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Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
Making it last: Timber durability and exterior wood finishes by Greg Nolan
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Making it last: Timber durability and exterior wood finishes by Greg Nolan

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Making it last: Timber durability and exterior wood finishes …

Making it last: Timber durability and exterior wood finishes

Assoc. Prof Greg Nolan, University of Tasmania, School of Architecture & Design

Durable timber design is a sustainability issue as good timber used outside shouldn't suffer due to poor selection and detailing. There are broad guidelines for balancing species performance, site conditions, architectural intent in finishes and maintenance, and an effective building service life.

Published in: Self Improvement, Technology
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  • 1. Greg NolanMaking it last: Timber durability and exterior wood finishes.
  • 2. Introduction
    Timber’s characteristics
    Its durability
    Design for durability
    Finishing exterior wood
    Key recommendation
    Designing timber cladding and exterior structures for durability is a sustainability issue
    Vertical board cladding
  • 3. Timber is a natural material
    Natural: existing in or caused by nature; not made or caused by humankind
    Timber is a natural product, drawn from the wood in trees
    Its character is determined by the species of tree and the form and growth of the wood in it over time
    Regrowth in a native forest
  • 4. Timber is a renewable material-conditionally
    not permanently depleted when used
    Renewable resources are:
    natural forces or
    organic materials - things that can be harvested and regrown
    Most renewable building materials are things grown
    They require both time and space to renew themselves
    They can be used sustainably if the rate of use is less than the rate of renewal
    Wood falls into this group
    Plantation pine
  • 5. Timber is variable (anisotropic)
    Anisotropic: having physical properties that have different values when measured in different directions
    It also varies with its original location in the tree, and the tree’s age, source, and species
    zone of wood property change
    zone of unchanging wood properties
    age of deposited wood – years
  • 6. Timber is hygroscopic
    Hygroscopic: the ability to lose or gain moisture content with fluctuations in environmental humidity
    When harvested, timber holds a significant volume of water
    After seasoning, timber absorbs or loses moisture to remain in equilibrium with the surrounding atmosphere
    As it absorbs moisture, it expands
    As it loses moisture, it shrinks
    Hardwood drying in racks
  • 7. Timber is biodegradable
    Biodegradable: capable of being decomposed by bacteria or other living organisms
    Wood can be broken down by:
    weathering
    fungi (or decay)
    insects and termites
    marine organisms
    The impact of these mechanisms vary with:
    exposure to hazard
    the nature of the wood
    Lyctid borer attack in sapwood
  • 8. Timber’s weathering
    The greying and minor cracking of timber due to mechanical or chemicalbreakdown of the surface by:
    light
    the action of dust and sand
    shrinkage and swelling due to moisture content changes.
    The breakdown rate is slow with effects often limited to the surface
    ~ 0.1 mm per year depending on species and board orientation
    Weathering affects appearance, the performance of finishes and eventually, decay rate
  • 9. Timber’s decay
    Decay is the decomposition of wood by fungi. Decay rates vary with:
    The wood’s character
    Its moisture content (20% MC and above)
    The ambient temperature (~ 5° to 60°C)
    • It can occur most readily in timber kept regularly moist
    It tends to attack the moisture-permeable end-grain most vigorously
    Decay hazard is often assessed above ground and in-ground contact
  • 10. Decay rate varies with climate:
    Zones of decay hazard
    Above ground decay hazard zones
    In-ground decay hazard zones
    Zone D has the greatest decay hazard potential.
    Source: FWPA 2010 Timber service life design guide
  • 11. Attack by termites
    Termite hazard zones
    Termites are cellulose-eating insects that occur in all parts of Australia
    They are rare in Tasmania and parts of Victoria
    Some species build nests in the ground, building cavities and other locations, travelling to edible cellulose in moist earth galleries
    Zone D has the greatest termite hazard.
    Source: FWPA 2010 Timber service life design guide
  • 12. Hazard classes for timber
  • 13. Timber’s resistance to hazards
    Timber resists hazards by
    its natural durability
    any applied treatment
    Natural durability:
    varies with species
    is rated in durability classes in-ground contact and above-ground
    All sapwood is rated Class 4
  • 14. Timber’s natural durability& life expectancy
    The ratings in this table are based on expert opinions and the performance of the following test specimens:
    (a) In-ground: 50 × 50 mm test specimens at four sites around Australia.
    (b) Above-ground: 35 × 35 mm test specimens at eleven sites around Australia.
  • 15. Treatment extends the product suite
    Material susceptible to biodegradation can be treated with preservative chemicals
    The toxicity and amount of chemicals retained governs the protection level
    The target chemical retention is set for the intended Hazard Level
    As the chemicals are carried in a liquid, wood’s permeability limits the effectiveness of treatments
    It is very hard to reliably achieve the target retentions in heartwood
  • 16. Design for durability
    Control moisture retention and exclude termites.
    Consider the specific project requirements
  • 17. Design for durability
    Decay and termites are the major contributors to timber’s breakdown
    Exclude termites
    To reduce decay, keep the timber dry
    Decay fungi needs the timber’s moisture content to be over 20% to survive
  • 18. Reducing decay
    Keep the timber dry
    Use a roof, eave or flashing
    Exclude & shed water
    Bevel horizontal surfaces.
    Use end-flashings
    Ensure the wood can dry out if it gets wet
    Allow ventilation around elements and joints
    Limit direct timber to timber contact
  • 19. Project-specific responses
    There is a relationship between:
    The hazard presented by the external application
    the conditions likely on the site
    the exposure of the element
    their detailing to limit water retention
    The natural or treated durability of the selected timber
    The consequence of deterioration
    the desire service life
    the criticality of failure
    the required appearance of the finished timber over time
    The intended management regime
  • 20. Balance the response
    Use durable species externally or treat selectively
    The most durable timber comes from mature material
    Treatment extends the resource and service life considerably
    Use coatings responsibly
    Coatings extend service life but require maintenance
    Use durable fasteners
    Fastener corrosion can be the limiting factor
  • 21. Finishing wood externally
  • 22. Finishing systems options
    Options include:
    • Natural (uncoated).
    • 23. Exterior clear coating.
    • 24. Semi-transparent oil.
    • 25. Pigmented exterior coating.
    • 26. Exterior stain.
    • 27. Paint system.
  • Finishing systems: natural
    The timber is exposed uncoated to the natural environment.
    Initially, water mobilizes soluble extractives in the timber.
    Surfaces weather.
    The rate is proportional to exposure.
    Decay can be avoided with detailing for moisture control and ventilation.
    Uncoated timber cladding
  • 28. Detailing is vital
    Use wide eave overhangsor verandas to exclude moisture.
    Keep surface exposure consistent.
    Provide adequate ventilation.
    Prefer vertical rather than horizontal cladding on exposed surface.
    Manage the splash zone from surrounding surfaces.
    Bevel upper surfaces of elements.
    Round arrises
    Project drip flashings over doors and windows.
    Minimise joins in horizontal cladding likely to retain moisture.
    Use the recommended size, pattern and quality of fixings.
  • 29. Mixing finishing systems
    The type and level of finish in a project should vary to suit the species, exposure and application
    Painted timber windows in the same building
    Unfinished Durability Class 1 sun screens
  • 30. Selections must suite the project
    Preferred species arrangement for windows and doors in commercial projects and Exposure Zone D residential projects
    Source: Nolan 2010, Timber Window and Doors Guide
  • 31. Key recommendations
  • 32. Key recommendations
    Specify, detail and finish carefully
    Bad detailing should not expose good timber to accelerated decay
    Detail timber in external applications to:
    Keep the timber dry
    Exclude & shed water
    Ensure the wood can dry out if it gets wet
  • 33. Key recommendations
    Use durable species externally or treat selectively
    The most durable timber comes from mature material
    Treatment increases the range of uses of low durability timbers
    Use coating responsibly
    Coatings extend service life but require maintenance
    Use durable fasteners
    Fastener corrosion can be the limiting factor

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