Glossary
A comprehensive guide to the most common timber terms.
The decomposition or deterioration of wood caused by fungi, leading to a loss of strength, structure, and appearance over time.
A flat surface or platform, typically made from timber, composite, or other materials, that is constructed outdoors and elevated from the ground. It is commonly used in residential, commercial, and public spaces as an extension of indoor living areas or as walkways, patios, or viewing platforms.
In timber and other forest products, the result of any process that lowers the value of the wood.
A dehumidifier kiln (also known as a DH kiln) uses a heat pump system to both warm the air inside the kiln and remove moisture from it by condensing water vapor—allowing timber to dry slowly and efficiently.
How It Works:
- Air inside the kiln is warmed by a heat source (typically an electric heat pump).
- The warm air absorbs moisture from the timber.
- The moist air passes over a cold coil inside the dehumidifier unit.
- Moisture condenses into water and is drained away.
- The now dry, warm air is recirculated through the kiln to continue drying.
The separation of plies or laminations through failure of the bond, visible at an edge.
As applied to timber, density is the mass of wood substance and moisture enclosed within a piece expressed in kilograms per cubic meter. As the mass will vary dependant on the amount of moisture in the piece, density is often expressed at a specified moisture content, usually 12%.
The difference between dry and wet bulb temperatures. It is a measure of humidity.
The temperature at which air becomes fully saturated with water vapour, causing condensation to form. In simpler terms, it’s the temperature at which moisture in the air turns into liquid water—like dew, fog, or condensation on surfaces.
Diagonal grain occurs when the natural grain direction of the wood runs at an angle to the edges or axis of the board, rather than straight along its length.
The change of a square or rectangular section timber to a diamond shape during drying. Diamonding occurs where the growth rings pass through diagonal corners of the section of the piece and is caused by the difference between tangential and radial shrinkage. It is a form of distortion.
In building and structural engineering—especially in timber or framed construction—a diaphragm is a horizontal structural element, like a floor or roof, that transfers lateral loads (such as wind or earthquake forces) to the vertical resisting elements (like shear walls or frames).
Movement of water through wood from points of high moisture content to points of low moisture content by molecular diffusion.
A measure of the rate of moisture movement through wood by diffusion as a result of differences in moisture content
In timber and construction, dimension refers to the measured size of a piece of timber, and it can vary depending on whether the timber is nominal, sawn, or machined (dressed). These terms describe how the size is defined and altered through processing.
- Sawn - The size of the timber after being sawn from the log, but before further processing like drying or dressing. It may still be rough and slightly oversized.
- Nominal - The original or theoretical size of the timber when it is first cut from the log, before drying or machining. It’s often a rounded figure and larger than the final size.
- Machined - The final size of the timber after planing or machining, which smooths the surfaces. This is the actual size used in construction and can be significantly smaller than the nominal size.
Changes in the size of a piece of dry timber as its moisture content changes to be in equilibrium with the surrounding atmospheric conditions.
Submerging timber in a dipping vat containing fungicides or other chemicals to prevent stain or decay.
Change in the colour of wood caused by fungal or chemical stains, weathering, or heat treatment.
A drying defect caused by the differential shrinkage along the three axes of a piece of wood. Distortion may take the form of cup, bow, twist, spring, or diamonding.
A cylindrical timber rod or steel bar generally without nut or thread driven into pre-drilled holes to make a joint.
A mechanical connection formed by inserting one or more dowels into corresponding holes drilled into two adjoining wood members, then gluing and clamping them for a tight fit.
Timber that has been surfaced by planing or machining to remove roughness and ensure consistent dimensions, making it suitable for visible applications or precision joinery.
A generic term for the decay of timber by fungi that at an advanced stage leaves the wood light and friable. The term is actually a misnomer as all fungi needs considerable moisture to grow.
The dry-bulb temperature is the actual air temperature, measured by a standard thermometer exposed to the air but shielded from radiation and moisture.
A dryer is a controlled environment system—such as a kiln or dehumidifier unit—used to reduce the moisture content of timber to a desired level for its intended application.
The process of removing moisture from timber to improve its serviceability in use. Also see: Seasoning.
An imperfection developing during drying that decreases the value of a piece of timber.
A reduction in timber grade and volume as a result of drying defects.
High-temperature drying involves drying timber at temperatures above the boiling point of water, often in the range of 100–130°C, to speed up drying time, reduce costs, and increase throughput in kiln operations.
The material or value lost during the drying of timber, caused by moisture removal and the physical or mechanical damage that may occur in the process.
The loss of moisture from timber or other wood products per unit of time. Drying rate is generally expressed in percentage of moisture content lost per hour or day.
The internal stresses that develop within timber as it dries, caused by uneven moisture loss between the surface and the core. These stresses can lead to defects such as warping, checking, case hardening, and even structural failure if not managed properly.
1. Durability is the natural resistance of timber to biological degradation such as fungal decay, termites, and borers, without the need for chemical treatment.
2. The capacity of a building or its components to perform as intended over time, resisting physical, chemical, and environmental deterioration, with minimal maintenance.
Durability class refers to a timber species’ natural resistance to biological degradation, such as fungi, termites, and borers, especially in untreated form.
Durability is expressed as one of four classes. The value for each species is based on trials of the resistance to both decay and termites of untreated heartwood in the ground. The classes are: Class 1- Timber of the highest natural durability, expected to have a life greater than 25 years in the ground and greater than 40 years exposed above ground; Class 2 - Timber of high natural durability, expected to have a life of about 15 to 25 years in the ground and 15 to 40 years exposed above ground; Class 3 - Timber of moderate natural durability, expected to have a life of about 5 to 15 years in the ground and 7 to 15 years exposed above ground; and Class 4 - Timber of low durability, expected to have a life of 0 to 5 years in the ground and 0 to 7 years exposed above ground. The sapwood of all species is regarded to be Class 4.