Shrinkage and Swelling Measures

What are the Shrinkage and Swelling Measures in Wood?

When we talk about wood, many people first think of warm colors, a pleasant feel, and a sense of nature. But wood can do even more - it “works.” This is precisely where the shrinkage measure and the swelling measure come into play. These two terms describe how much wood changes when it absorbs or releases moisture. And it’s not as complicated as it might seem at first glance.

Why Does Wood “Work” at All?

Wood is a hygroscopic material. That means it can absorb moisture from the surrounding air and also release it. Depending on whether it’s currently “drinking” or “sweating,” the wood swells (expands) or shrinks (contracts). If the air is very dry, the wood gives off moisture and contracts. In a humid environment, it takes on water and expands.

In short:

• Shrinkage measure (shrinkage): The wood contracts because it loses moisture.
• Swelling measure (swelling): The wood expands because it absorbs moisture.

What Exactly Is Happening?

Imagine the cell structure of wood as tiny tubes and chambers that can store water. When there is a lot of moisture, these tubes soak up the water and expand—the wood swells. If there is little moisture, they release water and contract—the wood shrinks.

Fiber Saturation and Its Limits

There is a point at which the cell walls are fully saturated. Beyond that, wood can still absorb additional water between the cells (for example in the form of free water), but the cell walls themselves are already “full to the brim.” The increase in volume eventually stops once the cell walls are saturated.

Differences Between Wood Species

The fact that some wood species warp more than others is mainly due to two factors: bulk density (how compact the wood is) and any “special ingredients” like resins or oils.

• Hardwoods (e.g., beech, oak) often have a higher bulk density and thus sometimes show greater swelling and shrinkage.
• Teak and other tropical woods are often permeated with oily, hydrophobic substances, which makes them “work” less overall.
• A classic example: spruce or pine shrinks and swells noticeably, but not nearly as drastically as some other wood with a higher density.

Directional Dependence: Wood Is Not the Same Everywhere

Wood swells and shrinks to different degrees depending on the direction in which it’s measured:

• Longitudinal direction (axial, parallel to the grain) Changes are minimal here. If you measure a board along its grain, you’ll hardly notice any difference.
• Radial direction (from the center of the tree outward, along the wood rays) Changes here are already more pronounced—possibly 10 to 20 times higher than along the grain.
• Tangential direction (along the growth rings) This is where it gets really interesting, because tangential changes can be 15 to 30 times greater than along the grain. If you’ve ever seen cracks that run across the growth rings, you know how dramatic that can look.

Key Figures at a Glance

Maximum Swelling and Shrinkage
The maximum measure indicates how much the wood changes from being fully saturated (maximum moisture content) to being bone-dry (oven-dry). This value is extremely important for planning since it describes the “worst case.”

Differential Swelling
Though it sounds complicated, it’s simply the percentage by which the wood expands or contracts for every 1% change in wood moisture content. This helps if you want to plan very precisely, such as for door frames or window frames that shouldn’t warp under any circumstances.

Swelling Pressure
When wood swells but can’t expand (for instance because it’s firmly fixed in a structure), an enormous pressure can build up. In fact, this pressure can become so great that it can even crack concrete or stone.

Practical Relevance

Avoiding Cracks and Deformations

If a piece of wood is prevented from expanding or contracting, internal stresses arise. This leads to cracks, warping, or breaks.

Optimal Wood Moisture

To prevent furniture in living spaces from suddenly sticking or wobbling, the wood’s moisture content should match the environment. For indoor settings, the ideal moisture level is typically between 8% and 12%. Construction timber can be a bit higher (12%–18%).

The Issue of Anisotropy

Wood with strongly tangential growth rings is more prone to warping. So if you need perfectly straight surfaces, you should pay attention to cutting patterns (e.g., rift or quartersawn) or choose a wood species that’s more dimensionally stable.

Treatment and Modification

• Thermally treated wood or chemically modified wood (e.g., acetylated) takes up less water and therefore “works” less.
• Surface treatments with lacquer, oil, or wax at least slow down the rate at which moisture is absorbed. It’s not a cure-all, but it’s quite effective against rapid and severe changes.

A Conclusion with Perspective

Swelling and shrinkage are perhaps the most important peculiarities of wood. That’s why carpenters, joiners, and DIY enthusiasts alike make sure their materials are stored, processed, and used under conditions as stable as possible. But that doesn’t mean wood is complicated or unpredictable - on the contrary: once you understand the basic principles and choose the right wood and the right construction method, you can create wonderful, long-lasting products that retain their completely natural “freedom of movement.