Garages and workshops get less attention than living areas when it comes to mould, but they’re often significantly worse, and mould in a garage affects the tools, vehicles, stored items and structural integrity of the space, as well as potentially spreading spores into the adjoining house through doorways and wall penetrations.
Illawarra’s humid climate means garages without active moisture management become mould incubators. This guide covers the specific causes, prevention strategies and treatment approaches for garage and workshop mould.
Why Garages Are High-Risk Mould Environments
Slab moisture. Most garages have a concrete slab without a DPC or vapour barrier. Ground moisture migrates upward through the concrete and evaporates into the garage air. This moisture wets stored items sitting on the floor, causes dust to become a mould-growth substrate on floor surfaces, and raises overall humidity in the space.
Temperature cycling. Garages heat up in summer afternoons and cool quickly in the evening. This cycling creates condensation on cool metal surfaces, tool boxes, car bodies, steel shelving, motorcycles, that appears as surface moisture. In humid conditions, this condensation sustains mould.
Poor ventilation. Most garages have a single large roller door and perhaps a small fixed window. When the door is closed, air exchange stops entirely. Any moisture produced, from a wet car, stored damp items, timber stored there, accumulates.
Stored organic materials. Cardboard boxes, timber scraps, old magazines, fabric items, canvas bags, garages are filled with the organic materials mould uses as food. In a moist, poorly ventilated space, these become mould colonies.
Condensation on vehicles. Cars driven in from rain bring surface moisture into the garage. As the car cools, condensation forms on the undercarriage and wheel wells, then drips onto the floor. In a poorly ventilated garage, this moisture doesn’t evaporate quickly.
Common Mould Locations in Garages
| Location | Why It’s Affected | Signs |
|---|---|---|
| Ceiling near rear wall | Poor ventilation, structural timber | Black staining on rafters/ceiling panels |
| Plasterboard internal walls | Shared with house, moisture from both sides | Discolouration, paint peeling |
| Stored cardboard and paper | Cellulose food source + floor moisture | Visible mould growth on boxes |
| Wall bases and skirting | Concrete slab edge moisture wicking | Band of mould at floor level |
| Timber frames around internal door | Moisture from both sides | Mould on door frame, door edge |
| Steel shelf legs and metal surfaces | Condensation | Orange rust + surface mould |
Prevention Strategies
Concrete Floor Moisture Control
Penetrating concrete sealer, applied to the garage floor, this reduces but doesn’t eliminate moisture migration. It also makes the floor easier to clean and more resistant to staining. Reapplication every 3-5 years maintains effectiveness.
Epoxy floor coating, a more complete solution. A properly applied epoxy coating creates a near-impermeable surface over the concrete slab, dramatically reducing moisture evaporation into the garage air. Quality two-part epoxy systems are durable and long-lasting. Preparation (grinding or acid etching the concrete) is critical for adhesion.
Anti-fatigue mat management, rubber floor mats are comfortable for working but trap moisture underneath. If rubber mats are used, lift and dry them regularly. Moisture trapped under a rubber mat for weeks is a guaranteed mould event.
Ventilation Improvement
Passive louvred vents, installing louvred ventilation panels in the side walls near the roofline and lower in the wall allows natural convective air movement. Hot air rises and escapes through the high vents, drawing cooler air in from below.
Exhaust fan with timer, a standard exhaust fan switched to run for 30-60 minutes after the car is put away removes the moisture spike from a wet vehicle. This is particularly effective in attached garages.
Ridge vent, for pitched-roof garages, a continuous ridge vent significantly improves air movement in the roof space, keeping rafters drier and reducing ceiling mould.
Storage Practices
Off-floor storage, items stored directly on the concrete slab accumulate moisture from below. Store items on shelving, pallets or plastic feet where possible.
Plastic bins instead of cardboard boxes, plastic storage tubs with lids protect contents from moisture and don’t provide a mould food source in the way cardboard does. A transition from cardboard to plastic is one of the most effective garage mould prevention steps.
Dehumidifier, for garages used as workshops where controlling humidity is important for tools, materials or projects, a 15-25 L/day dehumidifier run during working hours keeps humidity manageable. In Wollongong’s humid summer months, this is particularly worthwhile for woodworking shops.
Regular clearing, a cluttered garage has poor air circulation around stored items. Annual clearing and decluttering improves the ventilation patterns within the space.
Vehicle Management
Dry the car before storing where possible, wiping down a very wet car before closing the garage door makes a measurable difference to the moisture load, particularly for enclosed garages.
Leave the door slightly open after driving in, if security allows, leaving the roller door up 150-200 mm for an hour after putting a wet car away allows the moisture spike to disperse.
Drip trays, a drip tray under the car catch area catches water that runs off the vehicle and prevents it from spreading across the floor.
Treating Existing Mould in a Garage
Assessment
Before treating, identify all affected areas, garage mould is often more extensive than the visible patches suggest. Check:
- Roof void if accessible
- Stud cavities in any plasterboard walls
- Behind stored items (pull everything away from walls)
- Under any rubber or fabric floor items
Treatment Approach
Surface mould on concrete, scrub with a stiff brush and dilute bleach solution (1:10), allow dwell time, rinse and dry thoroughly. For large areas, a pressure washer is effective. Allow full drying before applying any sealer.
Mould on timber framing/rafters, treat with appropriate antimicrobial products and penetrating fungicide. See our dedicated guide on mould on timber framing for the detailed approach.
Mould on plasterboard walls (shared with house), this requires attention because mould on the garage side of a shared wall can colonise the wall cavity and emerge on the living area side. Professional mould remediation is appropriate for plasterboard wall mould that has any indication of spread into the cavity.
Fogging for widespread contamination, antimicrobial fogging is effective for treating widespread surface mould and elevated spore counts in a garage environment. Particularly useful when mould is distributed across many surfaces, rafters, wall faces, stored items in place, that would be difficult to manually treat individually.
Workshop-Specific Considerations
Wood dust and shavings, in a woodworking workshop, sawdust accumulates on surfaces and in corners. When damp, this dust becomes an ideal mould substrate. Regular vacuum cleaning with a HEPA filter vacuum is essential.
Paint and solvent storage, avoid storing these near areas with active mould, some solvents can interfere with antimicrobial treatment products.
Metal tools, surface mould on metal tools from condensation appears as fine grey-green growth on steel surfaces. Treat with an antimicrobial spray and a light machine oil coat to protect against recurrence. Corrosion occurs alongside mould on metal that has been repeatedly condensation-wetted.
Frequently Asked Questions
Does mould in the garage spread into the house? If the garage is attached to the house and shares a wall, yes, mould can colonise the shared wall cavity and eventually emerge on the living area side. Spores also migrate through the door between garage and house. Garage mould should be treated with the same priority as living area mould.
My garage has a musty smell but I can’t see any mould, where is it? Check the roof void (accessible through a ceiling hatch), behind stored items against walls, under rubber floor mats, and on the back face of any timber shelf uprights against the wall. Mould in these locations is very common and often not visible from a normal standing position.
Is it worth waterproofing a garage floor? For a garage used as a workshop, the answer is yes, moisture control dramatically improves the environment for tools, projects and the structure. For a basic vehicle storage garage, it depends on the moisture severity. An epoxy floor coating is a long-term investment that pays off in reduced mould treatment costs and preserved stored items.
Can I use the same mould sprays I use in the bathroom on a garage? For surface mould on non-porous garage surfaces, standard antimicrobial spray products work. For timber or mould with deep penetration into porous surfaces, you need a penetrating fungicide designed for structural applications. The bathroom sprays don’t have the penetration characteristics needed for timber.
My tools are rusting and I’m seeing some mould, is a dehumidifier enough? Rusting indicates the relative humidity in the garage is consistently above 70-80%. A dehumidifier will help, but make sure it’s adequately sized for the garage volume and sealed envelope. Also address the concrete floor moisture, a dehumidifier working against continuous moisture evaporation from the slab will run constantly and may not get humidity low enough. Seal the slab as well.
Mould in your Illawarra garage or workshop affecting tools, materials or your adjoining home? Contact Illawarra Mould Removal for a professional assessment and treatment.