How to Fix Mould on Plywood Bracing

We are often asked by customers:

"We are nearly ready for plastering, but have discovered white and black mould on the plywood bracing after the frame was exposed to the weather. What is the best way to treat it permanently, and will diluted bleach be effective?"

Discovering white or black mould on plywood bracing just before plasterboard is installed is something that should be addressed, not simply covered up.  The good news is that visible mould on plywood does not automatically mean the bracing has lost its structural integrity. In many cases, it is simply a sign that the timber remained damp for too long. The priority is to identify why the mould developed, confirm the plywood is still structurally sound, remove the contamination correctly, and ensure the frame is dry before the walls are enclosed.  
 

Is Bleach the Right Solution?

In short: No, not on plywood.

This is where a lot of advice online becomes misleading.

Plywood is porous. Bleach may lighten or remove the visible black staining while leaving fungal growth below the surface, allowing it to return. Authorities like Queensland Health state that bleach may not be effective on porous surfaces, while manufacturers specifically advise against bleach on wood for this reason. Chlorine-based products can also create corrosion concerns around nails, straps, and other metal connectors.  

NSW Health does permit diluted bleach as a secondary treatment for stubborn household mould, but that is general surface-cleaning guidance. For structural plywood bracing, the timber-specific construction guidance is more relevant.  

For structural plywood, treating the moisture problem is far more important than trying to sterilise the surface.

Step 1: Stop the Plastering

Do not install plasterboard until the issue has been investigated.

Once the wall is closed, inspecting or treating the plywood becomes far more difficult. More importantly, if moisture remains trapped behind the plasterboard, the plasterboard paper itself can become another food source for mould growth. Timber construction guidance recommends checking that framing has dried adequately before linings are installed.

Step 2: Find Out Why the Mould Appeared

Mould is a symptom, not the problem itself.

Common causes include:

• prolonged exposure to rain during construction
• delayed roof installation
• poor temporary weather protection
• water entering around windows or flashings
• plumbing leaks
• inadequate ventilation during drying

If the moisture source is not eliminated, no chemical treatment will permanently solve the issue. WorkSafe Queensland recommends fixing the water source as the first step in any mould remediation process.

Step 3: Check Whether the Plywood Is Still Structurally Sound

Surface mould is one thing. Structural damage is another.

Inspect every sheet carefully, paying particular attention to the edges and fixing points.

Look for:

• delaminated veneers
• swollen or expanded sheet edges
• soft or spongy timber
• loose or corroded fasteners
• lifting around nails or screws
• signs of prolonged saturation

If any of these are present, the plywood should be assessed by the builder, structural engineer or plywood supplier before plasterboard is installed. Cleaning cannot repair structural deterioration.

Step 4: Dry the Frame Properly

The objective is not simply to make the timber feel dry - it must actually be dry.

Use natural ventilation together with fans or dehumidifiers if required. Timber framing guidance recommends that framing should be at or below approximately 16% moisture content before internal linings are installed. A moisture meter should be used on the framing rather than relying on appearance alone.

Drying is the most important step in preventing mould from returning.

Step 5: Clean the Mould Correctly

For a small localised area, WorkSafe Queensland recommends:

• isolating the work area
• wearing a properly fitted P2 respirator, gloves, and eye protection
• cleaning using detergent and water or an appropriate cleaning solution
• drying the material completely afterwards

Avoid dry brushing, household vacuum cleaners, compressed air, aggressive sanding, and pressure washing.

These methods can spread mould spores throughout the building.

Step 6: Consider a Timber Mould Treatment

Once the plywood has been cleaned and dried, many builders choose to apply a timber-compatible mould treatment designed for porous timber products.

If you choose this approach:

• use a product specifically approved for timber
• follow the manufacturer's application instructions
• allow the plywood to dry completely before enclosing the wall

The treatment should be considered an additional protective measure - not a substitute for drying the structure.

When Should the Plywood Be Replaced?

Replacement is generally the better option if:

• veneers are separating
• the sheet has swollen significantly
• decay has started
• fasteners have lost their holding capacity
• the plywood remains persistently wet
• mould returns despite proper drying

Structural bracing is too important to gamble on. If there is any doubt about its condition, replacement is usually less expensive than rectifying problems after the house is completed.

The Biggest Mistake Builders Make

The most common mistake is focusing on killing mould instead of removing moisture.

No spray - whether bleach, vinegar or a commercial mould killer - can permanently prevent mould if the timber remains damp.

Long-term success comes from four simple steps:

1. eliminate the moisture source
2. verify the plywood is structurally sound
3. clean the visible contamination correctly
4. ensure the frame is completely dry before plasterboard is installed

Follow those principles, and the mould is unlikely to return once the building is enclosed.