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Glossary Of Timber Terms

Search A to Z of everything glossary of timber.

Air drying of timber is the process of stabilising by reducing its moisture content naturally. Timbers are stacked in racks allowing airflow to remove moisture slowly and evenly in a sheltered area.


ACQ (Alkaline Copper Quarternary) is a water-based timber treatment created to offer a safer alternative to the more commonly used CCA (Copper Chrome Arsenic) treatment. While also being used as an insecticide, it provides excellent protection against rot, insects, and fungus.


Backspan refers to the area of a cantilevered beam that provides the required rigidity to support a cantilevered load. This is usually fastened to a roof or floor structure and has a span ratio specified by an engineer for the necessary load.


Timber balustrading provides fall protection for decks, pergolas, and balconies. It is available in a variety of designs, spans, and sizes.

Batten, Ceiling

Ceiling battens are lightweight timber members designed for levelling ceilings and fixing sheet materials to plaster, cement sheet or lining boards. Timber ceiling battens offer superior strength and flexibility to lightweight steel options.

Batten, Roof

Roof battens are lightweight timber members designed to support roofing materials such as corrugated iron or tiles. Timber roof battens offer greater flexibility and superior fixing.


A timber beam is a dimensional piece designed to support roof or floor loads. An engineer specifies its strength, span and dimensions based on the structural requirements.

Beam, Ridge

A ridge beam is the central supporting beam in a pitched roof structure. It provides the connection point for rafters and the size of which is determined by roof load, span, and the pitch of the roof.


A bearer is a load-bearing timber member that supports floor joists in a subfloor or deck. It spreads the load of joists and transfers them to stumps or piers. A bearer’s size is determined by the span between stumps, the floor load and the dimensions of the joists in use.

Bird Mouth

A bird mouth is a cut made into timber rafters to allow a pitched, perpendicular beam to sit on the top plate of a wall or supporting beam. These cuts create a level surface for even load distribution and effective hardware fixture.


Bracing Timber bracing is used in most wall, floor, and roof structures to prevent lateral movement. Timber members are secured at angles to the structure and prevent unwanted movement.


A cantilever is a beam or structure that protrudes from a building but has no supporting structure underneath it. This is achieved by fixing most of the beam within the structure, providing load support for the cantilevered area. By this, a load applied to the cantilevered area is transferred upward into the supporting structure rather than downward into a footing.


On Center is a term used when developing a construction plan. The specified dimension in the plan represents the distance between the midpoints of the first and second timber members.


CCA stands for Copper Chrome Arsenic and is the chemical compound used to treat timber. Pioneered in the 1930s, it is highly effective in preventing timber rot, insect attack or fungal growth.


Defects is the term used to describe imperfections in timber that can cause structural or aesthetic concerns. Defects include cracks, knots, splits, twist, bows, cupping, and gum vein. While most timber has a variety of defects, select grades of timber are available with the lowest variations.

Durability Class

Treated timbers are rated with a durability class that defines their resilience to deterioration. A higher durability class will last longer when exposed to weather or when set into the ground. Different timber species also have inherent durability that is factored into their class.


Dressed is the term given to timber when it has been machined to a fine finish. The faces of the timber will be smooth and flat with perpendicular edges. These timbers are best used for architectural work, decks and furniture.


A fascia beam is that which wraps around the edges of a roof structure. It provides a mounting point for gutters and is often slotted to hold up even sheeting. Fascia beams are made of high-quality timber that is straight and precisely machined.


A gable is the perpendicular end of a roof structure where the ridge meets an exterior wall. It is often clad differently from the wall it rests on to create a desired architectural style. Gables are most common on smaller and simpler roof structures, such as sheds, pergolas, and carports.


Green refers to freshly milled lumber that has not yet been kiln or air dried. It still has high moisture content and will be prone to movement and shrinkage as it ages.


A timber hand railing provides fall protection on stairs and walkways while providing aesthetically pleasing details. These are constructed to provide support while walking across, up or down a structure such as stairs or a deck.


The term hardwood refers to timber that comes from slow-growing and deciduous trees. The nature of these trees creates a far denser timber that has superior strength and durability to that of softwoods.


Heartwood comes from the tree's centre, where the growth circles start. This timber is commonly used for posts due to its resistance to movement and density.

Joist, Ceiling

Ceiling joists are used to span from wall to wall and provide a load-bearing structure for the ceiling below. Ceiling joists are made from dimensional timber, specified by an engineer based on their span and required load bearing.

Joist, Floor

Floor joists are used to support wall structures and flooring. They are fixed to bearers and have their dimensions decided by an engineer based on their load bearing, span and spacing.

Kiln Dried

Kiln Drying is the process of removing moisture content rapidly from timber. Freshly milled timbers are stacked in racks and placed in a large kiln where the temperature and airflow are carefully controlled to remove the desired moisture content.


A timber lintel is a supporting beam placed above windows, openings, and doorways. It transfers the roof load to load-bearing studs or columns on either side of the opening. The size and material of a lintel are specified by an engineer and are decided based on the span and load required.

Load, Dead

Dead load is the force applied to a structure on a constant basis. This is applied by the structure's weight and other permanent items such as furniture, appliances, or cabinetry.

Load, Live

Live load is the force applied to a structure in a constantly variable nature. Factors responsible for the live load include weather or the movement of people and objects across a given surface.

Load Bearing

Load bearing refers to a timber member that supports the weight of a structure and the loads applied to it. Size, material, structural design, and more are all aspects that affect a timber member's capacity to support a structure's weight.

Lyctine Susceptible

Lyctine Susceptibility describes timber species at risk of being attacked by Lyctine beetles or Lyctus Borers. This causes a great deal of damage and loss of structural integrity.

Nail Laminated

Nail lamination is the process of joining multiple timber members in parallel by using nails. Usually performed using a nail gun, the spacing of the nails is often specified by an engineer, based on the beams' dimensions to be laminated and their load requirements.


Nogging is the process of placing perpendicular timber members between studs in a wall or floor frame. The noggings prevent bow and twist from developing in the studs and provide a perpendicular structure for fixtures such as cladding or cabinetry.

Non-Load Bearing

A Non-Load Bearing structure is one that does not have any loads applied to it by roof or floor loads. A typical example of a non-load bearing structure is an internal wall that is only required to support its own weight.


In reference to timber, overheating has a few different meanings. It can refer to the kiln drying process of removing too much moisture from timber by applying excess heat. Overheating can also mean that excess heat is being applied to timber by an external source, such as a fireplace. This can cause warping and distortion of the timber and even result in combustion.


Pitch is the measurement of the incline of a roof structure. This is achieved by dividing the rise and the span.

Plate, Bottom

The bottom plate of a wall is a piece of timber connected directly to a subfloor or slab. It provides the fixing structure for stud work and distributes the load evenly.

Plate, Top The

The top plate of a wall also referred to as a ribbon plate, is the timber member laminated to the wall plate. It allows for greater load distribution and roof loads to be indirectly applied to studwork.

Plate, Wall

The wall plate is the perpendicular timber member that holds studwork together. It provides a fixing structure for the stud work and roofing while distributing the roof load evenly to the studs.


A post is a timber member set in or directly connected to the ground. These have a variety of applications, including fencing, structural support, decoration and retaining.


A rafter is a timber roof member that spans from an external wall to the ridge line of a roof. It can be a stand-alone member or part of a roof truss.

Rail, Bottom

A bottom rail refers to the lowest of several fence rails that support palings and connections between posts.

Rail, Mid

A mid rail refers to the middle of several fence rails that support palings and connections between posts.

Rail, Top

A top rail is the highest of several fence rails supporting palings and connections between posts.

Roof, Pitched

A pitched roof comprises individually cut rafters that span from an external wall or beam to the ridge of the roof. Each timber member is responsible for a higher load bearing in a pitched roof.

Roof Truss

A roof truss is a lightweight engineered structure that uses internal bracing to cope with roof loads and distribute them to external walls. Roof trusses can be built to cover large spans and create multiple roof lines and shapes easily.


Sapwood is the outermost timber in the trunk of a tree. It is often the softest and least stable part of the tree and undesirable for use in construction or furniture.


Seasoned timber is that which has been aged for a period to achieve a desired aesthetic, stability, and moisture content. Decreasing the moisture within timber creates a high-quality product as seasoned timber provides higher longevity.


Shrinkage is the process of timber drying out and reducing in size. This happens predominantly along the grain of the timber and minimally across it.


Softwood is timber sourced from rapid-growing and evergreen trees. This timber has lower density, durability, and strength properties than that of hardwood sourced from slow-growing and deciduous trees.


Spacing refers to the distance between timber members. Spacing is determined by the size of timber members and the loads placed on them.


The span of a timber member is the distance between load-bearing points of connection. The span a timber member can support is determined by its dimensions and the loads applied to it.

Span, Continuous

A continuous span is a single timber member supported in more than two load-bearing locations. A constant span can allow for greater load bearing and spans between each load-bearing site.

Span, Single

Single span is the distance of a timber member between the supporting two load-bearing locations at either end. A single-span timber beam can only be reinforced by two points.

Structural Grade

The structural grade is the term given to high-quality timber. Structural grade means the timber is stable, consistent, and fit for load-bearing applications


A stud is a vertical timber member used in wall construction to support roof load and provide a fixing surface for internal and external cladding. Studs are an essential framing component, and builders can use timber or steel studs.

Stud, Jack

The framing members of a wall must be modified to account for the loss of wall studs when an opening is required, an example being windows or doorways. Jack studs can help in this situation. Jack studs are short vertical timber members and are weight-bearing studs that run vertically beneath a header to support the window or door frame.

Stud, Jamb

A jamb stud is a vertical timber member placed on either side of a doorway opening. These timber members need to be straight and without a twist to ease the installation of a door frame.


A stump is a timber or concrete member set into concrete under a floor structure. They provide load bearing and bracing to a subfloor and a level point of connection to bearers.

Sub-Floor Frame

A subfloor combines stumps, bearers, and joists to support a structure and its floor load. This includes house frames, decks, and walkways.


Sawn refers to the condition of timber once it has been cut to size. Sawn timber has a rough finish and still shows cut marks from the cutting blade used.


Tolerance is the acceptable variation in all dimensional aspects of a timber member. Tolerance applies to the dimensions, straightness, cup, and twist allowed in a piece.


Unseasoned timber is that which has not had any exposure to weather or climate. It may still be green or have been milled in a different environment than it now resides. Unseasoned timber is prone to instability and movement as it weathers and acclimatises.


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