The physical characteristics of timbers we are interested in are:
- Weight (density) - this will provide a useful indication of a wood's strength (heavier woods tend to be stronger).
Strength doesn't tell us much by itself and so is usually broken down further as follows:
- Hardness - this is how resistant the wood is to surface scratches and marking (hardwoods are generally more resistant).
- Compressive strength - this is how much weight can be applied to wood along the grain before it collapses.
- Bending strength - this how much weight can be applied to wood at right angles to it before it snaps.
- Stiffness - this refers to how much wood will be displaced when weight is applied above (or gravity).
These characteristics vary from tree species to tree species. If, for example, you are going to select a particular timber with which to construct bookshelves, you might be interested in the stiffness because you don't want your shelves to quickly start to sag.
Timber also varies in terms of quality characteristics. Quality includes such factors as:
- Resistance to degradation or rot - dry rot, termites, wet rot, etc. Some timbers last for hundreds of years, some need to stay dry, some are can withstand wet or damp conditions better than others.
- Resistance to fire - ability to withstand flames.
- Defects - these may diminish the usefulness of a timber.
- Aesthetics/appearance - some timbers have a more attractive grain and colour.
Resistance to Rot
Rot occurs because a timber is subjected to unfavourable conditions. Timber which is used in outdoor construction, for exterior doors and window frames, and for the posts of timber-framed buildings which are inserted into the ground are more vulnerable. Also, timber which is partially submerged in water is susceptible to rot.
- Dry rot - this name is a bit misleading because timber will not rot if it is dry. However, dry rot, is caused by a fungus which sucks the moisture from timber leaving it dry and crumbly. White fungal growths may be observed stretching across the timber and these usually lead to a pale brown fruiting body. Dry rots are promoted through damp conditions where there is insufficient ventilation, but unlike wet rot it does not require a significant amount of dampness to grow. It is the most destructive type of rot in timber, and can spread through brickwork and behind plaster. Dry rot has a distinct musty smell.
- Wet rot - this is also caused by species of fungi in conditions of excessive moisture. Timber affected by wet rot may become soft, cracked or distorted. Occasionally fungal growths may be observed. Like dry rot, wet rot may give off a musty smell.
Resistance to Fire
Generally speaking, harder timbers are more fire-resistant than softer ones. Also, larger cuts of timber will resist fire better. Often where a thick timber member has been used, in house construction for example, it may become charred on the outside in a fire but the inside is unharmed. For buildings there are strict local bylaws which dictate the timber thickness required for stairs, doors, etc. They will also specify which timbers are acceptable for use.
Defects in Timber
Some defects may be purposefully cut out when a log is converted into timber if they have been observed beforehand.
- Radial shakes - these are lengthwise splits along the outside of a log which may occur if a log is left for too long before being converted into timber.
- Heart shake - this is splitting lengthwise in the centre of a sawn piece of timber. If you were to cut a cross-section it will often be seen as a cross-shaped split. It is caused by shrinkage in the centre of the timber and may be because the tree was too mature when cut.
- Thunder shake - also known as 'upset', this is where there is a fine split across the grain usually observed as a wiggly line. It is not possible to detect this before the wood is converted and the resultant timber will have to have this section cut out.
- End splits - these usually occur when timber is seasoned and dried. It happens because the end of the timber dries out more quickly than the centre and so this uneven shrinkage cause the ends to split. This effect may be minimised through using more even drying practices.
- Split - splitting along the grain of the timber which carries through to the opposite side.
- Ribbing - this occurs where the spring wood shrinks at a different rate to the summer wood during seasoning and results in an uneven crimped surface.
- Check - splitting along the grain on the surface during seasoning. It does not carry through to the opposite surface.
- Warping - where timber becomes distorted from its original plane during conversion. Bow refers to distortion along the length, and cupping is curving of a cross section.
- Knots - these can compromise the strength of timber as well as aesthetic appeal. Loose or dead knots can fall out of timber, and if there are many of them then strength may be adversely affected.
- Bark pocket - where some bark has ingrown into the wood around a knot.
- Resin pocket (gum pocket, pitch pocket) - an area of liquid resin inside the timber.
Other defects may occur due to imperfect manufacture using cutting machinery such as cutter marks, chipped grain (wood breaks away below surface), and torn grain (tearing of wood below surface).
Timber is such a useful and versatile material
We use timber so much in our lives - from buildings to furnishings timber is so versatile and can be such an attractive material. To be able to select it and use it properly though, you need to know what you are doing. Our Carpentry course looks at the selection of timber and includes:
- Tools and tool selection.
- Cutting and Joining.
- Indoor and outdoor construction.
- Safety, and much more.
We have included (below) a link to our Carpentry course and some other courses you may find of interest. You can also view our directory of Permaculture and Self-Sufficiency courses. Whether you are looking to take an introductory course in Permaculture, or considering tackling a home renovation project, you will find a course suitable to your level of knowledge and experience.
If you have any questions, or want to know more, why not get in touch with one of our Permaculture and Self-Sufficiency tutors today? They will be pleased to answer any questions you may have about our courses or studying with ACS.