Design for Durability - Lunch & Learn

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This presentation, which includes termite management techniques, will help improve the specification and design performance of wood in construction. …

This presentation, which includes termite management techniques, will help improve the specification and design performance of wood in construction.

The presentation uses structural engineering criteria, years of experience and field trials and predictive modelling to provide guidance on methods to increase the service life of woods used in Australia.

Also covered are the affects of natural variation between wood species, the effectiveness of wood preservative treatments and climatic conditions.

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  • 1. Design for Durability
  • 2. Learn more about wood at UTAS
    Centre for Sustainable Architecture with Wood
    Graduate Certificate in Timber (Processing & Building)
    4 units, part time, online
    Areas covered include:
    Wood science
    Design for durability and service for life
    Timber as a renewable resource
    Sustainable design and construction
    Engineered wood products
    International technologies and developments
    Plus, selected topics of individual interest
    More information: Associate Professor Greg Nolan
    (03) 6324 4478 or
  • 3. Learning Objectives
    After this presentation you should be able to:
    Understand the key timber hazards
    Understand the fundamentals of timber durability
    Understand the design factors that need to be considered to achieve satisfactory timber durability performance.
    For architects - AACA Competency:
    Elliot River Fire Tower, QLD
  • 4. Presentation Contents
    Design process
    Main timber hazards:
    Decay, insects, corrosion of fasteners & weathering
    Natural durability & treatment
    Design detailing
    Other hazards:
    Marine borer, chemicals, fire
  • 5. Design Process: 1
  • 6. Design Process: 2
  • 7. Design Process: Discussion
    To determine performance requirements you may need to consider:
    Target life expectancy (minimum regulatory, standards or contractual requirements)
    Level of reliability (life safety, cost or consequence of failure)
    Costs (initial vs ongoing maintenance, repair or replacement)
    In the context of building regulations, the Building Code of Australia (BCA) has implicit durability performance expectations:
    see Durability in Buildings – Guideline Document at
  • 8. Design Process: BCA Design Life Guideline
    For normal buildings, the design life for most structural timber members is 50 years and for moderately accessible members 15 years.
  • 9. Timber Hazards
  • 10. From a durability perspective, the main hazards that need to be considered are:-
    • In-ground and above ground decay (including hazard or ‘H’ levels etc)
    • 11. Insects (inc. termites)
    • 12. Corrosion (of fasteners)
    • 13. Weathering
    • 14. Marine borers
    • 15. Chemical degradation (not usually an issue), and
    • 16. Fire (subject of other resources)
    Hazard Classes (‘H’)
    apply to these 3 agents
    Above ground durability trials, Beerburrum QLD
    Timber Hazards
  • 17. Decay
    The performance of timber with respect to decay is:
    • Highly related to the presence of free moisture in the wood above 20% moisture content (MC)
    • 18. Different with respect to decay when in-ground vs above ground (different timber durability ratings apply in each case)
    Macro climatic decay hazard maps have been developed to address these fundamental differences
    In-ground durability trials
    ‘Wedding Bells’, Nth NSW
  • 19. Decay: In-Ground Decay Hazard Zones
    In-ground decay hazard zones for Australia
    (Zone D has the greatest in-ground decay potential)
  • 20. Decay: Above Ground Decay Hazard Zones
    Above ground decay hazard zones for Australia
    (Zone D has the greatest decay hazard potential)
  • 21. Decay: Causes
    To thrive fungi need:
    • Moisture > 20% MC in wood
    • 22. Oxygen
    • 23. Temperature >25o to <40o (ideal)
    • 24. Food
    Remove any of these four key elements and fungal growth is either stopped or retarded
    e.g. preservative treatment renders the ‘food’ (wood) immune
  • 25. Decay: Absence of Causes Example
    Wood in an anaerobic condition (i.e. without access to oxygen) lasts indefinitely
    e. g. Kauri dug out from the ground after 10,000 to 50,000 years
    45,000 year old surf board
  • 26. Decay: Hazard Classes
    A decay Hazard Class system has been developed for Australia that:
    • Primarily relates to the use of preservative treated timber but is also used with natural durability
    • 27. Increases in severity from H1 to H6 Class
    • 28. Does not, at this stage, account for macro-climatic variations, but these can be addressed in the durability design process
  • Decay: Hazard Classes (H1 – H6)
    H1 - fully protected indoors, not termites resistant
    H2 – fully protected indoors plus termites resistant
    H3 -exposedto weather, aboveground,
    well ventilated
    H4 - in ground (landscaping)
    H5- in ground (more critical)
    H6 - marine piles
    30 deg
    from weather
    Exposed to weather
  • 29. Decay: Hazard Classes
    Hazard Classes as per AS 1604 for specifying preservative treatment
  • 30. Decay: Resistance to Decay
    can be
    treated to
    ‘H’ level
    cannot be
    treated therefore
    dictated by
    natural durability
    Sawn Section
    Natural Durability Ratings apply to heartwood (or true wood) only – not sapwood
    Only sapwood can be effectively treated
    Limit non-durable timber to 20% cross-section (max)
  • 31. Examples of In-ground Durability
    Class 1 (highly durable)
    ironbark, tallowwood
    Class 2 (durable)
    spotted gum, blackbutt
    Class 3 (moderately durable)
    brush box, rose gum
    Class 4 (non durable)
    all sapwood for all species, Victorian ash, Tasmania oak, Oregon, radiata pine, slash pine, hoop pine
  • 32. Examples of Above Ground, Exposed Durability
    Class 1 (highly durable)
    • ironbark, tallowwood, river red gum, spotted gum, blackbutt, cypress
    Class 2 (durable)
    • rose gum, jarrah, yellow stringybark, Sydney blue gum , brush box
    Class 3 (moderately durable)
    • Victorian ash, Tasmanian oak, huon pine
    Class 4 (non durable)
    • all sapwood for all species, Oregon, radiata pine, slash pine, hoop pine
    For an extensive list of natural durability ratings check AS 5604 Timber – Natural durability ratings, or download the Timber Service Life Design Guide, at
  • 33. Service Life of Heartwood
    Based on in-ground graveyard stake trials and above ground ‘L’ joint trials
    Greater service life can be achieved with larger sections, relevant maintenance and/or preservative treatment
  • 34. When to Treat Timber?
    No Treatment required
    Immunise Lyctus Susceptible Sapwood
    Preservative Treat in accordance with AS1604 or Timber Marketing Act (NSW)
    (1) Incising the timber may assist to obtain an envelope treatment
  • 35. Suitable Preservatives
    (1) Southern waters only
  • 36. Insects
    The main insects of commercial significance to the durability performance of timber are:
    • Lyctus beetles, and
    • 37. Termites
    Lyctus beetles only attack the sapwood of some susceptible hardwood species.
    Termites can attack any cellulose based material and are more active and prevalent in northern Australia.
    Furniture beetles are also present in Australia, but are low risk.
    Termite damage to a non-termite resistant post in ground contact
  • 38. Insects: Termites
    There are two types of termites that can cause commercial damage to timber
    Drywood termites
    Do not require contact with the ground. They are present in and north of Cooktown (QLD). Naturally termite resistant or preservative treated timber should be used where drywood termites are prevalent.
    Subterranean termites
    Are by far the most significant insect pest for timber. They require contact with the ground (for moisture), therefore a range of termite management options are available including isolation from the ground.
  • 39. Insects: Termite Hazard Zones
    Termite hazard zones for Australia
    (Zone D has the greatest termite hazard)
  • 40. Insects: Resistance
    Timber structures are best protected from insect damage through:
    • correct design and construction practices
    • 41. accurate specification and supply
    • 42. appropriate species selection
    • 43. preservative treatment (where necessary)
    Timbers natural resistance to different insects varies widely from species to species
    AS 5604 Timber – Natural Durability Ratingsprovides lyctus and termite resistance ratings for a wide range of species, as well as other natural durability ratings
  • 44. Examples from AS 5604
  • 45. Insects: Lyctus Beetle Protection
    Australian Standard grading rules, for decorative and structural timber, limit the amount of lyctus susceptible sapwood that can occur in different products. In addition, susceptible sapwood can be preservative treated to make it ‘immune’
    In NSW, consumer protection legislation prohibits the sale of lyctus susceptible timber, which if present, should be treated
  • 46. Insects: Termite Protection
    The Building Code (BCA) requires termite protection for buildings in designated termite prone areas
    Effectively all States/Territories except Tasmania and some Victorian local authorities, are designated termite prone areas
    In termite prone designated areas, the BCA provides two options:
    Either all ‘primary structural elements’ (definition varies from State to State) are to be termite resistant (for timber refer to AS 3660.1), OR
    Termite protection shall be provided in accordance with AS 3660.1 which provides options and combinations including isolation, termite shields, physical and chemical soil barriers, termite resistant materials etc.
  • 47. Corrosion: Fasteners
    There are two types of corrosion:
    Typical installation of fasteners embedded in wood subjected to corrosion
    Note: red marks denote where corrosion is possible
    Typical fastener installation subjected to atmospheric corrosion
  • 48. Most metal fasteners for timber have a part that is in the timber and a part exposed to the atmosphere
    Embedded portion
    Corrosion of the embedded part will be dictated by:
    • the moisture content of the timber
    • 49. the natural pH of the timber
    • 50. electrolytic action that may occur due to the presence of preservatives such as copper in CCA or ACQ treated timber
    The sapwood in this pole has been treated with CCA and is causing accelerated corrosion of the galvanized plate
    Corrosion: Fasteners
  • 51. Exposed portion
    Corrosion of the exposed portion of the fasteners will be influenced by:
    • all of the embedded factors
    • 52. air-borne contaminants such as salt or other chemicals
    Macro climatic hazard influences on corrosion need to differentiate between embedded and atmospheric corrosion, and apply separate hazard maps
    The sapwood in this pole has been treated with CCA and is causing accelerated corrosion of the galvanized plate
    Corrosion: Fasteners
  • 53. Corrosion: Fasteners
    Corrosion of fasteners needs to be considered for the following reasons:
    Breakdown of wood
    Integrity of fasteners and connections
    Aesthetics (rusting, stains etc.)
    40 year old hot dipped galv. bolts in cross-arms
  • 54. Corrosion: Hazard Zones – Embedded Corrosion
    Hazard zones for embedded corrosion
    (Zone C is the most hazardous)
  • 55. Corrosion: Hazard Zones – Atmospheric Corrosion
    Coastal Zones Related to corrosion due to airborne salt
    (Zone E has the greatest hazard)
  • 56. Corrosion: Resistance
    Resistance to corrosion is best provided by selecting and using material with the required resistance to corrosion, appropriate to the intended life of the structure – refer to the following table.
    Cross-arm King bolt
    (hot dipped galvanized)
    after 35 years exposure
    in power pole.
  • 57. Corrosion: Resistance (cont.)
  • 58. To help reduce deterioration of timber around metal fasteners where moisture is present, consider:
    Avoiding joint details that trap moisture
    Using non-corrosive or protected metals (galvanized, coated, stainless steel or monel metals)
    Avoiding placing dissimilar metals in contact with each other (e.g. copper, as in CCA or ACQ treatments, with zinc, as in galvanized coatings)
    Greasing, coating or sheathing fasteners in contact with CCA or ACQ treated timber with shrink wrap plastic or bituminous or epoxy paints
    Countersink and plug or ‘stop’ fasteners
    Polymer coated screws.
    Corrosion: Resistance (cont.)
  • 59. Weathering
    Unprotected timber exposed to moisture and sunlight will undergo physical and chemical changes known as weathering
    Weathering is the result of:
    • surface erosion (this is slow – 6 mm to 13 mm per century)
    • 60. wetting/drying (causing swellingand shrinking)
    • 61. chemical change (caused by sunlight and oxygen)
    • 62. freezing and thawing in alpine areas
    20 year old deck and retaining wall
  • 63. Unpainted or unfinished timber, when exposed for an extended period will:
    • discolour on the surface
    • 64. check, crack and splinter
    • 65. become rough on the surface
    Weathering may cause cupping and warping and colours to bleach to a silvery grey or show surface stains or mouldgrowth
    Horizontal surfaces are more prone to weathering than vertical surfaces
  • 66. Weathering (cont.)
    Although hazard maps for weathering have not yet been developed, it is generally accepted that the effects of weathering are more severe and accelerated in harsher climates with greater extremes in temperature and rainfall
    Note the delineation in degree of weathering owing to shelter provided provided by the roof.
  • 67. Weathering: Resistance
    Protection from weathering can be maximised by:
    Applying appropriate finishes, including paints, stains and water repellants
    Regular maintenance with appropriate finishes
    Architectural and design detailing that specifies overhangs, capping, verandahs and shading
    Considering ventilation, water shedding and drainage during design and construction
  • 68. Weathering: Paints, Stains and Water Repellents
    The following should be considered in respect of finishes:
    • Pale colours absorb less heat, therefore drying effects and accelerated decay due to raised temperature are minimised
    • 69. Oil based paints are better ‘vapour’ barriers than acrylics
    • 70. Paint systems should include quality primers and undercoats – they are designed to seal the timber and provide a key for top-coats
    • 71. Stains and water repellants require more frequent application to maintain their effectiveness
    • 72. Sawn surfaces provide a better ‘key’ for stains and water repellants than dressed surfaces – particularly denser species
    The following table provides a summary of different finishes and maintenance.
  • 73. NOTES:
    Table is compilation of data from many researchers
    Using top quality acrylic latex paints
    With or without added preservatives for mildew/mould
    Common water repellant preservatives including copper naphthenate
    Weathering: Paints, Stains and Water Repellents
  • 74. Weathering: Design Detailing
    Appropriate design detailing can help reduce the impact of weathering and improve durability performance by:
    Shielding timber with overhangs, pergolas, capping, flashing, fascias and barges
    Isolating timber using sarking, cladding and DPC’s
    Avoiding moisture traps by using adequate ventilation, free draining joints and water shedding surfaces, particularly when end grain is involved
    Ensuring ventilation and insulation addresses condensation potential in warm and cold climates by using vapour permeable sarking or foil
  • 75. Weathering: Good Details
  • 76. Weathering Hazards: Good Details
  • 77. Weathering: Other Detailing
    Make allowance for shrinkage and minimise shrinkage restraint to avoid splitting
    For weather exposed horizontal members (decking, handrails etc.) limit open defects on the top surface and keep members ‘stocky’ – preferred breadth to depth ratio approximately 3:1 to 4:1
    Limit the use of wide boards – narrow boards expand and shrink less in response to moisture changes
  • 78. Maintenance
    The satisfactory performance of timber structures will often depend on adequate maintenance programs.
    Maintenance regimes for finishes have already been covered.
    Other maintenance issues to consider are:
    Termite barriers, inspection and replenishment of treatments
    Maintaining sub-floor ventilation
    Tightening bolts and fasteners
    Re-applying supplementary preservatives and sealants
    Cleaning - sweep or blow, don’t hose away (moisture issues)
  • 79. Marine Borers
    Where timber (piles, braces etc.) is in contact with marine waters (ocean, bay and tidal), specific hazards such as marine borers and organisms need to be considered.
    Factors that affect the level of marine borer hazard include:
    • Macro-climatic hazard zone (warmer waters, higher hazard)
    • 80. Water salinity
    • 81. Sheltering effects
    • 82. Construction details
    Aquaculture is a valuable wood use.
  • 83. Marine Borers: Hazard Zones
    Marine borer hazard zones (Zone G is the most hazardous)
  • 84. Marine Borers: Resistance
    Marine piles and timber in marine contact are best protected by:
    Using species with high natural resistance such as turpentine or in cooler southern waters, swamp box, river red gum or white mahogany
    Using preservative treated timbers (‘H6’) with wide sapwood bands such as spotted gum and plantation softwoods
    Using mechanical barriers and floating collars
    Piles encased in sand filled concrete pipe
  • 85. Marine Borers: Resistance
    AS 5604 provides an extensive list of species with their marine borer resistances where known – as seen in the extract below.
  • 86. Chemicals
    Timber is resistant to mild acids but strong acids (pH<2) and alkalis (pH>10) can cause degradation.
    Timber is ideally suited to special applications such as:
    swimming pool buildings
    chemical storage
    reservoir roofs etc.
  • 87. Fire
    Fire is covered by two separate presentations:
    Using Wood in Bushfire-prone Areas
    Fire Safety and Performance of Wood in Multi-Residential and Commercial Buildings
  • 88. Conclusion
    Timber has the ability to deliver design service life in a wide range of applications
    There are significant issues that need to be considered to appropriately design, specify and detail timber structures to ensure satisfactory durability
    These issues are well known and understood
  • 89. More Information
    Technical Design GuideTimber Service Life Design
  • 90. Learn more about wood at UTAS
    Centre for Sustainable Architecture with Wood
    Graduate Certificate in Timber (Processing & Building)
    4 units, part time, online
    Areas covered include:
    Wood science
    Design for durability and service for life
    Timber as a renewable resource
    Sustainable design and construction
    Engineered wood products
    International technologies and developments
    Plus, selected topics of individual interest
    More information: Associate Professor Greg Nolan
    (03) 6324 4478 or