Timber Durability and Exterior Wood Finishes, presented by Professor Greg Nolan, University of Tasmania, School of Architecture & Design, September 2011.
This is the third in a series of three articles on the innovative use of timber in commercial and multi-residential buildings around the world.This information was collated from presentations at the WOOD SOLUTIONS 2011 SEMINAR SEPTEMBER 6TH, attended by John Scandurra from Scandurra Architects.
- The natural & treated durability of available species of timber;
- Effective Design & detailing for durability in exterior wood elements.
- Finishing exterior wood elements for enhanced appearance and service life.
References: “Design for durability” section of wood solutions website; AS1604: Treatments; AS5604: Durability.
Characteristics of timber:
- Timber is a natural material: its character is determined by species, form & growth over time. Timber has different properties which change even within the same species.
- Timber is a renewable resource: it requires both time and space to renew itself. The term sustainable means when the rate of use is less than the rate of renewal.
- Timber is anisotropic: it has physical properties which have different values when measured in different directions. It varies also in relation with its original location within the tree, in relation to the trees age, its source, and its species.
- Timber is hydroscopic: it has the ability to lose or gain moisture in response to fluctuations in the environment, i.e. it expands and contracts.
- Timber is biodegradable: it is capable of being decomposed by bacteria or by other living organisms.
- Weathering: Timber breaks down through weathering, fungi, insects, termites, marine organisms. Weathering is the greying or minor breakdown due to mechanical or chemical breakdown of the surface by: light, action of dust/sand, shrinkage/swelling due to moisture content change. The rate of breakdown is slow: its effects are usually limited to the surface at the rate of approximately 0.1mm per year. Weathering affects the appearance and performance of finishes and, eventually, its decay rate.
- Decay: decay rate varies with: wood’s character, moisture content (20% MC and over), ambient temperature (-5 to 60 degrees C). Decay rate varies with the climate. Termites are worst in Zone D areas.they are a warm weather species and so are rare in Tasmania and southern Victoria. Termites are cellulose eating insects.
- Hazard Classes for Timber, H1 – H6.
H3 Outside Above Ground: moderate wetting & leaching; decay borers and termites are biological hazards.
Timber resists hazards by:
- Natural durability, which varies with species & is rated in durability classes; and
- Applied treatment. Durability classification is rated 1-4, with Class 1 having most hazards resistance and Class 4 being sapwood.
To improve resistance to biodegradation, we introduce preservative chemicals into the wood. Chemicals are introduced in liquid form so the wood’s permeability limits its effectiveness. Fungus and insects are resisted by treatment, but treatment does not affect weathering.
Decay and termites are major contributors to timber breakdown. To reduce decay, keep timber dry, i.e below 25 degree moisture content.
Note: Heat treatment of timber improves its strength but this has not been proven in Australia.
All natural materials, such as stone, metal and timber, are not inherently stable materials (movement & decay). If specified, those properties need to be identified and the materials treated with a view to stabilising them, in order to enhance their durability and maintenance.
By researching more engineered versions of these materials, we should be able to manage out more of those maintenance problems. In Queensland, that especially applies to termites. For example, specifying LVLs in lieu of treated hardwood results in longer spans, straighter sections, and dimensional stability.
Their treatment for cellulose content is a subject for continued monitoring, with the view to a future when cellulose can be engineered right out of timber elements to the point where there is no need to specify termite barriers.
Project specific responses to external conditions.
Balance the response:
- Use durable timber species.
- Use coatings responsibly.
- Use durable fasteners.
Finish wood effectively:
- For uncoated timber: water mobilises soluble extractives in the timber.
- Surfaces weather: the rate of weathering is proportional; therefore, design surface exposure in response to aspect and prevailing site conditions (sun, wind, rain).
- Decay can be avoided by detailing for moisture control and ventilation.
Architectural Detailing strategies:
- Use wide overhangs to protect external walls from sun and rain exposure.
- Design façade surfaces to be consistent in their exposure, to prevent different rates of weathering along the same façade, resulting in different colouration
- Design for adequate ventilation to keep moisture content down.
- Detail cladding installation vertically, not horizontally, to prevent water settling on tops of cladding edges, and accelerating decay.
- Vary finishes according to suit the species, exposure and application.
- Refer the Zone D table for preferred species arrangement.
- Design and detail flashings to shed water away from the building facades.