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.
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
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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
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
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
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
