It depends on headcount and, in Ontario's case, project duration — and the threshold is not the same in every province, so don't carry one jurisdiction's number to another.
Short answer
In Ontario, a joint health and safety committee (JHSC) is required where a workplace "regularly employs 20 or more workers," per the Ministry's own guide, or on a "construction project on which 20 or more workers are regularly employed and expected to last three months or more." Below that, if more than 5 workers are regularly employed, you need a single health and safety representative instead — not a full committee.
The Ministry's guide sets it out as a table: 20 to 49 workers requires a committee with at least two members; 50 or more requires at least four. Workplaces where the worker count "regularly exceeds five" but stays under the 20-worker JHSC trigger must instead have the workers select one person as a health and safety representative. Sites with five or fewer regularly employed workers need neither.
Where a JHSC is required on a construction project of the size and duration above, the committee must also establish a worker trades committee, and at least two of its members — one worker-side, one management-side — must complete certification training, with refresher training required every three years.
This is where a lot of multi-province contractors get tripped up: the CCOHS jurisdictional summary shows most of Canada sets the JHSC trigger at 20 or more workers, matching Ontario — but Saskatchewan requires one at 10 workers, half Ontario's threshold. A crew that's comfortably under the requirement on an Ontario job can trip the requirement the moment the same headcount works a Saskatchewan site.
"Regularly employed" is doing more work in that threshold than the headcount on any given day. A site that peaks at 25 workers for two weeks of a six-month job, with a steady core of 12, is judged on the number regularly on-site, not the peak. It's also worth checking who counts as the constructor on the project before assuming the JHSC obligation sits with you rather than the party who hired you, and whether a worker raising the committee question is a protected activity if discipline is on the table around the same time.
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