PERTHStructural timber decay: a moisture problem in a timber costume
Fungal decay in structural timber in Perth: the 20% moisture threshold, where bearers and framing rot, AS 1684.2 durability requirements
Structural timber that stays wet, rots. Not quickly, and not visibly at first, but a bearer or a wall stud quietly losing strength to fungal decay is a material deterioration defect under AS 4349.1, and in a load-bearing member it becomes a major one. It is also, almost always, a moisture problem wearing a timber costume.
What timber decay is
Timber decay is fungal. Wood-destroying fungi need moisture, oxygen, and a food source, and structural timber is the food. Below about 20 per cent moisture content, timber is too dry for decay fungi to establish. Above it, sustained, they move in. That single threshold is why timber decay is really a water-penetration story: fix the water and you starve the fungus; leave it and no amount of new timber will last.
The two broad forms behave differently. Wet rot stays local to the wet area and stops when the timber dries. Some brown rots can transport moisture and spread into drier timber nearby, which makes them far more serious because the damage travels beyond the obvious leak.
Where it happens in a Perth house
Wherever timber meets sustained moisture. Subfloor bearers and joists over a poorly ventilated or damp void, which is why subfloor ventilation matters so much. Wall framing behind a failed wet-area membrane or a persistent leak. The base of stumps and posts in ground contact. Timber built in against damp masonry with no separation. Roof timbers under a long-standing roof leak.
AS 1684.2, the residential timber-framed construction standard, addresses this at the source: structural timber in ground contact, or unprotected from weather and moisture, has to be a durable species class under AS 5604 or preservative-treated under the AS/NZS 1604 series. Decay in those locations often traces back to the wrong timber having been used where durability was required.
Why it hides
Decayed timber can look sound. The surface holds while the interior softens, so a bearer that has lost much of its strength can present a convincing face until it is probed. Inspectors test suspect timber by sounding and probing precisely because appearance is unreliable, and because the members that matter most, under floors and inside walls, are the ones you cannot see from a doorway.
Decay and termites also travel together. Both need moisture, so the conditions that rot a bearer are the conditions that draw a termite colony, which is why a building and timber pest inspection reads them as one problem with two symptoms.
When it is a major structural defect
Under AS 4349.1 clause 1.4.10, decay is major once a load-bearing member has lost enough strength to threaten safety, utility or further deterioration. A rotten bearer under a main load path, a decayed stud in a bracing wall, or a post base that has lost its footing on the ground all qualify. And because decay is moisture-driven and progressive, it keeps advancing until the water is cut off, so it rarely stabilises on its own.
What an inspection tells you
We report decay found by probing and sounding accessible timber, the moisture readings that explain it, and the water source wherever it can be traced, because the source is the real finding. Where structural members are affected, the report says plainly that the extent and the repair need to be established, since a visual inspection cannot see inside a wall or under an intact floor. How it is fixed, and by whom, is covered in our guide on rectifying structural timber decay.
What decay actually is, and what it needs
Timber decay is the breakdown of wood by fungi, and understanding that simple fact tells you almost everything about where to look for it and how to prevent it. The fungi that rot timber need moisture to establish and grow; dry timber does not decay. This is why timber decay in a house is always, at root, a moisture story: find the timber that is staying wet, and you have found where decay will be, and find the source of the water, and you have found the way to stop it. Sound, dry structural timber can last the life of a building; the same timber kept damp will be broken down, and the loss of strength can be significant well before the damage is obvious.
It is worth distinguishing decay from the other timber problem people worry about, termite attack, because they are different things with different signatures, even though both destroy structural timber and both thrive in the same damp conditions. Termites eat the timber; decay fungi rot it. An inspection looks for both, and the conditions that invite one, moisture, poor ventilation, timber in contact with damp ground, tend to invite the other, which is why a damp subfloor or a persistent leak is a concern on two fronts at once.
Where a Perth house stays wet enough to rot
Decay concentrates wherever timber and sustained moisture meet, and in a Perth house that is a fairly predictable set of places. The subfloor of an older house with a suspended timber floor is the classic location: if the ground beneath is damp and the ventilation is poor or blocked, the bearers and joists can stay wet enough to decay, out of sight, for years. Wet areas are another: timber framing around a bathroom that a failed waterproofing membrane has been quietly feeding with water. Roof spaces where a leak has been getting in. External timber, decking, posts, window joinery, where exposure and poor drainage let water sit. And anywhere timber is in direct contact with damp ground or masonry that wicks moisture up into it.
The common factor is always sustained dampness, and the common cause is usually a failure of the things that are supposed to keep timber dry: drainage that has stopped working, ventilation that has been blocked, a leak that has been left, a detail that lets water sit against wood. This is why an inspection assessing decay looks at the moisture conditions as much as the timber itself, because the timber that is dry today but sitting in conditions that will wet it is timber that will decay tomorrow.
Why it hides, and what the inspection can honestly say
Timber decay is dangerous partly because it hides so well. It happens in the concealed parts of a house, subfloors, wall cavities, roof spaces, and inside the timber itself, so that a member can look sound on the surface while it is substantially decayed within. A visual, non-invasive inspection cannot cut into timber or open up concealed spaces, which means the decay it can confirm directly is the decay that has reached a visible or accessible surface, or that a moisture meter and a careful look at the conditions suggest. This is a real limitation, and an honest report is clear about it.
What a good inspection does within that limit is significant, though. It gets into the accessible subfloor and roof space where so much decay lives, rather than glancing from a hatch. It reads the moisture conditions and the drainage and ventilation that predict decay. It identifies the visible evidence and, where the conditions point to a high likelihood of concealed decay, it says so and recommends the further, more invasive investigation warranted. Where decay has reached the point of compromising a structural member's capacity, it is a major defect and is reported as one, with a recommendation for the appropriate specialist. The honest position is that a visual inspection finds a great deal of decay and flags the risk of the rest, and that where the stakes are high, it points clearly at the closer look that only opening things up can provide.
Common questions
What causes structural timber to rot?
Fungal decay, which needs timber sustained above about 20% moisture content. Below that, decay fungi cannot establish, which is why timber decay is really a moisture problem.
Where does timber decay happen in a house?
Subfloor bearers and joists over damp voids, wall framing behind failed waterproofing or leaks, post bases in ground contact, and roof timbers under long-standing leaks.
Is timber decay a major structural defect?
Under AS 4349.1 clause 1.4.10 it is major once a load-bearing member has lost enough strength to threaten safety or utility. Because it is moisture-driven and progressive, it rarely stabilises on its own.
This comes up on every building & timber pest inspection and pre-purchase building inspection we carry out.
Read next: the little grille in the brickwork is doing a job and fixing timber decay: stop the water, then the wood.
Sources
Read rather than recalled. Summarised here; the documents themselves are Crown copyright and are linked rather than reproduced.