Timber Roofing Glossary

A modern arsenic-free waterborne timber preservative that has largely replaced CCA for residential uses such as decking, framing and pergolas.

Australian Standard for Residential Timber-Framed Construction. Provides span tables, tie-down requirements and bracing rules for timber framing in housing and small buildings.

Australian Standard for Timber Structures. The engineering design code for all structural timber, covering material properties, design methods and durability requirements.

A thin strip of timber fixed horizontally across rafters to support roof tiles, shingles, or metal sheeting. Battens provide a fixing point for roofing materials and create an air gap for ventilation.

A V-shaped notch cut into a rafter where it sits on the wall plate. This joint provides a secure connection between the rafter and the supporting wall structure.

An Australian hardwood (typically F17–F22) widely used for structural framing, flooring and exposed roof timbers due to its strength and bushfire performance.

A traditional waterborne preservative giving timber a green tint. Restricted in Australia for residential use since 2006, but still used for poles and industrial timbers.

Horizontal timber members that span between walls to support the ceiling below and often tie the feet of opposing rafters together to prevent them from spreading outward.

An engineered wood panel made from layers of solid timber boards stacked crosswise and bonded with structural adhesives. Used for walls, floors, and roofs in mass timber construction.

A heavy-duty wood screw with a hex or square head, driven with a spanner. Used for high-load timber connections where nails are insufficient.

A horizontal timber member connecting two opposing rafters near the ridge, preventing the rafters from spreading apart under roof loads. Also called a collar beam.

Vertical timber strips fixed over the rafters, running from eaves to ridge. They create an air gap between the sarking and battens for improved ventilation and drainage.

Galvanised metal straps used in Australian cyclonic regions (C1–C4) to fix rafters or trusses to wall plates with much higher uplift capacity than standard ties.

The permanent self-weight of the roof structure and its coverings (timber, sheeting, tiles, insulation). One of the key inputs to span-table selection.

The amount a structural member bends under load. Span tables limit deflection to acceptable amounts (often span/300) to avoid sagging ceilings or cracked finishes.

Mass per unit volume of timber, usually expressed as kg/m³ at 12% moisture content. Higher density generally correlates with greater strength and durability.

A rating system (Class 1-4) indicating a timber species' natural resistance to decay and insect attack. Class 1 timbers can be used in-ground for 25+ years without treatment.

The lower edge of a roof that overhangs the external wall. Eaves provide weather protection to the wall below and can incorporate vents for roof space ventilation.

Wood products manufactured by bonding timber strands, veneers, or boards together to create structural elements with predictable, enhanced properties. Includes LVL, glulam, and CLT.

A structural stress-grade designation. F grades are not exclusively visual grades; confirm the applicable grading method, product certification and design properties with the supplier and a qualified professional.

Mid-range visual stress grade (11 MPa) typically applied to denser softwoods or lower-strength hardwoods. Used for higher-load framing, beams and bearers.

Higher-strength visual stress grade (14 MPa), commonly assigned to Australian hardwoods used in roof beams, lintels and structural framing.

High-strength visual stress grade (17 MPa) for premium hardwoods. Frequently specified for long-span rafters, ridge beams and structural timbers in Australian construction.

Very high-strength hardwood stress grade (22 MPa), used for heavy-load structural members such as large beams, columns and bridge timbers.

Top tier of common hardwood stress grades (27 MPa). Reserved for the densest Australian hardwoods such as ironbark and grey gum, used in heavy-duty structural applications.

Visual stress grade for low-strength structural timber (characteristic bending strength 5 MPa). Suitable only for light, non-critical applications such as short-span lintels or non-load-bearing framing.

Common visual stress grade for softwood structural timber in Australia (characteristic bending strength 7 MPa). Often used for rafters, joists and general framing where MGP timber isn't specified.

A vertical board fixed to the rafter ends at the eaves, providing a neat finish and often supporting the gutter. Typically made from treated pine or fiber cement.

Bent metal connector used to fix one timber member to another at right angles, common at rafter-to-plate and stud-to-plate connections.

Glued laminated timber - an engineered product made by bonding layers of dimension timber with durable adhesives. Used for beams, arches, and columns where long spans or curves are required.

A hazard class system (H1 to H6) specifying the required preservative treatment level for timber based on exposure conditions. H2 is for interior framing; H3 is for external above-ground use.

Preservative treatment for indoor timber protected from weather but at risk of borer or termite attack. Standard for roof framing in many Australian regions.

Preservative treatment for outdoor, above-ground timber exposed to the weather. Suitable for fascias, eaves, exposed rafters and pergolas.

A joint formed by cutting away half the thickness of each timber so they overlap flush. Frequently used at corners of plates and at bracing intersections.

Timber from broadleaf (angiosperm) trees, typically denser and stronger than softwood. Australian hardwoods like spotted gum and blackbutt are valued for their natural durability and appearance.

Australian classification of in-service exposure for treated timber. H1 (insect protection indoors) up to H6 (marine immersion). Roof framing typically requires H2 or H3.

The dense central portion of a tree trunk, no longer actively conducting sap. Heartwood typically contains natural preservatives that provide greater durability than sapwood.

A rafter extending diagonally from the corner of a building to the ridge at the intersection of two sloping roof planes. Hip rafters carry loads from jack rafters on both sides.

A native Australian softwood from Queensland and northern NSW with a fine, even texture. Used in plywood, joinery and some structural applications.

An engineered timber joist with an I-shaped cross-section: thin OSB or plywood web between solid timber or LVL flanges. Lightweight and stable for long spans.

Extremely dense, very durable Australian hardwood (F22–F34). Used for heavy structural timbers, ridge beams and bridge construction.

A short rafter extending from a hip or valley rafter to the wall plate. Jack rafters reduce in length as they approach the hip or valley.

A metal connector used to support the end of a joist or beam where it meets another member. Joist hangers provide strong, code-compliant connections without notching.

Timber dried in a controlled kiln to a specified moisture content. Faster and more consistent than air drying, and used for most structural pine in Australia.

Temporary loads on a roof such as people during maintenance, snow, or installed equipment. Combined with dead load and wind load to determine member sizes.

A solvent-borne timber preservative commonly used for indoor framing and joinery. Causes minimal swelling because it does not introduce water into the wood.

An engineered wood product made by bonding thin wood veneers together with all grain running in the same direction. Provides consistent strength and is available in long lengths for beams and headers.

A non-destructive grading method that mechanically tests each piece of timber for stiffness (modulus of elasticity). Produces more consistent and reliable strength values than visual grading.

Large-scale engineered timber products including CLT, glulam, and NLT used in commercial and multi-story construction. Mass timber buildings offer sustainability benefits and rapid construction.

Radiata pine graded by machine measurement of stiffness rather than visual inspection. MGP grades (MGP10, MGP12, MGP15) indicate the modulus of elasticity and predictable structural properties.

Machine Graded Pine 10 — the most common structural softwood grade in Australia, with a minimum modulus of elasticity of 10,000 MPa. Used for general-purpose framing, including most residential roof rafters and ceiling joists.

Machine Graded Pine 12 — a higher-strength structural pine grade (MOE 12,000 MPa) used where longer spans or higher loads are required. Stiffer and stronger than MGP10 but less common in stock yards.

Machine Graded Pine 15 — the premium MGP structural grade (MOE 15,000 MPa). Typically used in engineered designs, longer-span beams or where stiffness is critical. Special-order in many Australian regions.

A measure of timber stiffness — the ratio of stress to strain under load. Higher MOE means less deflection. Used to define MGP grades (e.g. MGP10 = 10,000 MPa).

The amount of water in timber expressed as a percentage of oven-dry weight. Seasoned structural timber should have 15% or less moisture content to minimize shrinkage and movement.

A traditional timber joint where a projecting tongue (tenon) on one piece fits into a matching slot (mortise) in another. Common in heavy-timber and exposed roof framing.

A mass timber panel made by nailing individual dimension lumber boards together on their narrow face. Often used for floors and roofs in heavy timber construction.

A structural panel made from compressed wood strands bonded with resin. Less common than plywood in Australia but used as sheathing and I-joist webs.

A vertical cut at the upper end of a rafter where it meets the ridge, ensuring full bearing against the ridge board or beam.

An engineered panel made from thin layers of veneer glued with the grain alternating at right angles. Used for sarking, bracing and structural sheathing in roofs.

A horizontal timber member spanning between rafters or trusses to provide intermediate support to the roof covering. Purlins transfer roof loads to the main framing.

The most common plantation softwood in Australia and New Zealand. Used for the majority of residential framing, treated to H2 or H3 for use in roofs.

Inclined timber members forming the main structural framework of a roof, extending from the ridge to the eaves. Rafters carry roof loads and determine the roof pitch.

The horizontal timber at the apex of a roof where the tops of opposing rafters meet. The ridge board provides a fixing point and alignment for rafter connections.

The horizontal distance over which a roof member supports load. Used in span-table lookups to account for the actual area of roof transferring weight to a beam or rafter.

The angle or slope of a roof, expressed in degrees or as a ratio of rise to run. Steeper pitches shed water faster but increase wind exposure and timber requirements.

An alternative way to express roof slope as rise over run (e.g. 4:12 means 4 units of rise per 12 units of horizontal run).

The outer, living portion of a tree trunk that conducts water and nutrients. Sapwood is typically lighter in color and more susceptible to decay and insect attack than heartwood.

A building membrane or board layer installed beneath the roof covering and over the rafters. Sarking provides secondary weather protection and can act as a vapor barrier.

An end-to-end joint used to splice two timbers into a single longer member, with the contact surfaces cut at matching angles for strength.

Timber that has been dried to reduce its moisture content, either through air drying or kiln drying. Seasoned timber has reduced shrinkage and improved dimensional stability.

The horizontal cut at the lower end of a rafter that allows it to sit flat on the wall plate. Forms part of the birdsmouth.

The reduction in timber dimensions as it loses moisture. Greatest across the grain (tangentially and radially) and minimal along the length.

Driving nails at an angle through one timber member into another, commonly used to fix rafters down to wall plates in low wind zones.

Timber from coniferous (gymnosperm) trees like pine, fir, and spruce. Generally lighter and faster-growing than hardwood, softwood is widely used for framing and structural applications.

The unsupported horizontal distance between two bearing points (e.g. wall plates). The maximum allowable span depends on timber size, grade, spacing and loading.

A reference chart showing the maximum allowable span for a given timber size, grade, spacing and load type. Required for compliant design under AS 1684.

Published reference tables that specify maximum allowable spans for timber members based on species, grade, size, spacing, and loading conditions. Essential for compliant design.

A dense, durable Australian hardwood with high strength (commonly F22–F27). Popular for exposed roof structures, beams and decking.

A classification of timber based on its structural properties, particularly strength and stiffness. Stress grades allow engineers to specify timber with known performance characteristics.

A compression member in a roof structure that transfers loads from purlins or rafters to a supporting wall or beam. Struts are often used in lieu of internal loadbearing walls.

A trade name for three Australian hardwood species (mountain ash, alpine ash, messmate). Used for structural and feature timbers including beams and posts.

Mechanical fixings (straps, brackets, bolts) that anchor a roof structure to the walls and foundations to resist wind uplift. Specified by wind classification.

Timber that has been impregnated with preservative chemicals to improve its resistance to decay, insects, and weathering. Treatment levels are specified by H-class ratings.

A versatile metal framing connector used to join rafters to top plates and other timber-to-timber connections at various angles. Provides strong multi-directional connections.

A prefabricated structural framework, usually triangulated, that spans the width of a building and supports the roof covering. Trusses are designed and manufactured off-site.

A timber beam running the length of a roof, supported by struts, that provides intermediate support to rafters. Also known as a strutting beam.

A rafter at the internal intersection of two sloping roof planes, forming a V-shaped valley. Valley rafters carry loads from jack rafters on either side.

A method of grading structural timber based on visual inspection for knots, slope of grain, splits and other defects. The basis for the Australian F-grade system.

A horizontal timber member fixed to the top of a wall that provides a bearing and fixing point for rafters, ceiling joists, or trusses. Also called a top plate.

A lightweight, naturally durable softwood with excellent dimensional stability. Used in Australia mainly for cladding and decorative roof features.

Australian site-rating system (N1–N6 for non-cyclonic, C1–C4 for cyclonic) that defines the wind loads a roof must resist. Affects timber sizing, fixings and tie-down requirements.