PERTHFixing timber decay: stop the water, then the wood
Rectifying structural timber decay in Perth: fixing the moisture source first, replacing to correct durability, the engineer and registered builder required
Replacing rotten timber without fixing why it rotted is a repair with an expiry date. The order matters more here than with almost any other defect: find the water first, then deal with the wood. Here is how structural timber decay is properly rectified, the trades required, and the standards that govern the work.
Step one is always the moisture
Decay fungi need timber above roughly 20 per cent moisture content to survive. Drop it below that and the fungus dies, whether or not you replace anything. So the first and non-negotiable step is to find and stop the water: the failed waterproofing, the roof or plumbing leak, the blocked subfloor vents, the drainage discharging against the structure, the ground contact drawing damp into a post.
Skip this and any new timber you install simply starts the same clock. A remediation that replaces a bearer but leaves the damp subfloor that rotted it has not fixed the defect; it has renewed it.
Repairing the timber
Once the source is dealt with, affected timber is cut back to sound material, allowing for the decay having travelled further than the visible damage, especially where a spreading rot is involved. Structural members are replaced or spliced with timber of the correct stress grade and durability for the location. In a genuinely structural member, a bearer, a lintel, a bracing stud, the replacement is sized and connected to carry the load it is taking, which is a design question, not a like-for-like swap.
Where the location is exposed to weather or ground moisture, the replacement timber has to be a durable species under AS 5604 or preservative-treated under the AS/NZS 1604 series, so the new work does not repeat the failure. Temporary propping is often needed while a load-bearing member is out.
Who is required
For a genuinely structural member, a structural engineer should specify the repair, because sizing, grading and connecting a replacement bearer or lintel to carry its load is engineering, and getting it wrong trades a decay problem for a structural one.
A registered building contractor carries out the structural work. In WA, building work over $20,000 must be contracted by a builder registered with the Building Services Board, and structural timber replacement is building work. Verify the registration, per our guide on checking a builder.
Because decay and termites share a cause, a timber pest inspection under AS 4349.3 is worth running alongside the repair, to confirm the moisture that caused the rot has not also brought termites, and to check the termite management system while the area is open.
The standards that govern the work
The controlling standard is AS 1684.2, residential timber-framed construction, which sets the selection, placement and fixing of structural timber for Class 1 and Class 10 buildings under the National Construction Code, and requires durable or treated timber where there is ground contact or moisture exposure. Timber species durability is classified under AS 5604, preservative treatment under the AS/NZS 1604 series, and timber structural design under AS 1720.1. The National Construction Code, published by the Australian Building Codes Board, calls up AS 1684.2 for residential timber framing. A repair carried out to those standards restores the member properly; one that reuses the wrong timber in a wet location is buying time.
What it means for a buyer or owner
If you are buying a house with evidence of structural timber decay, get the building inspection and, in most Perth cases, the timber pest scope with it, then price the repair with the moisture source included in the scope, not just the visible timber. If the work has been done, ask for evidence the water source was fixed and that correct-durability timber was used, because those two omissions are where a cheap repair fails.
Where decay traces to a builder's original use of unsuitable timber or a construction defect on a newer home, it can become a building service complaint or an expert witness matter. The WA disputes process sets out the sequence.
Why you fix the moisture before the timber
The single most important principle in dealing with timber decay is that you address the moisture before you address the timber, because timber decay is caused by fungi that need moisture to live, and replacing decayed timber while leaving the moisture source running simply sets up the new timber to decay in turn. Sound, dry timber does not rot; it is sustained dampness that allows the decay fungi to establish and progress. So the first step in any durable repair is to find and stop the source of the moisture that caused the decay: the leak that was feeding it, the drainage that was directing water against it, the ventilation that was too poor to let it dry, the ground contact that was wicking dampness into it. Stop the water first. That removes the condition the decay depends on. Leave it running, and any repair is temporary.
This is why a rush to replace the visibly rotten timber, without diagnosing why it rotted, is a false economy. The new timber goes into the same wet conditions that destroyed the old, and in time it goes the same way. The diagnosis, working out where the moisture came from and stopping it, is not an optional preliminary to the repair; it is the part that makes the repair last. A good repair sequence is always moisture first, timber second, because the timber repair is only as durable as the dryness of the conditions it goes back into.
Repairing the timber itself
Once the moisture is dealt with, the timber repair depends on how far the decay has gone and what the affected timber is doing. Decay that has compromised a structural member, a bearer, a joist, a stud, a roof member, is a structural repair, and the affected timber has to be either replaced or reinforced so that the structure regains the load-carrying capacity the decay took away. This is not cosmetic work; it is restoring structure, and where structural members are involved it is properly the domain of someone competent in structural repair, working to the relevant standards, because getting it wrong means a structure that looks repaired but does not perform.
The extent of the repair is governed by the extent of the decay, which is often greater than it first appears, because decay progresses inside timber and a member can be substantially decayed within while looking only marginally affected on the surface. A proper repair removes or reinforces back to sound timber, beyond the obviously rotten material, so that no compromised timber is left in place to continue failing. Where the decay is extensive or affects primary structural members, the assessment of how much has to be done, and the design of the repair, benefits from the involvement of someone qualified to make those structural judgements, rather than a patch that addresses only what is visibly rotten.
What it means for a buyer or owner
For a buyer, evidence of timber decay, or of a previous decay repair, is worth understanding properly rather than taking at face value. Active decay in structural timber is a major defect that needs proper repair, and the repair needs to have addressed the moisture cause or it will recur, so a previous repair that patched the timber without stopping the water may be hiding a continuing problem. The key questions are the same ones that govern the repair: what caused the decay, has the cause been stopped, and has the affected structural timber been properly restored. A visual inspection can identify the signs of decay and the moisture conditions that drive it, and flag where structural timber is affected, but the full extent within concealed or structural members can require a closer, sometimes invasive, look.
For an owner who has decay, the message is to resist the temptation to simply cut out and replace the rotten wood. Find and fix the moisture first, then repair the timber to the extent the decay actually requires, and where structural members are involved, get the assessment and the repair done by someone competent to restore structure rather than merely replace timber. It is more work than a patch. But it is the difference between fixing the problem and re-creating it. In Perth, where subfloors, wet-area framing, and roof spaces are the common decay locations and moisture is always the root cause, the discipline of moisture-first is what separates a repair that lasts from one that quietly rots again out of sight.
Common questions
What is the first step in fixing timber decay?
Stopping the moisture. Decay fungi need timber above about 20% moisture content, so cutting off the water source starves the fungus. Replacing timber without fixing the water simply restarts the decay.
Who repairs structural timber decay?
A structural engineer specifies the repair of a load-bearing member and a registered building contractor carries it out. A timber pest inspection under AS 4349.3 is worth running alongside, since decay and termites share a moisture cause.
What standard governs timber repair?
AS 1684.2, residential timber-framed construction, with timber durability under AS 5604, preservative treatment under AS/NZS 1604, and structural design under AS 1720.1, all under the National Construction Code.
This comes up on every building & timber pest inspection we carry out.
Read next: the name on the sign may not be the business building your house and structural timber decay: a moisture problem in a timber costume.
Sources
Read rather than recalled. Summarised here; the documents themselves are Crown copyright and are linked rather than reproduced.