This guide shows a variety of species and some of their different characteristics. Please note because timber is made from a natural product, each piece is different and may vary from the results listed below. Please use this as a guide only and speak to your timber supplier for more information.

Hardwood / Softwood

Timber that is used in the building and construction industry can be separated into two groups: Softwoods & Hardwoods. Timber merchants often refer to timbers that are easy to work with as softwoods. However, softwoods are not always a softer material than hardwoods. Strength is better determined by the density of the timber. Hardwood and softwood are actually categorised by the way they reproduce.

Hardwood (Angiosperms) – Hardwood trees flower and have seeds that form in ripening fruit or shells. They are deciduous which means they lose their leaves each year. They are typically denser than softwoods because they are slower growing. There are thousands of species of hardwoods.

Softwood (Gymnosperms) – Softwoods usually produce seeds that are uncovered in cone-like structures. This is where the name “Conifers” (a major source of softwoods) comes from. There are around 600 different species of softwoods.

Density – Green & Dry

The density of timber is determined by the weight of a set sample size. In Australia, kilograms per cubic meter (kg/m3) is used. The higher the density, the more difficult it is to work with. Less than 480kg/m3 is considered very soft. Between 480kg/m3 and 560kg/m3 is soft. Anything over 480kg/m3 is considered hard.

Green density is determined at the time the tree is harvested. The recorded values can vary depending on the time of year, weather conditions and the age of the tree when it is harvested.

Dry density is taken after the tree has air-dried and has a moisture content of 12%. Again, this figure can vary depending on the growth conditions and age of the tree.

Janka Hardness Rating

The Janka hardness rating shows how well a type of timber can resist scratching or denting. To determine the rating, an 11.28mm steel ball is forced into a sample piece of timber. Once half of the ball has been driven into the timber, the force required to drive the ball is recorded. The higher the Janka rating, the harder the timber. In Australia, the force is recorded in Kilonewton (kN).

Lyctid Susceptibility of Sapwood

Lyctids are pests that attack the sapwood of hardwood timbers. Sapwood is the outer layer of the timber. All softwood timbers are not susceptible to attack. Legislation governs the sale of timber that is susceptible to lyctids in New South Wales and Queensland. Australian Standards limit the use of luctid susceptible sapwood throughout Australia.

Termite resistance of Heartwood (Above Ground)

The heartwood of some species of timber is naturally resistant to termites. Heartwood refers to the inner part of the wood. There are some species of timber where the heartwood is resistant to termites but must still be treated. This is due to the amount of sapwood they have.

Durability Class – Above Ground & In Ground

Durability refers to the ability of the heartwood to resist insect attack and decay when used in ground. The performance is usually increased when above ground. Durability can vary in timber from the same species and even from the same tree. Because of this, durability ratings are a guide only. Durability ratings are as follows:

Above Ground

• Class 1: Very durable – Over of 40 years above ground
• Class 2: Durable – Between 15 and 40 years above ground
• Class 3: Moderately durable – Between 7 and 15 years above ground
• Class 4: Non-durable – less than 7 years above ground

In Ground

• Class 1: Very durable – Over of 25 years in ground
• Class 2: Durable – Between 15 and 25 years in ground
• Class 3: Moderately durable – Between 5 and 14 years in ground
• Class 4: Non-durable – less than 5 years in ground

Exposure to Marine Borers

There are many different kinds of marine borers. All live in seawater or brackish water (salt water and fresh water mixed together). Marine borers, as their name suggests, bore into and eat wood. Similar to durability, timber species are categorised into classes when referring to their ability to resist marine borers. Each class has a probable resistance expectancy listed in years:

• Class 1: Over 60 years
• Class 2: Between 41 and 60 years
• Class 3: Between 21 and 40 years
• Class 4: Less than 20 years (usually less than 5 years)

Strength Group

The strength of a type of timber refers to its ability to maintain stress without failing. The strength group is determined by identifying three factors:

1. Modulus of rupture: Also known as bend strength, it is the stress in the timber before it breaks in a bend test
2. Modulus of elasticity: Measures the resistance of the timber being deformed when stress is applied to it
3. Maximum crushing strength: Also known as compression strength, this shows how well the timber can withstand a compressive force

The above values are measured in megapascal (MPa). To identify the strength group of each species, 5 samples are tested and the average values collected. There are 7 strength groups for green (unseasoned) timber and 8 for dry (seasoned) timber. The lower the number, the stronger the timber. Green timber is identified with an S and dry is identified with SD.

Joint Group

The joint group is used to identify the bearing strength of the timber. It also shows the shear strength at the joins. There are 6 groups that are separated into green (unseasoned) – J, and dry (seasoned) – JD. J1/JD1 is the highest (strongest) rating and J6/JD6 is the lowest (weakest). Where a joint has two or more different species of timber, the lowest rated joint group should be used.

Radial & Tangential Shrinkage

Shrinkage is the difference in dimensions from when the tree is cut down to when it dries to 12% moisture content. Shrinkage is measured in radial (perpendicular to the growth rings) and tangential (parallel to the growth rings). Shrinkage can vary considerably within each species.

Stress Grade – Seasoned & Unseasoned

Stress grades are used for design purposes. They are only applied to structural timber products. The process of grading is to determine the stress limits that apply to timbers for structural applications. There are 10 F grades – F4, F5, F7, F8, F11, F14, F17, F22, F27, and F34. F4 represents the weakest and F34 represents the strongest timber. Where testing shows that a timber could come under more than one grade, the lower grade will be assigned.

Early Fire Hazard Indices – Ignitability, Spread of Flame & Smoke Development

Building materials, in some building applications, are required to have particular characteristics in the early stages of a fire. This is required by the Building Code of Australia (BCA). There are 3 indexes:

• Ignitability – Rated on a scale of 0 - 20 where 0 is the least likely to ignite
• Spread of flame – Rated on a scale of 0 - 10 where 0 is the slowest to spread flame. If ignitability is rated at 0, the spread of flame will be rated at 0
• Smoke development – rated on a scale of 0 - 10 where 0 develops the least amount of smoke

pH Levels

pH refers to the amount of hydrogen ions in the timber. Where the pH level is rated at 7, it is considered neutral. Anything lower than 7 is considered acidic. Anything higher than 7 is considered alkaline.