Greg NolanMaking it last: Timber durability and exterior wood finishes.<br />
Introduction<br />Timber’s characteristics<br />Its durability<br />Design for durability<br />Finishing exterior wood<br ...
Timber is a natural material<br />Natural: existing in or caused by nature; not made or caused by humankind<br />Timber is...
Timber is a renewable material-conditionally<br />not permanently depleted when used<br />Renewable resources are:<br />na...
Timber is variable (anisotropic)<br />Anisotropic: having physical properties that have different values when measured in ...
Timber is hygroscopic<br />Hygroscopic: the ability to lose or gain moisture content with fluctuations in environmental hu...
Timber is biodegradable<br />Biodegradable: capable of being decomposed by bacteria or other living organisms<br />Wood ca...
Timber’s weathering<br />The greying and minor cracking of timber due to mechanical or chemicalbreakdown of the surface by...
Timber’s decay<br />Decay is the decomposition of wood by fungi. Decay rates vary with:<br />The wood’s character<br />Its...
Decay rate varies with climate:<br />Zones of decay hazard<br />Above ground decay hazard zones<br />In-ground decay hazar...
Attack by termites<br />Termite hazard zones<br />Termites are cellulose-eating insects that occur in all parts of Austral...
Hazard classes for timber<br />
Timber’s resistance to hazards<br />Timber resists hazards by <br />its natural durability<br />any applied treatment<br /...
Timber’s natural durability& life expectancy<br />The ratings in this table are based on expert opinions and the performan...
Treatment extends the product suite<br />Material susceptible to biodegradation can be treated with preservative chemicals...
Design for durability<br />Control moisture retention and exclude termites.<br />Consider the specific project requirement...
Design for durability<br />Decay and termites are the major contributors to timber’s breakdown<br />Exclude termites<br />...
Reducing decay<br />Keep the timber dry<br />Use a roof, eave or flashing<br />Exclude & shed water<br />Bevel horizontal ...
Project-specific responses<br />There is a relationship between:<br />The hazard presented by the external application<br ...
Balance the response<br />Use durable species externally or treat selectively <br />The most durable timber comes from mat...
Finishing wood externally<br />
Finishing systems options<br />Options include:<br /><ul><li>Natural (uncoated).
Exterior clear coating.
Semi-transparent oil.
Pigmented exterior coating.
Exterior stain.
Paint system.</li></li></ul><li>Finishing systems: natural<br />The timber is exposed uncoated to the natural environment....
Detailing is vital<br />Use wide eave overhangsor verandas to exclude moisture.<br />Keep surface exposure consistent.<br ...
Mixing finishing systems<br />The type and level of finish in a project should vary to suit the species, exposure and appl...
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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

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.

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

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

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