Durability Guide

1.1 Durability in the National Construction Code (NCC 2022)

The NCC 2022 (Volumes 1 and 2) sets out performance requirements for durability. It states that building elements must be fit for their intended purpose for the minimum design life of the structure, accounting for:

• Material susceptibility to deterioration
• Exposure to environmental conditions
• Maintenance accessibility and intervals

Design Life Expectations

Where natural durability or preservative treatment does not meet the expected exposure conditions, additional protection, design justification, or maintenance plans must be provided to satisfy NCC requirements.

1.2 When is Durability Critical?

Durability should be addressed in every timber project, but it is especially critical when:

• Timber is exposed to the weather, including rain, UV, and wind-driven moisture
• Components are embedded, concealed, or inaccessible for inspection or re-coating
• Elements are in contact with moisture-such as soil, concrete, or steel
• Safety is involved-such as decks, balustrades, or load-bearing members
• Appearance retention is required-such as façades, soffits, and joinery

Timber can fail prematurely even if it has a high durability rating-if used inappropriately, poorly detailed, or left unmaintained.

Figure 1: Differential weathering and decay on a facade (WS TDG 10 p24)

1.3 How Timber Durability is Assessed

The durability of a timber element in service is a function of four interrelated factors:

1. Species Properties
    (natural durability of heartwood)
2. Exposure Conditions
    (moisture, fungal risk, UV, termites)
3. Preservative Treatment
    (applied hazard class and retention level)
4. Design Detailing and Maintenance
    (drainage, ventilation, inspection access, finish type)

These factors are formalised in Australian Standards including:

• AS 5604 - Timber - Natural durability ratings
• AS 1604 - Preservative-treated timber - Sawn and round
• AS 1684 - Residential Timber Framed Construction (detailing guidance)

Together, they inform both product selection and construction detailing.

1.4 Durability is Not Fixed

Durability is not a static material property-it is context-dependent.

Correct specification, proper detailing, and regular maintenance can significantly extend the life of even low-durability species-while poor detailing can lead to failure of even Class 1 hardwoods.

Key Takeaways

• Timber durability is a shared design and construction responsibility
• NCC 2022 requires materials to remain fit for purpose over the design life, which depends on both material and exposure
• Durability is driven by species properties, environmental conditions, treatment, and detailing-not one factor alone
• Early planning for exposure, inspection and maintenance is essential to successful long-term timber performance

Natural durability refers to the inherent resistance of a timber species to biological degradation-particularly fungal decay and insect (especially termite) attack-without the use of chemical treatment. It is a key material property of the heartwood and varies significantly between species.

Understanding natural durability is critical when specifying untreated timber in exposed or semi-exposed applications.

2.1 Durability Classes (AS 5604)

In Australia, natural durability is classified in accordance with AS 5604 - Timber - Natural durability ratings. This standard defines four durability classes, with performance benchmarks based on both above-ground and in-ground exposure scenarios.

Note: These values are based on optimal detailing and average exposure. Poor design, trapped moisture, or neglect can reduce performance across all classes.

Figure 2: Various exposure conditions (WS TDG 05, p27, p44)

For properties on timber species including durability in various scenarios, refer to sub-page species collection.https://www.woodsolutions.com.au/wood-species

Chart 1: General guide to probable life expectancy according to AS 5604. Source: AS 5604

Important Note on International Standards
Durability classifications in AS 5604 are not harmonised with international systems such as:

• EN 350 (Europe)
• AWPA (North America)
• JIS (Japan)

Do not assume a “Class 2” rating in one country is equivalent elsewhere. Always consult locally verified data when comparing international timber products.

This is particularly important for imported species or specification in international projects.

2.2 Heartwood vs Sapwood

Durability ratings apply only to heartwood, which contains natural extractives offering resistance to fungi and insects. Sapwood, which forms the outer growth rings of the tree, is not durable and is readily attacked by pests or fungi unless treated.

Implications:

• Even a Class 1 hardwood can fail prematurely if the element consists mostly of untreated sapwood.
• Treated timber products (e.g. H3 decking) must be verified for penetration depth-particularly where heartwood is present.

Figure 3: CCA treated sapwood outperforming durability class 2 heartwood (WS TDG 05 p39)

2.3 Durability and Use Context

Not all timber needs to be naturally durable - the choice depends on how the timber will be used. For example:

For safety-critical or concealed elements, higher durability or more robust protection is advised.

2.4 Common Durable Timber Species in Australia

Here are examples of Australian species commonly used for their durability:

Chart 2: Properties of common Australian timbers (WS TDG 10 p9)

Chart 3: Properties of common imported timbers (WS TDG 10 p9)

2.5 Design Tip: Don’t Over-Specify

Designers should resist the temptation to over-specify Class 1 species in every application. Using highly durable species where they're not required can:

• Increase cost unnecessarily
• Increase carbon footprint due to harder-to-source species
• Lead to inappropriate waste if appearance defects are rejected

Instead, focus on:

• Designing for exposure
• Using protective detailing and treatments
• Selecting species that are locally available and matched to use

Related: [Detailing Timber for Durability] · [Preservative Treatments and Use Classes]
Referenced in TDG 05, p. 7-9

Key Takeaways - Natural Durability of Timber

1. Natural durability is species-specific and determined by the resistance of the heartwood (not sapwood) to fungal decay and insect attack.
2. In Australia, AS 5604 defines Durability Classes 1 to 4, with Class 1 being the most durable and Class 4 the least. These durability classes have no correlation to international durability classes.
3. Life expectancy varies by exposure:
   • Above-ground and well-detailed: 7-40+ years depending on class
   • In-ground or high-exposure: <5 to 25+ years depending on class
4. Heartwood is naturally durable, but sapwood must be treated regardless of species if used in exposed conditions.
5. Designers should match the durability of timber to its intended use and exposure, rather than over-specifying high-durability species unnecessarily.
6. Sustainable design involves balancing durability with availability, cost, environmental impact, and design detailing.
7. Natural durability is just one part of a broader strategy - detailing, moisture management, and maintenance are equally important for service life.

To standardise treatment requirements based on hazard, the AS 1604 series defines Hazard Classes (H1 to H6), which describe increasing levels of biological threat.

3.1 Hazard Classes

A hazard class is a performance requirement, not a product-manufacturers must certify that treated timber complies with the appropriate class.

3.2 Geographic Decay Risk in Australia

The risk of timber decay depends on localised climate and exposure factors.

High-risk zones include:

• Tropical and subtropical regions (e.g. Northern QLD, NT)
• Coastal and estuarine zones
• Shaded or poorly ventilated sites
• Areas with high rainfall and seasonal humidity

Low-risk zones include:

• Arid or alpine regions with low humidity
• Fully sheltered interior zones
• Well-ventilated elevated structures

Figure 4: Above-ground decay hazard zones (WS TDG 05 p20)

CTIQ and industry maps show decay hazard zones across Queensland and Australia.

NCC 2022 and AS 1684 require detailing and treatment levels to reflect regional risk. Local authority requirements may vary-especially in bushfire-prone or termite-declared zones.

3.3 Building-Level Exposure (Detail Scale)

Even within a single building, exposure levels vary between elements:

• South-facing walls in cold/wet climates may retain moisture and be prone to decay.
• Decks over damp ground have higher in-service moisture content than wall cladding.
• Ground-level retaining walls face near-constant saturation and require in-ground durability or protection.

As a result, designers must evaluate each component (e.g. cladding, posts, sills, decking) based on:

• Orientation
• Drainage
• Likelihood of inspection and maintenance

This level of granularity is essential for correct species selection, treatment level, finish type, and installation detailing.

3.4 Linking Hazard Class to Treatment and Species

When specifying timber, hazard exposure class determines whether you can rely on:

• Natural durability, or
• Preservative treatment

Related: [Timber Durability & Treatment Classes] · [Specification and Fabrication]

Key Takeaways - Hazard Exposure Zones

• Exposure conditions are as important as timber properties in determining service life.
• Australia is divided into four hazard zones (A-D) based on regional decay risk - from arid to tropical.
• Localised microclimates such as shade, slope, and damp areas can significantly increase decay risk, even in lower-risk regions.
• Within a building, exposure varies - e.g. decks, sills, and posts require more protection than sheltered soffits.
• Understanding exposure informs timber selection, treatment requirements, finish type, and detailing strategies.
• Timber must be matched not only to the intended function but to the hazards it will face over time.

Not all timber is naturally durable - but many lower-durability timbers can be made suitable for use in exposed environments through preservative treatment. This allows cost-effective, readily available species such as Radiata Pine and Slash Pine to be safely used in above-ground, in-ground, or even water-contact applications.

In Australia, preservative treatments are classified according to the hazard class system defined in AS/NZS 1604 series. Each class represents the level of biological hazard the timber must resist in service. 

See Section 3.1 for the Hazard Class table.

4.1 Principles of Preservative Treatment

Preservative treatments are applied to timber through:

• Pressure impregnation (most effective and common)
• Soaking or diffusion
• Surface brushing or spraying (for cut ends or site repairs only)

The effectiveness of treatment depends on timber species and timber anatomy:

• Sapwood is porous and can be thoroughly treated.
• Heartwood is resistant to penetration and often cannot be reliably treated to required standards.
  • This means that in many species (e.g. Spotted Gum, Blackbutt), the heartwood cannot be brought up to H3 or H4 standards even with pressure treatment. These species must either rely on natural durability or be replaced with more treatable timber where treatment is required.

Effective treatment requires:

• Penetration to a defined depth
• Minimum chemical retention (measured in kg/m³)
• Compatibility with intended exposure class

Treatment is often visible via a colour indicator (e.g. green for CCA, red for LOSP), but colour is not a guarantee of compliance. Treated timber must carry a label or brand showing the treatment type, H Class, and certifier.

Related: [Preservative Treatments and Use Classes] · [Timber Durability & Treatment Classes]

Suggested image: Diagram of cross-sectioned timber showing treated sapwood vs untreated heartwood

4.2 Common Preservative Types in Australia

Some preservatives may corrode unprotected metal fixings-see Section 5 for guidance.

4.3 Treatment Application Methods

Treated products must be re-treated at cut ends to maintain protection-especially for in-ground or above-ground exposed elements.

4.4 Verifying Treatment Compliance

According to AS 1604 and CTIQ Book 2, treated timber must include:

• Treatment brand/label: Showing species, H Class, preservative type, and treatment plant
• Certificate of treatment: For QA and traceability
• End tags or face branding: Clearly visible on delivery

Inspection checklist:

• Label includes treatment standard (e.g. AS/NZS 1604.1)
• End-sealing applied to any site cuts
• Penetration depth verified (destructive or spot checks for H4+)
• Retreatment procedures in place on site

NCC 2022 requires that any treated timber used for structural or exterior purposes be traceable to a certified treatment process.

4.5 Treatment Compatibility with Other Elements

Fasteners and Fixings

• Some preservatives (e.g. CCA, ACQ) are corrosive to zinc-plated steel
• Use hot-dip galvanised (HDG), 304/316 stainless steel, or coated fasteners for treated timber

Related: [Specification and Fabrication] [Fastener Selection for Treated Timber]

Coatings and Sealants

• LOSP-treated timber may affect coating adhesion
• Always check compatibility with water-based finishes or adhesives

Key Takeaways - Preservative Treatments

1. Preservative treatment enables non-durable or sapwood timber to perform in high-risk applications
2. AS 1604 defines acceptable treatment types, H Classes, and performance levels
3. Only sapwood absorbs treatment-verify heartwood content where applicable
4. All treated timber must be labelled and traceable to a certified treatment process
5. On-site cutting and drilling must be followed by reapplication of preservative to exposed ends
6. Fixings, adhesives, and coatings must be compatible with the chosen treatment

The long-term durability of timber is not guaranteed by species selection or treatment alone. It also depends heavily on how the timber is detailed-particularly in how the design manages moisture exposure, drying potential, and biological hazard avoidance.

This section summarises the detailed discussion in 02 Moisture, Section 4.2 - Construction Detailing for Moisture Management.

5.1 Detail to Prevent Moisture Build-up

Durability failure most often results from:

• Moisture held against timber surfaces
• Water ingress into joints or end grain
• Trapped vapour in enclosed cavities
• Contact with moisture-retentive materials (e.g. concrete, masonry)

To prevent this, detailing must:

• Shed water away from timber
• Allow drying by air movement
• Avoid capillary action and water traps
• Separate timber from porous or wet materials

These principles apply regardless of timber durability class or treatment.

5.2 Referenced Detailing Practices

The following critical detailing practices are addressed in full in [02 Moisture, Section 4.2], and should be used for durability protection:

• Moisture-Shedding Design
  • Sloped surfaces and drip edges for water runoff
  • Avoid flat horizontal faces wherever possible
  • Chamfered beam tops; deck board spacing
• Protection of End Grain
  • Sealing of all exposed cuts, especially for treated timber
  • Avoidance of upward-facing grain or unsealed joints
• Joint and Interface Design
  • Drained and flashed joints
  • Back-priming or sealing of contact faces
  • Use of compressible seals or tapes in concealed interfaces
• Ventilation and Drainage Cavities
  • ≥20 mm cavity behind cladding systems
  • Ventilated subfloors as per NCC 2022 Part H2D6
  • Weep holes or perforated flashings in soffit returns
• Material Interfaces
  • Capillary breaks (plastic shims, EPDM, PVC) between timber and concrete
  • Avoidance of direct contact between timber and porous materials without ventilation

See also: [Moisture Management During Construction] · [Specification and Fabrication]

5.3 Capillary and Vapour Traps

Detailing must also prevent trapped moisture at:

• Floor-wall junctions
• Underside of exposed beams or lintels
• Unventilated boxed eaves
• Bottom edges of cladding returns

For these cases, ensure:

• Flashings kick water away from surfaces
• Timber is not enclosed on all sides
• Drainage paths are maintained

Decay fungi thrive in stagnant moisture zones-ventilation is as important as sealing.

5.4 Design for Inspection and Maintenance

Timber durability depends on maintenance access.

Ensure:

• Concealed timber (e.g. behind membranes) is verifiable via access panels, vents, or records
• Sealants and coatings are renewable: replaceable, re-coatable
• Flashings, joints, and overhangs remain visible for checking

Key Takeaways - Detailing Timber for Durability

• Good detailing reduces decay and insect risk even for low-durability species.
• Water-shedding, ventilation, and capillary separation are essential design tools.
• Revise all details carefully, in particular construction detailing of joints, cavities, and interfaces.
• Design for inspection and maintainability-durability depends on continued performance, not just installation.
• Detailing is most critical at horizontal surfaces, junctions, and mixed-material interfaces.

The durability of a timber element is only partly determined at the point of installation. Long-term performance depends on whether the element can be inspected, maintained, repaired, or renewed over time-particularly in response to environmental exposure, mechanical damage, and weathering.

NCC 2022 mandates that building elements be designed for a service life appropriate to their function, with consideration given to environmental exposure, accessibility, and required maintenance frequency.

6.1 Service Life Expectations in NCC 2022

The National Construction Code (NCC 2022, Volume 1: Part A5 and Performance Requirements BP1.1) and referenced standards (AS 5604, AS 1684) imply or state the following expected service lives:

These durations assume good detailing, correct installation, and planned maintenance.

6.2 Maintenance Requirements by Timber Type

Timber systems vary in their maintenance intensity based on their durability class, treatment, and exposure. A good design should balance performance, aesthetics, and accessibility.

Summary Guidance

6.3 Coatings and Finishes for Appearance Retention

Although most coatings are discussed under moisture management, some serve a visual durability role-protecting timber from UV greying, checking, and surface fibre degradation.

Appearance Maintenance

• Use UV-stabilised penetrating finishes for external applications
• Avoid film-forming finishes where movement is expected
• Select finishes compatible with preservatives (e.g. LOSP, ACQ)

Surface weathering does not always indicate structural degradation, but can reduce perceived quality and serviceability.

See: [02 Moisture, Section 4.4 - Timber Coatings and Moisture]

6.4 Inspection and Documentation

Ongoing inspections are key to verifying that timber is ageing as expected and that protection systems remain intact.

Inspection Best Practices

• Establish a maintenance log from the outset
• Include photographs of high-risk interfaces at handover
• Document:
  • Coating types and application dates
  • Sealant locations and renewal history
  • Replacement or treatment of elements (e.g. decking boards)

Suggested Inspection Intervals

6.5 Repair, Replacement and Redeployment

Where timber elements do degrade, a planned repair or replacement pathway ensures durability targets are still met.

Strategies Include:

• Designing for disassembly and element-by-element replacement (e.g. removable decking or soffits)
• Using modular panels or external cladding systems that can be refurbished off-site
• Avoiding concealed fasteners or fixed membranes in critical durability zones

Particularly relevant for CLT, Glulam, and engineered timber systems in external use.

Key Takeaways - Maintenance & Service Life Planning

1. Timber durability must be supported by ongoing maintenance, inspection, and documentation.
2. NCC 2022 requires that designers consider maintenance intervals and accessibility when determining service life.
3. Coatings used for appearance should be breathable, UV-resistant, and re-coatable.
4. All exposed timber should be inspectable and replaceable where degradation could compromise performance.
5. Well-maintained timber buildings regularly exceed their nominal service life.

This section summarises typical durability considerations for common timber building components, based on:

• Exposure condition
• Structural or appearance function
• Recommended species, treatments, and detailing
• Reference to relevant sections for detailed guidance

All design decisions should be validated against NCC 2022, AS 5604, AS 1604, and manufacturer data, with detailing and maintenance strategies documented.

Component Durability Summary Table

Notes:

• Always confirm treatment retention and penetration where required under AS 1604
• Use only durable heartwood for uncoated or unprotected applications
• Revise details for flashing, cavity, and sealing requirements
• Design maintenance plans in line with