Machined lengths of timber, depending on conditions during seasoning, storage, and transportation prior to being worked can often possess a number of different natural or manmade defects. Some render the length unusable, whereas some simply cause an inconvenience during installation. Here we shall discuss the most common of these defects.  

Crown

Crown is a characteristic of a length of timber in which a gradual curve can be observed down the length. This is present in most sawn boards simply due to the internal structure and stresses of the timber, as well as the seasoning process. With these varying internal stresses and densities within a length, it will tend to crown one way or the other. Very few boards are perfectly straight as they come from the merchants, with only a handful displaying no visible crown, though the internal stresses and potential future crowning of these boards are still present. 

With most boards displaying a definitive crown, during carpentry tasks such as rolling joists, framing studwork, or pitching roofs, we “cast our eye” down a length of timber, to determine the direction of the crown. In most operations, we install our timber with the apex of the crown facing upwards. Primarily, this reduces the overall undulation in our workpiece by ensuring all lengths are uniformly installed. Secondarily, we can ensure that over time as gravity and load are applied to our workpiece, the crown of each length of timber may settle straight or thereabouts as opposed to sagging the other way.  

Lengths of timber with an extreme crown can cause problem areas in certain aspects, such as creating a hump in a floor system or in the tiles of a roof. These timbers can often have their tops planed off in line with their surrounding members, or can simply be cast aside, being used as a length for cutting up instead.  

Cup

Cup within a board is a natural characteristic of dried timber, caused by the cellular structure of the wood, though it can be exaggerated due to an uneven drying of the faces of the length, either over time during seasoning, or rapidly due to exposure to the sun. This cup results in the board curling over its widest face, often, but not always in relation to its displayed end grain pattern straightening out. Similarly to crown in a length of timber, most boards will display a small amount of cupping, though in extreme cases the board may not be able to be installed correctly for its intended use. An example of this would be a leaning floor joist; nailed hard to the plate but leaning over at the top, causing a knock-on problem when installing solid timber blocking. 

For the most part the direction of the cup of a board does not matter as it does with the crown, however in a few situations such as forming double/ triple beams, or bolting timbers into steels, we require the external radius of the cupped boards to face outwards, as can be seen in the diagrams. This helps to combat the existing stresses within the timber, improving the overall stability of the component.  

As mentioned, boards cup naturally towards the “bark side” of the tree, as the severed annular rings attempt to straighten themselves. Exposure to moisture or extreme temperatures, either wholly or unevenly after seasoning can cause the length to cup further towards the bark side, or entirely in the other direction.      

Twist

Twist in a length of timber describes a change in the orientation of a board along its length. This can be caused by poor handling/stacking during seasoning, exposure over time to the sun, or undulating grain along the length, caused by poor milling. Twist is the most problematic affliction to deal with in a length of timber during installation in a site carpentry environment. For the most part all the criteria we work on involve some level of components being square, or straight. Twist in a board however can throw these criteria into disarray. For example, a twisted wall plate can affect a roof in terms of the fit of the bird's mouths of the rafters, as well as the height of the rafter backing. Another example would be a twisted rafter presenting itself poorly at the ridge.  

For all intents and purposes, twist in a length of timber generally creates additional work to overcome, and so these boards should be cast aside where possible or chopped up into smaller components.  

In joinery scenarios, the twist of a board can also be referred to as “winding”. The criteria and tolerances for finer workpieces are much smaller than in general carpentry, and so no degree of twist within a board can be tolerated. “Winding sticks” are instruments that can be used in conjunction with a length of timber to amplify the effects of a twist and identify a suitable course for correction. The winding sticks are placed at either end of a length. The user presents their eye level across the top of the winding sticks to identify any twist within the board. If the two sticks are perfectly parallel to one another, then the board is relatively flat. If the sticks are not parallel to one another, then the irregularity is amplified and displays the high and low spots of the board.  

Bow

Bow in a board refers to a curve along its length over its widest face. Of all the defects/ characteristics of lengths of timber, this is the least severe, as in most applications the bow can be straightened out. Through the use of fasteners such as coach screws on long nails, or through forcing other timbers into place against a bowed length, the timber can be straightened. Bow in a board rarely affects the creation of a workpiece and is not a defect to be concerned about.  

Shakes

Shakes are defects that occur within logs, caused primarily by the natural stresses that occur internally within a tree. These stresses can cause fractures and splits within the log, usually causing annular rings to separate from one another, or interrupting the rings completely, during the growth of the tree. These internal fractures can occur during the life of the tree, or more often will fully develop during the felling or seasoning process, which can exploit the internal stresses if carried out incorrectly. There are numerous classifications of shakes, all of which will present different issues when milling boards from an affected log. For the most part, shakes in logs have little to no effect on the useability or strength of the undamaged timber within the log. Good examples of fully formed shakes can be seen in the ends of traditional oak beams around heritage sites in England and Europe. Due to the traditional method of “green” timber framing, in which the logs were hewn and installed before being seasoned (making the wood easier to work with the tools of the time) the finished beams would dry over time, usually unevenly, causing these internal stresses of the logs to fully develop into shakes.  

Heart shakes

Heart shakes describe a split through the centre, or “heart” of the log. During the milling stage the boards should be cut parallel to this shake to allow fully sized boards to still be harvested whilst discarding the shake.  

Star shakes

Star shakes refer to a multi-pronged split that radiates away from the centre of the log but are still contained internally. These logs should be milled in a quarter sawn fashion, to eliminate the disruptions caused by the shake.

Radial shakes

Radial shakes, like star shakes, also radiate away from the heart of the log, but form visible external splits around the log. This is caused by a rushed seasoning process. These logs should also be machined in a quarter sawn or rift sawn pattern, avoiding the interrupted fibres of the timber caused by the shake.  

Cup shakes

Cup shakes refer to an internal separation of the annular rings, again caused by uneven or rapid seasoning in a log. This defect can cause splits and weakness along the annular rings of a milled board, or as is often the case will completely disrupt the ability to harvest full sized boards from a log. Occasionally, boards that have been milled with partial cup shakes arrive on site and are unsuitable for use. These boards feature a semi-detached portion of the timber that is cleanly separated from the main body of the board. Bark, or a discoloured protective layer may often fill the void between the detached layers.   

Ring shakes

Ring shakes are very similar in presentation to cup shakes, however, are more extreme in their appearance, completely separating one ring from another internally within the log.   

Split

A split in a board describes a fracture along the length of the timber, parallel to the grain of the board. This is often caused by an extreme external force such as a drop or an extreme load being applied to the end of the board, or by another defect within the timber to begin with, such as a shake. A split is often very easy to identify within a board, as the annular rings and grain at the end of the board can be observed to be separated. A “split” can also be known as a “check”. 

Wane

Wane, or waney edge in a length of timber describes an area of the board in which the bark or the raw edge of a log has not been sawn off properly at the mill. This will often occur in the lengths of timber that are milled from the outside of the log where the manufacturer is trying to produce as little waste as possible. Wane on a board is a not uncommon, as boards with small amounts are often passed through the quality and inspection stages. These timbers usually have a corner or an end missing. These timbers are still usable in most applications.  

Knots

For the most part, knots in site carpentry rarely have a negative effect on the construction process. Knots are categorised into 2 descriptors: dead knots and living knots. Living knots occur in a board where a limb once grew away from the centre of the tree, causing a change in the direction of the grain and the annular rings within the board. These branches are still connected to the tree when it is felled, with the changes of direction in grain being suitably woven together. 

In some instances, during the life of a tree, a limb can become damaged or detached before the tree is felled. The tree will continue to grow in width and size, and as it grows its annular rings will continue to expand outwards, enveloping the end of the detached limb. This completely encapsulates the once limb within the growing tree, disrupting the annular rings of the limb. Over time, the part of the limb or branch that remains within the tree will slowly die, causing separation of the fibres of the limb from the annular rings of the main trunk. This type of knot is known as a dead knot, or loose knot. 

In a length of milled timber, a dead knot, due to its fibres having been detached from the rings of the tree, will often fall out of the knot hole, leaving a hole in the length of the timber. This again rarely causes any structural issues within site carpentry, but in second fix applications, as well as joinery and cabinetry, this defect can be very unsightly, often having to be filled.  

Singular small knots do not affect the stability or strength of the length of timber, though large clusters or particularly large knots in the middle of a board can reduce its load bearing capacity due to the grain disruption. Knots can be unsightly in some work pieces when it comes to joinery or cabinetry, and can be problem areas when planning or chiselling, due to the change in direction of the grain. The difficulties of working with/around knots should be considered on site when producing finished components such as cutting housings in door lining or chopping locks into doors. Knots in these areas should be avoided where possible, by flipping components or by substituting materials.

Upset

Upsets or cracks in a length of timber or a log refer to the compressed or crushed fibres perpendicular to the length of the timber, often caused by a harsh impact during felling, or by extreme stress caused to the tree by strong winds. An upset or crack along the length of a timber severely weakens its strength and ability to take load and can often present in a visible crack across the timber.

Snipe

Snipe refers to a few different defects caused by the milling process and the tools involved. Sometimes boards can pass through the blades of a saw or a planer at an improper angle or presentation, causing irregular widths to occur within the finished length. These boards come few and far between from the merchants, and for all intents and purposes do not need to be accounted for in construction. Snipe when it comes to joinery and workshop situations often refers to thickness planing timber, with snipe occurring on the ends of boards as they pass through a planer. During regular operation, the even downwards pressure of the planer rollers maintain the stability of the board, allowing for an even thickness to be taken. However, when the board enters and exits the planer, the exact alignment of the board can be disrupted due to uneven contact with the rollers, causing the cutter to take an uneven cut at both ends. During planing operations, this can be counteracted by leaving the boards long, allowing the snipe to occur on the waste timber, or by consecutively feeding lengths of timber through the cutter to ensure even pressure on the boards.  

Case hardening

Case hardening occurs in timber that has been incorrectly seasoned and describes an affliction in which the internal moisture levels of a length of milled timber are much wetter than the dry exterior. This leads to an extreme amount of conflicting tension within the length, which can be released in a dangerous fashion when the timber is cut. In most instances of case-hardened timber being cut, the kerf behind the blade will close, pinching the blade and either damaging the tool, or explosively pushing or pulling the timber away from or into the blade.