Attic Mould

Attic mould is fungal growth on the underside of roof sheathing, rafters and trusses. It usually appears as black, grey or white spotting or staining, often heaviest on north-facing slopes, near the eaves, or around a bathroom fan or other moisture source. Mould needs moisture, a food source (wood and paper facings) and suitable temperatures; in attics the controllable factor is moisture.

Sources of moisture

• Indoor air leakage. Warm, humid air rising from the house through gaps in the ceiling, light fittings, attic hatches, plumbing and wiring holes and top plates. In cold weather it condenses on the cold sheathing.
• Exhaust fans and dryer vents that discharge into the attic, are disconnected, or have leaking ducts.
• Roof leaks at flashings, penetrations and damaged roofing.
• Blocked ventilation, for example insulation pushed against soffit vents.
• Damp construction materials or high humidity from crawlspaces and basements.

Ventilation alone is not the cure

Building-science writers and home inspectors repeatedly note that adding ventilation does not reliably stop attic mould if humid indoor air keeps leaking in. In some climates, extra ventilation brings in humid outdoor air that can make conditions worse on cold nights. The recommended order is to stop the moisture source first: air seal the ceiling plane, route all exhaust ducts outdoors, fix leaks, and then make sure intake and exhaust vents work as designed.

Assessment and remediation

Light, old staining on dry wood may be inactive; a moisture meter helps check whether the wood is currently wet. Active growth, odours or extensive coverage usually call for professional assessment. Remediation methods include cleaning, abrasive or dry-ice blasting and treatment with appropriate products, followed by fixing the moisture cause. Badly damaged sheathing is replaced, often during reroofing. People with health concerns should seek guidance from local health authorities before disturbing mould.