Key takeaways
- Hazard levels are assigned by the biological threat at the point of use, so the same species and grade can carry different treatments depending on whether it is above ground, in ground or in a wet area.
- Preservative retention is the number that matters, not the label on the pack, and retention requirements differ between hazard levels and sometimes between products.
- Every cross-cut, notch, bore and planed face exposes untreated timber, and the protection you lose at those points depends on whether the treatment is a surface envelope or penetrates the full section.
- Storage on site is where a lot of treated timber quietly degrades, because ponding, ground contact and poor air movement can let decay establish before the material is even installed.
- Cut ends and fixings are the two most common points of failure in treated timber work, and both are cheap to get right at the time and expensive to rectify after the frame is closed.
Treated timber is one of the few materials on a residential site that carries its own specification printed on the piece. If you look along the length of a treated board or a piece of treated framing, you will usually find a stamp or a tag that identifies the treatment and the hazard level it has been produced to. That stamp is not decoration. It tells you what biological threats the piece is expected to resist in service, and it tells the certifier and the supplier what the timber was bought for.
The problem is that hazard levels get treated as a purchasing detail rather than a site practice. A builder orders treated timber, the yard delivers treated timber, the frame goes up, and everyone assumes the durability question is answered. It is not answered until the cut ends have been dealt with, the fixings are compatible and the material has survived storage without sitting in a puddle for a fortnight.
This article is about the durability and handling side of treated timber by application. It is not a termite management article; the termite management system, the chemical barrier and the physical management of the slab edge are a separate subject. Here we are looking at what hazard level means, how it maps to where you use the timber, what happens at a cut end, and how to store treated timber so that the treatment you paid for is still doing its job at practical completion.
What a hazard level actually tells you
A hazard level is a statement about the biological threat the timber will face at its point of use. It is not a strength grade, it is not a finish, and it is not a measure of how wet the timber was when it was treated. It is a classification that says, in effect: this piece is intended for a location where the hazards are of this kind and this severity, and it has been treated to a retention that is considered adequate for that location.
The hazards that drive the classification are the ones that destroy timber in service: fungal decay, insect attack including termites, and in some locations marine borers. The severity of those hazards depends on moisture and access. Timber that stays dry and well ventilated is at low risk. Timber in contact with the ground, or in a location that is persistently damp, is at high risk because decay fungi need moisture to establish and termites can move through soil to reach it.
That is why the same species and the same structural grade can be sold at different hazard levels. A piece of framing timber intended for an enclosed wall is not exposed to the same threats as a piece of retaining wall timber that will sit in soil. The treatment specification changes, and so does the retention of preservative in the wood.
When you are checking a delivery, read the stamp for two things: the treatment type and the hazard level. If the pack does not carry that information clearly, or if the stamp is illegible, treat it as a supply problem and resolve it before the timber is cut. Once you have cut treated timber to length, you have removed the evidence and you have created an exposed end at the same time.
Matching hazard level to application on a house
On a typical residential job, treated timber turns up in a handful of places, and the exposure conditions at each of those places are different. Walking the job and listing them before you order is the simplest way to avoid both over-ordering and under-specifying.
Start with anything that touches the ground. Posts, bearers in a subfloor that is close to the ground, retaining walls, garden edging, and any structural member that is embedded or in contact with soil are the highest-risk applications on the site. These need the hazard level that corresponds to in-ground or persistently damp exposure, and the retention that goes with it.
Then look at anything that is exposed above ground but still weather-exposed: deck framing, pergola beams, carport posts, bracing that sits outside the envelope, and fascia or barge boards in some designs. These members get wet, but they also dry because air moves around them. That is a different hazard level again, and it is usually a lower retention than in-ground work.
Finally, look at the enclosed applications. Bottom plates, some wall framing, and certain structural members inside a building envelope are treated for termite management or for moisture risk, but they are not exposed to the weather. The hazard level here is lower again. Getting this split right matters commercially: specifying in-ground retention everywhere is expensive and unnecessary, and specifying an enclosed-level product for a member that sits in soil is the kind of mistake that shows up years later as a failed post or a soft bearer.
The termite management article covers how the treatment interacts with the overall termite management system. For ordering purposes, the point is that hazard level is application-specific. A single delivery of ‘treated timber’ is not a specification.
Retention, penetration and what the label does not tell you
Two pieces of timber can carry the same hazard level and still behave differently if the preservative has been applied differently. The key distinction is between treatment that penetrates the full cross-section and treatment that forms a protective envelope around the outside.
Penetrating treatments are driven into the wood so that the preservative is distributed through the section. If you cut a piece like that, the newly exposed face still contains preservative, because the treatment was not confined to the surface. Envelope treatments, by contrast, rely on the outer zone of the timber being treated. The core may be effectively untreated. Cut through an envelope-treated piece and you have opened a path into unprotected wood.
This is why the handling advice differs between products. For some treated timber, a field-applied supplement at a cut end is a belt-and-braces measure on top of a treatment that already extends through the section. For other products, the supplement is the only protection the cut face will ever have. The manufacturer or supplier should be able to tell you which category your product falls into, and the supplement they recommend for it.
Do not assume that a higher visual retention, more preservative smell, or a darker colour means deeper penetration. The only reliable source of that information is the product documentation and the treatment stamp. If you are unsure, ask before you cut, not after.
It is also worth remembering that retention requirements are not uniform across hazard levels. A product sold for an enclosed application is not automatically suitable for an in-ground application just because it is treated. The retention specified for the more severe exposure is higher, and the piece either meets it or it does not.
Where each hazard level belongs
| Exposure condition | What it means for the timber |
|---|---|
| Fully protected, dry in service | Timber that stays dry and out of the weather; decay and insect attack are unlikely. |
| Protected but with some risk of moisture | Timber inside a building envelope where occasional wetting or high humidity is possible. |
| Exposed above ground, weather-exposed | Timber outdoors with air movement around it, so it dries between wettings. |
| In ground or persistently damp | Timber in soil, in contact with the ground or in a continuously damp location. |
| Severe or marine exposure | Timber in the most aggressive conditions, where the highest retention is needed. |
Cut ends, notches and penetrations
Every time you cross-cut a piece of treated timber, you create an exposed face. Every notch, every bore for a bolt or a pipe, and every planed or sanded surface does the same thing. The question is not whether the exposure matters; it is how much protection has been removed and what you are going to do about it.
The practical response has three parts. First, reduce the number of cuts in critical members by ordering accurate lengths and setting out carefully. A bearer that is cut twice has two exposed ends where one would have done. Second, make the cuts clean. A clean, square cut presents a face that a supplement can wet and penetrate, whereas a ragged or crushed end is harder to treat properly. Third, apply the supplement the product requires, at the time of cutting, not at the end of the week when the frame is up.
There is a temptation to think of the supplement as a complete answer. It is not. A brush-applied supplement at a cut end is a local measure. It does not restore the retention of the parent piece, and it does not compensate for using an under-specified product in the first place. If a member is in a high-hazard location and it has been cut, the supplement is part of good practice, but the starting point still has to be a piece that was specified for that location.
Fixings are the other half of this. Treated timber can be more corrosive to metal fasteners than untreated timber, depending on the treatment chemistry, and the wrong fixing can corrode, stain the timber and lose capacity. The article on galvanic corrosion between dissimilar metals explains the mechanism; for treated timber the practical step is to use the fixing system the manufacturer or supplier nominates for the treatment type, and not to substitute whatever is in the ute.
Where a cut end will be concealed—inside a wall cavity, under a floor, behind a lining—it is easy to skip. That is exactly where it matters, because nobody will see the decay start.
Handling cut ends and penetrations
- 1Set out to limit cutsOrder lengths and set out so you minimise cross-cuts in critical members.
- 2Cut clean and squareA clean cut takes a preservative supplement better than a ragged one.
- 3Identify the treatment typeConfirm whether the product is envelope-treated or penetrated through the section.
- 4Apply the correct supplementUse the supplement the manufacturer or supplier specifies for that treatment.
- 5Do not rely on the supplement aloneA field-applied supplement is not a substitute for the specified retention in the parent piece.
Site storage: where treated timber quietly loses its advantage
Treated timber is not immune to degradation before it is installed. It has been treated to resist biological threats in service, but it can still be damaged by the way it is stored, and wet timber that stays wet is a risk regardless of its treatment.
The basic rules are the same as for any timber on site, with a bit more emphasis on moisture. Keep the pack off the ground on bearers or dunnage so that air can move underneath. Do not leave packs sitting in a low point where water pools after rain. If the timber is covered, cover it so that water runs off and air can still circulate; a tarp draped directly over a pack can trap moisture and create exactly the damp conditions you are trying to avoid.
Think about how long the material will be on site. A pack that is delivered and installed within a few days is a different proposition from a pack that sits through a wet month while the slab is being poured and the frame is being set out. If the programme means the timber will be stored for a while, plan where it goes before the delivery arrives.
In flood-prone areas there is a further consideration. The Australian Building Codes Board guidance on construction of buildings in flood hazard areas and the NSW flood risk management manual deal with the design side of flood exposure, including the choice and detailing of materials that may be inundated. If you are building in a flood hazard area, the durability requirements for timber may be more demanding than the standard application, and the hazard level appropriate for a member in an ordinary house may not be appropriate there. Check the project documentation and, where the requirement is not clear, ask the designer or certifier before you order.
Storage also interacts with handling. Timber that has been dragged through mud, dropped on the ground repeatedly, or used as a temporary prop will have damaged surfaces and dirty ends. Those are the places where decay and insect attack will get a start.
Other site hazards that sit alongside treated timber
Treated timber is not a hazardous substance in the way that some other building materials are, but it is still a material with handling requirements. Cutting and machining treated timber produces dust and offcuts, and the Safe Work Australia model code of practice for construction work sets out the general duty to manage risks on a construction site, including dust and the safe handling of materials. Follow the manufacturer’s safety information for the specific product, and keep the general site controls—dust management, housekeeping and personal protective equipment—in place.
Offcuts and treated timber waste should be managed so they do not end up in the wrong place. Burning treated timber is not an acceptable disposal method, and burying offcuts on site is not either. Follow the waste arrangements for the project.
There is also a lesson to take from how other materials on site are handled when they carry a health risk. The Safe Work Australia code of practice for the safe removal of asbestos exists because a material that is harmless when intact can become a serious hazard when it is disturbed, cut or broken. Treated timber is not in that category, but the underlying principle is familiar: the way you cut, handle and clean up a material determines the risk it presents on site. Apply the same discipline to treated timber as you would to any material with a product safety sheet.
Worked example: a builder in western Sydney
A builder in western Sydney is putting up a ground-floor extension with a timber deck. The deck is attached to the house, part of it is close to the ground, and the design has a couple of posts that land in garden beds. The builder orders treated timber for the deck framing and the posts in one delivery, and the yard supplies a single treated product across the whole order.
On site, the frames are cut to length, the posts are cut to height, and the offcuts go in the bin. The pack sits on the ground beside the house for a week while the slab for the extension is poured. It rains twice. The posts are set, the deck framing goes in, and the job is closed up.
Two things have gone wrong, and neither is visible at practical completion. First, the posts that land in the garden beds are in a higher-hazard location than the deck framing above ground. If the single product supplied was specified for the above-ground application, those posts do not have the retention appropriate for their exposure. Second, the cut ends on the posts and the framing have been left untreated, and the pack spent a week on wet ground. The supplement that should have gone on the cut ends was never applied.
The cost of fixing this is not the cost of the supplement or a tube of sealant. It is the cost of discovering the problem after the deck is finished and the garden is planted, when the only remedy is to replace members. The commercial lesson is that the specification has to be split by application at the ordering stage, and the handling has to be planned at the same time. A treated timber delivery is not a single line item; it is a set of pieces with different jobs and different exposure conditions.
What to check before you order and before you cut
The checks that prevent most treated timber problems are simple, and they happen before the material is in the frame.
- Walk the job and list every location where treated timber will be used, grouped by exposure: in ground or persistently damp, weather-exposed above ground, and enclosed.
- Match each group to a hazard level, and record that on the order so the yard supplies what you asked for.
- Confirm the treatment type for each product, and whether it penetrates the full section or forms an envelope.
- Confirm the field-applied supplement and the fixing system that the manufacturer or supplier recommends for that treatment.
- Plan where the pack will be stored, how it will be kept off the ground, and how it will be covered so water runs off and air still moves.
- On delivery, check the stamp on the timber against the order before anything is cut.
- At cutting, apply the supplement to every cut end, notch, bore and planed face as the work proceeds.
None of this is complicated, and most of it costs nothing but attention. The difficulty is that the consequences of getting it wrong are delayed. A treated member that is under-specified or has an untreated cut end will usually look fine at handover. The failure comes later, and by then the cost falls on whoever has to open up the work to fix it.
If you want to go deeper into the material and system choices that sit around this topic, the comparison of steel and timber framing covers the framing decision, and the guide to how NSW builder CPD works explains how these subjects fit into your continuing professional development. The full course catalogue lists the modules that cover materials, durability and site practice in more detail.
Questions builders ask
What does a treated timber hazard level actually mean?
It describes the biological threat the timber will face at its point of use, such as decay fungi, termites or marine borers. It is not a strength grade and it does not tell you how deeply the preservative has penetrated. Two pieces with the same hazard level can still behave differently if they were treated differently.
Can I use the same treated timber for deck framing and for posts in the ground?
Not necessarily. Timber in ground contact or in a persistently damp location faces a more severe hazard than timber that is exposed above ground, and the retention required for that exposure is higher. Split the order by application so the yard supplies the right product for each location.
Do I have to treat cut ends on treated timber?
Yes, cut ends, notches, bores and planed faces all expose untreated or less-protected timber and should be treated with the supplement the manufacturer or supplier recommends. On envelope-treated products the cut face may have no protection at all until you apply the supplement. A field-applied supplement is not a substitute for specifying the right product in the first place.
How should I store treated timber on site?
Keep packs off the ground on bearers so air can move underneath, and avoid low points where water pools. If you cover the pack, arrange the cover so water runs off while air can still circulate, rather than draping a tarp directly over the timber. Plan the storage location before the delivery arrives, especially if the programme means the timber will sit for a while.
Is treated timber hazardous to work with?
Cutting and machining treated timber produces dust and offcuts, so follow the manufacturer’s safety information for the product and keep the general site controls in place. The Safe Work Australia model code of practice for construction work sets out the general duty to manage risks on site, including dust and material handling. Dispose of offcuts through the project waste arrangements, not by burning or burying them.
Does flood exposure change the treated timber I should use?
It can. Buildings in flood hazard areas may have more demanding durability requirements because materials can be inundated, and the hazard level appropriate for an ordinary house may not be appropriate there. Check the project documentation and the relevant flood guidance, and where the requirement is not clear, ask the designer or certifier before ordering.
Sources
- Construction work: code of practice · Safe Work Australia
- Construction of buildings in flood hazard areas · Australian Building Codes Board
- Flood risk management manual · NSW Government
- How to safely remove asbestos: code of practice · Safe Work Australia
Published 18 September 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
