Mould Assessments in Commercial Properties

What Triggers One and What Happens Next

Steve Little

6/30/20263 min read

A tenant reports a musty smell in the southwest corner of the their unit. Maintenance looks around, doesn't see anything obvious, and replaces a stained ceiling tile. Six weeks later, the tenant is back, this time with photos of visible mould growth and a letter from their lawyer.

This scenario plays out more often than it should. Not because property managers are negligent, but because mould in commercial properties is genuinely misunderstood and can be a complex issue and can be a complex issue when it needs to be formally assessed, what the assessment involves, and what the owner's obligations are.

Here's a practical guide.

What Triggers a Formal Mould Assessment?

Not every mould situation requires a formal assessment by a qualified professional. But several circumstances do:

Visible mould growth beyond a minor area. Health Canada guidelines generally treat mould growth covering less than 1 square metre as minor and potentially manageable without professional involvement (depending on the material affected). Growth beyond that threshold typically warrants professional assessment before remediation.

Tenant health complaints. When occupants report symptoms (respiratory irritation, headaches, fatigue) that correlate with time spent in the space, air quality and mould are among the first things to investigate. A documented health complaint changes the legal calculus significantly.

Moisture events. Roof leaks, plumbing failures, HVAC condensation issues, and flooding events all create conditions where hidden mould growth is possible. A formal assessment after a significant moisture event establishes a baseline and identifies remediation needs before they become larger problems.

Pre-construction. Disturbing mould impacted building materials without proper precautions and containment creates airborne exposure for workers and can spread contamination. A pre-construction assessment is good practice and increasingly expected under contractor health and safety requirements.

Post-remediation verification. After mould remediation is completed, a clearance assessment confirms that the work was effective and that the space is safe for re-occupancy.

What Does a Mould Assessment Involve?

A formal mould assessment is conducted by a qualified indoor environmental professional (IEP) or industrial hygienist. The process typically includes:

Visual inspection. A systematic examination of the affected area, and often areas beyond it, looking for visible mould growth, water staining, damaged materials, and conditions that support mould growth (elevated humidity, condensation, organic material).

Moisture mapping. Using moisture meters and thermal imaging, the assessor identifies areas of elevated moisture in building materials (walls, floors, ceilings) that may harbor mould growth that isn't yet visible.

Air sampling. Airborne mould spore sampling, analyzed at an accredited laboratory, provides a baseline for indoor air quality and indicates whether elevated spore levels exist relative to outdoor control samples.

Surface sampling. Bulk or tape-lift samples from mould impacted surfaces confirm the presence and type of mould. This is particularly useful for identifying mould species that may have greater health implications (certain Stachybotrys and Aspergillus species, for example).

Written report. The assessment produces a report documenting findings, mould species identified, affected areas, likely moisture sources, and recommended remediation scope of work.

The Owner's Obligations

In Ontario, the Occupational Health and Safety Act imposes obligations on employers to maintain a workplace free from hazards, and mould that creates health risks falls squarely within that. Property owners who lease to commercial tenants have obligations both as landlords and, where they have employees on the premises, as employers.

For multi-tenant commercial buildings, the Residential Tenancies Act doesn't apply, but lease agreements and the general law of landlord and tenant create obligations to maintain premises in a habitable condition and to respond appropriately to known defects.

The key word is "known." Once a property owner has been made aware of a potential mould issue (through a tenant complaint, a maintenance report, or a previous assessment), inaction creates compounding liability. Documenting the response (investigation, assessment, remediation) is as important as the remediation itself.

After the Assessment: Remediation

Mould remediation in commercial properties should follow established protocols: Health Canada's guidance on indoor mould, IICRC S520 (Standard for Professional Mould Remediation), and New York City Department of Health guidelines, Environmental Abatement Council of Canada (EACC) Mould Abatement Guidelines, and Canadian Construction Association (CCA) Mould Guidelines for the Canadian Construction Industry are widely referenced.

For larger remediation scopes (areas greater than 1 square metre), containment of the work area, HEPA filtered negative air pressure fans/vacuums, worker PPE, and clearance testing before re-occupancy are standard requirements. The remediation should be performed by a contractor with documented experience in mould remediation, not a general maintenance contractor working outside their expertise.

RiskCheck's Hazmat team conducts mould assessments for commercial and industrial properties across Ontario and Canada, from initial investigation through clearance verification.