Species Classifications Explained

This technical information is designed to breifly explain the various classification systems used on the species detail page to allow a user to better understand the qualities of various Ghanaian Timber.

Click on the classification you wish to know more about from the list below:

Weight

The weight of a species is a broad guide to its performance. Lighter weight species are softer, less durable and less strong. Very heavy species can exhibit very high levels of strength, natural durability and toughness.

To assist in gaining a general idea of the nature of a species, the descriptions provide the average weight of that species when dried to 12-15% moisture content. In practise there is always variation.

Strength

The strength of a species can be assessed by various criteria, including stiffness, compression parallel to grain, resistance to impact and bending strength. Strength is also related to moisture content and the presence of defects. This publication bases assessment of the general strength of a species on the modules of elasticity. The classification is indicative only and is not for the purposes of calculation of working stresses.

When forms of lamination are used - for example in plywood, finger jointing and laminated beams - strength properties are usually improved because the influence of defects is minimised.

Natural Durability

Natural durability classification relates to fungal decay and is for heartwood in contact with the ground and based on the following broad convention.

It has to be noted that the extreme weather test conditions of total open air exposure in contact with the soil may be more arduous compared with longer life expectancy of well designed and maintained exterior components in actual buildings. This classification does not indicate resistance to insect/termite attack. The sapwood of all species is particularly vulnerable to biological hazard.

Treatments with wood preservatives also provide assurances of higher resistance, although degree of penetration of preservative varies according to the system of treatment, the species concerned and the presence of sapwood. Preservative treatment may be required at different stages from the freshly felled log through to the finally dimensioned wood product. The need for preservation will be governed by the environment in which the wood product will be used.

Marine borers Timber used in sea defence and dock work face marine borers like Teredo and Limnoria. Untreated timber species have degrees of vulnerability but some have conspicuously higher levels of durability, including Denya, Kaku, Kasia and Odum.

Shrinkage

Freshly felled logs have a very high mositure content. The wood gives up much of this moisture in drying to an equilibrium with the relative humidity of the surrounding atmosphere. This initial drying out from the green state to air dried is accompanied by shrinkage in dimension. Air dried material in temperate regions is of the order of 18-20% moisture content, but somewhat higher in the tropics. The classifications used in these instances relates to test specimens further dried down to 12% moisture content. Many end uses require, at some stage, kiln drying to further reduce moisture content.

Movement in Service

Subsequent to initial reduction in dimension from the green state, timber achieves a relatively stable condition but this is subject to variations in environment moisture content, which can cause swelling and shrinking when the wood is in service. This 'movement in service' is a useful measure for those seeking timbers where the dimensional stability is important, for example in relation to the ease of opening and closing windows. Data on movement in service is not available for every species.

Indication of Forest Availability

The natural species associations in the tropical forests of Ghana vary from one geographic/climactic zone to another. Afrormosia, Avodire and Niangon, to take three random examples, are species associated with specific regions. Some species are widespread. This simplified classification is therefore somewhat generalised. Small availability is not to be confused with risks of endangering the existence of a species. The silvicultural prescriptions of the Forestry Department with Forest Reserves are designed to ensure perpetuation of commercial supply, and hence of young growing stock.

Historical Production Volumes

This classification gives an indication of past relative quantities of logs harvested commercially. It relates to levels in Ghana. Other tropical countries may have larger or smaller outputs. The popularity of some species may relate heavily to some specific end uses. For example, logs of low to medium density species are particularly suited to the production of veneer for panels. The utilisation of small volumes may relate for example to those specialised timbers prized for their decorative appeal and mainly used for sliced veneer. Some end products permit the use of a range of species with physical similar characteristics.

Lesser known species. Those species classified as 'Low' or 'Insignificant' may be regarded as Lesser Known Species.

Working Properties

The information given on this site is of necessity limited. Some further general comments can be made. High density species are by their nature usually slow to saw and have higher blunting effect on saws and cutting edges. An assessment classifying their performance, for example as 'Satisfactory', is made in awareness of this.

Interlocked grain requires specific well recognised techniques for machining and finishin. Appropriate cutting angles and hardened cutting edges are universally recognised as requirements in processing many hardwoods. References to irritant dust highlight the importance of health and safety in factories, including the installation of efficient dirt extraction equipment.