Mould on timber framing, floor joists, roof rafters and joinery is a different problem from surface mould on tiles or plasterboard. Timber is porous, mould hyphae (root structures) penetrate into the wood, not just across the surface, and the structural integrity of the timber is at stake alongside the health issue. Treating it correctly requires understanding the depth of penetration, the species present and whether the timber has been structurally compromised.
This guide covers assessment, treatment approaches and the decision point between treating and replacing affected timber.
Types of Timber Affected and Their Risk Profiles
Not all timber in a home carries the same mould risk. The most significant structural concerns arise in:
Floor bearers and joists, horizontal structural members that support floor loads. In Illawarra’s older homes with subfloor voids (Wollongong suburbs, Dapto, Thirroul), bearers and joists are directly exposed to ground moisture and poorly ventilated air. Mould on structural floor timbers is common and can progress to structural decay (wood rot, which often accompanies severe mould growth) if left untreated.
Wall plates and bottom plates, the horizontal timber members at the base of wall frames, sitting on or near the concrete slab or subfloor. These are the first to be affected by rising damp and ground moisture migration.
Roof rafters and ceiling joists, in homes with inadequate roof sarking or roof leaks, rafters develop mould from condensation or water ingress. Mould on roof timbers is less visible than subfloor mould but equally structurally significant.
Timber joinery, window frames, door frames and skirting boards in wet areas or on external walls. These are lower structural risk but affect aesthetics and seal integrity.
Structural posts and columns, timber posts supporting decks or subfloor voids can develop mould and decay where they contact the ground or sit in water-collecting post holes.
Assessing the Extent of Mould in Timber
Surface mould, mould that has colonised the timber face without penetrating deeply, looks fluffy, powdery or discoloured and can be scraped off with a knife, revealing clean wood underneath. This is treatable.
Deep penetration mould, mould with hyphae 3-10+ mm into the wood, may show as staining that extends below the surface, and the wood in these areas has often started to break down. Test with a bradawl or sharp screwdriver: if the point penetrates easily to more than 10 mm depth, the wood has been softened by fungal decay.
When assessing structural timbers, moisture meter readings are essential. Timber above 20% moisture content (wood moisture equivalent) supports active mould growth. Readings above 25-28% indicate the timber has been wet for extended periods and structural assessment is needed.
| Timber Condition | Moisture % (WME) | Treatment Approach |
|---|---|---|
| Surface mould, sound timber | 14-20% | Clean, treat with antimicrobial, address moisture source |
| Moderate penetration, firm timber | 18-25% | Aggressive cleaning, penetrating fungicide, monitor |
| Deep penetration, softening timber | 25-30%+ | Treat if load is not critical; consider replacement |
| Significant decay, structural compromise | Any | Replace, no chemical treatment restores structural capacity |
Treatment Protocol for Mould on Timber
Step 1: Dry the Timber
Treatment is most effective when timber moisture is reduced below 20% first. Accelerate drying with dehumidifiers, fans and ventilation, addressing the moisture source simultaneously. Treating mould on actively wet timber is an exercise in futility, new growth will immediately follow.
Step 2: Remove Loose and Surface Mould
Wire brushing (HEPA vacuum to capture released spores) removes the loose surface mould layer. This step is essential before applying treatment products, applying fungicide over a thick mould layer reduces penetration to the wood surface.
Use P2 respirator, nitrile gloves and eye protection for this step. In enclosed spaces (subfloor voids, roof voids) additional PPE may be warranted.
Step 3: Apply Antimicrobial Treatment
Quaternary ammonium compounds (QACs): Water-based antimicrobial solutions that kill mould on contact. Apply by brush, spray or sponge to fully wet the surface. Allow dwell time per manufacturer instructions (typically 10-30 minutes) then wipe or allow to dry.
Borax solution: Borax (sodium tetraborate) dissolved in water (1 cup per 4 litres) is effective and relatively safe for the operator. Apply to timber and allow to dry without rinsing, residual borax continues to inhibit regrowth.
Penetrating fungicides: Products specifically formulated to penetrate timber and kill subsurface mould. These typically contain copper or zinc compounds and are applied by brush with sufficient product to wet the surface and penetrate the grain. Particularly important for structural timbers with known deep penetration.
Encapsulants: After treatment and drying, a paint-on encapsulant seals treated timber surfaces, contains any residual spores and slows future moisture uptake. Used routinely on treated subfloor timbers.
Step 4: Address the Moisture Source
This must happen in parallel with treatment, not after. Without fixing the moisture source, groundwater, roof leak, failed vapour barrier, inadequate ventilation, treated timber will be recolonised within months.
Step 5: Monitor and Re-Inspect
Structural timbers that have had significant mould treatment should be re-inspected 3 months after treatment to confirm no regrowth. Include a moisture reading to verify the moisture source has been effectively controlled.
When to Replace Rather Than Treat
Treating mould on timber is appropriate when:
- The timber is structurally sound (firm, not soft or punky)
- The mould does not penetrate more than 3-5 mm
- The moisture source can be definitively addressed
Replace timber when:
- Structural capacity is compromised. Decay alongside mould reduces load-bearing capacity. A structural engineer should assess timbers showing significant decay in load-bearing positions.
- Mould penetration exceeds 20-30% of timber cross-section. At this point, the structural contribution of the member is reduced.
- The timber species is not naturally durable. Radiata pine framing (common in post-1960 homes) has minimal natural decay resistance and deteriorates faster than hardwood species. Heavily mould-affected radiata pine structural members often cannot be saved economically.
- The timber cannot dry below 20% moisture. If the moisture source cannot be addressed, persistent rising damp, groundwater, chronic condensation, replacement into a better-detailed position is necessary.
Specific Situations in Illawarra Homes
Subfloor joists in Wollongong and Dapto homes: Very common treatment scenario. Access via the subfloor hatch. Treatment is cost-effective for light-to-moderate mould on sound joists. For heavy mould in homes with poor subfloor access (very low clearance, steep sites), the labour cost of treatment may approach or exceed replacement cost for affected members.
Roof rafters from roof leaks: Roof void access is typically via ceiling inspection hatch. Treatment of rafters near previous leak points is standard after the leak is fixed. Confirm the leak is resolved before treating, testing the repair through a rain event or hose test before spending on remediation is sensible.
Window frames and reveals: Painted hardwood or aluminium-wrapped timber frames are usually treatable. Raw or poorly sealed timber on exposed southerly-facing windows, common in older Thirroul and Austinmer homes, often has deep penetration and may need replacement along with improved window design.
Frequently Asked Questions
Can I paint over mould on timber framing? No. Painting over mould does not kill it, the mould grows through paint and the paint coating is simply wasted. Treat and allow to dry fully before any paint or encapsulant application.
My subfloor timbers have a lot of mould, will treatment stop them from decaying? Mould treatment addresses mould fungi. Wood decay (rot) is caused by different fungal species, specifically brown rot and white rot fungi, that work at lower moisture levels. If your subfloor has significant mould, the moisture conditions also support decay fungi. Treating mould and fixing moisture stops both. A structural assessment by a building inspector is warranted if there’s any concern about decay having already progressed.
Is it safe to stay in the house while subfloor timbers are being treated? For treatment of the subfloor void specifically, the main risk is product vapour migration upward into living spaces. Professional remediators seal subfloor access hatches and ensure ventilation during treatment. For DIY treatment in a subfloor void, keep the home well-ventilated and use appropriate products.
How do I find mould in roof rafters without accessing the roof void? Thermal imaging during a mould inspection can identify moisture-affected areas in the roof structure from below, without requiring access. Any moisture-rich areas identified can then be confirmed by inspection through the ceiling hatch or by a roofer from above.
Do I need a structural engineer report if my floor joists have mould? For significant mould on load-bearing floor joists, particularly in older homes where timbers may already be undersize by modern standards, a structural engineer’s opinion on the remaining capacity is worthwhile before deciding treat vs replace. This is particularly the case if floors feel springy or soft, which suggests advanced decay alongside the mould.
Mould on structural timbers in your Illawarra home needs specialist assessment. Contact Illawarra Mould Removal for a professional inspection that evaluates both the mould and the structural condition of affected timbers.