Wood classification

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By the end of this unit you should have developed a knowledge and understanding of:

the specific names and classifications of woods
the processing from source into a usable commercial material
the performance characteristics of different types of timber
why different types of timber are suitable for different applications
stock forms of wood and timber

Wood and timber can be put into three categories:

Soft Woods

Timber from coniferous trees

Pine, Douglas Fir, Spruce

Hard woods

Timber from deciduous trees

Oak, Beech, Mahogany, Teak, Ash

Manufactured (man-made) boards

Boards made from wood pulp, chips or layers e.g. Chipboard, Conti board, MDF, Plywood Stirling board

Woods can either be deciduous or coniferous.

Deciduous trees

have broad green leaves which are shed in theautumn in a temperate (different seasons) climate. These are known as Hardwoods. Hardwoods grow in warm countries such as Africa, Asia and India and can take 100 years or more to mature, although there are faster growing hardwoods.

In tropical countries hardwoods often do not shed their leaves as there is often no noticeable change in the seasons like there is in the UK.

coniferous trees

have needle type leaves which stay on the tree all year round which is why they are known as Evergreens. These are Softwood trees. They generally grow in colder countries such as Russia and Scandinavia and take around 20 years to mature.

The UK has a temperate climate which is why so many different types of tree grow there and why some lose their leaves in winter.

Deciduous conifer - Larch example

'Siberian Larch timber is a softwood that comes from a tree native to western Russia and that cold climate leads to a timber that is more durable than many other softwoods, with a natural resistance to decay.' From wikapedia

Sources and Origins

The source of natural timber, paper, card and manufactured boards is trees which grow in a variety of climates and Britain, with its temperate climate, is able to support a wide variety of different trees.

Hardwoods

Hardwoods tend to grow in warmer countries around the equator

such as Africa, India and Asia.

Softwoods

Softwoods grow in countries nearer to the two poles such as

Scandinavia, Russia and the Baltics.

Wood structure

Woods are fibrous materials made up of plant cells and resins. The main structure is Cellulose fibres held together with Lignin which gives wood its strength but needs to be removed to make wood pulp for paper (see L9-10: Paper and Boards)

Wood is a natural material and one that can be used in its green state i.e. when it has been recently felled or cut down.

Wood is hygroscopic meaning it gives up moisture in a dry environment and takes in moisture in a wet environment. As a result Wood is always prone to be unstable as it can shrink and expand depending on the moisture conditions.

Anatomy of a tree

Trees grow from the outside inwards making a new layer each year (hence

annual rings). As a result the weakest wood is just inside the bark while the

heartwood is the most stable and dense section of the tree.

Eventually the heartwood may begin to die and is know as deadwood.

Wood Conversion

In order to use wood we inevitably need to cut down trees!

Trees are cut a few feet from the ground leaving a stump with the roots still attached in the ground as it is much too hard to dig them up.

However, some trees can re-grow from their stumps and the process of doing this on purpose is known as coppicing.

The trees then have their branches removed and these are then chipped and used to make chipboard, paper and other wood-based products.

Where the branches join the trunk, they leave a knot (a defect).

We are then left with the tree trunk ready to be converted into usable wood.

Conversion of timber

As soon as possible after felling the tree should be converted into usable timber. Natural wood is only available at the maximum width of the tree it came from!

There are two main methods of converting timber: Through and through (or Plain or Crown sawn) which produces tangential boards and Quarter Sawn which produces radial boards.

Through and through

Through and through produces mostly tangentially sawn timber and some quarter sawn stuff. (see diagram)

Tangential timber is the most economical to produce because of the relatively less repetitive production methods. It is used extensively in the building industry.

Quarter Sawn

The Quarter sawn is far more expensive because of the need to double (or more) handle the log. There is also more wastage. It is however more decorative and less prone to cup or distort.

*Note also there are two ways of sawing the quarter.

Seasoning

Wood is full of moisture when felled and is referred to as green timber. In order to make it stable and usable, it is necessary to reduce the moisture content in a controlled way. This is known as seasoning.

Air Seasoning:

The traditional way of doing this is simply to allow air to pass around the wood in order to reduce the moisture content over time. The wood is stacked on bricks to keep the wood from wet floors and a roof is placed over so rain doesn’t reach the wood. Spacers between the planks of wood allow for circulation and the whole process requires little or no supervision.

However, it can take years for the wood to season properly!

Because wood is needed daily, it would be impossible to use traditional methods in industry as it would simply take too long for wood to become usable so a kiln drying method is used.

Kiln Seasoning

In this method, wood is placed in a kiln and heated up using steam. This means that the wood does not dry out quickly, but the moisture content can be reduced in a controlled way resulting in a process that can take days or weeks rather than months and years.

Hardwoods

Hardwoods are more expensive than softwoods for one main reason and that

is the time it takes for a hardwood to reach maturity i.e. when the tree is fully

grown and can provide the most quality wood.

Hardwoods typically take over 100 years to reach maturity although there are

faster growing species such as Bamboo.

* Note that bamboo is not a tree but classified as a grass.

Hardwood is more expensive than soft wood and is used in more quality joinery jobs, expensive flooring, or outside areas which are exposed to the weather like front doors.

It is often used on expensive window frames as it looks great and lasts for years.
On floors it's very hard wearing.
Being harder it's much harder to work with but you can get a great finish on.
It doesn't split as easily as pine.

Examples of hardwoods are:

Beech

Modern and sleek

Mahogany

rich and classic

Oak

traditional and cosy

walnut

exquisite and striking

Teak

Ash

Cherry

Zebbrano

Applications for hardwoods

softwoods

Softwood is the cheapest of the two varieties of timber due to the fact that trees reach maturity in 25 years or less. Compare this to hardwoods which can take 4-5 times longer to grow and you can see a similar increase in prices.

It also helps that the UK is geographically nearer to softwood producing countries so shipping costs would be less. The UK produces a reasonable amount of softwood for its own use. All of these factors mean that softwoods are more readily available and therefore cost less in the UK.

*This may not be the case in Thailand but remember that you are sitting an exam from a UK exam board so the above is relevant!

Softwood is used in a huge range of general constructional jobs. In the UK it's usually Pine or Spruce but other types include Douglas Fir and Western Red Cedar. It's cheap and very easy to work with, although some care is required to avoid it splitting when nailing, screwing into, sawing or drilling. It comes in several forms:

rough sawn treated

Rough Sawn treated -quite roughly presented - this is used for outdoor use and is pressure treated with preservative to help prevent rotting (tanalizing).

Used in fencing a lot. You can usually tell it's treated by it's slightly odd colour usually a brown or green tinge to it depending on the color of preservative.

rough sawn

Rough Sawn - quite roughly presented often use where you won't see it like stud walls which are covered with plasterboard.

Planed

Planed- nice smooth surface used for precision joinery or where you see the surface. e.g. door frames, skirting. Referred to as PSE (Planed Square Edge).