Natural Wood Drying: How Solid Wood Dries Properly

Wood keeps moving, long after the tree has been felled. How it is dried determines how dimensionally stable it will later be as furniture, flooring or a home accessory. In this article you will learn what happens during natural wood drying, how long it takes, how it differs from kiln drying and why wood warps as it dries.

What happens when wood dries?

Freshly felled wood contains a great deal of water. After felling, the trunk is cut into planks and boards of the required thickness at the sawmill. From that moment on, the wood releases moisture into its surroundings until its moisture content has adjusted to the surrounding air. This state of balance is known as the equilibrium moisture content.

Solid wood changes in two ways as this happens: it shrinks when it loses water and it swells as soon as it absorbs moisture again. That is why careful drying is so important. If wood is dried too quickly or unevenly, internal stresses can build up that later show as cracks or warping.

Natural and kiln drying compared

Natural air drying

For air drying, the boards are stacked with spacer sticks so that air can circulate evenly around them. A roof or covered shed protects the wood from rain and direct sun. The wood dries slowly over months or years and needs no additional energy to do so. How fast it goes depends on the wood type, thickness, season and climate.

Outdoors in Central Europe, air-dried wood usually reaches a moisture content of around 12 to 18 per cent. For outdoor applications that is often sufficient. For furniture and furnishings in heated interiors, the wood is usually dried further afterwards.

Kiln drying in a drying chamber

In kiln drying, the wood dries in a chamber where temperature, humidity and air movement are precisely controlled. This makes it possible to reach the desired moisture content, for interior fit-outs for example, in a targeted way and in a short time. Modern drying programmes are matched to the wood type and thickness to keep stresses in the wood low. Another advantage: the heat also kills insects and their larvae in the wood. The process does, however, require energy.

In practice, the two methods are often combined: the wood is first pre-dried in the open air and then brought to its target moisture content in the kiln.

How long does natural wood drying take?

For hardwoods such as oak, a tried-and-tested rule of thumb applies: allow around one year of drying time per centimetre of thickness. A five-centimetre oak plank therefore needs around five years in the open air. Softwoods such as spruce or pine dry considerably faster. Storage also plays a major role: well-ventilated, covered stacks dry more evenly than tightly packed wood.

Why wood warps as it dries

During drying, it is not only the moisture content that changes but also the shape of the planks. As it loses water, the wood gets smaller. But it does not shrink equally in all directions.

Shrinkage along the growth rings

Lengthways, along the grain, wood hardly shrinks at all. Across the grain it is a different story: along the growth rings, wood shrinks roughly twice as much as it does across them. The position of the growth rings in a board therefore determines how much it cups as it dries.

Earlywood and latewood

Each growth ring consists of two zones. In spring the tree grows quickly and forms light, open-pored earlywood. In late summer growth slows down, producing darker, denser latewood with smaller pores. The denser latewood is what you see as the line of the growth ring. Because the two zones differ in density, they also respond differently to moisture.

Side boards and centre boards

In side boards, the growth rings run flat and over a long stretch through the board. In centre boards they stand more or less upright. That is why side boards warp considerably more during drying than centre boards, which are considered particularly dimensionally stable.

How dried solid wood takes up moisture again

Even dried wood continues to respond to its surroundings. If it comes into contact with water or the humidity rises, the wood cells absorb moisture, the wood swells and changes shape and size. This is especially noticeable in wooden items that get wet regularly, such as cutting and serving boards:

• Always wash both sides: if a board only gets wet on one side, it swells unevenly, bows and starts to rock.
• Rescuing a warped board: submerge it completely in water for a few days and then let it dry slowly in an airy place.
• No heat: do not stand the board next to an oven or radiator to dry, or it will dry on one side again and warp once more.
• Oil regularly: a wood oil or wax protects the surface and slows down moisture absorption.

What moisture content does wood need indoors?

In heated living spaces, wood usually settles at an average moisture content of around 8 to 12 per cent over the year. In winter the heated air is dry, in summer the humidity rises again. Furniture and furnishings made from solid wood should therefore be dried to a suitable moisture content before they are worked, so that they shrink or swell as little as possible later on. A stable indoor climate with a relative humidity of around 40 to 60 per cent also does solid wood good.

You can read more about the properties of different wood types in our article Wood guide: an overview of solid wood types. Further information on wood drying is also available on Wikipedia.