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TIMBER
WOOD PRESERVATION
WOOD AS A BUILDING
MATERIAL
surrounded by a lighter rim of sapwood
(much lighter in some eucalypts), and then
the bark. Sapwood conducts fluid, but the
Softwoods and hardwoods
heartwood is generally drier and is
Trees are either softwoods (gymnosperms,
biologically dead. Metabolic by-products
or conifers) or hardwoods (angiosperms, or
(extractives) are deposited at the
flowering plants). Softwoods are not always
sapwood/heartwood boundary which
softer than hardwoods. The Australian
expands as the tree grows. Extractives
native cypress pine (Callitris) is a softwood
darken the heartwood and also make it
of greater density and hardness than
naturally durable due to their toxicity to
many hardwoods, while balsa (Ochroma)
fungi and to some termites and borers.
Wood identification
Wood structure
The characteristic cell structure of different
The microscopic structure of wood
woods can be seen with the aid of a
resembles a bundle of straws glued
microscope, enabling the genus or species
together. Each straw represents a cell
of tree to be identified from a 10 mm cube.
with a cellulose wall and a hollow centre
Correct identification allows accurate
(lumen), and lots of fine perforations
matching of colour and other physical
through the walls. The cell walls are
properties in replacement timbers.
impregnated with lignin, a natural polymer
that glues the cells together and gives
wood its strength. Most cells are oriented
longitudinally (parallel to the trunk of the
tree), with some radially aligned.
Wood density, hardness
and strength
Cells of dense woods have small lumens
and thick walls. When the wood is dry, the
In softwoods the longitudinal cells
lumen is largely air, and the proportion of
(tracheids) provide both structural strength
air to solid material is a measure of density.
and transport of fluids, while in hardwoods
It ranges from 25–30 per cent for grey or
these functions are performed by separate
yellow box (eucalypts), to about 70 per
cells, fibres providing strength and vessels,
cent for a low density wood such as red
fluid transport. The presence of vessels is
cedar (Toona australis), and nearly 90 per
one distinguishing feature of hardwoods.
cent for balsa.
Growth rings are most apparent in some
Hardness and strength generally increase
softwoods where the latewood of the
with increasing density. Wood is very strong
previous season differs greatly from the
in tension, when measured along the grain,
earlywood of spring. Latewood is harder,
because of the long chain cellulose
darker and denser than earlywood because
molecules in the walls of the longitudinally
latewood cells have thicker walls and
arranged cells.
smaller diameter lumens. While a growth
ring usually represents a year's growth,
climatic extremes and bushfires can disrupt
normal growing patterns.
WOOD DETERIORATION
There are four main agents of wood
deterioration: fire, fungal rot, borers and
termites. The effect of ultraviolet light is
Sapwood and heartwood
relatively shallow; it degrades exposed
A cross-section of a tree-trunk has
surfaces, turning them grey and weakening
heartwood (generally darker) in the centre,
the surface cells.
TIMBER
is a hardwood.
Wood preservation
Fire
are permanently damp and at risk.
Powderpost beetles (Lyctids) attack only the
Surprisingly, wood is not very flammable.
These should preferably be made from
sapwood of some hardwoods. They are so
It does not ignite readily except in small
naturally durable timber. The sapwood of
common that susceptible timber is
twigs, or when very dry and at high
most species is not resistant to fungal rot.
generally attacked in the first few years of
temperatures. Large timbers char slowly
and form a layer of charcoal which
protects the wood beneath and slows
the rate of combustion. Some timber
structures may therefore remain
serviceable after a fire.
To repair rotted structural timber, remove
the visibly decayed zone together with any
surrounding area affected to be certain of
removing all active fungi. Apply fungicides
to the remaining timber as a precaution.
service. Since sapwood is generally only a
small proportion of a timber section (it is
limited by law in New South Wales and
Queensland), Lyctids generally cause little
structural damage even if most of the
sapwood has been thoroughly burrowed.
Recently, a solid form of fungicide based
Powderpost beetle damage can be found
Fungal rot
on boron and fluorine has been devised
years after the attack has ceased, and
All rotting of wood is caused by fungal
instead of the normal liquid or paste, and
though initially alarming, once positively
attack. Fungi are minute organisms which
proved effective in treating wooden
identified it rarely requires further action.
live on and within wood and slowly digest
transmission poles. Inserted as small rods
the cell wall materials leading to softening
into pre-drilled holes, the fungicide diffuses
The name Furniture beetles (Anobiids)
and decay. All forms of fungal rot –
through the wood only when it becomes
suggests a preference for furniture but
including brown rot, white pocket rot, soft
wet and fungal attack is likely. Fungicidal
these beetles can also attack flooring and
rot, the simply named decay, and the
rods are suitable for these large section
framing timbers. They mainly attack the
confusingly named wet and dry rots –
timbers, but may weaken joints in smaller
sapwood of softwoods, preferring old well-
cause severe damage to timber in service
sections such as window frames.
seasoned timbers such as Baltic pine, and
damp humid conditions such as poorly
and are often not discovered until the
decay is well advanced.
Although fungal spores are common in the
air they cannot develop and attack wood
unless it has a moisture content in excess
of about 20 per cent by weight. Dry wood
will not rot. Preventing fungal attack is
Borers
Damage to timber by wood borers is
generally minor and rarely needs treating.
However, some borers can cause
considerable destruction. Knowing the
difference can save unnecessary treatment.
ventilated floors. Furniture beetles are a
hazard to old buildings and active
infestations must be treated. The loose
frass they produce is gritty like fine salt,
unlike that of powder post beetles which
is compacted in the galleries and is fine
and powdery.
simply a matter of keeping wood relatively
Most borers are beetles with a four-stage
dry. This, of course, is easier said than
life cycle beginning with eggs, which are
Pinhole borers infect only green timber and
laid in the vessels or in cracks in the wood
standing trees, and die out as the wood
surface. These hatch into the larval or grub
dries. All the damage they do is done
stage, and then burrow into the wood
before normal use of the wood, so no
producing a network of galleries which
action is required. Common among the
done; some timbers, such as fence posts,
may considerably weaken the timber.
pinhole borers are the ambrosia beetles
Adults develop from a pupal stage, burrow
whose galleries have a characteristic
out of the wood and fly off to breed.
dark stain.
The exit or flight holes of the adult are
generally the first signs that borers are
present, often with a fine dust (frass)
which is borer excrement.
The European house borer (Hylotrupes
bajulus) is an introduced pest which is
occasionally found in imported timber.
It attacks the sapwood of seasoned
When evidence of borers is discovered
softwoods and can cause major structural
three questions should be answered:
damage. Quarantine controls apply to
1 Is it still active? If not, control measures
will not be required.
An example of dry rot or fungal decay under a
staircase showing the fruit bodies that appear
when the fungus is mature and the waterconducting fungal strands.
imported timber to prevent further
infestations.
2 What type of borer was or is
responsible? Some are benign.
Control measures for active borer
3 What is the extent of damage and are
structural repairs required?
injection of insecticides; and fumigation
Some of the more important borers found
buildings. Handling and applying the
in Australian building timbers are:
chemicals involved need specialist skills.
infestations include spraying, coating and
with toxic gases, sometimes of complete
Wood preservation
Termites
piers and stumps and against the base of
Drywood termites
Unlike borers, termites are a major
masonry walls. Organochlorines, once
Unlike subterranean termites, drywood
hazard to wooden building structures and
commonly used for this purpose, are being
termites do not require continuous
contents. Often incorrectly called white
phased out around Australia owing to their
contact with the ground as they can obtain
ants, termites are social insects living in
high toxicity and persistence in the
moisture from the wood in which they
colonies numbering more than a million.
environment. In NSW they have been
feed and live. Native drywood termites are
Mature colonies have three main types
illegal since 1 July 1995. Less hazardous
found in northern Australia where the
of termite: reproductives, soldiers
chemicals (organophosphates) can still be
climate and wood are wetter: here they
and workers.
used, but require frequent reapplication.
can be a problem for buildings.
Wood-eating termites are classified
Physical barrier systems are an alternative
according to their living and feeding habits:
to chemicals. Two types now in use are
dampwood, tree-dwelling, subterranean
stainless steel mesh and crushed granite
and drywood termites. The last two are
screenings. The holes in the mesh and
the most common types found in buildings.
between the granite particles are too small
for termites to crawl through, and the
Subterranean termites
Subterranean termites live largely
underground, building their nests in old
tree stumps and root systems. Some forms
build prominent nest mounds which may be
steel and granite are too hard for the
More significant are several introduced
species, particularly the West Indian
termite which has been found in several
places between Sydney and Brisbane.
Whereas subterranean termites produce a
dusty frass, drywood termites have
distinctive faecal pellets. They are usually
treated by fumigating the whole building
termites to chew. Though principally
with methyl bromide, a procedure
designed for use in new buildings, these
requiring specialist advice and skills.
methods may have some applications in
protecting existing buildings.
up to three metres high. From the nests,
tunnels or galleries radiate outwards to
When termites are discovered
sources of food which may be trees, fallen
If active termites are discovered they
logs, posts, or stumps of timber buildings.
should not be disturbed. Cover any breach
Subterranean termites need a dark, moist
in their shelter tubes with mud, and seal
environment, such as a poorly ventilated or
breakthroughs into galleries in timber
drained sub-floor area, and cannot survive
sections immediately with masking tape
for long in the open sunlit air.
or similar adhesive. Then seek specialist
Termites can extend their tunnel system
above ground by constructing shelter
tubes of mud in search of more food.
These tubes may run up the side of a
advice from someone with appropriate
skills (such as an entomologist) to identify
the termite species and hence likely
nesting habits.
Subterranean termite damage in floorboards
building pier or stump or over the base
If the nest can be located (relatively
of masonry walls to a timber floor above.
straightforward for mound-building
From the flooring, termites may invade
species) it should be destroyed or treated.
wooden components throughout a
Colonies whose nests cannot be located
building, including the roof framing.
are best treated with arsenic dust, a slow
The first sign of activity is often the
acting poison that is carried back to the
Keep it dry!
collapse of floorboards.
nest by the foraging termites and will
Keeping wood in buildings dry will
slowly kill the whole colony. Household
significantly reduce the risk of:
PREVENTIVE
CONSERVATION
Termite control
insecticides may kill termites locally but
Good building construction incorporates
are most unlikely to reach the nest and
• swelling and shrinkage cracking
termite shields (ant caps) on stumps and
eradicate the colony.
• fungal rot
piers. These are not intended to prevent
termites entering the timber above, but to
Arsenic dust is highly toxic and should only
• attack by termites and some borers.
be handled by someone with appropriate
Ensure that roof drainage, guttering and
training. After treatment the building should
stormwater disposal systems are working
be carefully monitored for signs of termite
properly, that there are no plumbing leaks,
activity. Once activity is extinct, the
and that any surface water is drained well
For many years the standard barrier
damaged timbers should be conserved or
away from walls. Underfloor spaces should
against termites has been insecticidal
replaced, and consideration given to the
be kept well ventilated.
chemicals sprayed into the soil around
installation of some form of barrier.
slow them down and force them to build
shelter tubes out and around the shields,
where the tubes can easily be seen.
Wood preservation
This edition was adapted by Heritage
Keep it coated
Beesley, J. 1987, Wood deterioration in buildings:
Victoria for the Heritage Council Victoria
Coatings such as paints, varnishes, waxes
A guide to the identification and treatment of
and oils are the principal means of
wood deteriogens, Technical Bulletin 7.1,
controlling swelling as well as protecting
Australian Council of National Trusts,
Heritage Victoria
Level 22, 80 Collins Street, Melbourne.
Tel. 9655 6519
and enhancing timbers. Most coatings act
as barriers and prevent water penetration
National Trust of Australia (Vic.), Melbourne.
Bootle, K.R. 1983, Wood in Australia:Types,
properties and uses, McGraw-Hill, Sydney.
www.heritage.vic.gov.au
into the porous cellular structure of
www.doi.vic.gov.au/heritage
wood. End grain, a cross-section through
Creffield, J.W. 1991, Wood destroying insects:
the original fluid transport system of the
Wood borers and termites, CSIRO
wood, is especially vulnerable and requires
Publications, East Melbourne,Vic.
Acknowledgments
thorough coating. However, no coatings
This publication was produced in
are totally impermeable to air or water
accordance with resource-sharing
vapour and so moisture can still enter and
arrangements approved by the Australian
other common timber pests, New South Wales
leave the wood slowly. Small checks or
University Press, Kensington, NSW.
Heritage Information Network.
splits in paint coatings are a sign that the
It was first published as Wood Preservation
by the Heritage Council of New South
wood is swelling and shrinking excessively
and that repainting is needed.
Phillips, M.W. & Selwyn, J.E. 1978, Epoxies for
wood repairs in historic buildings, Technical
Preservation Services Division, Heritage
Wales Technical Advisory Group for
Material and Conservation March 1998.
Hadlington, P.W. 1987, Australian termites and
Conservation and Recreation Service, US
Regular maintenance inspections
Department of the Interior, US Government
Wooden items need regular maintenance,
Printing Office, Washington, DC.
and should be inspected every six months.
Standards Association of Australia 1993
Disclaimer
Look in subfloor spaces for signs of rot
Any representation, statement, opinion
and termites in the flooring and framing.
or advice expressed or implied in this
In roof spaces, look for evidence of leaks
publication is made in good faith but on
that may promote fungal growth.
the basis that the State of Victoria, its
Any sign of breakage or distortion of
Stumes, P. 1979,W.E.R.– System manual: Structural
agents and employees are not liable
roof structures should be investigated
rehabilitation of deteriorated timber, Association
(whether by reason of negligence, lack of
by a structural engineer. Check external
care or otherwise) to any person for any
paint finishes for splitting or cracking that
damage or loss whatsoever which has
may indicate water penetration to the
occurred or may occur in relation to
wood beneath.
representation, statement or advice
referred to above.
December 2001
Association of Australia, Homebush, NSW.
for Preservation Technology, Ottawa.
Townsend, A. & Martyn, M. n.d. The repair of
wood windows, Technical Pamphlet 13,
Society for the Protection of Ancient
FURTHER READING
Verkerk, R. 1990, Building out termites: An
Ashurst, J. & Ashurst, N. 1988, Wood, glass
Australian manual for environmentally responsible
and resins; and technical bibliography, Practical
control, Pluto Press Australia, Sydney.
building conservation, vol. 5, English
Heritage Technical Handbooks, Gower
ISBN 0 7311 8726 1
buildings from subterranean termites, Standards
Buildings (SPAB), London.
that person taking or not taking (as the
case may be) action in respect of any
Australian Standard AS 3660–1993; Protection of
Technical Press, Aldershot, UK.
Wilcox, W.W., Botsai, E.E. & Kubler, H.
1991, Wood as a building material: A guide for
designers and builders, John Wiley & Sons,
New York.
www.heritage.vic.gov.au