Wood from Trees to Houses.

We often hear people grumbling about money and all kinds of other things that "don't grow on trees"; the great thing about wood is that it does grow on trees—or, more specifically, in their trunks and branches.

Hardwoods and softwoods

• Hardwoods are ones that come from broad-leaved (deciduous) trees (those that drop their leaves each fall, also known as angiosperms because their seeds are encased in fruits or pods). Examples include ash, beech, birch, mahogany, maple, oak, teak, and walnut.

• Also they have lovely, attractive grains and are used for such things as making fine furniture and decorative woodwork

• Softwoods come from evergreen (coniferous) trees (those that have needles and cones and retain them year-round, also called gymnosperms. Examples include cedar, cypress, fir, pine, spruce, and redwood.

• And it often come from very tall, straight trees, and are better suited for construction work (in the form of planks, poles, and so on).

What's wood like?

The inner structure of a tree makes wood what it is—what it looks like, how it behaves, and what we can use it for. There are actually hundreds of different species of trees, so making generalizations about something called "wood" isn't always that helpful: balsa wood is different from oak, which isn't quite the same as hazel, which is different again from walnut.

Let's just say, different types of wood have more in common with one another than with, say, metals, ceramics, and plastics.

Strength

Physically, wood is strong and stiff but, compared to a material like steel, it's also light and flexible. It has another interesting property too. Metals, plastics, and ceramics tend to have a fairly uniform inner structure and that makes them isotropic: they behave exactly the same way in all directions. Wood is different due to its annual-ring-and-grain structure.If you've ever chopped wood with an ax, you'll know it splits really easily if you slice with the blade along the grain, but it's much harder to chop the opposite way (through the grain). We say wood is anisotropic, which means a lump of wood has different properties in different directions.

Traditional wooden buildings are supported by huge vertical poles that transmit forces down into the ground along their length, parallel to the grain. That's a good way to use wood because it generally has high compressive strength (resistance to squeezing) when you load it in the same direction as the grain.

On the other hand wooden poles are much weaker placed horizontally; they need plenty of support to stop them bending and snapping. That's because they have lower tensile strength (resistance to bending or pulling forces across the grain). Not all woods are the same, however. Oak has much higher tensile strength than many other woods, which is why it was traditionally used to make the heavy, horizontal beams in old buildings.

Wood and Water

It's hygroscopic, which means that, just like a sponge, it absorbs water and swells up in damp conditions, giving out the water again when the air dries and the temperature rises. Like wooden windows, you'll probably notice that they open much more easily in summer than in winter, when the damp outdoor conditions make them swell into the frames (not necessarily such a bad thing, since it helps to keep out the cold).

Why is it so?

Well, the trunk of a tree is designed to carry water from the roots to the leaves: it's pretty much a water superhighway. A freshly cut piece of "green" wood typically contains a huge amount of hidden water, making it very difficult to burn as firewood without a great deal of smoking and spitting.

From Tree to Roof.

It's a longer and more complex journey than you might think that takes in harvesting, seasoning, preserving and other treatment, and cutting.

Harvesting

Growing plants for food is called agriculture; growing trees for human use is silviculture—and the two things have a great deal in common. Wood is a plant crop that must be harvested just like any other, but the difference is how long trees take to grow, often many years or even decades. How wood is harvested depends on whether trees are growing in plantations (where there are hundreds or thousands of the same species, generally of similar age) or in mature forests (where there's a mixture of different species and trees of widely differing ages).

Seasoning

A freshly cut tree is a bit like a sponge that comes presoaked in water, so it has to be completely dried out or seasoned before it can be used.

• It is less likely to rot and decay.

• It's easier to treat with preservatives and paint.

• It's much lighter and easier to transport.

Dry wood is also much stronger and easier to build with. Typically wood is dried either in the open air (which takes anything from a few months to a year) or, if speed is important, in vast heated ovens called kilns (which cuts the drying time to days or weeks). Seasoned wood is still not completely dry: typically its moisture content varies from about 5–20 percent, depending on the drying method and time.

Preserving

Creosote (another popular wood preservative) is a strong-smelling, oily brown liquid usually made from coal-tar. Unlike paint, it is a fungicide, insecticide, miticide, and sporicide: in other words, it works by stopping fungi, insects, mites, and spores from eating or growing in the wood.

Different kinds of treatment help to protect and preserve wood in other ways. It's a great irony that wood can be used to build a fine home that will last many decades or burn to the ground in minutes. Wood is so plentiful and burns so well that it has long been one of the world's favorite fuels. That's why fire-protection treatment of wooden building products is so important. Typically, wood is treated with fire retardant chemicals that affect the way it burns if it catches fire, reducing the volatile gases that are given off so it burns more slowly and with greater difficulty.

Cutting

How much cutting a tree needs depends on the product that's being made. Something like a utility pole or a fence post is not much more than a tree stripped of its branches and heavily treated with preservatives; that's an example of what's called roundwood.

Trees need a bit more work in the sawmill to turn them into lumber, timber, or sawnwood.

Flat pieces of wood can be made from trees by cutting logs in two different directions. If you cut planks with the saw running in lines parallel to the length of the trunk, you get plainsawn wood.

If you fell a tree, cut the trunk into quarters, then slice each quarter into parallel planks, you get quartersawn wood.

Other wood products

Some woods are very rare and expensive, while others are cheap and plentiful, so a common technique is to apply an outer layer of expensive and attractive wood to a core of cheaper material.

Veneer is a thin decorative layer applied to cheaper wood made by turning a log against a blade, much like peeling an apple.

Plywood is made by taking layers of wood (or plies) and gluing them together with an outer coating of veneer.

Laminated wood is a weaker kind of plywood in which the grain of each layer runs in the same direction.

Particle board (often called chipboard) is made by taking the waste chips, flakes, and sawdust from a mill and forcing it under high pressure, with glue, in a mold so it sticks together to make planks and panels.

Fiber-board is similar, but made with wood-pulp fibers instead of wood chips and sawdust.

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