Greg Nolan

Making it last: Timber durability and exterior wood finishes.

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

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

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 groupPlantation pine

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 changezone of unchanging

wood properties

age of deposited wood – years

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

Timber is biodegradable

Biodegradable: capable of being decomposed by bacteria or other

living organismsWood can be broken down by:• weathering• fungi (or decay)• insects and termites• marine organismsThe impact of these mechanisms vary with:• exposure to hazard• the nature of the wood

Lyctid borer attack in sapwood

Timber’s weathering

• The greying and minor cracking of timber due to mechanical or chemical breakdown 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

• 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

Timber’s decay

Decay rate varies with climate:Zones of decay hazard

Zone D has the greatest decay hazard potential.Source: FWPA 2010 Timber service life design guide

Above ground decay hazard zonesIn-ground decay hazard zones

Attack by termites

Zone D has the greatest termite hazard.Source: FWPA 2010 Timber service life design guide

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

Hazard classes for timber

Hazard Class Exposure Service Conditions Biological Hazard

H1 Inside above ground

Fully Protected, Well ventilated Borers Only

H2 Inside above ground

Protected from Wetting, Nil leaching Borers and termites

H3 Outside above ground

Moderate wetting and leaching

Decay borers& termites

H4 Outside in ground

Severe wetting & leaching

Severe decay, borers & termites

H5 Ground contact

Extreme wetting, leaching &/or critical

useVery severe decay,

borers and termites

H6 Marine waters Nth & Sth

Prolonged immersion in sea water

Marine wood borers and decay

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

Timber’s natural durability& life expectancy

Class Probable in-ground life expectancy (years)

Probable above-ground life expectancy

(years)Example species

1 Greater than 25 Greater than 40 Ironbark, Tallowwood

2 15 to 25 15 to 40 Spotted gum, Blackbutt, WRC

3 5 to 15 7 to 15 Brush box, Southern blue gum, Messmate

4 0 to 5 0 to 7 Vic ash, Radiata pine, Douglas fir

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.

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

Design for durability

Control moisture retention and exclude termites.

Consider the specific project requirements

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

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

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

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

Finishing wood externally

Finishing systems options

Options include:• Natural (uncoated).• Exterior clear coating.• Semi-transparent oil.• Pigmented exterior

coating.• Exterior stain.• 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

Detailing is vital

• Use wide eave overhangs or 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.

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 buildingUnfinished Durability Class 1 sun screens

Selections must suite the project

Element Relative Exposure Building Exposure External Finish Timber

Sill High Normal Painted or stained Durability Class 1 or 2 timber

Sill High Normal Painted Durability Class 1 or 2 timber or commercially treated LOSP hardwoods

Frame (except sill)

Medium Normal Painted or stained Durability Class 1 or 2 timber, commercially treated LOSP hardwoods, orVPI boron treated hardwoods

Frame (except sill)

Medium Normal Painted Durability Class 1 or 2 timber, or commercially treated LOSP hardwoods, orVPI boron treated hardwoods, orH3 treated softwood

Sash or door Medium Sheltered Unfinished Durability Class 1 or 2 timber

Sash or door Medium Normal Painted or stained Durability Class 1 or 2 timber, or commercially treated LOSP hardwoods, orVPI boron treated hardwoods, orH3 treated softwood

Sash or door Medium Normal Painted Durability Class 1, 2 or 3 timber, orH3 treated softwood

Preferred species arrangement for windows and doors in commercial projects and Exposure Zone D residential projects

Source: Nolan 2010, Timber Window and Doors Guide

Key recommendations

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

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