Wood Works No.2 – Timber terminology explained

You know the feeling. You go to the plumbers or electricians and if you are not in the trade then you can feel (or be made to feel!) somewhat inadequate as you don’t know the lingo? Well, wood is no different, so here we explain some of the common timber terminology…

Softwood or hardwood?

As with any industry, we deal with terminology every day and sometimes forget that what is obvious to us, is not so for everybody.
The commercial division of timber into hardwoods and softwoods bears little relation to the hardness or softness of the timber; did you know Balsa wood used for model making is a hardwood?
Softwoods are produced from coniferous, mostly evergreen trees such as pines, firs, spruce, cedar and larch.
Hardwoods come from deciduous, broad-leaved trees. These can be further sub-divided into temperate hardwoods, such as oak, beech, ash and walnut and tropical hardwoods, such as teak, iroko, meranti and sapele.

Sapwood and heartwood

In any tree, the sapwood refers to the outer, living part of the tree that is responsible for conducting water and nutrients from the roots to the leaves. The heartwood provides the tree trunk strength and is the inner part of the log that gives the timber its ‘character’, i.e. its appearance, colour, smell, durability etc.
The sapwood in all species can be considered perishable so must not be used for external purposes unless it is pre-treated with preservative. In some species, though by no means all, the sapwood can be a lighter colour so visually distinct from the heartwood. In furniture or flooring, if this contrast is not desirable, say in oak or walnut, then this should be specified to ensure only the darker heartwood is supplied.

Would you like that flat sawn or quarter sawn?

Appearance and properties of timber vary depending on how it is cut from the log

These terms relate to the manner in which the log is sawn whereby the boards are either cut in a way which follows the growth ring (flat sawn, sometimes called tangential cut) or across the growth rings (quarter sawn, or radially cut).
The significance of this is two-fold. Firstly, movement across the tangential width of the boards is approx. twice that of radial boards, so allowances may need to be made to accommodate this. However, a more obvious factor is that the face of the boards look different, with flat-sawn boards showing the ‘flame’ of the growth rings and thus often have a more attractive appearance, or ‘figure’. There is always the exception of course, one of which is that the ‘silver figure’ visible on quarter sawn oak which is traditionally used for fine cabinet work and violins; sometimes referred to as ‘fiddle-back’ figure.

Do you need it treated?

If the timber is to be used in a location where it may get wet, i.e. external use, then you will need to consider the natural durability of the timber. Durability refers to the natural resistance to biodegradation by insect and fungi of the heartwood of the timber: Remember, all sapwood is perishable.
As a result of many decades of ongoing timber research, the natural durability of different species has been assessed and assigned to one of 5 categories:

Durability ClassDescription
Class 1:Very durable
Class 2:Durable
Class 3:Moderately durable
Class 4:Slightly durable
Class 5:Not durable

The durability of timber can be increased using preservative treatments.  These range from superficial surface brush application (remember Tommy Walsh applying a brush treatment to the cut ends of all those deck boards in BBC’s Ground Force under the watchful eye of Alan Titchmarsh?!) to commercial treatments using pressure and vacuum cycles to force the preservative into the timber.

All well and good, but how do you know whether your timber needs treating and how do you specify it if it does?

Well, this depends on the ‘Use class’ of the service situation where the timber is to be used, defined as:

Use class   Service situationDescription of exposure to wettingExamples of service situations
Use class 1Interior, coveredDryNormal pitched roofs; internal joinery; floorboards
Use class 2Interior or coveredOccasionally wetTiling battens; timber framed house framework; ground floor joists
Use class 3Coated: exterior, above groundOccasionally wetExternal joinery, cladding, external load-bearing timbers
Uncoated: exterior, above ground, unprotectedFrequently wet (above damp proof course)Fence rails and panels; garden decking not in contact with ground
Use class 4Exterior, in contact with ground or fresh waterPermanently exposed to wetting (below dpc)Fence posts & gravel boards; decking timbers in contact with ground
Use class 5In salt waterPermanently wetMarine piling, piers & Jetties, sea defences

The Use class system is actually quite useful as it means that you only need to specify the Use class, rather than have to concern yourself with the specific treatment that is used to achieve it.

Are you thinking of painting or staining it?

Gulp! Is there a difference?
The paint or the stain is the finish applied to the surface to enhance or maintain the appearance of the timber. It should not be relied upon as a substitute for the preservative treatment and vice-versa. Without any finish, the timber will ‘weather’ to a grey colour and the surface fibres will break down and become rough and woolly to the touch. Exterior wood stains are available in a wide variety of colours and opacities, from ‘paint like’ finishes to more translucent stains that allow the grain of the timber to show through and are generally recommended for exterior use.

Do you need the timber graded, C16? C24?

Sounds good, what’s the difference?
Well, when you purchase structural softwood timber, such as joists (also referred to as ‘carcassing timber’) it is commonly offered as either C16 or C24. This means that it has been visually or machine graded, based on its inherent strength reducing characteristics such as knots, slope of grain, fissures etc. The ‘C’ tells us that it is a Coniferous species (softwood) and the number indicates its strength class based on its bending strength (N/mm2); the higher the number, the higher the grade. So, in this case, C24 would be the higher grade of timber with less knots, splits etc. than a C16 section.

A grade stamp on C24 timber

Graded timber should be marked with a grade stamp, which also contains information on species and the person or body responsible for the grading assessment.
Although C16 and C24 are the most commonly available from timber merchants, I should add for completeness that there are currently 12 strength classes of softwood (C14 – C50) and 14 classes of hardwood, which are prefixed with a ‘D’ for Deciduous (D18 – D80). Thankfully, the number of commercially available grades of timber is somewhat less – 4 softwood and 6 hardwood.

References:
BS EN 350:2016. Durability of wood and wood-based products. Testing and classification of the durability to biological agents of wood and wood-based materials
BS 8417:2011+A1:2014. Preservation of wood. Code of practice.
BS EN 338:2016. Structural timber. Strength classes.

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Roger

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