It can be a nasty shock to learn that all wood will move, regardless of species or location.  But as the saying goes, being prepared is half the battle.  When working with and defining wood, it is essential to comprehend why and how it moves. 

Learn how to prevent wood movement and design furniture that can withstand wood expansion and contraction. Our expert tips will help you avoid common mistakes and finish your project with confidence.

What does "movement" mean? 

Timber movement, often known as warping, occurs when unequal shrinkage or expansion causes the wood to curve in one plane.  Typically, it affects planks and other tiny pieces of wood, though it can also impact bigger profiles.  Movement is a general term that covers a variety of particular movements.  These are covered in greater detail in the list below: 

• Bow: A curvature that runs the full length of the wood's edge. 

• Crook: A curve running the full length of the wood's face. 

• Kink: A small warp that's frequently brought on by a knot 

• Cup: U-shaped, with the sides of the face higher or lower than the wood's middle. 

• Twist: A change in the timber's length that prevents the two ends from sitting flush on a flat surface.
  

Why does wood move? 

Until it establishes equilibrium with its environment, timber acts as a "hygroscopic" substance, absorbing moisture from or releasing it into the atmosphere. 

A dynamic exchange of moisture is constantly taking place between the wood and the surrounding air.  Diffusion is the method through which the moisture is moved in the form of water vapour.  Differences in the amount of moisture in the air are what cause this dispersion.  Therefore, moisture is moved from the area of greater concentration to the area of lower concentration when an area with more is placed in an area with less moisture. 

Here, the idea of "equilibrium moisture content" (EMC) comes into play.  At EMC, the moisture content of the wood is in equilibrium with the humidity or moisture content of the surrounding air. 

Fundamentally, the wood fibers stretch and contract as a result of absorbing and releasing moisture, respectively.  Relative humidity rises in the summer and falls in the winter in the northern hemisphere. 

As a general rule, timbers will enlarge or contract by 1% for every 4% variation in the moisture content of the air around them.  

The timber might have a moisture level of over 50% when a tree is first cut down.  However, summer humidity in the UK is 15%, so as the timber adjusts its moisture content from 50% to 15%, a significant change will occur.   

Where warping takes place

However, not all of the lumber has undergone the same alteration.  Uneven fluctuations in moisture are caused by a variety of factors.  Primarily, the outside of the wood will dry out more quickly than the inside, however the direction of the grain will also be important.  The movements mentioned earlier in this blog are the result of these unequal movements, which cause one region to dry and faster than another.

The moisture content change from freshly sawn to seasoned timber is the main reason for visibly warped wood.  Although the seasonal changes won't be obvious in outdoor furniture, the diffusion process never ends. In the UK, summer humidity is 15% and winter humidity is 20%, so the timber will continue to alter. 

The changes are concentrated over the breadth and height of the grain, therefore even under conditions of high moisture content fluctuations, the length of the timber will not change significantly.  This is due to the fact that all types of wood are formed of tiny individual fibers, which expand or contract along their breadth rather than their length.  

when seen in the diagram below, the longitudinal direction, parallel to the grain, of the timber will only shrink by 0.01% of its length when the moisture content changes from freshly sawn to EMC.  But the size of the timber will shrink against the grain by 4% to 8%. 

Timber movement factors include species. 

Different species of wood move in various ways.  Tropical hardwoods like Opepe and Iroko are among the species that are considered to be resistant to warping. Straight grain patterns and a generally low fresh sawn moisture content are the main causes of this. Softwoods are typically more prone to warping than hardwoods. Once the first moisture content drop has taken place and the wood has found equilibrium with the atmospheric moisture content, Douglas fir is a softwood that is known to be stable. 

The risk of warping is thought to be particularly high for oak wood. Read this blog post to learn more about Oak and risk management strategies.

How is the wood cut? 

The most popular and economical way to cut wood is with a flat saw.  In this case, the entire log is split longitudinally and horizontally, resulting in a board with horizontal grain as opposed to vertical grain.   

Quarter-sawn and rift-sawn are the other techniques for cutting wood.  Vertical grain boards are produced.  With the same change in moisture content, a flat sawn board will move around twice as much as a vertical grain board.  Due to this, lumber that has been quarter- or rift-sawn is significantly less likely to warp. 

However, quarter-sawn and rift-sawn timber are costly and occasionally difficult to locate since cutting them is significantly less effective.  The majority of wood is flat sawn; therefore, quarter sawn must be mentioned. 

Heartwood versus sapwood 

Heartwood and sapwood are the two types of wood found in every trunk.  The core section of the trunk, known as the heartwood, is considered to be "dead wood" because nothing can pass through it.  The smaller portion of the trunk outside of the heartwood known as sapwood is responsible for moving sap up and down the trunk.  Always deeper in color than sapwood is heartwood. 

Given that sap wood actively conducts the water and heartwood passively does not, this is extremely relevant to timber warping.  As a result, the sapwood is more susceptible to warping because its fibers expand and contract much more quickly than those in the heartwood. 

Timber's profile size 

As a general rule, large timber sections (end profiles larger than 150mm square) are more resistant to warping than small ones. (end profile less than 80mm square).  Large portions of lumber have a natural strength that accounts for this.  The fact that the grains are travelling in multiple directions in huge parts means that they virtually balance each other out because uneven grain patterns are a major cause of warping.  However, a dominant grain angle will be present on smaller portions, which will increase warping. 

Grain Structures 

Straight and closely spaced grain tends to make wood more stable.  Hardwoods are typically more stable than softwoods, however this isn't always the case.  Hardwoods have closer and denser growth rings than softwoods because they grow more slowly.  

Wood-Made products 

Plywood and MDF are examples of wood-made products that won't move or distort noticeably.  They will work, but only at an insignificant 1/10th of the price of real wood.