What is Timber Checking?
Timber checking refers to the cracks that form along the grain of wood as it dries and loses moisture. This is a natural consequence of the wood's seasoning process. When a tree is cut down, it can contain up to half its weight in water. As the timber dries, this moisture evaporates, causing the wood fibers to shrink. However, the outer layers of the wood dry faster than the inner core, due to their greater exposure to air. This uneven drying process creates tension within the wood, and eventually, cracks develop along the grain, relieving this tension. These cracks are what we call timber checks.
Types of Checks:
Click On The Boxes Below to See Details About Each Type
Surface Checks
Surface checks are the most common type of check, appearing as fine hairline cracks that run parallel to the wood grain on the surface. They typically have a shallow depth, rarely exceeding a few millimeters.
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Cause: Uneven drying is the culprit. As the outer layers of wood dry faster than the inner core, this creates tension that manifests in these shallow cracks.
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Impact: In most cases, surface checks have minimal impact on the structural integrity of the wood. They might affect the aesthetic depending on the desired look, but for many applications, they add a touch of rustic charm.
Shake Checks
Shake checks are separations within the wood that occur along the growth rings, often extending deep into the timber. They can be single or multiple cracks running parallel to the grain. Unlike surface checks, shakes can sometimes have a noticeable gap and might be accompanied by a slightly rough texture along the crack.
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Cause: Unlike surface checks caused by drying, shakes are typically present in the living tree due to internal stresses, like wind or rapid growth. These stresses cause separations along the growth rings.
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Impact: The impact of shakes depends on their severity. Small, shallow shakes might have minimal structural impact, but extensive or deep shakes can weaken the wood and require special considerations during construction.
Split Checks
Split checks are separations that run along the grain, often starting from the end of the timber and extending inwards. They can vary in depth, sometimes reaching the full length of the board. Unlike surface checks, which are typically hairline, splits can have a noticeable gap.
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Cause: Similar to surface checks, split checks are caused by uneven drying. The ends of the timber are more exposed to drying than the sides, causing tension that can lead to splits.
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Impact: The impact of splits depends on their depth and length. Minor splits extending a short distance might not be a major concern, but deep splits running a significant portion of the length can compromise the structural integrity of the wood, especially for load-bearing applications.
Why Does Timber Checking Happen?
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Uneven Drying: The outer layers of wood dry faster than the inner core. This difference in moisture content causes the outer wood fibers to shrink at a faster rate, creating tension and leading to cracks.
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Wood Species: Some species of wood are more prone to checking than others due to their density and structure.
Freshly sawn timber begins drying.
Drying is unproportional leading to tension in certain areas.
Eventually the tension is too great and the wood fibers give way.
Freshly sawn timber begins drying.
Is Timber Checking a Problem?
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Mostly Aesthetic: In most cases, timber checking is primarily a visual change and doesn't significantly affect the wood's structural strength.
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Structural Considerations: The design values used in engineering calculations already account for a certain level of checking present in timber.
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Extreme Cases: In rare instances, very deep checks or splits that extend through the wood might raise structural concerns and necessitate inspection.
Minimizing Timber Checking
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Free of Heart Center (FOHC) Timber: Using timber cut specifically to exclude the center of the tree (FOHC) significantly reduces the likelihood and severity of checking. The heart center dries at a different rate than the surrounding wood, and its exclusion minimizes the uneven shrinkage that causes cracks. While FOHC timber is more expensive due to the larger logs required, it offers superior stability and a reduced chance of checking, making it a valuable choice for applications where minimal checking is desired.
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Timber Selection: Choosing wood species less prone to checking can help, although some level of checking is expected in almost all timber.
Embracing the Beauty
Timber checking is often seen as a desirable aesthetic feature, adding character and a sense of natural beauty to timber structures and furniture. These cracks, like the wrinkles on a well-worn leather armchair, tell the story of the wood's life and transformation from a living tree into a functional and enduring building material. The variations in color and texture that can accompany checking further contribute to the unique visual character of timber. In some design styles, such as rustic or farmhouse, timber checking is a celebrated feature that enhances the overall look and feel of the space.
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