Mould in Garages and Sheds: Why It Matters and What to Do

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Mould in a garage or shed is common across the Illawarra, and for most homeowners it stays an inconvenience rather than a serious building problem. But when the garage shares a wall with a bedroom or living space, or when belongings stored there will return to the house, mould in a garage deserves more than a quick spray-and-forget. If you are noticing a musty smell in a room next to your garage, see our guide on musty smells, headaches and fatigue at home before deciding whether to act. The good news is that garage mould is usually solvable with improved ventilation and sealing, and professional remediation is only warranted in a minority of cases.

Why garages grow mould

Garages and sheds are not built to the same standard as living spaces. They typically have a concrete slab floor, minimal or no wall insulation, large unweatherproofed openings, and little to no mechanical ventilation. In the Illawarra’s coastal climate, that combination reliably produces mould in a garage at some point.

The humid outside air moves freely through gaps around roller doors and corrugated sheeting. Concrete slabs absorb moisture from the soil below and release it as vapour into the air above. In a sealed space with poor air movement, that vapour has nowhere to go.

Common contributing factors in Illawarra garages:

• Water entering under or around roller doors after heavy rain or hosing down the driveway
• Washing machines or hot-water systems installed in the garage releasing steam and condensation
• Wet cars driven in after rain, with moisture evaporating once the door is shut
• Irrigation water splashing against the external wall
• Subfloor drainage failing on a sloped block and ponding near the slab edge

Surface mould on timber wall framing, stored cardboard boxes, fabric items, and the concrete floor itself near the perimeter is the typical result.

When garage mould becomes a house problem

A standalone detached shed with mould is primarily a storage concern. An attached garage is a different situation.

In many Illawarra homes, the garage sits directly under or beside a bedroom, with a shared plasterboard or masonry wall in between. If that wall has penetration gaps, failed seals, or missing cavity insulation, a musty smell from the garage can migrate into the living area.

If a bedroom or lounge beside the garage has a recurring smell without an obvious source of its own, the garage is worth investigating. Hidden mould behind shared walls between a garage and a bedroom is a pattern that turns up regularly in double-brick and brick-veneer homes in suburbs like Fairy Meadow and Corrimal.

We are not overstating the risk: spore migration through a solid wall is not automatic or dramatic. But gaps, penetrations for pipes and cabling, and poorly sealed door frames create pathways worth closing regardless, both for moisture management and fire safety reasons.

Our mould inspection service includes checking shared wall conditions as part of a whole-property review. If you want a professional look at whether the garage is feeding mould into the house, that is the starting point.

Belongings: what to save, what to bin

Deciding what to do with mould-affected belongings stored in the garage is often harder than cleaning the garage itself. The table below gives a practical framework.

The general rule: hard, non-porous items can be cleaned. Soft, porous items with visible mould are rarely worth the effort to restore, particularly for anything that will later come back into the house or into contact with family members.

Ventilation and sealing fixes

Fixing mould in a garage starts with airflow and moisture control, not cleaning product. Cleaning the visible mould without fixing why it appeared gives you a clean-looking garage for a few months before mould returns.

Practical steps that work:

Improve airflow. Fixed louvred vents in opposite walls create passive cross-ventilation. For a fully enclosed garage, a small inline exhaust fan on a humidity sensor is a low-cost option that runs only when needed. Simply propping a side door open for 30 minutes after wet weather makes a measurable short-term difference.

Replace the roller door bottom seal. Brush seals on most roller doors degrade within 5-10 years. A new seal strip meaningfully reduces water and humid air entry and is a straightforward replacement job.

Address concrete floor moisture at the perimeter. A concrete sealer reduces the rate of moisture vapour release from the slab surface. On sloped or low-lying blocks common in Dapto and Albion Park, confirm that surface drainage directs water away from the slab edges rather than towards them.

Check the subfloor if it extends under the garage. In some older Illawarra homes, a partial subfloor void runs under an attached garage section. Our subfloor and roof void service covers these combined jobs where the garage and subfloor moisture problems are linked.

Seal penetrations in shared walls. Any pipe, cable or conduit passing through the wall between the garage and the house should be sealed with an appropriate product. This limits moisture and odour migration as well as improving fire safety.

When professional cleanup is worth it

Professional remediation for a garage makes sense in these scenarios:

• Mould has spread across large areas of framing, wall sheeting or roof structure, not just surface spots on stored items
• The garage shares a wall or ceiling with a sleeping room used by someone with respiratory sensitivities
• There is a suspected moisture source behind the wall (rising damp, subfloor drainage, plumbing penetration) that needs investigation before cleanup
• Remediation is part of a water-damage insurance claim and you need documented scope and clearance

For lighter cases, proper cleaning practice combined with the ventilation fixes above is often sufficient. If you are unsure of the extent, a brief mould inspection before you start cleaning can save you doing the job twice.

For broader guidance on living with the Illawarra’s coastal climate, our preventing mould in coastal homes guide covers ongoing maintenance habits that also reduce garage mould.

The NSW Health mould factsheet is consistent on the fundamental point: mould needs moisture to grow. In a garage, fixing the moisture pathway is the foundation of any lasting solution.

FAQs

Is it safe to use the garage if it has mould on the walls?

Brief, occasional use is generally fine for most people. Spending extended periods in a heavily moulded, enclosed garage is not ideal, particularly for anyone with respiratory sensitivities. If you can smell it strongly, improve ventilation before continuing to use the space as a workshop or storage area.

Why does my garage concrete floor always grow mould spots near the edges?

Concrete slabs in the Illawarra often sit close to natural ground level, and moisture from the soil migrates upward through or around the slab perimeter. The edges and corners are typically the dampest points. Improving perimeter drainage and applying a concrete sealer can reduce this, though full elimination depends on site conditions and proximity to the water table.

My shed is separate from the house. Do I still need to worry about the mould?

If it is fully detached, the mould is primarily a storage and materials concern rather than a health-of-the-house issue. The main practical question is what gets stored there. Items that will be brought back into the house, such as camping gear, seasonal clothing, or tools, should be checked before returning them.

Will anti-mould paint on the garage walls stop it coming back?

Not if the moisture source is still active. Anti-mould paint additives discourage surface fungal growth on an already clean and dry substrate, but they cannot prevent mould returning if the wall is staying damp. Fix the moisture first, then consider a moisture-resistant coating as a finishing step.

Related reading

• Preventing mould in coastal homes
• Hidden mould behind walls: warning signs
• Mould on mattresses, carpets and soft furnishings
• Mould in kitchen cabinets
• Musty smell, headaches and fatigue at home
• Mould behind wardrobes and furniture
• Professional mould remediation service
• Subfloor and roof void mould service
• Mould inspection and testing service