Education - Hazards

Hazards

Hazards are anything that may cause deterioration of timber during service, and therefore reduce the long-term expected performance of the timber. They include a range of biological and physical or environmental hazards. Timber can be effectively protected from all hazards through appropriate detailing, treatment and maintenance.

Further information on improving timbers durability for all hazards can be found in the section on strategies to improve durability .

Mechanical degradation

Insects - Termites

Fungi

Marine organisms

Hazard classes

Environmental conditions have been classified according to how quickly timber is likely to deteriorate in that environment. This classification addresses the question of whether or not the wood is in contact with the ground, exposed to the elements or a potentially damaging industrial environment. They range from Hazard class 1, a completely protected environment, to Hazard class 6, marine water. A range of hazards will be examined first.

Weathering

Continued wetting and drying of timber causes cyclic swelling and shrinkage , which can lead to cracking and accelerated deterioration of timber, as the cracks allow water deeper inside the timber.

The strategy to minimise the impact of weathering is to protect the timber from moisture and direct sunlight. This is achieved using effective sealants - usually paints. Detailing so that crucial elements are shielded from moisture and direct sunlight also helps.

Continued maintenance is a vital part of protection against the effects of weathering. Where paint has cracked so that water can get under the paint, it can impregnate the wood, cause swelling, and remain trapped there by the paint that is still intact. The swelling not only causes cracking in the wood, but further damages the paint. Paint must remain a watertight seal to remain effective as protection from weathering for wood.

Chemicals

Compared with other building materials, timber has quite effective resistance to chemical attack. This makes it a suitable material for use in acidic or salty environments, provided fasteners can be protected against degradation.

Timber has been used in many industrial environments, in the chlorine-rich environment of swimming pool enclosures, and in marine environments provided marine organisms can be kept at bay. Other examples of timber applications where its resistance to chemical attack is of benefit include:

Waste landfill piling (soils and ground water is usually acidic)
Tanneries
Vats
Piles or facings in swamps
Chemical storage buildings
Water treatment works

There are two main methods of chemical degradation:

Interaction between the chemical and the constituents of wood – Timber is resistant to mild acids. However, strong acids (ph less than 2) and strong alkalis (ph greater than 10) can cause degradation of the timber. The degree of degradation is dependent upon the species of timber (softwoods are more resistant to attack by acids and alkalis than hardwoods), type of chemical (oxidising acids are worse than non-oxidising) and exposure conditions.
Chemicals that cause swelling of the wood may lead to mechanical degradation – Chemicals that do not swell timber, such as petroleum oils and creosote have no appreciable effect on timber properties. However, water, alcohol and some other organic fluids that swell but not attack timber may reduce some properties. Although, when these liquids are removed, swelling will be reversed, and properties regained. Constant wetting and drying may cause the timber to crack.

It is advisable to receive expert advice on the specific conditions in which the timber will be used, as many factors to be considered when designing structures in adverse chemical environments.

Timbers natural chemical resistance can be enhanced further by impregnating with:

Phenolic resins (improves acid resistance)
Furfuryl alcohol (increases alkaline resistance)
Monomeric resins followed by polymerisation (greatly improves chemical resistance)

Fire

Wood burns! Critical elements need to be protected from fire so that they can continue to perform their safety roles during a fire. The three main fire protection methods for timber elements include:

Having sufficient cross-section of timber in critical members to give strength during fire in spite of the loss of some timber from the outside due to burning. The degree of over-size can be calculated from expected charring rates, fire duration and load reduction during the fire.
Insulation of hidden members with fire-resistant and insulating materials such as fire rated plaster-board. Systems can be designed and tested to prevent burning of hidden framing members under fire conditions. <link to MRTFC>
Prevention of spread of fire by using intumescent coatings or fire retardant treatments. Sprinkler systems also lower the temperature and may prevent fully developed fires from occurring. These strategies seek to prevent fires getting hot enough to reach “flash-over” temperature. This will prevent ignition of many timber elements.

Any of these methods must be included at the design stage. It is often too late to fit them once the building is completed.

Protection with sprinkler systems gives some concessions on passive systems for certain classes of buildings within the BCA.

For a system to return satisfactory performance it must have insulating qualities, be able to function at elevated temperatures itself and resist the impact of flame without deterioration for some time. It is possible to have timber systems that satisfy all of these requirements.

Fire ratings

The Building Code of Australia (BCA) addresses fire ratings in the same way for all building materials. Similar systems are used elsewhere in the world as well.

The Fire Resistance Level (FRL) is a set of three numbers. Each number represents the number of minutes that an element can withstand a “standard” fire while still delivering appropriate performance for the fire situation. The objective is to provide a building in which the fire can be contained in one fire compartment separated from others by fire rated partitions. The fire authorities must be able to access the burning building after the fire has taken hold to evacuate occupants and to fight the fire. They must be able to do this for a given period of time in relative safety.

To establish the FRL, standard furnace tests are carried out on building systems. The systems are loaded, and thermocouples measure the temperature on all of the surfaces. Timers are used to measure the time it takes for the system to cease to perform satisfactorily.

The first number is the number of minutes that the system can carry the appropriate fire load without collapsing.
The second number is the number of minutes before fire breaks through the cladding of the system. This represents a breach of the fire compartment.
The third number is the number of minutes before the temperature on the non-fire side of the system reaches an unacceptable level.

Corrosion

Most timbers are slightly acidic (ph 3 to 6), therefore when moisture is present and the metals in contact with the timber have a low resistance to corrosion (eg. unprotected steel), chemical reactions occur that cause a loss of strength in the surrounding timber. Dark staining of the timber around steel fasteners is evidence that this has happened.

Particular care is needed when fastening to some treated timbers. Impregnated chemicals can change the electronegativity of the timber so that it may precipitate corrosion of even galvanised steels. Special inert coatings are required for these fasteners.

To prevent deterioration around metal fasteners:

Use non-corrosive or protected metals ( eg galvanised or plated)
Countersink and plug fasteners to prevent moisture traps
Avoid the use of dissimilar metal for fasteners
Grease, coat or sheath fasteners in contact with CCA treated timber

Mechanical degradation

Many timber applications may require specific design to resist mechanical degradation caused by the friction and knocks of general use. These applications include:

Stair treads, door sills
Flooring
Decking
Subbing strips/fenders
Water cooling tower baffles

Species with high densities (in excess of 800 kg/m³) and hardness qualities can provide satisfactory performance for many of these applications.

Insects

Insects that may cause damage to structures or contents include

Termites
Lyctids
Furniture beetles

Termites

Termites have existed for approximately 50 million years, and are commonly known as white ants in many parts of the world. There are more than 300 species of termites in Australia, however as most eat only grasses, only around 15 species are of economic importance in respect of the damage to timber they may cause. There are two types of termites that may attack buildings in Australia:

Subterranean mastotermes
Dry wood cryptotermites

Local advice on termite prevalence and protection is available from local government agencies or from local departments of Agriculture.

The subterranean termites are distributed throughout most of Australia and are the most voracious. They require warm, moist conditions to survive, and most often nest in soil. Attack on buildings is usually initiated from a nest in the ground from which the termites build galleries over piers or walls to attack the cellulose in the building. Usually the nest is outside the building perimeter, but occasionally a nest may be buried in the soil beneath the building. Brick, steel and concrete construction by itself does not offer protection against termites, which may gain access via wall cavities and fine cracks in mortar or slabs. Termites in the building must remain in contact with the soil (for moisture) and with the central nest and are generally most active in the summer months.

In rare cases, a nest may be established inside the buildings as an offshoot from the original colony. This can occur where a source of permanent moisture is available to the termites within the building (eg. leaking plumbing) and in this situation there may not be contact between nest and soil.

The best protection against them is to provide simple and inexpensive measures during construction to eliminate the presence or trapping of moisture, to provide proper ventilation to enable the drying out of timber, and enable inspection of cavities, floor and roof systems. <Link to protect timber from pests>

Drywood termites do not require contact with the ground and as there is little external evidence of infestation, they are difficult to detect. Although prevalent in the Pacific region, preventative measures against drywood termites are only necessary in the tropical northern and north east coastal and adjacent tableland areas. The use of termite resistant species such as cypress, ironbark and other Durability Class 1 species <Link to natural durability>, regular inspection and/or preservative treatment <Link to strategies to improve durability> provides protection in these susceptible regions.

Lyctids

Although widespread throughout Australia, this beetle which lays its eggs in empty timber cells so that their lavae can feed on the wood, doesn’t usually cause concern to users of timber. Softwoods are immune from attack and it is only the sapwood of some hardwood species is susceptible to attack. The cells have to be both open and large enough for the beetles to be able to lay their eggs in them. They physically cannot access the full cells in the heartwood, and in some species, even the sapwood cells are too small.

Australian grading Standards (both structural and appearance grades) limit the amount of sapwood that can be present in timber products, and some state legislation may require all sapwood to be treated to at least H1, to prevent it being susceptible to lyctid and other borer attack.

Furniture beetles – Anobium and Queensland Pine Beetle

These insects also pose only a minor threat to timber members.

Anobium occurs in southern Queensland and NSW. It may attack all softwoods and some lower density hardwoods.
Queensland Pine Beetle is restricted to an area east of the dividing Range in S-E Queensland.

If protection is required for critical timber elements, then preservative treatment should be considered for susceptible timber species.

Fungi

Fungi can be broadly grouped into two main types:

Moulds and stains (blue stain) which usually only affect appearance
Rot fungi (white, brown or soft) can significantly reduce strength properties as they penetrate and break down the cell walls of wood.

Fungi spores are wind borne and are almost everywhere. Even so, fungi do not attack timber unless the conditions for its growth are ideal. Fungi require the following four conditions to be present simultaneously (removing one of these conditions will prevent fungal attack.)

Moisture – 25%+ moisture content
Oxygen – completely submerged or saturated timber, or timber that is 600mm or more below ground is rarely attacked by fungi.
Temperature – 25º to 40ºC is ideal.
Nutrients – particularly sapwood which contains sugars and carbohydrates

Timber is best protected from fungal attack by eliminating contact with moisture, specifying a species with appropriate natural durability, or treating with preservatives to a level appropriate to the hazard.

Marine organisms

The greatest threats to timber in marine environments (usually piles in marine or tidal waters) are bivalve molluscs (toredo borer or shipworm) and crustaceans (Limnoria or wood lice), particularly in warm waters, The molluscs tunnel into the timber for shelter and their damage can go unnoticed until it becomes extensive. Crustaceans only burrow to beneath the timber surface, and their damaging effect would be negligible except for the surface breakdown caused by wave and mechanical action. This is followed by additional burrowing until an ‘hourglass’ shape is formed.

It should be noted that marine piles are subjected to three zones of durability hazard. The bottom ends are usually embedded in mud and are free from hazard while the section above high water mark must withstand weathering and mechanical degradation. The greatest hazard occurs in the zone between the mud line and the high water mark where the previously mentioned marine organisms are most active.

Protection for marine piles or timber in contact with sea water includes:

Using species with high natural resistance eg turpentine, satinay, cypress pine and jarrah. These species are naturally resistant either because of high silica content or naturally toxic nature, and may provide up to 5 years additional protection if the bark is left intact.
Impregnate timbers with chemical preservatives
Using chemical and/or mechanical barriers

Hazard classes

Environmental hazard levels can be rated. All environments can be assessed for their exposure to biological hazards and given a classification from H1 to H6.

These classes are detailed in AS1604, and the classification is essentially underpinned by studies of fungal attack on timber in various environments. Study buildings in all Australian states have been used to confirm the hazard classes. Similar hazard classes for timber are used in most other parts of the world.

H1 - is an environment in which the timber is completely protected from moisture (an essential agent for biological growth) and is protected against termite attack; typical of the timber used internally and protected by a termite barrier.

H2 - is an environment in which the timber is protected from moisture, but where there is the possibility of termite attack. It is typical of external, but shaded timber.

H3 - is an environment in which the timber is exposed to rain and sun, but is well above ground and well ventilated. This is typical of most externally used timber in buildings.

H4 - is an environment in which the timber may be in contact with the ground, and in a low hazard environment. The low hazard environment simply means dry – similar to most conditions under houses. This classification is typical of external timber used in sub-floor structures, but not horticulture and fencing.

H5 - is an environment in which timber is continually in ground contact. This is appropriate for buried timber elements such as fence posts, piles and utility poles, retaining walls and bridge abutments.

H6 - is an environment in which timber is in contact with seawater. This includes wharf structures, and here the risk is from marine organisms.

Hazard Level	Exposure	Service Conditions	Biological Hazard	Typical Uses
H1	Inside above-ground	Completely protected from the weather, well ventilated and protected from termites	Insects other than termites ie. lyctid or anoiid	Framing and flooring, furniture and interior joinery
H2	Inside above-ground	Partially protected from wetting. Nil to slight leaching	Borer, termite and/or moderate decay	Framing and flooring used in dry or damp situations
H3	Outside above-ground	Subject to periodic wetting and leaching	Moderate decay and/or borers and termites	Weatherboard, fascia, window joinery, exterior framing and decking
H4	Outside, in-ground	Subject to severe wetting and leaching	Severe decay, borers and termites	Fencing, greenhouses, pergolas, and landscaping timbers
H5	Outside, in-ground and contact with or in water	Subject to extreme wetting and leaching and/or where the critical use requires a higher level of protection	Severe decay, borers and termites	Retaining walls, piling, house stumps, building poles, cooling tower fill
H6	Marine water exposure	Subject to prolonged immersion in sea water	Marine wood borers and decay	Boat hulls, marine piles, jetty timbers

Water can accelerate degradation. The low hazard classes are for environments which are continually kept dry. The high hazard classes are for service conditions that can involve significant contact with water. A key point for the wise use of timber is to keep it dry.