Timber Glossary

The A to Z of everything timber.

The following terminologies are offered for your convenience. The definitions have been presented with the layman in mind so may not reflect the technical definitions detailed under the Building Code Of Australia (AS1684).

Simply click on the ‘+’ symbol alongside the relevant term to read more about it.

Air-Dried
The process for drying timber where the material is typically stripped (timber separated to allow even air flow) and left for periods of 3-12 months to dry to a level suitable for use – which may or may not be seasoned.
Backspan
The full span from which an overhang or cantilever extends.
Balustrading
The vertical timbers used on handrail running between the top handrail and the bottom rail.
Batten, Ceiling
The timber attached to the bottom of the rafters, ceiling joists, or trusses to support the ceiling.
Batten, Roof
The timber attached to the top of the rafters or trusses to support the roofing.
Beam
In general it refers to any timber used to support heavy loads. In particular it refers to a timber member used to support rafters.
Beam, Ridge
The timber at the peak of a roof where the rafters meet.
Bearer
In general it is used to describe any timber used to support heavy loads. In particular it refers to the member used to support floor joists.
Birds Mouth
The name given to the triangular cut made to the underside of rafters so they sit properly across the wall or beam supporting them.
Bracing
Where most timber members are designed to support weight bearing down from above, bracing is used to support a structure from lateral (sideways) forces such as the wind. Bracing may be achieved in many ways from using timber to sheet material (plywood, etc.) to steel strapping.
Cantilever
Where the timber extends beyond physical support (similar to overhang) but where the extended area supports additional load.
Center
Where a measurement is taken from the center of one member to the center of the next. Typically used when working with parallel repeating members such as joists, rafters, or studs.
Defects
There are a range of defects that can occur in timber that may affect the appearance and/or structural.
Dry See
Seasoned.
Durability Class
All timbers commonly used in Australia are classified by one of four durability ratings known as Durability Class (and often referred to as just Class – e.g.: Class 1). The rating refers to the untreated heartwood of a species and as such does not reflect additional protection offered from chemical treatment. In general sapwood is always non-durable regardless of species.

Durability Class Durability / Suitable for Class 1 Very Durable – for external, in-ground use Class 2 Durable – for external, above ground use Class 3 Moderately durable – for external, covered and protected Class 4 Non-durable – for internal use only.

Durability ClassDurability / Suitable for
Class 1Very Durable – for external, in-ground use
Class 2Durable – for external, above ground use
Class 3Moderately durable – for external, covered and protected
Class 4Non-durable – for internal use only
Fascia
Timber member used at the end of rafters generally used for attaching guttering.
Gable
The triangular section at the end of a roof.
Green
See Unseasoned.
Handrailing
Handrail is typically assembled from a top rail, a bottom rail, balustrading, and possibly a mid rail, depending on the style wanted. Handrail is usually assembled around the edge of decks or balconies to act as a protective barrier.
Hardwood
In the literal sense the term hardwood is used to describe heavy density or ‘hard’ timbers. It actual refers to the botanical classification of the timber. Hardwood species are always deciduous trees and when a log is cut in cross-section will generally show a smaller band of sapwood surrounding the larger heartwood area. Also see Softwood, Sapwood, and Heartwood.
Heartwood
The central portion of a log that is surrounded by the sapwood. The heartwood is higher density than the sapwood and as such cannot be effectively preservative treated. It is this portion of the log that durability classes refer to. Hardwood logs are predominately heartwood with a narrow outer ring of sapwood. By contrast, softwoods are predominately sapwood with a smaller heartwood area.
Joist, Ceiling
The timber used in the roof structure to support the ceiling battens and ceiling only. Ceiling joist do not carry any weight from the roof.
Joist, Floor
The horizontal timber members used for supporting flooring or decking and in turn supported by the bearer(s).
Kiln Dried(KD)
Often used to describe seasoned timber although it actually refers to one of the drying processes used. Kiln-dried material is timber that has been seasoned using large ovens (or kilns). Also see Air-Dried and Seasoned.
Lintel
The horizontal member used over window and door openings to distribute the load out to the adjacent wall frame.
Load, Dead
Dead load covers the loading placed on a timber structure from forces that are generally expected and remain fairly consistent. This would include such things as the weight of the structure itself and any roofing, ceiling lining or floor covering.
Load, Live
Live load covers the loading placed on a timber structure from forces that are inconsistent and variable. This includes such things as wind loading or someone walking/jumping/running on the structure.
Load Bearing
Used to refer to any component of a structure, or the structure itself, that needs to carry load. Load bearing members need to be structurally rated subject to specific loads applied.
Lyctid Susceptible
The sapwood of some species of timber is classified as lyctid susceptible and as such must be treated for use in Queensland under the TUMA Act.
Nail Laminated
Where two smaller pieces of timber are fastened together using a specified nailing pattern to create a member stronger than the sum of its parts.
Nogging
The horizontal pieces of timber placed in-between the studs of a wall frame to help keep the studs straight and for fastening wall lining or cladding.
Non-Load Bearing
Used to refer to any component of a structure, or the structure itself, that does not carry any load. Non-load bearing timbers do not necessarily need to be structural in nature.
Overhang
Where the timber extends beyond physical support (similar to cantilever) but where the extended area does not support additional load.
Pitch
The angle at which the roof slopes.
Plate, Bottom
The timber around the bottom of a wall frame on to which the studs are attached.
Plate, Top The
Timber around the top of a wall frame on to which the studs are attached.
Plate, Wall
The piece of timber attached to a wall for supporting joists or rafters. Wall plates are not used to carry weight over spans. They are used as a point to transfer load from the joists to the wall the plate is attached to.
Post
A vertical member to transfer loads to ground level from the main structure. The term posts usually implies a square (or near square) end section as opposed to stumps or poles, which are round.
Rafter
The horizontal members usually supported by a beam and designed to support the attached roofing and roof battens. May also support a ceiling.
Rail, Bottom
The bottom member of a handrail which, in conjunction with the top rail, supports the balustrading.
Rail, Mid
The intermediate rail that can be included in a handrail which sits just below the top rail.
Rail, Top
The top member of a handrail which, in conjunction with the bottom rail, supports the balustrading. The top rail is usually a breadloaf or a ladies waist profile although other styles are commonly used.
Roof, Pitched
Old style of construction where a roof structure is erected manually as opposed to trusses which are used almost exclusively today. For older houses with a pitched roof care must be taken if moving/removing any internal walls to ensure they are not carrying roof load from the pitched roof structure. Due to the limited of use of pitched roofs this glossary does not encompass many of the members used in this area.
Roof, Truss
A roof truss is a manufactured frame designed to spread roof load to the outside walls of a structure generally resulting in all of the internal walls being non-load bearing.
Sapwood
The outer portion of the log that surrounds the heartwood. The sapwood is typically non-durable regardless of species but benefits the most from preservative treatment. Softwood logs are predominately sapwood surrounding the heartwood where hardwoods tend to only have a smaller band of sapwood.
Seasoned
Refers to timber that has been dried to a level of moisture considered stable with its surroundings (referred to as Equilibrium Moisture Content). For South-East Queensland this is from 10 to 15% moisture content. The advantage of seasoned timber is that some of the shrinkage that occurs with timber has already been done offering a more stable timber product. The drying process can be done via either air or kiln drying, or a combination of both.
Shrinkage
Unseasoned timber is subject to shrinkage as it dries. Shrinkage will occur in both the radial (across the end section) and the tangential (along the length) of the material. The shrinkage rate varies subject to the individual species although the tangential shrinkage tends to be uniformly marginal.
Softwood
In the literal sense the term softwood is used to describe lower density or ‘soft’ timbers. It actual refers to the botanical classification of the timber. Softwood species are always coniferous trees and when a log is cut in cross-section will generally show a small heartwood core surrounded by the larger quantity of sapwood. Also see Hardwood, Sapwood, and Heartwood.
Spacing
The distance between repeating members of timber such as studs, rafters, and joists and usually with a measurement based on center to center. Also refers to the likes of palings and balustrading, in which case the measurement usually applies to the gap between the members.
Span
The distance that a timber member has to support itself and any other loading from one support to another.
Span, Continuous
Where a single timber member spans across three or more points of support.
Span, Single
Where a single timber member spans between two points of support only.
Structural Grade
All timber species typically used for construction are given structural ratings, which relate to the materials strength and ability to carry load. Depending on the timber various techniques are used for grading the material from visual grading (manually identifying each piece of timber and grading based on the species and any defects that are present) commonly used for unseasoned hardwood to machine grading (machinery that puts weight on the timber and measures deflection) used for pine framing.
Stud
The vertical members of a wall frame.
Stud, Jack
The studs placed under window openings and above window and door openings.
Stud, Jamb
The studs used on either side of window and door openings.
Stump
Typically refers to round posts. In particular round posts used under a building to support the sub-floor frame.
Sub-Floor Frame
Refers to the structure that goes under flooring or decking and is made up of posts, bearers, and joists.
Tolerance
For all timber products there are tolerances that are allowed for in relation to end section and length. This tolerance allows a certain amount of variation to the dimensions a timber member is sold as. The amount variation depends on the actual material inquestion with sawn unseasoned timber having a wider allowance than dressed seasoned and engineered products. The tolerance for the end-section is usually expressed as something like +/- Xmm which would mean the piece of timber could be plus or minus Xmm.
Unseasoned
Refers to timber that still retains a level of moisture and has not been dried to a level considered seasoned. Allowance has to be made for shrinkage that will occur in unseasoned timber as it dries although much of this can be accounted for with the use of good building practices.
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