PERTHThe 25mm gap that decides whether your termite barrier works
AS 3660 governs termite management in Australian buildings. What it covers, what it deliberately does not, and the inspection zone that gets built over on
Perth sits in one of the higher termite-pressure regions in the country. Most people know that. What far fewer buyers know is that the standard governing termite management says nothing at all about inspecting for them, and that the barrier protecting their house probably has something built over it.
Three standards, and they do different jobs
The termite management family is AS 3660, and the parts matter.
AS 3660.1 covers new building work — the design and construction of subterranean termite management systems for new buildings. It is referenced in the National Construction Code, so it is not advisory. The current edition is 2014, reissued with an amendment in 2017.
AS 3660.2 covers the treatment of existing buildings. Different problem, different document.
AS 3660.3 covers assessment of systems and components that the first part does not detail.
None of them is the inspection standard. That is AS 4349.3, which is what a timber pest inspection reports against, and it is a separate exercise with a separate scope from the building inspection under AS 4349.1. Three standards, three jobs, and an operator who blurs them is telling you something about how they work.
AS 3660.1 is also explicit that maintenance and inspection procedures sit outside its scope. The standard builds the system. Keeping it working is somebody else's chapter.
What the system is actually protecting
Subterranean termites reach timber from the soil. Where they cannot get at it directly they build mud-walled shelter tubes across surfaces they cannot eat, including up the face of a concrete footing. Those tubes are the thing an inspector is hunting for, because they are frequently the only visible evidence of a colony that has been working inside a wall for a year.
They can also do without the soil entirely. Where there is a permanent moisture source inside a building — a leaking shower waste, a dripping cold water line under a slab — a colony can establish inside the structure with no ground contact at all. That is why a plumbing leak in a timber-framed wall is treated as a pest problem as well as a water problem.
The 25 millimetre rule, and why it matters to you
Here is the part that turns up on inspection after inspection.
AS 3660.1 requires that attachments and items adjacent to a building be separated from it by a gap of at least 25 millimetres, so there is a clear, uninterrupted view across the inspection zone. The list is long and it is ordinary: steps, verandas, porches, access ramps, carports, trellises, decks, hot water systems, air conditioners, downpipes, service pipes.
The reason is simple. The barrier works by forcing termites into the open where they have to build a visible tube to get past it. Cover that strip and the barrier is still there, still installed correctly, and completely useless, because nobody can see the evidence it was designed to produce.
Now think about what happens to a Perth house in its first fifteen years. A paved alfresco poured hard against the wall. A raised garden bed. Retaining that brings soil up over the slab edge. A concrete pad for the air conditioner. A deck. Every one of those is a normal, sensible improvement, and several of them quietly bury the inspection zone.
We check that strip on every timber pest inspection, and where it is covered we say so plainly, because "no evidence of termite activity" means something quite different when a third of the perimeter could not be seen.
If you are building
The system goes in early and gets covered by everything that follows, which makes it one of the strongest arguments for staged inspections. At slab stage the barrier is visible. At handover it is not.
Ask for the documentation. A compliant system comes with paperwork identifying what was installed and where, and that paperwork should be handed over. If it was not, that is worth chasing while the builder is still contactable.
If you are buying
A building and timber pest inspection runs the two scopes properly and reports them separately. What you want out of it is not only whether termites are present today, but whether the house has been set up so that anyone could tell.
What the termite management system is really protecting
It is worth being clear about what a termite management system does and does not do, because the misunderstanding of it is where a lot of grief comes from. The system does not make a house termite-proof, and it does not kill or keep away the colonies that live in the surrounding ground. What it does is manage the pathways by which termites can get from the soil into the timber of the house, so that they either cannot get in undetected or are forced into the open where their activity can be seen and dealt with before serious damage is done. It is a management system, in the truest sense: it does not eliminate the threat, which is always present in a high-pressure region like Perth, but it manages the risk to a level a homeowner can live with, provided the system stays intact and is monitored.
This framing matters because it explains why the system is never a set-and-forget installation. A management system that is installed and then never inspected, never maintained, and gradually compromised by landscaping and additions is a management system that has stopped managing. The colonies do not go away; the region's termite pressure does not diminish; and a house whose management system has quietly lapsed is a house whose protection has lapsed with it, even though nothing visible has changed. Understanding the system as ongoing management rather than a one-off barrier is the key to keeping a house protected in a region where the threat is constant.
The clearance rule, and why it matters to you
One of the practical details that a termite management system depends on is maintaining a zone that allows termite activity to be detected, and this is where a specific and easily-defeated requirement comes in. The system is designed on the assumption that certain parts of the structure, particularly the edge of the slab or the base of the walls, remain visible so that any termite workings, the mud leads termites build to bridge from the soil to the timber, can be seen. When that inspection zone is maintained, a termite attempting to get in has to build a visible lead across it, and that lead is the warning sign an inspection or a vigilant owner can catch. When the zone is covered, the termites can cross it concealed, and the whole detection principle is defeated.
This is why something as ordinary as landscaping can undermine a termite management system, and why homeowners defeat their own protection without realising it. A garden bed banked up against the wall, paving laid over the slab edge, soil built up across the inspection zone, mulch piled against the base of the house, any of these can bridge over the very zone the system relies on remaining visible, giving termites a concealed path in and removing the ability to detect them. The management system was designed with a visible inspection zone in mind, and covering that zone, however innocent the landscaping intention, hands the termites exactly the concealed route the system was built to deny them.
What it means whether you are building or buying
If you are building, the termite management system is something to get right at the start, installed properly at the correct stages of construction, documented, and designed so that the inspection zones it relies on can actually be maintained. A system installed well but in a way that makes the inspection zone impossible to keep clear, or that is not documented so that future owners do not know what they have, is a system set up to fail later. The build is the moment to get the system in properly and to record what was installed, because both the installation and the documentation are far harder to reconstruct after the fact.
If you are buying, the termite management system is part of the due diligence, particularly in Perth given the region's high termite pressure. The questions worth answering are what system the house has, whether it has been maintained and inspected, whether the inspection zones have been kept clear or compromised by landscaping, and whether there is documentation to establish all of this. An inspection assesses the house for evidence of termite activity and for the conditions, including compromised inspection zones and moisture problems, that raise the risk, and it will flag where landscaping or additions have bridged the protection or where the conditions invite termites. Combined with the system's documentation obtained from the seller, that gives a buyer a picture of how well the house is actually protected, which in a high-pressure termite region is worth knowing before rather than after purchase.
Common questions
What is AS 3660?
The Australian Standard family for termite management. AS 3660.1 covers new building work, AS 3660.2 covers treatment of existing buildings, and AS 3660.3 covers assessment of systems not detailed in the first part.
Is AS 3660 the inspection standard?
No. Timber pest inspections report against AS 4349.3, which is a separate standard with a separate scope from the building inspection standard AS 4349.1.
Why does a 25mm gap matter around a house?
AS 3660.1 requires attachments and adjacent items to be separated from the building by at least 25mm so the inspection zone stays visible. Covering that strip leaves the barrier installed but unable to show the evidence it is designed to produce.
This comes up on every building & timber pest inspection and building stage inspections we carry out.
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Sources
Read rather than recalled. Summarised here; the documents themselves are Crown copyright and are linked rather than reproduced.