck materials dry before and after installation.

The Heart Pith is the spongy center of the tree that appears on the surface of the board. The true pith is the size of a pencil or ¼" in diameter, but it may move around a lot in crooked trees. In addition to the pith the first 3 or 4 growth rings are pretty immature and should be treated with caution. As these underdeveloped sections dry out they can open up and curl at the edges. You will probably want to cut out this section.

Ring Shake

Ring Shake is caused by parasitic bacteria that weaken the wood and results in cracking between and parallel to the trees growth rings resulting in lower strength. This is much like a large straw with a smaller straw inside of it. If it is stood upright it can fall out of the bottom. Ring Shake can also cause a dark discoloration and an unpleasant vinegar or rancid odor. This defect usually occurs in wood with high moisture content. It takes a long time for shake to spread so it is usually found only in the lower part of trees and can be cut out.

Some boards appear to have discolored streaks. They are commonly referred to as mineral streaks. This happens when dark colored minerals are drawn into the tree from its water source or when mold grows in poorly ventilated areas. The stains are generally blue or brown. Stains can be cut out or covered by a dark colored wood stain. Some commercial wood cleaners containing oxalic acid can be effective to clean off stains from mold.

Many types of insects feed on and make their home in wood. Beetles, moths and wasps are the most common types. Most insect borers are "secondary invaders" or are attracted to wood that is already weakened, damaged, or dying. The insect’s presence usually means the tree is already in poor health. Wood boring insects produce sawdust like frass (excrement). Their holes are usually round and are scattered in a random pattern across the wood. Wood content must be over 30% moisture content for insects to thrive. Pin wormholes are wormholes that are smaller than ¼" in diameter.

Pitch, Gum and Sap

These conditions are seen as a pocket of bleeding resinous material found on a board and occur when the tree is injured. It sometimes happens when a feeding insect damages the living part of the bark. Resin or sap can stain lumber and may cause difficulty when applying finishes.


The presence of bark or the absence of wood on the corners of a board is called wane. This defect will only be found in lower grade lumber. You may be able to turn the board over to conceal this kind of defect in some applications.

Shelling is a severe raised grain on the pith side of lumber. Because trees growth rings are made up of earlywood and latewood layers repeated cycled of wetting and drying can cause the earlywood to separate from the latewood. This is the most common in southern pine lumber.

Crook or Crown

A crook or crown is a defect found in a board that is bent end to end in the direction of the tall ends of a board as it continues down the length of the board. It can occur from uneven drying or if the trees pith (soft core at the center of the tree) is at the edge of the board. You may be able to rip the board to recover a narrower straight board, or make multiple passes through a jointer.


A bow is a defect in a board that bends along the grain lines. If the board were laid across a flat surface both ends would be in the air. There is no warp across the grain. This can be caused by uneven air circulation during as the wood dries. Any bowing causes internal stresses that will make the board difficult to cut. Ripping a board that is bowed can be dangerous. Crosscutting should be done with the bow facing upwards.


Cupping occurs when the board bends edge to edge across the face of a board and where the ends of the boards will look like the letter U. It is common in boards that are cut close to the pith. Cupped boards can be ripped into smaller boards and planed. Trying to force it flat can cause cracking.


Twisting is a general term for a board that that bends in any variety of directions and cannot maintain a straight line. Sometimes one corner doesn’t line up with the others. It usually occurs when the wood grain pattern is not parallel to the edge.


A knot is a defect in a piece of wood caused by the presence of a branch. In grading lumber knots are classified for their size, form, soundness, and the firmness which they are held in place. Knots are not only unattractive looking they also can weaken the board strength and often act as the starting points for checking. Higher grades of lumber will have fewer and smaller knots.

Dead or Loose Knot – During the development of a tree lower branches usually die off and are surrounded by subsequent growth around it. Dead branches produce knots in this way that are not attached to the wood and are more likely to fall out.

Tight Knot or Closed Knot– Tight knots do not affect the woods strength and has a flat face free of openings on the board’s surface. These knots were produced from a living branch that was incorporated into the tree as it grew.

Spike Knot – Spike knots are the result of a branch that runs into the edge of a piece of lumber, growing larger as it nears the edge. These can become raised on a deck surface and can be very sharp and dangerous to step on with bare feet. Do your best to cut these sections out as they can pose the worst problems.

Pin Knot – A knot that is less than ½". Smaller knots are usually sounder and are less visible. They may be present on some higher grades of wood.

Manufacturing Defects For Decking

These are defects found in lumber that are caused by sawing and dressing lumber. The machining process to produce dimensional lumber puts a large amount of stress on wood. In trying to reshape a large uneven log composed with complicated grain patterns into usable lumber can cause reactions that produce low quality lumber.

Roller Check – Roller check occurs when a cupped board is mechanically flattened between roller planes. A light roller check in up to 2’ long, a medium roller check is between 2’ and 4’ long, and heavy roller check is more than 4’ long.

Skip – Skip is referred to as any area where the planer missed the surface leaving it roughed up. Skip is caused when the width or thickness of a piece is too small for the planer to remove all the rough surfaces.

Machine Burn – Machine burn is the visible darkening of wood due to the overheating of the knives usually because they are too dull. This defect can usually be removed by sanding.