Now that we understand the characteristics and terminology associated with the living tree, it's important to understand the characteristics of the timber that is processed from it. Looking at the through section of a log again, we can begin to understand the physical properties of processed timber. The fibres of the wood that provide strength and structure to the tree grow parallel with the trunk or branches. When we look at the through section of the trunk, we are looking at the ends of the fibres. These fibres are held together with lignin, a natural substance produced within the tree. Whilst this bond is strong enough to support the tree and maintain the stability of the wood, it is not suitable for resisting splitting and wedging forces applied to the end grain fibres. Mechanical wedges in the form of sharp tools such as chisels or axes are easily able to split the end grain fibres apart, breaking the lignin bonds under the high pressure that is exerted. This is how firewood splits so easily along its grain when chopped with an axe. These characteristics can be used to our advantage during carpentry and woodworking practice, such as removing bulk waste by splitting it down the grain or creating interesting joinery components such as wedged tenons.

Based on these qualities, processed structural timber is machined from the length of the tree, taking advantage of the natural strength created by the continuous fibres of the wood. The continuous nature of these fibres gives the timber excellent load resistance in comparison to its own weight, capable of spanning long unsupported distances. In fact, sharpened tools such as saws and axes must be employed to actively sever these fibres when cutting across the grain.  

Timber is strong and flexible when stressed perpendicular to the grain, thus making it a good material to use for floor joists and rafters. It also possesses very appropriate compressive resistance when load is applied downwards, parallel to the grain, making it a good choice of material for use in timber posts, and for upright studs in walls.  

Knots in timber occur where branches once grew away from the main trunk of the tree. These branches grow organically in the same way that the main trunk of a tree would, with the grain running parallel to the branch, and a series of annular rings within. Within a length of machined timber, the direction of the grain changes where a branch once grew away from the trunk. This is known as a knot.  

In a perfect scenario, a length of machined timber would possess perfectly parallel grain in relation to the edges of the timber. When shaping or sawing lengths of timber like this, the shavings and cuts made are consistent along the length, making the work easy. However, not every tree grows with perfectly straight fibres, and not every log is machined perfectly parallel to the grain. Often, we are left with machined timber that possesses waving and irregular grain, at least to some extent. This effect is more noticeable in slower growing hardwood timber, as opposed to the tall and fast-growing softwood timbers. Looking at the diagram of these exaggerated characteristics, we can see how the grain of the tree grew within the shape of the trunk, and how the timber has now been machined from it. The grain waves its way down the length, exiting the top edge of the timber, and then swooping back down into the length. When we look at this top edge of the timber, we have end grain and continuous grain present alongside each other. This can create a problem when working with the timber, as continuous grain and end grain are receptive to cutting and shaping in different capacities. We will look at how to deal with these instances later in this publication.